Category: Tech Tips


In order to maintain combustion (burning) you need three things, fuel, heat and oxygen. If you have all three in the proper proportion you can maintain a continuous state of combustion.

Remove one (or reduce one sufficiently) and the triangle of combustion can collapse.

In a common NG gas furnace the heat is the igniter, the fuel is Natural Gas and the oxygen is provided by combustion air.

Combustion air is literally just the air needed to provide a continuous supply of air for proper combustion (burning). In the case of burning fuels like natural gas our goal is to achieve complete combustion where the end products being vented are CO2 and H2O and this requires the right mix of air and fuel.

For perfect combustion you need about a 10:1 ratio of air to fuel with safe levels of extra air or “excess air” putting us more into the 13.5:1 to 15:1 range.

All gas fired appliances must have both a flue / chimney to exhaust the leftover products of combustion (outlet) as well as combustion air to provide the oxygen for burning (inlet).

In high efficiency furnaces the combustion air is generally piped in, directly from the outside straight into the combustion chamber. This creates a dedicated source of oxygen and also a cleaner install as no other provisions need to be make for combustion air.

In 80% furnaces the burners usually have “open” combustion and they rely on air being drawn into louvers on the furnace cabinet. In this design the space on which the furnace resides must have open communication to the outdoors or other “uncontained” space.

The International Fuel Gas Code requires the following combustion air openings for a room containing combustion appliances:

Vertical opening – One-inch free area for each 4,000 Btu/hr. input of gas burning appliances in the room.

Horizontal duct opening – One-inch free area for each 2,000 Btu/hr. input of gas burning appliances in the room.

Mechanical fan – One CFM of air for each 2,400 Btu/hr. input of gas burning appliances in the room.

Indoor air –  50 cu. ft. of area for each 1,000 Btu/hr. of the appliances.

Not to get into the specifics of code becasue there are lots of specifics that you need to pursue beyond a tip like this, but you must have a dedicated method to get significant air to the furnace to ensure safe and complete combustion.

If you do not, the real possibility exists that the furnace could begin burning improperly creating an unsafe condition for the occupants due to Carbon-monoxide.

Different parts of the country provide combustion air in different ways, but you MUST have some method of providing unlimited fresh air to a furnace or to the room in which the furnace is located. This means when a furnace is in a tight space, ensure you have some sort of significant combustion air.

— Bryan

carrier_defrost_thermostat

When you work on a heat pump system and you want to test defrost there are many different test procedures to follow to test the board and sensors.

Most involve “forcing” a defrost by shorting out pins on the board or advancing the time of the defrost initiation and installing a factory provided pin jumper.

Lots of pins and jumping involved.

But one thing to need to be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost.

A thermostat is an open and closed switch, they are usually round in shape like the one shown above and they open within a set temp range and they close within a set temp range. The one shown above is a Carrier Defrost Thermostat and it closes at 30 degrees +/- 3 degrees and it opens at 65 degrees +/- 5 degrees. In this case, because this particular sensor closes in colder than 32-degree temps you can’t even use a (freshwater) ice bath to test it.

If it is below 32 outside it is easy to test (duh) otherwise you can just run it in heat mode with the outdoor fan off and see when it closes by using an Ohmmeter and testing against a line temperature clamp in the same location.

On a defrost thermostat you can also easily jump it out to test the board since it is just open and closed.

A defrost “sensor” is generally a thermistor. A thermistor changes resistance based on the temperature it is exposed to. In order to test you can measure the ambient temperature, make the sensor is removed and acclimated, measure the Ohms of resistance and compare to the manufacturer chart.

Thermistor

You CANNOT jump out a thermistor with a typical jumper to test.

— Bryan

P.S. – A podcast about Heat pumps is available HERE

belly band crankcase heater

When I first started in the trade as an apprentice we worked on a lot of Trane heat pumps that used crankcase heaters that slid into the compressor sump on the big orange Tyler reciprocating compressors like the one below.

