- Tech Tips
This article is written by regular contributor, experienced rack refrigeration tech and RSES CM Jeremy Smith. Thanks Jeremy. Also… There is a podcast out about what kills compressors HERE
A technique that you can use to diagnose compressor problems and to help differentiate them from other possible issues is the use of compressor performance analysis.
Manufacturers do extensive testing of their compressors before they sell them, and a part of that testing is available to you as a troubleshooting tool. The compressor performance chart. I’ll primarily refer to Copeland compressors as they are what I service most, but I’ve been able to find charts and data from other manufacturers through their websites and tech support lines.
Let’s look at a real-world example. I went to do a follow-up check after a major leak and recharge on a set of freezers. On arrival, the cases, which had been running now for 14 or 15 hours since having been repaired, weren’t as cold as expected. Checking the unit, here is what I found:
209v (average of all 3 legs)
Unit at 18-20°F
The suction line was cool to the touch and the sight glass had a thin ‘river’ of refrigerant in it. The high suction pressure really jumped out at me here as worthy of more consideration.
Now, a high suction pressure in this instance can be caused by high load (note the high unit temperature) or it can be caused by a compressor problem. Looking over the data here, I was concerned about the health of the compressor and its ability to pump properly. I did a quick “pump down test” and found it inconclusive. The compressor pulled to 24” Hg easily and held there. Still, I wasn’t happy with this, so I pulled out my smartphone and opened the Copeland Mobile app.
A quick note on pump down ‘tests’. They really aren’t effective on most modern compressors. I performed the test and included the results here to illustrate exactly that fact.
Entering the model of the compressor leads you to select the application (R502 low temp which is closest to R404a low temp). Selecting the “Diagnostics” tab brings you to a screen where you and input pertinent data and the app then outputs both the expected amperage at your conditions and the percent deviation
from the norm.
In this case, my expected amperage was significantly higher than my observed amperage, so the high suction was definitely caused by a compressor problem.
I recovered the refrigerant from the machine and removed the compressor head and valve plate for internal evaluation.
Finding a single broken suction reed, the rest of the internals were intact making this a good candidate for a new valve plate. I Installed new valve plates, evacuated and restarted the machine and re-evaluated operation.
Had this been a hermetically sealed compressor, I would have had no choice but to condemn and replace the compressor. This time, the amperage was within 5% of specifications (sorry, didn’t get a screenshot) and I continued to monitor unit operation until equipment reached 0° F, verified and completed proper
charging of the unit and called it a day.
Why not use RLA (Rated Load Amps) (? Or use LRA÷6 (Or is it 8?) to diagnose?
The simple answer is that they just aren’t sufficiently accurate enough for me dealing with high stakes, high dollar equipment and they shouldn’t be accurate enough for you, either.
Let’s return to my real-world example…
The compressor has a listed RLA of 25.8 and an LRA (Locked Rotor Amps) of 161.0. Now look back at the original screenshot of the app. It calls for an amp draw of 17.09A at that set of conditions. If we compared that to the RLA, even the correct amperage looks low. If we use common LRA divisors 161 ÷ 6 gives us 26.83A and 161 ÷ 8 gives us 20.125A. Maybe a little better than the RLA method but still off by a significant amount. Enough to cause concern and possibly lead to an incorrect diagnosis.
Not one of these methods gives us an accurate expected amperage for this machine. That inaccuracy can lead us to draw a bad conclusion and potentially wasting time and money pursuing a “bad” compressor that is in fact, working exactly as it should.
Like most things in HVAC/R using a fixed operational target without considering the specific conditions can lead to misdiagnosis and a lot of wasted time. You would be surprised what is available within manufacturer specs if you take the time look.
— Jeremy Smith, CM
Let’s use a bit of imagination for a minute.
Imagine you have two totally identical 3-ton systems. One of them is completely normal and the other has no fins at all on the evaporator coil.
They both have the same charge, airflow and compressor capacity. What will be different in terms of readings and performance of the one with no fins? Think about it yourself before moving on.
