Month: February 2019

This tech tip is written by experienced tech and VRF / VRV specialist Ryan Findley. Thanks, Ryan! (Note: Ryan refers to VRV rather than VRF because he specializes in Daikin and these articles are written from a Daikin VRV perspective)

This tech tip will cover the different modes of operation of a VRV system along with some of the unique control features of the VRV machines.

First, before we discuss how the machines accomplish the actual heating and cooling of the space, let’s define a few terms that we use frequently.  

Target Evap/Target Cond– VRV machines have target condensing temperatures and target evaporator temperatures that change based on load.  It’s not uncommon to see some of these numbers get pretty far off what a standard a/c will run. The only reason to hook gauges to one of these machines is to verify the accuracy of the pressure transducers.

4 Way Valve– Reversing valve.  There are multiple 4-way valves on these machines.  VRV 3 has 2 (a heat exchanger 4-way valve and the dual pipe 4-way valve), and VRV 4 has 3 (2 heat exchanger (outdoor coils are split coils), and one dual pipe).

Thermistor– A temperature sensing device that alters its resistance based on temperature. There are multiple different types of thermistors used across the product lines. Refer to your service manual or service app for temperature->resistance charts.

A1P Board– The A1P board is the conductor of the machine.  It is the one talking to all the other boards and also the board where the majority of the thermistor inputs and solenoid coil outputs come from.


Before we discuss the individual modes of operation, I’m going to cover how the bs boxes work to set the mode of operation for the fan coil.

With this diagram from the service manual, we can see how the refrigerant is routed through the bs box.  The top is showing a unit in cooling mode and calling for cooling, the middle unit is in heating mode and calling for heating while the bottom unit is in heating mode, but the thermostat is satisfied.


Now that we have the foundation let’s get into the modes of operation.

Cooling Mode– No need to make it any more complicated than it is.  The liquid line is always the liquid line, the dedicated suction line is active, and the dual gas line is also a suction line.  EEVs in the indoor units are in are controlling to a target superheat at the outlet of the fan coil. This number is typically 9 degrees, but there are instances where that number can change.  The farther off setpoint, the target superheat will be lower. The opposite is true the closer you get to setpoint. The outdoor unit(s) will leave the heat exchanger 4-way valves de-energize and energize the dual gas four-way valve to use the dual gas line as an additional suction line.  Outdoor fans will modulate to maintain a steady discharge pressure. Here’s a visual aid from the VRV 3 service manual. This is taken from a heat recovery machine.

Parallel Operation– This mode of operation is used when both heating and cooling are needed.  The liquid line and suction line remain dedicated to those tasks, so in this mode, the dual gas line will be a discharge line.  It accomplishes this by leaving the dual gas pipe 4-way valve de-energized. Outdoor heat exchanger control is determined by the load.  

Heating– During the heating operation, the system will energize the heat exchanger 4-way valve and leave the dual pipe 4-way valve de-energized.  The dual gas line carries discharge gas to the bs boxes. The suction line is inactive in this mode. During this mode, the indoor EEVs control to subcooling control. Target subcooling in the indoor units is typically 9 degrees.  The farther off setpoint it gets, the lower the target subcooling goes. The opposite is true as it approaches setpoint.

Defrost- Defrost is initiated based on demand.  The machine is looking for at three conditions that will cause a defrost:

  1. Small temperature drop across the outdoor heat exchanger.
  2. If there is a temperature drop at the outlet of the heat exchanger.
  3. If the low pressure stays low enough over a 2 hour period.

Defrost will terminate if:

  1. The max defrost time is 5 minutes and 30 seconds or 6 minutes have elapsed (depending on model).
  2. Heat exchanger temp rises above 57.8 degrees for more than 90 consecutive seconds.
  3. Condenser pressure rises above 440.8 psi.

During defrost, the modules maintain whatever mode they were in when defrost was initiated.  If this is a multi-chassis system, the units will alternate who is in defrost to continue to provide heating.  As each module finishes, the next one initiates. Fan coils continue to operate during this period. Fan coils in heating mode take their fan speeds to the lowest speed, if any fan coil happens to be in the off mode then the fans will turn off.  EEVs move to 160 pulses or 224 pulses depending on the model of the outdoor unit.

Oil Return– Oil return is used to bring oil back to the outdoor units.  Since oil is an insulator along with a lubricant, oil return is necessary to maximize the efficiency of the indoor heat exchangers. Oil return is initiated if:

  1. Run time of 8 hours or 2 hours after a power cycle.
  2. On-demand based on target evap, target condenser, and compressor speeds.

Oil return terminates in cooling mode either after 5 minutes or if the suction line temperature minus the target evaporator becomes less than 41 degrees.  

Oil return terminates in heating mode if evaporator pressure drops below 31.9 psi or 9 minutes.

