Tag: air conditioning

This is a piece about oversized air conditioners.

Though the symptoms and consequences of oversized heating equipment are similar to those of air conditioners, you’ll notice that the focus throughout the article will be on the cooling side. Specifically, from the perspective of climate zone 1 (hot and humid).
I’m gonna skip right through the lecturing about proper equipment sizing, selection, and duct design. There are trained professionals for that and I’m not one of them. Instead, we are going to riff from the perspective of a system that has already been installed and is doing damage.
We are gonna go over some of the symptoms, their characteristics and why making improvements to oversized HVAC it’s a slippery slope.

So, what does an oversized system looks like?
Like any other one, you’ve worked on. Except, these systems:

• Can’t keep the occupants comfortable throughout various rooms in the house.
• Comfort complaints are intensified at night.
• It short cycles periodically, but it specifically does so when it’s less than 94° outside and still feels warm inside. Even when the thermostat it’s showing 67° as the room temperature.
• The relative humidity is consistently high (over 55%) or, at best, goes through big swings throughout the day. These swings will normally track with the operation cycles.
• Light films of condensation might be visible on supply vents.
• Duct work sweating.
• Excessive noise from vents. Returns, supplies or both.
• The temperature feels (noticed I said feels, not reads) significantly warmer around the perimeter areas of the space (larger exposure to exterior walls) than on the interior ones (hallways and such).

If you pull up to a service call and any meaningful combination of these symptoms are the reason you’re there, put the gauges back in the truck. There is no need to worry about subcooling or superheat. I promise.

But why? What’s so wrong with oversized equipment anyways?

Run time is the obvious place to start. The lack thereof that is.
Oversized equipment will naturally result in larger and colder air volume being moved throughout the space. Invariably, the wall control will reach its setpoint faster and the system will cycle off before it had the chance to do its job.

What is its job exactly?

Let’s start with the mean radiant temperature. The linked article explains it very well but in short, human comfort has as much to do with the temperature of the surfaces around us as the one displayed by the thermostat.

Our body temperature is normally 98 degrees, our skin is closer to 94. So, if we were to stand by a wall with a surface temperature of 75 degrees our bodies will cool off by radiating heat to it at a more comfortable rate than if we were to stand by a wall at 85 degrees. And the same goes for couches, beds, kitchen counters, etc.

An AC system must run long enough to keep a cooler and consistent temperature on all the surfaces of a home. If the outdoor temperature it’s in the ’90s and yet, the system runs for only 10 to 15 minutes each cycle, this won’t be enough to keep the mean radiant temperature of the surfaces in your home under control. Therefore, you’ll be uncomfortable despite the thermostat reaching, and “maintaining” an indoor temperature in the 60’s. This phenomenon is worsened at night when the outdoor temperature drops and the AC runs even less.

Apparatus dewpoint (ADP) is next. ADP is the effective surface temperature of the cooling coil. Or as we call it, coil temperature. I will use these 3 terms interchangeably.

While a system is off, the evaporator coil will be at a temperature close to that of the return air path and its surrounding surfaces. This temperature is much higher than that of when the system is running. Once the system cycles on, the return air temperature will dictate the evaporator saturation temperature based on the DTD and it will reach the ADP.

But just because the refrigerant entering the evaporator is at 40 degrees doesn’t mean that all of the coil will immediately drop to this temperature. This process takes time. The cold refrigerant has to make several passes before it can first, absorb the heat from all of the evaporator’s body mass, for it then, come down to the design ADP.

If we are having average run cycles in the 10 to 15 minutes range, this won’t be enough to ensure that the whole evaporator surface reaches its design operating temperature, and dehumidifies the air before the system cycles off. Therefore, the dehumidification capacity of the system will be consistently and greatly compromised, resulting in poor relative humidity control in the space.

This phenomenon is seriously worsened when dealing with high-efficiency systems. To increase SEER ratings manufacturers have found ways to drop the compression ratio and therefore power consumption. To achieve this, they have increased the suction saturation temperature through the use of larger coils. So, not only does the evaporator starts out warmer, but now it has more surface to bring down to temperature. The shorter run times of oversized systems will accentuate the otherwise negligent consequences of having a larger and warmer cooling coil surface temperature.