It was very common for these heaters to break off where the wire entered the rod and short against the bottom of the condensing unit. Some of the old timers I worked with would say “This is Florida, we don’t need those things here”, disconnect it and move on.

I later learned that isn’t the correct approach

Systems that have crankcase heaters, have them for a reason and while outdoor ambient temperature is one factor it isn’t the REASON crankcase heaters exist. Refrigerant is attracted to the refrigerant oil in the compressor when the system goes into the off cycle, the amount of refrigerant in the oil and the rate at which it moves into the oil depends on the type of refrigerant and oil and the temperature of the compressor.

When the compressor is off for a while a significant quantity of refrigerant can migrate to the compressor and condense. When the compressor comes on the refrigerant rapidly expands and foams the oil, forcing it out of the compressor and into the system. This is called a “flooded start” and will eventually result in compressor damage due to lack of lubrication, it also decreases system efficiency due to the oil in the system inhibiting the transfer of heat.

Strategies like hard shut off expansion valves, liquid line solenoids help to keep liquid refrigerant out of the compressor and oil separators help to keep the oil in the compressor and out of the systems but the trusty old crankcase heater is still a simple and commonly used strategy to prevent flooded start. If you find one that is failed you would be better off replacing it instead of taking the word of techs who tell you just to cut it out, like I once did.

— Bryan

 

 

 

Take a look at the specs from this Copeland scroll compressor pulled from the Copeland Mobile App (which is an incredible app by the way).

This is a single-phase compressor so the amperages listed are based on an amperage reading from the wire connected to the common terminal.

LRA is locked rotor amperage which is the expected measurable starting amperage and RLA is rated load amps, meaning the amperage it will draw when running normally at its rated load. You may wonder why there are two different RLA ratings here… that’s not what this tech tip is about but if you get the app and click the i with the circle around it you can find out.

The point is we are always taught to measure amperage on common with single-phase motors, but do you know why?

A single-phase motor like the one shown above has three terminals (Common, Start and Run) but only two actual windings (Start and Run). The common terminal is just the “common” point between both of the windings so when we measure the amperage on common we see the total current of both windings.

In tradeschool we learn Ohms law which teaches us

VOLTS = AMPS X OHMS

However, when we try to apply that in the field we realize some things pretty quick that get in the way of applying that neat little formula

Namely –

  • Voltage (and therefore amperage) isn’t fixed in an alternating current so we measure RMS values not ACTUAL peak values
  • The total resistance (impedance) in an inductive (magnetic) load isn’t fixed and is a combination of the static resistance of the windings and the inductive reactance that builds as the magnetic fields expand and collapse and as back EMF is generated when motors spin.
  • Even in a simple DC light bulb circuit we cannot simply measure the resistance of the bulb with a meter and apply ohms law because the resistance of the filament increases as the filament heats up (try it sometime).

So to summarize….

YOU AREN’T GOING TO BE ABLE TO ACCURATELY APPLY OHMS LAW IN THE HVAC/R FIELD

When we measure the ohms of windings from terminal to terminal it is mostly meaningless because the readings are often very low anyway… sometimes so low that your meter becomes inaccurate.

Notice how low the resistances are of this same compressor.

The real resistance of the motor only shows up when it is energized with alternating current and the magnetic fields begin to interact, this total resistance when energized is called impedance.

We do know that the start winding has a higher static ohm value than the run winding and that when we add start to common and run to common together that it will equal run to start (which is a fairly obvious statement since common is just a center point) and that if the thermal overload is open we will measure OL between C-R and C-S but will read the combined value R-S.

These are all true and are reasons to pull out the meter but this still doesn’t tell us anything about the title of this article and you are probably wondering what the heck I’m driving at.

I’m making sure we are all on the same page before I drop a start winding fact bomb on you…

But one more thing we need to come to an agreement on.

The run winding is connected “Across The Line”, in other words with one leg of split phase power connected to Common and the other to run. The current that travels through that run winding is completely a function of the total impedance of that winding which has several factors including the static winding resistance, the inductive reactance of the windings and the back EMF that builds as the motor starts running.