Backup Operation– If a unit faults out on a compressor related fault, if the indoor units are turned off then back on by the centralized controller the machine will go into automatic backup operation.  The unit will run with the offending compressor disabled for 24 hours then try to run it again. To get out of backup operation, cycle power to the outdoor units.

Emergency Operation- This is used when there is a need to lock out a compressor until you can get back to it to replace it.  In VRV 3, the entire module will be locked out. With VRV 4, you can lock out individual compressors or the entire module.  Emergency operation can be found in mode 2->38, 39, and 40.

— Ryan

Remote Ice Machines

Why would someone want to take an already complicated machine and make it even more complicated? The answer: human arrogance and engineer’s hubris. (just kidding) Fortunately, thousands of these ice machines have been installed and serviced- let’s take a little overview of a few of the critical differences and things to consider when working on remote ice machines versus self-contained models. We’ll be covering Manitowoc remote series in this article, as other brands usually have very similar setups.

What’s the difference?
Remote Ice Machines are a good option in places where water-cooled ice machines are not a good option due to poor water quality, and self-contained air-cooled machines are neither desirable nor practical- such as restaurant lobbies, kitchens saturated with grease, or in areas where you cannot reject heat without affecting the air conditioning load.

There are several different types of ‘split system’ ice machines. Some have only the condenser coil, condenser fan, and head pressure control valve in the remote unit. Others have the entire traditional condensing unit including the compressor, receiver, and accumulator outside. Higher capacity systems require larger compressors and components which can take up valuable space, which is something many restaurants have a limited amount of.

How do they work?
Almost the same as regular self-contained ice machines. The refrigeration cycle does not change (much) with the addition of a remote condenser or condensing unit. There are, however, some key components added to the system which are not typically present in smaller self-contained ice machines.

Here is a refrigerant piping schematic from a self-contained Manitowoc S-Series Ice machine:

Notice that the refrigeration piping is very similar to a typical refrigeration system, but with the addition of a hot gas bypass line from the compressor discharge line to the evaporator inlet after the TXV. This method is used in the harvest cycle to divert hot gas from the compressor to the evaporator grid to allow the ice to drop. In a remote ice machine, there are several more components.
Some extra components often include:

⦁ Head Pressure Control Valve (Headmaster)
⦁ Accumulator
⦁ Receiver
⦁ Liquid Line Solenoid Valve
⦁ Harvest Pressure Regulating Valve
⦁ Harvest Pressure Regulating Solenoid
Note the differences in the following piping schematic for Manitowoc Remote S-Series machines:

In this diagram, you can see there are some extra considerations to take into account when troubleshooting these remote machines. Since the remote condensing units often are exposed to varying ambient conditions, they must be equipped with a Head Pressure Control Valve (headmaster) to maintain a minimum pressure in the liquid line to the expansion valve (typically 180 psi) in low ambient conditions. The liquid receiver stores “extra” refrigerant to be used by the system in low ambient conditions. If charged correctly, the receiver is designed to maintain sufficient charge in the system to operate down to approximately -20°f ambient. These components are typical in any refrigeration system exposed to wide outdoor ambient temperature swings, but there is a set of parts that are not typically encountered: the Harvest Pressure Regulator and Solenoid.

HPR Valve and Solenoid

During the harvest cycle, the hot gas valve is energized, and discharge gas is piped directly to the evaporator inlet via the hot gas bypass line to assist in harvesting the ice. For this to work, there must be a sufficient amount of heat maintained to be rejected long enough for the ice to drop within the 3-minute harvest cycle. At times, there may not be enough heat to keep the hot gas bypass line warm enough to efficiently harvest the ice in the evaporator, and the high-temperature, high-pressure vapor can begin to condense into a liquid in the evaporator- causing liquid to enter the suction line and the compressor. As we all know, compressors are not fond of trying to compress a liquid; so Manitowoc solved this problem by adding the HPR valve.

During the harvest cycle, the HPR solenoid is energized, allowing vapor from the top of the receiver to meet the HPR valve. The HPR valve is similar to the Headmaster, in that it modulates refrigerant to maintain a minimum pressure. If the suction pressure falls too low during the harvest cycle, we lose the ability to transfer sufficient heat to the evaporator- the HPR keeps the suction pressure high enough through the harvest cycle to maintain the heat required for proper harvest. A simple enough concept, yet often overlooked by many technicians not familiar with remote systems.

Not so different after all

As with any system you are unfamiliar with or don’t have much experience working on, never be afraid to consult tech support or search online for a service manual to read through. Service manuals for all the major ice machine brands can be found online. When it comes to ice machines, knowing the sequence of operations is the key to successfully troubleshooting any make or model. Remote ice machines are not much different from typical refrigeration systems in the way they function. Patience is a must for any technician who has the pleasure of working on ice machines- remember always to be thorough and observant.