Did you just say SEER?! At no other time, an AC system is more efficient than when is not running, right? Because is not using any energy. So, wouldn’t it make sense to provide the consumer with a system that cycles off more often then? Nope. To begin with, the upfront costs of having larger equipment installed are normally more than that one of smaller capacity.

Also, and more importantly, the single, highest point of energy consumption for an AC system is when it turns on. Once a system cycles on and off more times than necessary throughout the day, the presumed savings of not having it run for a given amount of time go out the door.
And to top it all off, the clients are ticked off! Not only did their electric bill not go down much if any, but now they are also uncomfortable.

So how can we fix it?

Well, to fix it we would have to replace the system with one of the appropriate capacity. But that’s probably not gonna happen right away is it? Not until the consumer has enough pain to motivate the expense anyways.

Before we invariably end up talking about extending runtime and/or lowering airflow I want to make a quick stop on static pressure.
When there is an oversized system connected to existing, older ductwork. As soon as you start diagnosing the issue, you’ll run into a high external static pressure reading. At this point, a light bulb will go off in your head “it’s the ductwork”!

You’ll carry on to quote duct improvement solutions that will drop the TESP, maybe even throw some return air path upgrades. Let’s say the customer agrees, and once the work is done you perform a complimentary (or not) test and balance and ultimately confirmed that the TESP is now within acceptable levels.

“I’m going to be a hero” you may say to yourself. Well, if you did in fact improved the duct system to a point where the equipment is now moving more air than before, then the problem just got worse.

I get that it’s a controversial stance but, next time you realize you are in front of one of these situations ask yourself:
More, colder air. Do I really want to make this oversized system run better?

About extending run time

If the envelope doesn’t change, then the alternative left would be extending runtime. There is a number of ways to achieve this:

• Strategically place remote temperature sensors on the warmest areas of the house that report to the thermostat and therefore will trick the system into running more. The thermostat may feature dehumidification specific algorithms.

• Purposely de-balance the airflow distribution throughout the house, so there is more air hitting the exterior surfaces and as little as possible on the interior areas where the wall control may be located – the ceiling on this strategy is pretty low in my experience.

• And all of the above plus reducing the airflow to its minimum possible setting to run a colder coil temperature and run a lower SHR. Therefore, the dry bulb temperature as sensed by the wall control won’t drop as fast…maybe.

Doesn’t sound that bad, does it? Except these will also result in colder supply air temperatures. This is the leading cause of sweating ducts and vents in these scenarios, but that’s not the worse part.

This will directly result in localized, colder surfaces throughout the envelope as well. Condensation on vents and ductwork you can notice fairly early, before they become a problem. But what about the condensation you can’t see? The one that had been forming on building materials for a while and wasn’t a problem until now that a coconut tree sprung out of one the walls. A “moisture” remediator gets called next and what follows it’s an unfortunate tale of lawsuits and bad reviews.

I am not saying that improvements to ductwork and runtime shouldn’t be made to an oversized system but…
Have you ever heard of the bull in a china shop metaphor?

The china shop is the house and the oversized HVAC is the bull.

Genry Garcia
Comfort Dynamics, Inc.

One trait I’ve seen with good technicians is that they take their jobs VERY seriously, but they learn not to take themselves too seriously. A few months ago I had someone tell me online that I must think I’m the A/C “god” because I’m always telling everyone the “right” way to do things. This got me thinking….

I don’t want to be an A/C god, too much pressure, and heaven knows I’ve broken all of these rules more than once. I’ll settle with being an A/C Moses, descending Mount Sinai with the oracles of truth from on high

The problems with this metaphor are many, but let’s roll with it. The truth is there are many “prophets” like Jim Bergmann, Dave Boyd, Dan Holohan, Jack Rise, John Tomczyk, Bill Johnson, Dick Wirz and Carter Stanfield that I have taken these “commands” from, and they likely learned these from those that came before them. Just DON’T build a golden calf to poor workmanship or we will smash the tablets and make a big mess… Ok here are the commands.

1. Thou Shalt Diagnose Completely

Don’t stop at the first diagnosis. Check everything in the system, visually first if possible, and then verify with measurements. Sometimes one repair must be made before other tests can be done, but often you can find the cause of the initial problem as well as other problems BEFORE making a repair which helps save time, provides better customer service, and creates a better result.