The fins attached to the tubing increase the contact time and turbulence of the air over the lower temperature coil surface
This increased air to metal contact time and surface area allows more heat to leave the air and enter the refrigerant. The pressure and temperature of the refrigerant in the evaporator coil (assuming it is changing state from liquid to vapor) is a function of the amount of refrigerant being moved through the coil (by the compressor) and the amount of heat entering the refrigerant from the air.
If the evaporator coil had no fins we would see these results in the system running with no evaporator fins compared to the normal system –
Eventually the suction pressure / temperature would stabilize once the amount of heat being picked up in the coil and the reduced compressor capacity reached an equilibrium but by the time the tubing would likely be freezing over and the system would be VERY inefficient.
A coil with no fins has a high BYPASS FACTOR and a low CONTACT FACTOR
As a technician you will likely never calculate bypass or contact factors but you will see their impacts all the time like in the extreme thought experiment above. It is easier to wrap your head around the impact of bypass factor by considering the extreme edges of design with a coil with no fins being an extreme example of high bypass and a massively over-sized evaporator with very dense and deep fins being the opposite side of the spectrum.
If you have more surface contact between air and metal and when the air is moving at lower velocity (slower) the air will impart more heat to the coil and will get closer to the coil temperature.
If you have less surface contact (like no fins) and the air is moving at a higher velocity (faster) the air will impart less heat into the evaporator coil and will stay warmer when compared to the coil temperature.
The part that can make this tricky to imagine is once you realize that changing the bypass factor simultaneously impacts the difference between the air temperature and coil temperature AND changes the coil temperature itself because the coil temperature is partially dictated by the amount of heat entering the coil FROM the air.
Look back up at the bullet list of impacts that occur with a coil that has no fins. Now flip the results for an over-sized evaporator coil (assuming the air flow remains constant)
Now this last point about the humidity has a caveat, the coil still needs to be well below dew-point so this correlation between increased coil size and increased dehumidification isn’t absolute. When you couple a larger coil with LOWER airflow you can get the best of both worlds by having lower bypass factors without driving the coil temperature up too far resulting in improved efficiency and latent capacity.
I’m not saying you should go out and put larger coils on everything, but as a design consideration you may want to take a good look at manufacturer expanded performance data when choosing a coil match and you may find an up-size gives you better performance even if you dial back the airflow a bit.
I know this may be a little bit of a brain mush concept but keep going back to the impact of the evaporator with no fins to help set your mind straight. It certainly has helped me.
I get questions all the time about performing “load calculations” and “rules of thumb” as well as how to do it properly. This article isn’t about load calculation but the only good answer is to find a quality ACCA approved Manual J software and get used to using it.
You may have heard from others in the field that Manual J tends to “undersize” the equipment. If you are an engineer or designer you may be shocked at how “oversized” most equipment is when compared to Manual J. Like most things, the truth can be found somewhere in between and here are the reasons for this.
System selection is just as important as Manual J because if you don’t match the proper equipment to the BTU load or if you fail to consider the factors you can easily end up with a system that is undersized or that does not deal with the humidity load of the space.
Here are some common factors that contractors & designers fail to properly factor into their Manual J
Duct leakage rate
We can guess, but the only way to really KNOW the rate of leakage is to do a duct leakage test like the one shown below by Corbett Lunsford.
Building Envelope Leakage Rate
The rate of leakage in and out of a structure is one of the most overlooked aspects of a load calculation. Once again, we can guess based on the age and construction type but the true leakage rate can vary wildly. The only true way to test leakage rate is by measuring it with a blower door.
Even insulation is often a guess in areas where you cannot access walls or portions of the attic. It takes a combination of experience and thermal imaging or other R-value measuring tools to truly calculate heat loss / gain through insulation.