— Austin Higgins

How an AC System got to be Oversized (Maybe)

But My Old Unit Worked Fine?!

Most of us have heard this at some point. This complaint comes typically from a particularly unhappy customer after the installation of a brand new AC system. Throughout this article we’re going to explore the possible root causes of this situation, but first some ground rules:

  • We are going to navigate this issue from the perspective and comfort needs of climate zone 1…in other words it’s hot and humid!
  • Also, the intention is to describe the impact that progressive accumulation of a set of factors can have on the comfort (or lack thereof) of people within a building envelope over time, which means that, while a hypothetical story, it is based on actual events plus some reasonable assumptions.

So, going back to our complaint.

The complaint comes from whom we’re going to call homeowners C. The C’s just bought their first house about seven years ago. The home was ready to move in, but they did have to replace the old dark shingle roof shortly after. The new roof is much more cosmetically appealing with the light tile they used instead. It is a 1980 production home, and with the converted garage it adds up to about 1,450 square feet. Is not much, but it’s home, and they’re proud of it. So much so that when the old AC system started having some problems a few months back, they decided it was time for a new one.

Through the reference of friends/family, they got your number and requested a free estimate. You showed up on time, wore your booties, petted their dog and had the right answers for all their questions. “You had me at 0% interest!” said the husband while the wife asks “What are all these returns you said we need?”. She only asked because you’ve pointed out how all three bedrooms and the converted garage have only the undercut on the doors as return air paths.

The existing unit is a 14-year-old, 4-ton split system. “Do you guys feel this system keeps the house comfortable?” you asked. “Sure,” they said, “but we can probably use a bigger one. It gets warm in here every time we have a party”.  But you know better than that, you convince them to stay with the 4 ton, but as an act of good faith, you’ll run a new refrigeration line set. It’s only 50 feet, but the existing suction line is 3/4,” and you know that the manufacturer recommends a 7/8” for better system capacity and performance. The liquid line will, of course, be 3/8”.

So, the system gets replaced including the new lines and the returns are added for all four spaces. That’s when the fun starts! For months you’ve had to go back every 2 or 3 weeks, all to address the same chief complaint. They feel uncomfortable no matter how low they set the temperature. You’ve tried replacing and relocating the thermostat. You’ve also been in touch with the manufacturer a few times and confirmed that the subcooling and superheat are within range. The TESP is high, but that’s “OK” because this new air handler has a constant CFM motor (2.3 or 3.0) and according to the fan performance table the unit is indeed moving enough air.

“What did I do wrong??” you say to yourself. Nothing, but the C’s are very upset! Right now, that 0% interest doesn’t feel as sexy as it did back when you were petting their dog. What’s worse, they are blaming you for it! They are convinced that all their predicaments started when the new system was installed, but little do they know that it was many years in the making.

From the top

It’s 1980 and the house is brand new. It’s a slab on grade, 1,200 square feet initially with 9’ ceilings, 3/2 with an adjacent garage. A dark shingle roof housing the ductwork in a ventilated attic and the back of the house faces West. The back yard can be admired from inside the house since there are some seriously large glass sliding doors. A load calc gets done on the house, and an engineer estimates that a 3-ton system will work (perhaps a smaller one would have worked too). The indoor unit it’s installed in a closet with a louvered door inside the conditioned space.

Enter homeowners A. The A’s are a mild-mannered semi-retired couple and some peace it’s all they’re after at this stage in their lives. They like the new house and add a back porch where they can hang out and BBQ occasionally. But the now porch covered back of the house that faces West gets too hot in the afternoon. “Let’s plant a couple of trees dear,” she says while the husband changes the subject with the hope that she’ll forget about it. The trees get planted and the perspective of enjoying some shade in the afternoon looks promising.

Fast forward 15 years and among other things it looks like it’s time for a new AC system. Mrs. A has been thinking about converting the garage into a room so her husband can watch sports and drink beer undisturbed. She hasn’t however decided on the wallpaper yet, but it’s happening. I addition, as the system has aged, it has decreased performance and can’t seem to keep up on the hot summer days which can only mean one thing…they need a bigger unit!

A 3 ½ ton system gets installed. Same ductwork but a new 8” supply duct gets added to the garage. “That’s 200 CFM for you, Mr. A,” said the installing contractor as he got into his truck and drove off. The garage finally gets converted into a room. The garage door gets removed, and a block wall goes up instead. A Brand new TV, minibar and a nice comfortable chair but no dedicated return air path to go with that new supply vent. Just the door undercut. The conditioned space is now roughly 1450 square feet (don’t know the aspect ratio of the garage; let’s use round numbers).