2. Thou Shalt Not Make Unto Thee Thine Own Reasons

Jim Bergmann often talks about how when techs don’t understand something, they start making up their own reasons that something is occurring, and then train other techs in these made-up reasons. If you don’t understand something, a bit of research and study goes a long way.

3. Thou Shalt Not Change Parts in Vain

In other words, DON’T BE A PARTS CHANGER. Never condemn a part on a guess or make a diagnosis out of frustration. Get to the bottom of the issue no matter how long it takes. This is better for the customer, the company, the manufacturer, and your development as a tech. If you aren’t confident, call someone who is fundamentally sound and get a second opinion BEFORE you leave the site. Better yet, send them a text with all the readings, model and serials, conditions, photos, type of compressor, type of controls, type of metering device, and what you have done BEFORE you call them. Get the diagnosis right the first time.

4. Remember the Airflow and Keep it Wholly

So much of HVAC/R system operation has to do with evaporator load, with LOW load being most commonly caused by LOW AIRFLOW, and low airflow being most commonly caused by dirt buildup. Keep blowers, fans, filters and coils clean and unobstructed. Check static pressure when duct issues are suspected in order to verify and properly setup blower CFM output to match the requirements of the space and outdoor environment.

5. Honor Thy Trainers and Mentors

New techs will often learn a few facts and cling to them as though they are the end all and be all of system diagnosis. I have met techs who get over-focused on everything from suction pressure (most common), to superheat, subcool, static pressure, delta T, and amp draw. A good tech continues learning from older and wiser techs and trainers who see the whole picture. When you are new, it’s hard to remember all of the factors that go into system diagnosis and performance. More experienced techs who have kept up on their learning develop a “6th sense” that can rub off on you if you “Stay Humble” (to quote the great philosopher Kendrick Lamar). Listen more than you talk, and learn the full range of diagnostic and mechanical skills.

6. Thou Shalt Not Murder The System by Failing to Clean

A good technician learns the importance of keeping a system clean early on and never forgets it. Condenser coils, base pans, drain pans, drains, evaporators, blower wheels, filters, return grilles, secondary heat exchangers and on and on… A system that is set up properly initially and cleaned regularly will last much longer, cool or heat better, and use less energy. In my experience, techs that don’t believe in maintenance don’t perform a proper maintenance themselves. Use your eyes, and clean what’s dirty.

7. Thou Shalt Not Commit Purgery without Vacuumy

Proper evacuation is one of the most overlooked disciplines of the trade. Jim Bergmann says again and again, a proper vacuum is performed with large diameter hoses connected to core removal tools. The cores are removed from the ports, the hoses have no core depressors, the hoses are connected directly to the pump (not through gauges). The vacuum (micron) gauge is connected on the side port of the core removal tool, not at the pump. The pump has clean vacuum pump oil and the pump is run until the system is pulled below 500 microns (exact depth depends on the system). The core tools are then valved off and the “decay” is monitored to ensure that the system is clean and tight.

Purging with dry nitrogen prior to deep vacuum helps with the speed of evacuation, and installing line driers assists in keeping the system clean and dry, but neither are a substitute for a proper deep vacuum and decay test.

8. Thou Shalt Not Steal (from the customer)

Good techs provide solutions for their customers to get a broken system working, as well as other repairs or upgrades that result in optimum performance. Most techs don’t INTEND to lie to a customer, but their lack of understanding on the products they are OFFERING, along with strong incentives to OFFER these upgrades can result in dishonest practices. A good, profitable technician has a deep understanding of all the repairs and upgrades they perform, as well as a sense of empathy for the customer.

9. Thou Shalt Not Bear False Witness Against Other Technicians

This all comes down to a witch’s brew of ego and insecurity all mixed together. You have either done this yourself, or you know of someone who has gone to a customer’s home or business and thrown the previous technician or company under the bus in front of the customer. In some cases it may be nothing but bravado, and in other cases it may have a measure of truth in it (or may be undisputed). Either way, talking negatively about other techs and companies does nothing but make you look petty and angry. Demonstrate your skill and knowledge by discussing the courses of action you intend to take, and if required, you can COMPARE these actions to previous actions taken; just stay away from personal attacks. Let the customer be the judge about the last guy.