This one is very challenging to calculate. You may have two homes with nearly identical construction, orientation, and layout, one with significant shade from trees and another without. This shade can represent a significant decrease in radiant heat transfer to the walls, windows and roof and will vary seasonally based on the season and various times of day. Shade is something you can factor in using common sense. While I wouldn’t suggest “under sizing” just because of shade, you can be sure that a well-shaded structure will have lower radiant gains which will have an impact in all seasons.
For every cubic foot of air you move out of a building you are also moving one cubic foot of air into the building, either through a designed path, through cracks and gaps, or when a door or window opens. One way or another when you move air out you are also moving it in. It is always better to move that air into the building through a designed path where the air can be controlled, measured and likely treated (ERV, HRV, Dehumidification) instead of through cracks and gaps that can be in any number of undesirable places. In addition to this, there are new standards being enforced surrounding ASHRAE 62.1 & 62.2 that mandate mechanical fresh air be brought into all structures. This fresh air needs to be considered as to heat gains and losses both sensible and latent.
Because of all these factors and industry pressures I have found that design professionals have a tendency to underestimate heat gains and losses (on existing, untested structures) while contractors and field personnel tend to oversize equipment based on “experience” or “rules of thumb” and usually a combination of both. This happens because it is much more common for a contractor to get a complaint of a system “not keeping up” than humidity, or power consumption issues because the thermostat displays the temperature in big bold numbers while humidity and power are a bit more abstract.
Design professionals are under pressure not to oversize equipment by the industry (and rightfully so) but may not be fully aware of all of the “as built” conditions that exist.
But for sake of argument, lets say the heating season losses and cooling season gains have been perfectly calculated
We now bump up against some of the most common areas of misunderstanding by contractors and field staff which is properly matching the equipment to the load. here are some of the biggest mistakes.
In order to properly select equipment, it is recommended that you use ACCA manual S to ensure that you don’t miss any of the steps. ACCA has a great quick guide on system selection you can read HERE
Here are some tests you can apply to your residential design to help double check that your selection matches the design.
All of these need to take into account the manufactures specifications matching the load calculation conditions to the specifications of your equipment at the same conditions. While a furnace may produce the same heat output no matter the conditions, air conditioning and heat pump equipment output will change depending on indoor and outdoor conditions.
The final step is configuring the equipment to the proper air flow levels so that the sensible/latent capacity will match the design. If the system was designed for 400 CFM per ton then ensuring that the equipment is set to output that airflow is critical.
The year was 2002 and I was 20 years old. I had a helper named Clay and he was a really nice guy, probably 15 years my senior, already with streaks of grey in his hair with a way of making you feel at ease… so very at ease.
OK, he was sleeping most of the time…
He fell asleep in my van between most calls and even fell asleep in company meeting leaning against a condenser box.
Now maybe I’m just being judgmental and Clay was a narcoleptic, or maybe he was staying up all night doing superhero work, but I think the guy just liked to grab a few , or MANY winks.
It didn’t really bother me much when Clay would sleep, I could tell he wasn’t learning much and didn’t care much, but he was a pleasant sort of bloke and I didn’t hire or manage him so it really wasn’t my problem.
Nowadays I do employ and train lots of people and now it is my problem and trainees who sleep in vans DRIVE ME CRAZY! Instead of focusing on my evil capitalist anti-sleep, judgy ways right off let’s ease into with some more trainee tales and let’s see if you can catch a thread of commonality.
The Nasty Dudes
When I was a helper I rode with a few old techs that took it as their life mission to teach me all about every possible nasty thing they could pump into my mind. I started in the trade at 17, home-schooled, wide eyed and as sheltered a Christian kid as you can get (almost). These guys started in using words and pushing mental images I didn’t even understand at first.
Once I got into my own van I was glad to have a break from it… until I started getting trainees. I had three guys that rode with me that I remember in particular who ONLY wanted to talk about what they did, saw or thought about last night. One of them used to be a “tech” (used loosely) in Miami and if I took him at his word he slept with 120% of the of the occupants of South Florida. Needless to say, the fantasy game is strong with most of these guys and I didn’t take a word they said seriously.