As the turn of the century approaches, traffic has been getting hectic, and there were a couple of near hits last hurricane season. The A’s have had it and are ready to sell the house and move on! By now those young trees that Mr. A reluctantly planted are towering over the back of the house providing cool shade.

The cosmetic inclined home remodel

Cue homeowners B. Now the B’s are in it to win it, they have some capital to burn and show up sledgehammer in hand. It’s time for some serious remodeling.

They want the house to look modern but also want it to be safer. As such they rip out all the windows and replace them with low SHGC, low E, impact glass windows. As for the sliding glass doors, they replace them with smaller, wood framed double doors and a similar type of glass as the windows. Now they don’t have to board up the house three times every hurricane season, they thought.

For the interior, among other things they get a new kitchen and add a 100 CFM exhaust hood. To give the house the ultimate modern look, they replace all the existing lighting with ceiling recessed lights. About two dozen of those evil things throughout the house! No LED bulbs, they are the vented cans type so they can dissipate the heat up into the attic by convection.

When all the construction work is done, they move in and crank down the air conditioner. But something’s wrong…”We made all these improvements to the house, new, more efficient windows and doors and the thermostat won’t go below 76?! It must be time for a new AC unit.”

As they’re walking through the local big chain store of home goods one day, they see a kiosk where they are promised a better life by way of installing a new “Comfort System” for their home. “Just what we need!” they said. The B’s secure a conveniently prompt, free visit from the ‘Comfort Advisor/Specialist/Commission Based Former Car Salesman.’ This so-called specialist shows up at their house, and after listening to their problem, he concludes that their unit must be too small.

“You need a 4 ton for this house I am sure. Not only that, because today is Wednesday we are going to give you a complementary UV Light kit that is going to clean the air in your house” LOL!

The system gets replaced with a 4 ton. The owners still have some comfort issues, but it does feel more refreshing in the house, and so, they move on with their lives.

Fast forward a decade or so, and now they no longer like it there, one of them got a job offer out of town, they don’t like their local Sedano’s supermarket, traffic got worse – I don’t know, you pick the reason.

The C’s buy the house from the B’s and here you are, with a brand new, perfectly operational 4 ton AC system and a very unhappy customer.

Where did it go wrong?

By now you probably know where I’m going with this, but let’s try to break it down anyway:

1 – How the block load decreased:

Beware of the shade. The West and south facing sides are the two with the most heat gain incidence. By building a porch over the Westside and adding trees, a good chunk of the block load got chopped off.

In addition to that, the fenestration factor of the load was profoundly affected by upgrading the windows and doors. The more energy efficient windows and doors installed have also decreased the amount of heat that the AC system was supposed to handle.

Lastly, there is a new roof. As compared to shingles (especially dark colored ones), roof tiles will reflect some heat as opposed to absorbing it. This keeps the roof surface cooler and therefore, also the attic. A cooler attic will result in a cooler ceiling and of course, less heat gain. I highly recommend this excellent research study by the FSEC on the subject.

2 – How the cooling load increased:

As the AC system kept getting bigger, the lack of return consistently worked to offset the decrease of the block load.

The table below can be found on page 28 of ACCA’s Manual D.

You can also read this article by Allison Bailes quick before you keep going that way it’ll make more sense.

Remember when in anticipation of the garage being converted into a conditioned space, how the system was oversized by the ½ ton? How come that didn’t cause any trouble?

Because we lost about 100 CFM of return air from the former garage and replaced it with unconditioned infiltration air coming into the envelope through every crack and crevice. If a supply vent brings in 160-180 CFM of cold air and all you have as return is a door undercut of 1” on a 36” door, on an isolated room you’ll only get 80 CFM back. Because this is not a tight envelope, the balance will likely come from the attic through unsealed access doors and ancillary spaces. Therefore, this tonnage increased was pretty much a wash and an extra expense on the utility bill.

But remember how the car salesman/comfort specialist sold homeowners B a 4 ton? And they upgraded all those windows and doors…how come that didn’t cause any trouble?

Let’s start with the consequences of connecting a 4-ton air handler to the existing ductwork designed for a 3 ton.

The old 4-ton unit had a PSC evaporator motor. Connecting the larger air handler to the smaller air distribution system will cause the TESP to go up, but it’s also going to move more air than the previous, smaller unit. It will likely not move enough air for the system to deliver its 4-ton nominal capacity, but it will move more air none the less.

If the duct configuration remains unaltered, this, in turn, will proportionally increase the airflow out of each vent. Meaning that if you had 100 CFM coming out of a given supply vent and the total system airflow is increased by 20%, then there’d be 120 CFM approximately being delivered by that same vent.