10. Thou Shalt not Covet Thy Neighbor’s Job

Many good techs start to do poor quality work when they get burned out… and buddy let me tell you- I HAVE BEEN THERE. It is important to remember that every job from maintenance tech to business owner has good things and bad things about it. There are good days and bad days, great customers and total jerks, 16 hr days and 8 hr days. You may hit a spot where you decide to change jobs, and that is totally fine and may be a great decision. Just don’t make a rash decision because the grass looks a little greener. ALWAYS do quality work no matter where you work, or how bad it gets. Doing poor quality work because your job is getting you down is like a cancer that will grow and do harm to you and your career.

Take pride in your work, keep your eyes and ears open, learn something new every day and the HVAC/R gods will smile warmly upon you.

What commands would you add or remove?

— Bryan

 

 

These are two separate emails that I sent to our customers and staff in preparation for what could be a very hot Memorial day weekend (2019). I’m sharing it here so that you can use parts of it in your business as you see fit. I hate seeing techs get beat up on hot holiday weekends so hopefully this helps ease the pain if even just a little. Stay safe out there!


We Have a HEAT WAVE COMING in a Few Days!

I’m not sure if you’ve taken a look at the weather forecast for Memorial Day weekend but it is looking like it’s going to be BLAZING!

There are a few things to consider that could pose an issue for some of you that you should be aware of

  • If it hits the forecast temperatures it could be the hottest days we’ve seen in over a year. Really hot days can reveal issues with air conditioning equipment that don’t usually show up
  • These FIRST hot days of the year are coming on a holiday weekend. A/C companies will have limited staff working and supply houses will be closed.
  • In Central Florida A/C systems are generally designed to maintain 75° inside on a 93° – 95° day. When the temperatures get up to near 100° you may not be able to keep your home below 80° inside. If there is ANYTHING wrong with the A/C this can be even worse.

Take Action This Week Before The Heat Hits

You are getting this email because at one point in time you’ve done business with Kalos. Rest assured this email isn’t about marketing, if you have another company you work with now then the same applies with them. In the A/C business we are used to dealing with a hot “first day of Summer” as well as a hot holiday weekend but NOT both at the same time. Please read below and consider taking these actions on Monday 5/20 before the heat hits.
  • If you have ANY concern that your A/C may not be working the way it is supposed to then get a service call to check it out early in the week so it can be addressed (or given a clean bill of health) before everyone is swamped. We charge ***Redacted*** for a diagnostic service call which is cheap insurance and you can either call or text us at ***Redacted*** (Or use whomever you prefer). It is too late to schedule a tuneup / cleaning / maintenance for next week unless it is already on schedule but you can still get a full checkup which we call a diagnostic service. 
  • If you are sure your A/C is working OK make sure your air filters are all fresh and clean and as the days start to warm up make sure doors stay closed and blinds, curtains are closed.
  • If the weekend comes and your thermostat starts to rise and the the A/C runs all the time, try to stay calm. Don’t call an A/C contractor UNLESS the thermostat rises above 80°. I know it can be frustrating but this is how systems are designed in Florida because we cannot oversize them due to humidity.
  • If you think your A/C is on it’s last leg and needs replaced we can get that setup for a free consultation and estimate during the week next week. Keep in mind the installation schedule will fill up REALLY quick for the week after Memorial day.
There are a few other practical things you can do to keep your home cooler that don’t involve the air conditioner at all.
  1. Replace Halogen and Incandescent light bulbs with LED, they produce less heat. Led recessed can trims can also reduce attic air leakage into the home.
  2. Don’t cook on the stove or oven during really hot days, Hot days = PIZZA!
  3. Run fans when you are in the room, don’t run them when you aren’t. Leaving fans running in rooms you aren’t using actually INCREASES the temperature of those rooms. Fans cool you via convection and evaporation they don’t decrease the air temperature.
  4. Get insulation added to your attic. If you call an insulator early this week they may be able to get it done before the weekend hits. That’s a win -win because it helps with your power bill and comfort on hot days.
This next piece of advice is worth the price of the email so pay close attention. If you get into the heat of the weekend and your thermostat starts to rise there are two secret HVAC technician life hacks you can use to make things a little better for a short period of time.
  1. FIRST, go outside and place your hand over the outdoor fan. If you can hear it running and the air blowing out the top feels hotter then the outdoor air then the compressor is running. If not then shut the system off and you will need to schedule a service call.
  2. Look all around at the pipes going into the unit. If you see any ice, go inside and turn the unit to OFF and the fan to ON at the thermostat. Leave it this way for 8 hrs or so to defrost the unit. If you call out a tech and they show up to a frozen unit it will result in delays and/or extra charges. Keep in mind that when the unit defrosts the part inside (the furnace or fan coil) may leak some water so be prepared with towels around the inside part of the air conditioner. If there was no ice then go to step 3 
  3. If the unit is blowing out warm air outside and there is no ice you can see anywhere and your filter is new and clean you can use a lawn sprinkler to mist water onto the side of the outdoor part of the air conditioner. Make sure it is low pressure and only do it for a maximum of 6 hrs a day and only for a few days a year during the hottest parts of the day. This helps to improve the capacity of the air conditioner and takes some of the load off of the compressor. DO NOT DO THIS LONG TERM, CHLORINE AND MINERALS IN THE WATER CAN AND WILL DAMAGE YOUR AIR CONDITIONER. 