Does it shock you that none of them worked out as techs?
The Phone Lookers
I’m thankful that I started in the trade during a time of radio dispatch before cell phones were popular and even before Nextel chatter became a thing (remember Nextel). Back then our favorite entertainment was listening to little dispatch dramas and techs helping one-another with diagnosis.
Nowadays many helpers / trainees / apprentices and techs alike turn to their phones for entertainment between calls. If you have had a young person working with you who was on there phone all the time how did that work out for them?
The Random Guesser
I once rode with a guy who would guess what the next problem would be before we got there.
“This one is gonna be a coil leak” he would state confidently
This was before detailed, web based job histories were a thing so it was really just a total guess based on the type of equipment common in a particular community.
He actually wasn’t a bad tech and this was just his way of entertaining himself, but his method of pre-diagnostic guessing was all based on the dreaded “I’ve seen that before” rather than a solid diagnostic approach based on observation and measurement.
The common thread? All of these techs were busy engaging (or disengaging) their minds in lot of things that weren’t making them better at their jobs.
The Diagnosis Game
There were a few techs that really taught me and this teaching often happened on the drives between calls and it came in four flavors
Different people do the diagnosis game differently and probably also call it different things (some of you may not be comfortable with “games” at work) but it has some common elements and is designed to challenge the helper to think and imagine their way through a diagnostic scenario.
Here is how I do it –
Me: Ok, I’ve got one
Trainee: You’ve got one what?
Me: I’ve got a diagnosis for you
Trainee: OK, what is it?
Me: I’m a service call so you can ask me questions as the customers or choose to check various things and ask me the findings until you make a diagnosis
Trainee: OK so what’s the suction pressure
Me: Is that how you start a service call? Come to screeching halt in front of the home, hop out and check suction pressure?
Me: Ok, so what do you do first?
Trainee: Check the thermostat I guess
Me: So you come to a screeching halt, jump out, slam open the customers door and start checking their thermostat?
Trainee: (rolling eyes) No… I guess I talk to the customer
And I just keep going and going and going like this, picking on every detail of the call and diagnosis process they use, detail by detail until they gather enough information to make a diagnosis.
Along the way they will inevitably say something like –
Trainee: Is it a bad capacitor?
Me: Can you ask an air conditioner questions?
Me: I only answer yes or no questions about observable facts, provide you tool readings or answer questions as the homeowner
Trainee: OK, does the contactor have power?
Me: What does that mean?
Trainee: Is there power going into the contactor?
Me: What is power?
Trainee: Is there VOLTAGE at the CONTACTOR!
Me: Where at the contactor?
Trainee: The HIGH VOLTAGE!
Me: Measured with what?
Trainee: MY METER!
Me: What setting on your meter?
Me: OK, Where are the leads placed
Trainee: Across the bottom of the contactor I guess?
Me: Don’t guess, what is the bottom side called
Some of you are reading this and getting annoyed thinking I’m a pedantic nerd who just like to antagonize helpers (which is somewhat true I will admit). Some guys hate this at first, but they usually come to enjoy it EVENTUALLY and it teaches imagination, reasoning and precise language.
Along the way you can throw in little scenarios where they die or get injured because they forget to double check their meter to a known power supply or fail to wear PPE etc…
Yes, it’s a little dramatic but the human brain is hardwired for story and competition and the diagnosis game used both of these human traits to help teach HVAC/R. It is better than classroom training because they get to apply and test what they are learning as they are thinking about it, it works best if they actually think you are wrong on something and have to come up with proofs or experiments to try and prove you wrong.
So I posted on social media a few weeks ago about how I don’t like it when trainees sleep in the vans and I got pretty well roasted for being a judgmental sleep hater and after all “what do I expect trainees to do while riding along?”