But what does that mean? Remember how we can only get about 80 CFM worth of return air? Now there is more airflow going into each room with a door closed, but we are getting less a percentage of it back. If for example there were four bedrooms with the doors closed, and the total combined airflow for these rooms was 700 CFM then we are getting 320 CFM of it back or 46%. If with the bigger unit the total airflow went up by 20% then there are 840 CFM going in but still, only 320 CFM coming back which is now 38%. Consequently, the pressure in the central open area is even lower and there is a higher pressure differential driver bringing unconditioned air into the building envelope. If there isn’t a low resistance path for the return air to travel from the isolated rooms to the core/main return area, then upsizing the system will result in more infiltration, not necessarily more cooling of the space.

I get that these doors may not always be closed, but when they are this is going to happen. These rooms not being isolated all the time will intermittently mask this condition and make it much harder to diagnose.

To add insult to injury, the reverse stack effect has kicked in.  Remember what else the modern homeowners B added? High hat can lights! And these aren’t the LED sealed can ones available nowadays. No, these are the evil ones, full of little openings. Not only is the pressure differential between the attic and the conditioned space higher, but now there is a highway for the reverse stack effect of bringing nasty, humid attic air into the space.

Lastly, there is the occasional use of the kitchen exhaust hood at 100 CFM without makeup air and the more than likely small percentage of duct leakage to factor in. At this point, it wouldn’t be unthinkable to have at least 5 – 10% duct leakage. Duct leakage has the same space depressurization effect as the lack of return mentioned above (for different reasons). If you want to look at some hard numbers on its consequences, check this article by Neil Comparetto.

But wait for a second! Isn’t this an air conditioner?! If the air conditioner is bigger, then it should be able to handle more significant loads.

Correct, higher loads of design conditions at 80 °F DB and 67 °F WB, not of infiltration air at 75 °F Dew Point! Imagine that you were trying to thread the needle and every time you miss the hole somebody kept handing you a thicker thread.

The capacity of the system went up

So now is the present day and it’s your turn to screw up. Of course, you meant well and wanted to do the best possible job but, you were doomed right out of the gate. If you see a 4-ton system and the consumers tell you that it has been keeping the house comfortable, then what do you replace it with? A newer, shinier and more efficient 4-ton system…with a constant CFM ECM (not the same as a continuous torque) evaporator motor…and a larger diameter suction line…and added low resistance return air paths to all the isolated rooms.

The new ECM motor will deliver the design airflow against up to 1” WC for most manufacturers. This means that though at an energy penalty, now the system is moving as much air as it was designed to do, so the system capacity improved over the old unit.

With the larger diameter suction line, you got about 2.5% of total capacity back over the old system with the 3/4 line. About 1,000 BTU/H, not much but the system capacity did go up.

Adding returns to all the infamous isolated rooms put it over the top. Now the pressure differential between these rooms and the core return area is nearly if not zero. Because of this, the system is no longer pulling in as much infiltration air as it was before. All the volume that is being delivered by the supply vents is now smoothly making it back to the air handler in the form of cool, crisp return air. And this is where it gets hairy.

Because the block load of the house was decreased by the new roof, new windows and doors and the shading of the trees, and now, because there is less infiltration of unconditioned air, there is more air moving across the evaporator and the total system capacity went up, there is less of a load for the AC system to handle which makes it…(drum roll)… oversized!

Sure the ceiling recessed lights are still there and the reverse stack effect is still doing its thing but, in light of the new circumstances, the occupants will have double down on those to have a chance at the system, not short cyclin – as much.

By now you, the installing contractor have figured it out. You take a big gulp, brace yourself for impact and walk up to poor homeowners C. After an elaborate but oversimplified explanation of the situation, you gather the courage and confess to them: “Mam, this unit is oversized” …. pause for effect… “But My Old Unit Worked Fine?!”


— Genry Garcia

A refrigerant is anything we use to move heat from one place to another using the compression refrigeration circuit, however, the history of refrigerants and the different kinds is quite diverse and interesting.

Have you ever noticed how your skin feels cool after you apply rubbing alcohol to it? For a long time scientists and inventors experimented with substances that evaporated easily at atmospheric pressure like ether and alcohol, they noticed that these substances cooled the surface they left when they evaporated away. It was understood that substances remove heat as they boil (change from liquid to vapor) because that is one way our bodies reject heat while sweating. As the sweat evaporates it removes heat from our skin leaving us cooler.

This is known as an “open” process, the alcohol, ether or sweat leave as it cools so you always need more to keep the process going. The trick was to create a process that could be done over and over without losing the “refrigerant” to the atmosphere.

A physician named John Gorrie built one of the first compression refrigeration machines and it used air as the refrigerant. By compressing the air it would increase in temperature and heat could be rejected out of it, he would then “rarify” or depressurize the air which would drop the temperature and allow heat to be absorbed into the air from the water and could… eventually… produce ice.