Finally, If you do need a service call you can call or text us at ***Redacted*** but PLEASE be patient. We are doing everything we can to get ahead of the weather but the technicians who will be working during Memorial day weekend are going to be under a lot of pressure. Some of them are Veterans who have served our country and all of them care deeply about doing a good job for our customers.

Sometimes your best option may be to get a hotel for a night or two if your A/C goes down during a time like this while waiting to have it repaired. You may also consider getting a portable A/C from a big box hardware store to keep the master bedroom cool.

As a customer of Kalos you are a huge priority to us but caring for our team members and their families are still priority #1. For us this means shutting down the schedule for the day at midnight so our techs can get a few hours of sleep before they start the next day.

Rest assured that if you need us we will do everything we can do to help while maintaining the health and sanity of our staff.

Thanks for being a Kalos Customer, I really appreciate you.

— Bryan Orr


 

Hello Everyone,

I may be overreacting but as they say “Fate favors the prepared”… Don’t they say that? I’m sure somebody does.

The current forecast shows high temperatures of 98° on Memorial Day Weekend. This is not ONLY high temperatures on a weekend when supply houses won’t be open on Monday (although we can always make them open if need be) but it will be the hottest day of the year and MAYBE the hottest day in several years.

This means a few things,
Monday 5/27 will be a full staff work day for service employees just don’t forget to honor those who gave the last full measure of devotion to our Nation as we serve our customers.
All managers will need to be ready to assist on 5/25 & 5/26 (Including Myself)
Anyone who is willing to help out on the evening of 5/24 – 5/26 will be most appreciated from installers to refrigeration techs to CSRs to parts quoting. Anyone who can grab a service call or a phone call will be appreciated.
Have your trucks stocked and ready to rock. If a real emergency occurs and we need a part from a supply house, especially for Warranty or contract customers then almost all suppliers have emergency lines and we should use them, if we are miserable then they can be as well.

Keep a cooler or case of water on your truck, grab it from the shop or use your card to get it if need be. STAY HYDRATED.

If it is as hot as forecast we may need to prioritize calls. The first thing is that from Saturday 5/25 – 5/27 we will be charging a ***Redacted*** diagnosis fee for non-contract / non-warranty residential customers for the holiday weekend. If the customer asks why just say it is the holiday charge.

We will prioritize customers in this way for service

*** Customer Name(s) Redacted *****
Commercial Refrigeration Warranty Work & Callbacks
Residential Warranty & Callback Customers
Residential Contract Customers
Regular COD customers of all types
Ductless Lanai / Florida Room Units (Schedule for the week)
When we schedule over the weekend we need to use VERY wide windows for any COD customers we put on schedule. Do not schedule any calls for ductless systems on the holiday weekend, these are not emergency calls and should only be scheduled during regular hours on weekdays. If these customers get cranky keep in mind that we NEVER promise 24hr or weekend service to residential customers unless they have a specific contact to that effect.