In other words… the amperage starts high because the resistance starts low in the run winding and amperage goes down as the motor gets up to speed because the total impedance increases.

Remember, ohms law teaches us as resistance goes up, amperage goes down if the voltage stays the same.

The start winding is connected through a run capacitor and potentially some other start gear and not connected “across the line” like the run winding. This means that the current that moves through the start winding is limited by BOTH the total impedance of the winding AND the capacitance of the run capacitor and any other start gear.

 

Here is an image from an oscilloscope on this very same compressor referred to above with 197V applied, a proper run capacitor and no hard start kit…

Take a long close look.

Notice the blue line is the RUN WINDING CURRENT and the red line is the START WINDING CURRENT.

Notice that ALL of the true inrush current occurs on the run winding and the start winding current doesn’t go up until the run winding current starts to go down?

That’s because unless the start winding has some form of start capacitor it cannot draw any amperage higher than what the run capacitor will allow. In essence, the run capacitor becomes a ceiling or current limiter that allows only so much stored current per cycle and no more.

Try it sometime.

Measure the running amperage on the start winding with a capacitor slightly larger, slightly smaller and then with none at all. You will see higher amps, lower amps and then (obviously) no amps.

Try taking an inrush reading on the start wire of a compressor with no hard start and see what you get.

Then try it with a hard start.

Notice anything different on the start winding amps? Can you see the moment the back EMF removed the hard start from the circuit? Was the TOTAL amperage actually lower with a hard start or was the time to start decreased and more current shifted to the start winding?

— Bryan

 

 

 

 