I expect nothing
One of my political heroes (who will remain unnamed) was once asked how he kept his sanity working in DC with so much corruption and lunacy, he responded
It’s Simple, I’ve just learned to have low expectations
While I fully expect trainees to fall asleep, look at their phones, guess at diagnosis, fail to read manuals and share inflated escapade stories my STANDARD is that they will actually engage their brains and learn about HVAC/R (and hopefully some life lessons) when they work with me. This is a process and the diagnosis game is one of my favorite tools to get them there.
I firmly believe that anyone who really wants to learn this trade can become usefully proficient much faster than most think possible if they just spend their work day engaged rather than relying on osmosis to fill their brains with experience.
Many of us learned this business because we really had no choice but to make it work and necessity forced us to turn on our minds and make it happen. Some of the people who train with you may not have those pressures and may not have influences in their lives that require then to be on the ball.
You don’t need to be a jerk to hold new people to a high standard, I would suggest step one is to have low expectations and step two is start requiring more of their attention during drives investing in them and bringing them up to your standard.
And trainees… Don’t sleep in the van or stare mindlessly at your phone. Trust me.
A good leak detector is a big investment and one of the more important tools a tech has on the truck. I’ve had the same leak detector for years and I’ve replaced everything on it from sensors to pumps to the probe.. and no, the one shown above isn’t mine.
One thing that I have learned is that with leak detectors care, maintenance and testing is a huge part of finding leaks the first time and will also save you a lot of money. This little tutorial covers the Bacharach H10 series but many of the tips apply to most detectors.
Keep it Clean and Dry
Leak detectors don’t like moisture and dirt. Make sure to keep the rubber tip and filter on the end to help prevent creating a seal that pulls in moisture and keep the detector off the floor and out of the dirt. If you ever DO get your detector wet, shut it off quickly, pull out the batteries (if it has them) and put them in a degassing chamber and pull a vacuum on it. This will dehydrate the detector and can often save it.
Store it in a Safe Place
Leak detectors have sensitive pumps and sensors in them that can be damaged if they are handled roughly. Also, keep from kinking the hoses or probes as this can cause leaks in the tube that will impact your reading as well as restrict flow to the sensor.
The H10 series of leak detectors has a red ball flow indicator in the probe. The first thing I do when I start the detector and allow it to warm up is to check the flow through the probe by pointing it down and seeing is the red ball floats.
Check your Sensor
Make sure your sensor is properly connected and on the H10 you can physically feel the heat from the sensor guard door when the unit is running.
Adjust the Sensor
On the H10 you can use the adjustment on the bottom to increase the current through the sensor as it ages to maintain performance. Make sure to adjust it back to the starting point when you install a new sensor to extend the life of the sensor.
Use a Reference Leak
I see many techs attempt to use a bit of refrigerant out of a tank to test their leak detector. With most detectors having a published leak detection accuracy of 0.10 oz per/yr this is a really rough way to test a detector. The best way is to use a tiny calibrated leak or a leak reference bottle like the one shown above to ensure that your detector is going to find small leaks as well as large ones.
If you treat your detector well and confirm the operation of it every time you use it, you should get great results and a long life.
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!
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
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
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.
I recently received a message asking for a discussion of RH. WB and DB. Time and time again I hear techs say that condensation occurs when “hot meets cold” which may be true in some cases but that is only a shorthand way to describe it and doesn’t really address what is going on when we see condensation and undesirable growth.
RH = Relative Humidity as in the percentage of humidity relative to the amount of moisture can hold at that temperature.
I like to think of it like sugar in a cup of coffee. The hotter the coffee the more sugar the coffee can hold.
When we say the air is “humid” we can mean it contains a lot of absolute moisture in grains or pounds or we can mean it is high “relative” humidity which is the more common meaning. 95 degree air at 50% RH contains far more moisture in lbs per lb of air than 65 degree air does at 50% RH does.