There were several issues with Dr. Gorrie’s design, one big issue was that while he was using compression and expansion he wasn’t making use of the power of evaporation to greatly increase the amount of heat that could be moved.

It wasn’t long before others began using refrigerants like ammonia, CO2, sulfur dioxide and methyl chloride using the same compressing and expanding that Dr. Gorrie used but with the added benefit of boiling (evaporating) the refrigerant in the evaporator to absorb a maximum amount of heat as well as the change back to liquid (condensing) in the condenser.

As times have progressed refrigerants have changed in order to make them more safe for humans and for the environment. Nowadays refrigerants and refrigerant handling in the USA are regulated by the EPA Section 608. In order to legally handle and service air conditioning and refrigeration in the USA you need to pass the EPA 608 exam and carry the certification card.

So what makes a good refrigerant? 

A Good Refrigerant –

  • Has high latent heat of vaporization (it moves a lot of heat per lb when it boils and condenses)
  • Boils and condenses at temperatures we can easily manipulate with compression
  • Mixes with the oil appropriately so that the oil can do the job of lubrication in the compressor as well as return.
  • Doesn’t blow stuff up or catch on fire
  • Doesn’t poison people
  • Doesn’t hurt the environment
  • Isn’t too expensive

Because we have seen increased environmental regulations over the last 25 years there has been a push to find good refrigerants even if it means going into the flammable and toxic spectrum.

Thankfully, refrigerants are well marked and so long as we pay attention and follow best practices there shouldn’t be any issues.

The markings are pretty simple

A refrigerants have low toxicity

B refrigerants have high toxicity

1 refrigerants have low flammability

2L refrigerants are only “mildly” flammable

2 refrigerants are low flammability but higher than 2L

3 refrigerants are highly flammable

The most common toxic refrigerant is Ammonia and you would generally only find it in old appliances or in large industrial applications.

Propane (R290) is a flammable refrigerant and it is becoming quite popular in small self-contained refrigeration units like vending machines and reach in coolers. These propane units will be very clearly marked and should be handled with extreme caution, especially when electrical sparks or open flame are or could be present.

— Bryan



I got this question via email (edited slightly for length)

Some things I’ve done because I’ve been taught to do them yet I don’t know why I do them. One of those things is putting a jumper between w1/e and w2. Sometimes, in the case of a Goodman for example, I’ve been taught to combine the brown wire along all the whites at the air handler. Do you mind just clarifying the whole situation with w1/e jumped to w2? And also maybe x2 on some stats? Thanks for your help.
— J

Back in the early 2000’s when I was the lead trainer for another company some of the most common miswiring issues has to do with electric heat. So much so that I created a bunch of different wiring diagrams with a fancy program called “Microsoft Paint” to illustrate how to wire different combinations of equipment. Here is one of them.

In older thermostats (older than the diagram shown here) there were no installer setup programs in the thermostat where you could designate the type of system the thermostat was connected to. Each terminal performed a particular universal function and you would configure the operation based on how you wired it up. Which terminals you connected where, which you left open and sometimes, which ones you jumpered out.

So first, let’s give a quick look at the meaning of each terminal

W – When you see a W terminal it just means heat. Usually, you will only see W when the control only has one stage of heat

W1 – Means first stage heat. In a heat pump first stage heat is the same as the first stage cool. It just means the contactor/compressor is turning on. Whether that is heat or cool is actually dictated by whether or not the O/B terminal is energized. This is why on many old thermostats you would jumper Y1 and W1 in a heat pump application.

W2 – Means second stage heat. This could be the first stage of heat strips in a common southern heat pump, the gas furnace backing up the heat pump in a modern “hybrid heat” application or just a second heat strip bank in the case of a straight electric system. W2 is generally called on based on a temperature differential between setpoint and space, outdoor temperature and/or run time.

W3 – Is just the next stage of heat after W2

E – Is emergency heat, usually just a way to manually drive on what would normally be the secondary form of heat without stage 1 heating.

Emergency heat only makes sense when there is some sort of secondary heat source and really even then, it only helps if the secondary heat source is sufficient to heat the space as in the case when the secondary is a furnace, Hydronics or a large heat kit. In Florida, most of our units have 5KW auxiliary heat which will never be sufficient to heat a home in an “emergency”.

Many of these other terminal designations are a holdover from a time when all the controls in the thermostat and defrost board were electromechanical, and much of it was for indication/trouble lights and some of it was for the thermostat to be able to perform staging based on outdoor temperature due to the fact that run time logic was not available. So for your X2 question, have a look at the thermostat and diagram below.