  Have you ever wondered why your old refrigerator never needs service, gauges installed, and can run for 30 years that way maybe only needing an occasional cleaning of the condenser?  For crying out loud, utilities are buying these old energy hogs through some programs because they never seem to die. Why do they last so long?  A good evacuation, a correct refrigerant charge, and maintaining a sealed system. Evacuation is the most important part of an installation followed by charge and airflow to assure the efficiency, reliability, and longevity of equipment.   When it comes to evacuation, in an industry plagued with bad information, I do not know why I was surprised to learn that many technicians think it is OK to measure the system vacuum at the vacuum pump. Now I wouldn’t say it’s like driving from the back seat of a car, only because it’s much worse than that. It’s more like driving blind from the trunk.  In my opinion, a senseless practice, and simply poor practice when it comes to proper evacuation. If you hate your micron gauge and think that evacuation is impossible or dark magic,  I think we have stumbled across the reason.   Now I am not going to blame technicians for this, as somewhere along the line, (likely when the marketing department took over-engineering)  the 1/4″ test port on a vacuum pump became a service port for evacuation and at times a mounting point for the vacuum gauge and the pump blank off valve became the isolation valve for vacuum. I know this is true because I have talked to many salesmen at tradeshows who have confirmed this to be their understanding. But here is what is important to understand, the 1/4″ test port is nothing more than that, it is a test port. It is designed to provide a port to test the ultimate pulldown vacuum of a vacuum pump. It is not, and was never intended for evacuation or a permanent location for the vacuum gauge, and the blank-off valve is not intended to isolate the vacuum from the system. Let’s start with some basics. Pressure and vacuum are two completely different sciences and cannot be treated the same. Here is a fundamental example. Given a straw, you could easily blow out a candle from more than a foot away. Take that same straw and try to suck the candle out, and even inches away it is not possible. Vacuum is not directional, if you have ever used a cracked straw, you know they are pretty much useless for drinking. The vacuum pulls from wherever it can. A vacuum is simply a reduction in pressure, and it is strongest at the pump inlet and gets weaker and weaker as it moves away from the pump toward the system. This is due to friction and leakage, which means the vacuum is weakest at the furthermost point away from the pump. The pressure differences are extremely small that create the flow back to the vacuum pump. These pressures are typically as low as .002 psi. Remember, a vacuum is limited by physics. The deepest vacuum we can achieve is -14.996 PSIG. That said, the only way to reduce the friction and increase the flow is larger hoses.  A 1/4″ hose has such a low conductance speed that it should never be used for evacuation. Using a 1/4″ hose chokes the vacuum pump—no matter how big—down to about 0.5 CFM at 1,000 microns. Hoses that are 3/8″ or 1/2″ inside diameter are the smallest that should be used for evacuation. That said, the vacuum at the hose inlet can be much more different than the vacuum at the pump connection, and again, much more different at the far end of the system. But wait, there’s more! Schrader cores provide additional pressure drops, and we all know there are times when
the core depressor either does not open or barely opens the Schrader valve. This will also play havoc in the evacuation process. These pressure drops, again, will make the vacuum at the pump much deeper than it is in the system. The reality is your pump could be at 250 microns while the system is still well over 2,000 microns. This could lead to catastrophic failure of the refrigeration system over time. Vacuum pump blank off valves leak! Vacuum pump blank-off valves are designed for nothing more than keeping the oil in the pump when the pump is turned off with the hoses connected, in the case of a tip-over, and to keep the oil from absorbing large amounts of moisture while the pump is stored. They are not vacuum rated, and if tested with a micron gauge, most–if not all–will creep toward atmospheric pressure in a matter of minutes. This is the primary reason for vacuum trees with vacuum rated ball valves. Those are intended for that purpose. They are not there only to provide a connection point for multiple hoses. The blank-off valve on the evacuation tree, or evacuation manifold, are less than ideal for system decay testing as it also does not isolate the core tools and hoses that are also a significant source of potential leakage, if for no other reason than simply the huge amount of connections. The reason the blank-off should be closed during isolation is to keep the oil from sucking out of the pump when the vacuum rig is under a vacuum and the pump is off. If you are testing the ultimate pulldown of the pump with a micron gauge, closing the blank off prior to shutting down the pump is critical as you will easily pull oil into the vacuum gauge if you shut the pump off before doing so. So what is a technician to do? After all, there is no other convenient place to install the vacuum gauge on a typical system, there are only two ports on the entire residential system. The answer is vacuum rated core tools. Using core tools allows two important things to happen. First, removal of the core which is a significant restriction, and second, it provides a place for the vacuum gauge that allows isolation of the vacuum pump and hoses. While a core tool is not 100% leak-free, down to about 20 microns, a good core tool will not be a significant source of leakage when the valve is closed and the system is isolated. Core tools should be cycled several times during evacuation to release any trapped air around the valve, but aside from that, the only source of leakage during isolation at that point is between the ball valve and the service port on the system. If a non-permeable connector is used for the vacuum gauge, the source of leakage not has been significantly reduced if not, for all intents and purposes, completely eliminated. Just recently I was reading a post online that “vacuum rated” is nothing more than an industry buzz term. Nothing is further from the truth. Everything leaks, even solid copper lines! It is the leak rate that we are concerned with. Vacuum rated defines the performance under a vacuum and clarifies the leak rate of the core tools or the hoses tested. It tells the user the ultimate vacuum that the tool will perform to without the leakage rate overcoming the ability of the pump at the rated micron level. Vacuum rated means it was tested for the process of evacuation and can be expected to perform adequately down to the rated level. This does not mean that standard core tools cannot be used, but it does mean that the user should test them for the intended purpose to assure that they are tight enough to perform without being a significant source of a leak in a vacuum. Because of the hoses, vacuum pump blank-off valve, manifolds, and core tools leak, we need to either remove or isolate as many of these components as we can from the system during the decay test. Just to be clear, evacuating through a manifold is not a great idea either, as the small porting and hoses also are a significant restriction. Core tools and a proper evacuation rig allow a good evacuation to happen. Making a few small changes in your approach to evacuation will make a huge difference in your frustration level, and make the system last years longer. — Jim Bergmann / MeasureQuickPS. Open the gas ballast if equipped before starting the pump, close it when you hit 1500 microns, then open it again after you have isolated the pump/rig for a few seconds prior to the pump shutdown. This will help keep your oil dry, and assure that you achieve maximum vacuum during the evacuation. It will also help your pump last longer.