TECHS & CSRS PLEASE READ CAREFULLY BELOW

Whenever we have days where the outdoor temperatures get above 93° you will have some customers that call in because their A/C is not keeping up or because it “runs all the time”

In Florida we have ACCA (Air Conditioning Contractors of America) guidelines for design and 93° – 95° are used as the design outdoor temps. This means that on a 98° day many systems may not maintain 75° and may go up as high as 80°. When a customer calls in and says the temperature is going up in the house we can ask what the thermostat says the indoor temperature is. If it is 80° or lower at 2PM – 9PM of a 98° Summer day then there is likely nothing wrong with the system so long as it is running. For the skeptical client you may send them this link **redacted**

If as a technician you go to a call on a callback or warranty job for not keeping up or running all the time you need to do the following steps

Perform all of your normal visual inspections and measurements ( Filter, coils, thermostat calibration, subcool, superheat, delta T, Amps, Voltage) if all this check out then
Check static pressure and visually inspect ducts and attic insulation – if all this checks out then
Do a full MeasureQuick PDF report and send it to ***Redacted*** so we can be aware and upload to the file…. What? You say you DON’T KNOW HOW TO DO THAT? well… you have a full week to figure that out.
If the customer is COD then we do the basic tests and OFFER the Attic inspection for ***Redacted*** to check ducts and insulation on a basic small house or ***Redacted*** if it will take over 30 mins.

When we find a house where the A/C is working properly (based on all these tests, not “BEER CAN COLD” or suction pressure) then we do NOT tell them their A/C may be undersized… The resolution is almost ALWAYS in decreasing heat load on the space not increasing system size. This includes items like –

– Sealing Gaps to the attic
– Replacing incandescent light trips with sealed LED
– Upgrading Attic Insulation
– Closing Blinds and Drapes
– Keeping doors closed
– Upgrading Ducts

If a system is LEGITIMATELY undersized it’s generally better to carve out a room and add ductless rather than trying to up the tonnage of the old A/C and upgrading ducts + trying to make it fit in the same space.

MeasureQuick system reports will be your best friend in helping you PROVE that the A/C is working properly in cases where the customer is stuck on the idea that there is a problem and it only takes about 10 mins to do completely.

The goal is all pitch in to get our customers taken care of without kicking a bunch of callbacks or cranky customers down the road into the next week. Weeks like this can either be a profitable kickoff to the Summer or a miserable mess depending on how we approach it.

My Cell is ***Redacted*** if you need me. I will have it on and with me.

Thanks!

Bryan

 

The most common and often most frustrating questions, that trainers and senior techs get goes something like this. “What should my ______ be?” or “My _____ is at ______ does that sound right?

Usually, when the conversation is over both the senior and junior techs walk away feeling frustrated because the junior tech just wanted a quick answer and the more experienced tech wants them to take all of the proper readings and actually understand the relationships between the different measurements.

In this series of articles we will explore the, “What should my _______ be?” questions one at time and hopefully learn some things along the way.


So what should the superheat be?

First, what is superheat anyway? It is simply the temperature increase on the refrigerant once it has become fully vapor. In other words, it is the temperature of a vapor above it’s boiling (saturation) temperature at a given pressure.

The air around us is all superheated! Head for the Hills!

How can you tell that the air around us is all superheated? Because the air all around us is made of vapor. If the air around us were a mixture of liquid air and vapor air, first off you would be dead and secondly, the air would be at SATURATION. So the air around us is well above its boiling temperature (-355° F) at atmospheric pressure which means it is fully vapor and SUPERHEATED. In fact, on a 75-degree day, the air around you is running a superheat of 430°

But why do we care?

We measure superheat (generally) on the suction line exiting the evaporator coil and it helps us understand a few things.

#1 – It helps ensure we are not flooding the compressor

First, if we have any reading above 0° of superheat we can be certain (depending on the accuracy and resolution of your measuring tools) that the suction line is full of fully vapor refrigerant and not a mix of vapor and liquid. This is important because it ensures that we are not running liquid refrigerant into the compressor crankcase. This is called FLOODING and results in compressor lubrication issues over time.