In the modern thermostat, they have usually relegated these staging configurations and terminal designations into the installer setup and every thermostat is a little different. In general, in the south we jumper w2 to E because they truly are the same, in some cases, this does nothing, in others, it just ensures that the aux heat comes on quicker if the user chooses emergency heat.

Are there some cases where emergency heat could be totally different than aux heat? I’m sure… I have just never seen one personally. Like usually, it all comes down to knowing your particular piece of equipment and your controls, reading the installation instruction is a good first step.

— Bryan



This article is written by HVAC contractor and Building Science Whiz, Michael Housh. Thanks Michael!

For a while, I’ve fallen into this camp where I feel Manual J overshoots heating loads. I would like to first off say, that Manual J is only as good as the information you give it, but we often run into this problem where we have to make educated guesses in the field. Most of us will likely assume insulation values in the wall assemblies (based on age, house type, etc.), windows are often an educated guess (they all suck anyway), and the huge infiltration rate (without a blower door number you’re guessing).

Through some conversations with friends, I was inspired to look at my utility bills and see what knowledge I could gain, and I figured I could take you along for the ride. In my experience utility companies have a fairly easy way to look at your usage rates for given periods of time. I pulled up my gas usage for 2018 and start slicing and dicing.

The first thing I did was determine what it costs to heat my domestic hot water by averaging the usage during the summer months, which came out to 15.2 CCF (CCF = volume of 100 cu. Ft. of natural gas). I was then able to adjust the data to a usable format, which is the Therm.

1 Therm = 100,000 BTU’s. The CCF measurement actually equals a Therm if the average heat content of gas is 1,000 BTU in your area, however national averages are typically higher than that. For where I am in Ohio our average heat content of gas is 1,070 BTU’s per CCF, so I was able to calculate the number of Therm’s used for a given month.

For the sake of brevity, I did a quick generic block load (as I normally would) on just the first floor of my house (which is @ 2400 sq. ft.), I also have some basement area that’s finished, but haven’t put it into the model yet. I will spare you on the exercise, but the concept is important to understand, that Manual J is rather linear based on outdoor temperature (a good exercise on your own is to generate a load, manipulate the design temperature to several different values and plot them on a graph).

If looking at just usage vs. Manual J there’s a huge difference, however, my design temperature is 5° and 2018 was a pretty mild winter, so I looked at the historical weather data for 2018 and added them to my spreadsheet. I also did some calculations (gracefully giving my boiler 70% AFUE), to show the approximate output.

My home was primarily built in the 1950s with an addition that was likely in the 1980s. For a home of this nature (and especially without a blower door number), I default to “loose” for an infiltration rate. I then took my worst case month (January), I manipulated the design temperature to 31° in my model, which was the average for that month, and began to adjust the infiltration rate up to “average” and came out with a pretty similar result to my actual usage.

Once satisfied that the model was pretty close to usage, I set the design temperature back to 5°.

This is a 10-15% difference from the original load/model. Now, all of this stuff is just a little thought experiment I decided to let play out, I hope to expand on these ideas further in the future and hopefully spark some more thoughts on this. DO NOT go out and start doing this, but DO start challenging the norm and come to a deeper understanding for yourself.

— Michael

The point of this article is to give you a full understanding of the role fuses, overloads and circuit breakers play in the protection of HVAC/R equipment. If you skim read or jump to conclusions you will be tempted to argue. Be patient, if you want to understand you will need to read all the way through and possibly even watch the videos at the end. This topic is WIDELY misunderstood so the odds are when you first read it you may think I’m crazy. Do your own detailed research once you get to the end if you still dispute what is contained here.

There are a few topics in HVAC/R that get widely confused and result in a lot of misinformation because of the similarity of the concepts. If you have two terms that have SIMILAR meaning but get used interchangeably you can come to completely logical sounding (though totally incorrect) conclusions.

For example, a tech could say a particular circuit is reading “no Ohms to ground” and by that he could mean zero Ohms or he could mean the meter is reading OL which means infinite Ohms.

In the same way, I often hear people say something is “shorted” and what they really mean is it’s not working, or something inexplicable is happening. So let’s define some terms starting with one of the most often confused. Here is the dictionary definition.

Short Circuit

In a device, an electrical circuit of lower resistance than that of a normal circuit, typically resulting from the unintended contact of components and consequent accidental diversion of the current.

When a professional uses the term short or short circuit, they can mean an electrical path with lower than designed resistance or they can also mean any unintended path.
For example, if two conductors in a cable are compromised and touching one another a tech will often say they are shorted even if there is not a low resistance overall in the circuit.
Because of this the term “Short” has become a broad term and must be used carefully.


Place too much a load on

Pretty simple, when you put too much load in the bed of your truck it bottoms out, when you place too much load on an electrical circuit or device, it fails. In the case of a conductor, this load is in the form of amperage, more amperage than designed and the conductor will fail due to overheating.