If you are used to simple, straight cool split systems you know that the low voltage to the outdoor unit is usually VERY simple with just a Y (contactor power) and a C (common) connected to the outdoor unit in many cases. When the condensing unit controls are strictly two-wire low voltage there is no continuous low voltage power so there are also no timers or other logic in the condensing unit. Usually, in these cases, the LV wires connect directly to the contactor coil.

A heat pump needs to be able to switch between heat and cool and defrost which brings in the necessity for more control conductors and complexity.

A heat pump defrost board like most modern controls contain both loads and switches to control different functions.  because it has timers and some basic “logic” the board requires a power supply and for most residential split system boards this power comes from the C (common) and R (hot) terminals from the indoor 24v transformer.

The defrost board also utilizes the constant power on the defrost board R terminal to back feed voltage through the W2 wire back to the secondary heat inside whether it be heat strips, furnace or hydronic secondary heat.

This helps to counteract the cooling effect that occurs when the heat pump when it shifts from heat to cool mode for defrosting. This function is an important thing to test on heat pumps to reduce cold draft complaints during the winter.

Simply force the board into a defrost and check for 24v between w2 and c at the outside board to confirm proper operation or check the secondary heat via ammeter or visual confirmation during the defrost cycle.

— Bryan

As the evening approaches on this All Hallow’s Eve, Reformation day or Halloween (depending on your preference), let us take a moment to focus on some of the truly terrifying elements of our trade, because the Scariest stories are TRUE.

Real Ghost Stories 

The year was 1921 and a wealthy family purchased a new home in quiet part of town. It was a large, old building and the family was excited to live in such a majestic home.

The trouble started almost right away and the lady of the home (referred to only as Mrs. H) began to recount her experiences in the home to her doctor in letters that were later published.

This house was lit by gaslights and had servants quarters and passageways, a perfect house for a haunting. From Mrs. H’s account to her doctor:

“One morning, I heard footsteps in the room over my head. I hurried up the stairs. To my surprise, the room was empty. I passed into the next and then into all the rooms on that floor, and then to the floor above to find that I was the only person in that part of the house. Sometimes after I’ve gone to bed, the noises from the store room are tremendous, as if furniture was being piled against the door, as if china was being moved about, and occasionally a long and fearful sigh or wail.

“Sometimes as I walk along the hall, I feel as if someone was following me, going to touch me. You cannot understand it if you’ve not experienced it. But it’s real. As I was dressing for breakfast one morning, B, who is four years old, came to my room and asked me why I’d called him. I told him I’d not called him, that I’d not been in his room. With big and startled eyes he said, ‘Who was it, then, that called me? Who made that pounding noise?’

“I told him it was undoubtedly the wind rattling his window. ‘No,’ he said, ‘It was not that. It was somebody that called me. Who was it?’ And so on he talked, insisting that he’d been called and for me to explain who it had been.”

The hallucinations continued, with the family feeling the presence of the unknown. They experience hauntings, rattling beds, lethargy, and temporary paralysis. Even the plants began to wither and die.

Mrs. H continues:

“Some nights, after I’ve been in bed for a while, I’ve felt as if the bed clothes were jerked off me. And I’ve also felt as if I’d been struck on the shoulder. One night I woke up and saw, sitting on the foot of my bed, a man and a woman. The woman was young, dark and slight and wore a large picture hat. I was paralyzed and could not move.”

After speaking with different people about their malady with the spirit realm, a relative suggested that they are being poisoned. He had heard similar accounts from people poisoned by combustion gases experiencing similar symptoms.

It turned out that the gas lighting and the furnace were dumping carbon monoxide in the home. As soon as the furnace was properly vented their ghosts disappeared and life returned to normal.