It’s a common misconception that hot air is more humid. While it’s true that hotter air can hold more moisture in the same way that hotter coffee can hold more sugar it does not mean hotter means more humid. In fact, if you heat a mass of air and the amount of moisture doesn’t change the RH will go down as the air gets warmer. This is why the RH coming out of the top of a furnace is lower than the RH going in. No change has occurred in the actual amount of moisture present, the air is just hotter and therefore lower in humidity relative to how much it can hold.
DB = Dry Bulb and is the temperature of air without taking account for evaporation / relative humidity
WB = Wet Bulb and is the temperature of the air with the evaporative effect of a “wet bulb” taken into account. Quite literally wet bulb temperature is the temperature a thermometer bulb will be when covered in a wet fabric and whirled in the air or placed in an air stream.
If the RH is below 100% the WB will always be lower that the DB. The differential between the DB and WB illustrates the RH. The higher the differential the lower the RH. The lower the differential the higher the RH. When DB and WB read the same then the RH is 100% and the air is “saturated” and no more evaporation can occur.
When air hits 100% RH the dry bulb and wet-bulb temperatures are the same and this point is known as “dewpoint”
A friend of mine was telling me that a utility had the idea that they could reduce energy consumption by turning HVAC equipment on and off rapidly during times of high load. In their minds wouldn’t it be better to spread out the off time rather than keeping the system off for longer periods and allowing the space to become uncomfortable?
The answer is a HARD NO on that one!
Short cycling is a condition where equipment goes on and off more than is optimal, each time it goes on and off is called a cycle and we (almost) always to keep run times long and cycles to a minimum. Sometimes short cycling occurs due to a system fault and sometimes it occurs due to a mismatch between system capacity and load (cooling or heating too quickly).
Short cycling is a problem for many reasons including poor temperature control, inadequate dehumidification, rapid component failure and the list goes on and on. RAPID short cycling can quickly cause contact and relay failure due to arcing and can be very damaging to motors.
Let’s look at some common conditions that cause short cycling –
Safeties, Limits and Pressure Switches
Anytime there is an issue with the equipment that causes high temperatures or and high/low system pressures there are often controls that shut the system off before a catastrophic failure occurs. Some common examples would be –
When one of these safety controls turns the system off there will often be a time delay that prevents the equipment from coming right back on. In most of these switches there will be a gap between the make and break (on and off) points in the switch which will naturally help to prevent rapid short cycling.
Loss of Power
When power is rapidly cycled (turned on and off) the components may go off then back on quickly if there is no time delay. Here in Florida this happens often during thunderstorms but it can also be caused by flipping a breaker on and off rapidly or bumping a condensate switch. These sorts of rapid short cycling events are hard on motors and controls and can even cause scroll compressors to run backwards (on rare occasion).
Oversizing / Low Load
The way we control temperature with most appliances is by running them until they hit setpoint and then shutting them off. This can result in short run times when the load is low or when the equipment is oversized. The best designs result in the equipment running non-stop when during peak load, this isn’t intuitive for most customers and they will often complain that the system “never shuts off”, you can reassure them that so long as they are staying comfortable, never shutting off is a good thing for system longevity and power consumption.
Controls Design & Setup
The controls play a big part in run time depending on how they are setup. It is almost never as simple and turning on and off at a set temperature because that would almost certainly result in short cycling unless the system has variable capacity or the capacity is perfectly matched to the load. In most real world conditions the controls will need to manage a deadband or gap between on and off to balance comfort and short cycling.
Controls do this by maintaining and on off dead-band and maintaining a maximum CPH (cycles per hour) like the Honeywell thermostat shown above. This means that a thermostat with a 3 degree dead-band with a cooling CPH set to 3 and a set-point of 75 degrees would come on at 76 and shut off at 74 while turning on and off a maximum of 3 times per hour.
So these are some of the factors that impact short cycling but what are some of the issues associated with short cycling? here is an incomplete list –
I’m sure you can think of many more.
The goal is to run long and steady cycles without any cutting in and out safeties and a good match of system capacity with the load. This helps us provide comfort, efficiency and system longevity.
What are some short cycling issues you have seen?