In the case of a motor, this same thing holds true but actual load (opposing force) on the motor results in increased amperage load which causes increased amperage and overheating.

This is why a compressor with failing bearings will draw higher amperage, the motor slips due to the additional mechanical load, this drops the impedance (resistance) in the motor windings, resulting in higher amperage.

Ground Fault

The momentary, usually accidental, connection of a current carrying conductor to ground or other point of differing potential

A ground fault occurs when an electrical conductor or device that is electrically charged comes in direct contact to ground, a grounded assembly or substance, usually resulting in large current spikes until either a protection device opens the circuit or the circuit itself fails open (breaks) due to heat.

I say USUALLY  because there are cases when a ground fault may exist with no spike in amperage, such as when you are using an ungrounded, two-prong appliance like a hair dryer or an old drill (or a drill that you cut off the ground plug in order to use on a two prong cord). If the internal windings on the device short to the casing there will be no path from the casing to the ground unless something else makes a path, like say , YOUR BODY. Then when your hand touches the drill casing and connects to ground, some current will leak to ground through the very high resistance load that is your flesh and organs. The circuit will not “overload” because it will not be drawing abnormally high amps but you may still die from the incident. This is why ground fault circuit interrupters (GFCI) are used in some high-risk applications, to break the circuit when a ground fault exists, even if that ground fault does not result in an overcurrent condition.

Overcurrent Protection

A form of protection in an electrical circuit that prevents excessive current usually at a predetermined value – Usually refers to a type of protection designed to deal with instantaneous spikes in current

Over-current protection can be used as a broad term that can include circuit breakers, fuses, etc… basically anything that prevents a current from rising above a predetermined value. It CAN be a pretty broad term in some circles, HOWEVER, in the electrical community when overcurrent protection is used it is generally referring to short circuit or ground fault conditions.

Any condition that results in quick, massive spikes in current is addressed by overcurrent protection. If you want to argue read THIS from Siemens.

Overload protection

Overload protection is a protection against a running overcurrent that would cause overheating of the protected equipment

Overload protection deals with higher current resultant from too much current being pulled by a load. When the compressor goes out on overload after one second because it is locked, that is an example of overload.

When a condenser fan goes off after running with a blade that it has too steep of a pitch, that is an example of overload. Overload in a motor is dealt with by the overload in the motor, not by the overcurrent protection/circuit breaker/fuses.  In the case of motor loads specifically, if the overload were to fail, the overcurrent protection would usually break the circuit eventually, but that is not it’s primary design function in most cases. Again, read THIS from Siemens if you are getting riled up at this point.

When a manufacturer writes their system specs and prints their equipment labels they use guidelines provided by the National Electrical Code (NEC) and they refer to articles 430 and 440 of the code to calculate the required minimum conductor size and maximum overcurrent size.  This is how they come up with the MOCP, or Max Breaker / Fuse size and the MCA or minimum circuit ampacity/conductor size required.

Here is some manufacturer electrical data from a Carrier 25HCC condensing unit –

Notice the maximum breaker or fuse is 40 amps on the 4 ton and the MCA is 26.1 with an allowable wire size of  #10 on an assembly rated at 75°C and a 60°C circuit


On this system it is perfectly acceptable by the National Electrical Code and the manufacturer to run #10 wire and a 40 amp breaker so long as the wire run is a properly rated copper conductor under 70 feet (on this specific unit because of spec shown above). Wire length has an impact on voltage drop which is only addressed as a suggestion in the code but is clearly laid out by the manufacturer either directly or based on the minimum voltage if you do a voltage drop calculation. In this case, the voltage must by 197v or above

This is because the circuit breaker or fuse is providing the overcurrent protection (as well as some backup overload protection) and the motor overloads are providing the overload protection.

Now.. If you would like, you are allowed to put in a lower rated breaker than the max so long as you don’t go below the MCA rating because the breaker itself also needs to be able to handle the rated capacity. Just be aware that the lower you go the more likely you will be to have nuisance tripping.

You can also install larger wire if you like, just be aware that in some cases the equipment lugs may not be rated to hold/connect the larger wire. Also, keep in mind that you must also upsize the grounding conductor when arbitrarily upsizing the current carrying conductors.

If you would like some more discussion on the topic you can see the three videos I did on this HERE, HERE , HERE and HERE

Finally, before you start leaving comments about what is written, please see the two videos below. If you watch these videos, read the reference material and STILL think that what I’m saying is false in some way, feel free to join in the conversation.

— Bryan

P.S.- Mike Holt (shown below) is a friend of mine and considered the authority on NEC and electrical training in the US.

In this episode we speak with Embraco about R290 (Propane) refrigerant, Hydrocarbons and what you need to know about them.


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