This preceding story is one of my favorites from the Podcast and Radio Show This American Life which was brought to their attention by Albert Donnay, toxicologist and CO expert.

It makes me wonder how many of the hauntings in these old homes is due to CO rather than the spirit realm.


The Deadly Gift 

The year was 1938 and Walt Disney was just off of his first blockbuster success with his film “Snow White”.

Walt and his brother Roy decided to buy a new home for their parents in North Hollywood, finally moving them down from Oregon to be near their now famous sons. In November of 1938 their mother complained to Walt that the furnace smelled strange so he sent some of his studio repairmen over to have a look.

Several days later the housekeeper found both of Disney’s parents unconscious in the home, with their mother Flora dying shortly after. Their father recovered shortly after, but many accounts say that Walt never forgave himself and was later heard mumbling

“I told those techs to buy a BluFlame combustion analyzer from TruTech tools before they went out. Heaven knows if they used the coupon code getschooled they would have had significant savings!”

All of my facts in that story are definitely, 100%,  maybe true.

In all seriousness, testing combustion and using low level CO monitors in homes and for yourself while working around combustion appliances can save many lives as well as undiagnosed illness and even a haunting now and then. See anything wrong with the furnace above?


Roofs and Ladders 

We were called out to a new high rise condo building in our area to maintain a bunch of rooftop equipment and what we found was an acrophobic nightmare. No guard rails, no parapet wall… just equipment, with much of it a few feet from the edge with sure death awaiting below.

Our service manager promptly called the customer and let them know that we would be back once they had measures in place to make the equipment safe to service.

Guess what they responded?

Nobody else has a problem with it

Whether it’s equipment that cannot be safely serviced according to OSHA 1910.1 like the ones above or extension ladders put up through scuttle holes 20′ straight up we need to start making customers responsible for providing us with safe working conditions rather than just doing it because “Nobody else complained”.

Maybe a harness tied off can work the first time until they get a proper permanent ladder or guardrail or WHATEVER WORKS, but just going back time and time again and putting ourselves in danger is the definition of insanity.


Moisture Problems 

The pager went off at 2 AM… I was on call AGAIN because the guy who WAS on call quit right in the middle of his week… he just couldn’t take this thing beeping at all hours. I grabbed the on call cell phone that was as long as your forearm and dialed the after hours voicemail line… YOU HAVE ONE NEW MESSAGE… the familiar robotic voice chirped at me.

The man in the recording sounded panicked “You were all out here earlier today and replaced an evaporator coil and now my WHOLE CEILING JUST FELL IN!”

Well… It wasn’t his ENTIRE ceiling, just a large portion of his master closet ceiling over his suits and ties and patent leather shoes. All of this happened because the tech out earlier that day hadn’t paid attention to how he strapped drain and there was a newly formed sag resulting in a double trap. Add in the fact that he had “moved” the pan switch out of the way and forgot to reinstall it properly.

Water damage, mold and mildew, lawsuits and 2 AM service calls can be prevented by paying attention to –

  • Drain pitch
  • Float Switch Location and Testing
  • Drain Cleaning
  • Pan Positioning
  • Proper configuration of drains in horizontal applications
  • Drain Cleaning

Many of the biggest nightmares in my career have been due to drain issues and moisture due to surfaces hitting dew-point. Keep the moisture where it belongs and the pager will stay quiet… who has pagers anymore anyway?


The Tiny Plug

I was sitting on the couch the other evening watching football when my oldest son who rarely has much to say piped up and said

“Dad, what happens if you test gas pressure and forget to put the plug back in”

The hair raised on parts of me where hair shouldn’t raise.

Turns out he was just curious and hadn’t actually forgotten to put the test plug back in on a valve but it did get me thinking that there is nothing quite so scary in our trade as a combustible gas leak and none more odious than “forgetting” something that critical.

When working on gas appliances always make sure to leak check connections and for gas bypassing the valve during the off cycle using a combustible gas leak detector…. Trust your nose as well… if you smell gas odorants then investigate.