Image courtesy of Parker / Sporlan

#2 – It gives us an indication as to how well the evaporator coil is being fed

When the suction superheat is lower it tells us that saturated (boiling) liquid/vapor mixture is feeding FURTHER through the coil. In other words, lower superheat means saturated refrigerant is feeding a higher % of the coil. When the superheat is higher we know that the saturated refrigerant is not feeding as far through the coil. In other words higher superheat means a lower % of the coil is being fed with saturated (boiling) refrigerant.

The higher the % of the coil being fed the higher the capacity of the system and the higher the efficiency of the coil.

This is why on a fixed orifice system we often “set the charge” using superheat once all other parameters are properly set. Adding refrigerant (on a fixed orifice / piston / cap tube) will feed the coil with more refrigerant resulting in a lower superheat. Removing refrigerant will increase the superheat by feeding less of the coil with saturated (mixed liquid and vapor) refrigerant.

This method of “setting the charge” by superheat does not work on TXV / TEV / EEV systems because the valve itself controls the superheat. This does not negate the benefit of checking superheat, it just isn’t used to “set the charge”.

#3 – We can ensure our compressor stays cool by measuring superheat

Most air conditioning compressors are refrigerant cooled. This means that when the suction gas (vapor) travels down the line and enters the compressor crankcase it also cools the motor and internal components of the compressor. In order for the compressor to stay cool, the refrigerant must be of sufficient volume (mass flow) and low temperature. Measuring superheat along with suction pressure gives us the confidence that the compressor will be properly cooled. This is one reason why a properly sized metering device, evaporator coil, and load to system match must be established to result in an appropriate superheat at the compressor.

#4 – Superheat helps us diagnose the operation of an active metering device (TXV / TEV/ EEV)

Most “active” metering devices are designed to output a set superheat (or tight range) at the outlet of the evaporator coil if the valve is provided with a full liquid line of a high enough pressure liquid (often at least 100 PSIG higher than the valve outlet / evaporator pressure). Once we establish that the valve is being fed with a full line of liquid at the appropriate pressure we check the superheat at the outlet of the evaporator to ensure that the valve itself is functioning properly and /or adjusted properly. If the superheat is too low on a TEV system we would say the valve is too far open. If it is too high the valve is too far closed.

#5 – Superheat is an indication of load on the evaporator 

On both TEV / EEV systems and fixed orifice systems (piston / cap tube) you will notice that when the air (or fluid) going over the evaporator coil has less heat, or when there is less air flow (or fluid flow) over the evaporator coil the suction pressure will drop. However, on a TEV / EEV system as the heat load on the coil drops the valve will respond and shut further, keeping the superheat fairly constant. On a fixed orifice system as the load drops so will the superheat. It can drop so much on a fixed orifice system that when the system is run outside of design conditions the superheat can easily be zero resulting in compressor flooding.

When the load on the evaporator coil goes up a TEV / EEV will respond by opening further in an attempt to keep the superheat constant. A fixed metering device cannot adjust, so as the heat load on the coil goes up, so does the superheat.

When charging a fixed orifice A/C system you can use the chart below to figure out the proper superheat to set once all other parameters have been accounted for or you can use our special superheat and delta t calculator HERE

Using this chart requires that you measure indoor (return) wet bulb temperature so that the heat associated with the moisture in the air is also being accounted for as well. This is one of MANY target superheat calculators out there, you can use apps, sliderules etc… Here is ANOTHER ONE

Remember, this chart ONLY applies to fixed orifice systems.

So what should your superheat be in systems with a TEV / EEV? The best answer is… like usual… Whatever the manufacturer says it should be.If you really NEED a general answer you can generally expect

High temp / A/C systems to run 6 – 14 degrees of superheat

Medium Temp  – 5-10 

Low Temp – 4-10

Some ice machines and other specialty refrigeration may be as low as 3 degrees of superheat

When setting superheat on a refrigeration system with any type of metering you often must get the case / space down close to target temperature before you will be able to make fine superheat adjustments due to the huge swing in evaporator load. Once again, refer to manufacturer’s design specs.

— Bryan

P.S. – Trutech has a really great resource on charging best practices

Try out our new, simple superheat calculator for fixed metering A/C systems

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