Most of all…

Please…

Whatever you do…

Don’t forget to put the little plug back in after testing the gas pressure.

Also watch out for razor blades in your apples tonight… or better yet… don’t eat fruit being handed out during Halloween. What sort of demented psychopath hands out fruit on Halloween?

— Bryan

This article is written by technician and HVAC School community star Kenneth Casebier… Thanks Kenneth!


When looking at replacing a single phase A/C motor with an aftermarket motor from your van, there’s a few things you should know and pay attention to.

First, the factory OEM motor is always going to be the best option especially when talking about blower motors in a furnace or fan coil unit as that motor was specifically designed for the static pressure and application of the unit. Sometimes this isn’t always an option and for those times there’s a few guidelines that will aid in ensuring the motor of choice will be a good decision for both the tech and the consumer.

The first hard fast rule when selecting a motor is going to be frame and size. The frame of the motor needs to match the application, the last thing you want to do is modify a piece of equipment just to install a motor that may fail because of improper installation and now the equipment may not accommodate the right motor because of the modifications made. The actual depth of a condenser fan motor is very important as an aftermarket motor where the body of the motor is taller than the original, it can create a situation where the blade wont be positioned properly in the cabinet/shroud leading to incorrect amounts of airflow and potentially causing issues with obtaining the correct amp draws for reliable performance. Blade position can be EXTREMELY important to condenser airflow and should be carefully considered especially when up-sizing the HP of a motor.

The next most important consideration is amps. The amp draws need to be similar to the factory motor. Always check the data plate as the motor you’re removing may have been changed with an improperly matched motor thus why you are there now. A good rule to follow is keep amp draws within + or – 5% of the original but as close as possible or exact is a best practice. You have to keep in mind the blade, blower wheel, and duct work or shroud are going to affect the ability to properly load up a motor. If you choose a motor too far out of specs for the application you may find yourself in a potential situation for a prematurely failing motor.

(Note from Bryan: If you are replacing an OEM motor with a more efficient motor such as replacing a PSC with an ECM the amperage may go down in those cases and still be acceptable)

Horsepower is the biggest value that there seems to be some confusion on. An easy way to make a wise choice when selecting an aftermarket motor is NEVER DECREASE HORSEPOWER! Keeping the HP the same or increasing by no more than 1 value is a safe practice that will keep you from going back and replacing the motor again.

An example of this would be if you have a failed OEM ¼ hp motor, a “like” 1/3 hp would be an acceptable option, however a ½ or ¾ hp motor may work but will likely cause size issues and will be more costly to operate for the end user and therefore a bad choice.

The last major consideration when selecting a replacement motor is RPM, in PSC motors you want the match to be exact. A 1075 RPM PSC motor is 6 pole motor with a synchronous speed of 1200 RPM and a 825 RPM motor is an 8 pole pole motor with a synchronous speed of 900 RPM. Some motor manufacturers will use slightly different RPM ratings such as 1100 vs. 1075 but this is still a 6-pole motor and the 1075 can replace the 1100.

Additionally it is good to look at the bearing type used when replacing with ball bearings having a longer life but often noisier than sleeve bearings. Also consider the ambient temperature rating of the motor and chooser higher temp rated motors in more extreme ambient conditions where appropriate.

Always use the proper sized capacitor when replacing a motor and it is a good idea to replace capacitors with motors as a precaution.

Remember, no after market motor is going to be an “EXACT” replacement and for that reason I always recommend the factory OEM when possible. In extreme temperatures I know getting the equipment operational can be a driving factor in the decision making process and I’ve even “loaned” a motor to someone until I could order and return with the OEM. This can incur extra costs to the owner but it’s still better and sometimes cheaper in the long run than leaving an improperly applied motor in a system.

— Kenneth Casebier

First, a thermocouple is not a flame rectifier like a modern flame sensor. A thermocouple actually generates a milivolt potential difference when it is heated by a flame.. Just to get that out of the way for any of you newer techs who are used to modern flame sensors.

With higher efficiency gas fired equipment being the norm for replacement systems, thermocouples and standing pilots are becoming a thing of the past. Newer appliances do not typically utilize a standing pilot, opting instead for hot surface or spark to pilot ignition. These types of ignition systems have benefits over standing pilot, from increased reliability and longevity to higher efficiency numbers. But there are many appliances in the field that still use a standing pilot, and a good service technician should be able to diagnose a thermocouple issue.

Many of you will say-

“Why even check the thermocouple? It’s a 5 dollar part, just throw a new one in!”

“Why are you so lazy? Do you even HVAC in real life or just on the internet?”

Yes, I know thermocouples are cheap and I am all for replacing them when they need to be replaced, or while replacing a gas valve or pilot assembly. But over the years I have seen a lot of guys ( me included) go on calls for pilot issues, find a pilot blown out, relight the pilot, and then because it’s the easiest, quickest fix, replace the thermocouple, only to have the same customer call in a day or two later with the pilot being out AGAIN. And when the tech goes back and relights the pilot, then what? Is that brand new thermocouple bad after a few days? Probably not. There is probably some other issue, but checking the thermocouple millivolt production is the first step for a proper diagnosis.

So how does a thermocouple work? Well, I’m no scientist ( I’m barely a writer), but I’ll tell you what I know. When different metals are joined, and there is a temperature difference between them, a magnetic field occurs between the joints where the different metals meet. The heat of the pilot flame is the source of the temperature difference in a normal pilot system. Through this process, a small amount of current is produced, generally around 30 millivolts. This voltage is sensed by the gas valve and is used to keep the pilot valve internal to the main gas open. If the pilot goes out, the heat that is generating the potential (voltage) is lost, thus current stops flowing to the gas valve, and the pilot valve is closed, closing off fuel to the pilot assembly. The thermocouple is a safety device. If the pilot flame goes and the pilot valve doesn’t close, the burner compartment and potentially the room the equipment is in can fill up with gas. That the consequences of that would require a different article.

When should you check a thermocouple? I am in the habit of checking thermocouples when I encounter them, whether it’s on a maintenance inspection or a service call. If you are in the habit of checking them, it usually doesn’t take more than a few minutes. If the millivolt measurement is less than 26-27, I typically recommend replacement.

To check a thermocouple, you need a multimeter that is able to measure millivolts. It is typically shown as mV or is just the third decimal over on the DC voltage reading. Remember, the meter should be set to DC voltage.

It’s also helpful to have a extra set of hands, but it is very possible to perform this check by yourself if you hold your tongue correctly (or just use alligator clips). First, disconnect the thermocouple from the gas valve. Then light the pilot. Most gas valves have a turn knob that has to be set from On/Off to Pilot. There usually is a push button that is pressed to manually open the pilot valve, sending gas to the pilot assembly in order to light the pilot. The trick is to light the pilot, and position the meter leads in the proper place to read the voltage. The push button must be depressed through the whole check. With the thermocouple being disconnected from the gas valve for checks, the pilot valve should not stay open and the flame should go out when the push button is let up.

Put on meter lead directly on the gas valve side of the thermocouple. Put the other lead on the copper line as shown by my right hand in the picture above. While holding the meter leads in this position, light the pilot. The thermocouple needs to heat up for 30 seconds to 1 minute in order to obtain a proper reading.

30 millivolts is the desired reading, with a swing of plus or minus 5 millivolts. If the readings are in that range, and you have been having pilot failure issues, more than likely there is some other cause. Dirty pilot assembly/ orifice is the most common other issue I encounter, but it could be down draft/flue or combustion air issues, fuel pressure problems, or a failing gas valve. But as stated above, the thermocouple should be eliminated as a potential issue before moving on with a proper diagnosis. Don’t throw parts at a problem and see what sticks. With thorough troubleshooting, you can save a lot of time, headaches, and maybe the customer a little bit of money and frustration.

— Justin Skinner

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