Short cycling has nothing to do with time between normal on-off cycles. Short cycle occurs when unit is rapidly turned off then back on within less than 3 minutes at t-stat or power interruptions such as during lightning storms. Such off then back on shortens compressor life.

Typically under 6 cycles an hour, is a fair number with single stage equipment. With 2 stage probably 3 an hour would be good and the switch from 1 to 2nd stage or from 2nd to 1 is not considered a 'cycle'.

Cycling is also dependent on ambient conditions, loads in the room of where the controls are located and so on.

Realize a quote unquote 'properly sized' AC system is 'oversized' 90% of the time it runs. There will be some 'short cycles.'

You have to look at the WHOLE picture.

Short cycling is a detriment, because the system doesn't run long enough to return oil to the crankcase of the compressor. Without proper amount of oil, your poor compressor will have a short life.

First of all, most condensers (AC or HP) have a 5 min lockout preventing the compressor turning off and back on again too quickly. Secondly, if your house was built in 2005, it should have pretty good insulation. You need to time the on/off cycles carefully and get an accurate assessment of whether your system(s) is really "short cycling". If it is, that's generally a symptom of an oversized system but I wouldn't think 4.5 tons in a 3600sq ft home is oversized. You may be able to increase the anticipation and/or cycle time at your thermostat.

I'm still not understanding. Is short-cycling based on how many cycles per hour, or on the time of each cycle? My understanding is the latter, and that under 10 minutes per cycle would be considered a short cycle, no matter the lag between cycles. Which is correct?

As evidenced by above comments by various people,"short cycling"mean's different things to different people. To me it describes a malfunction or condition relivant to non-performance. To others it has something to do with run time of fully functional units. I've already answered based on my discription in an above post so will not do so again. As to what you are calling possible short cycle. The amount of time a unit run's before reaching set point and turning off and how long it is until t-stat turn's it back on is largly dictated by a combination of things outside the mechanics of unit. Some of those things are building insulation, number of windows in building,location of windows,quility of windows,caulking throughout interior and exterior of building,number of people inside building,size of people inside building,how many and type of lights,other appliances such as cook stove in operation and,,,,,,,,well I don't think we need to continue. One major exception to the preceeding is proper size and type hvac equipment. Austin Air covered all this but you evidently still don't understand. Why don't you tell us what you are worried about and what causes you to worry,other than random comments on the net? Is the unit not keeping the place comfortable? Do you think your electric bill is too high? What make's you ask this question? Perhaps if you rephrase your question. This I can tell you with absolute certainty,there is no prescribed minutes and secounds between cycles and anyone that say's different is blowing smoke up your ductwork.

The only reason I even thought about it was after reading another thread a guy said his was running for 8 minutes then shutting off for 5. Different setup different situation but I guessed mine was about the same or similar so I was curious. Every one was stating he needed to figure out why it was short cycling and I had never really heard of that.

Today I got my stopwatch out and started checking mine. When I got home from work the unit was running. Outside temp was 83 degrees and 60% humidity thermostat is set at 77 degrees. When the AC shut off I started the timer and it was off for 15 minutes and 8 seconds. Then it started back up and was running for 9 minutes 36 seconds. Shut off again, this time for 12 minutes 21 seconds and it then ran again for 9 minuets 32 seconds. During this entire time the thermostat never changed temps, it stayed on 77 the entire time so it must kick on at a half degree drop or something similar. Again I am not noticing any problems and have no issues with the comfort level I was just curious if there is a magic number that would be considered an issue.

There is a specialized branch of hvac engineering that can give you a close estimate,,,,IF,,,,you first tell them exact details on above mentioned plus a few wildcard conditions. I will defer to our in house member called "energy-rater"who know's far more than I about this.

I'd echo what flyin-lowe said. When I read about HVAC short-cycle is referring to a negative result from oversized equipment which causes it to run for several shorter cycles rather than fewer longer cycles, not some type of mechanical malfunction as klem1 considers it - as a matter of fact I've never heard of it in that context, only as it relates to over-sized equipment. Coincidentally, I also took out the stopwatch today and got very similar numbers to flyin - about 8.5 minutes on followed by 12.5 minutes off, with an outside temperature of 84 and the thermostat set on 75. And similarly I've had no issues with the comfort inside the house - neither the temperature nor the humidity - though I don't have a humidistat to measure the latter.

Short cycling technically is related to short run times, not short time between run cycles ... although the two can be related.

The issue with short run times is the system doesn't run long enough to remove sufficient humidity ... typically meaning it's oversized ... cools too fast. A properly sized air conditioning system typically can be expected to run almost continuously at extreme/high outdoor temperatures. Cooler outdoor temperatures of course it'll run for shorter periods since there's less of a heat load to battle.

Short time between cycles (frequent cycling) is more related to how fast exterior/ambient heat filters back into the cooled space, triggering the thermostat to turn the system on again. Other specific factors can cause frequent cycling ... such a cold air register blowing on a poorly-located thermostat. That doesn't mean the system is oversized ... just that a blast of cold air on the thermostat triggers it to shut off prematurely.

I've previously fretted over "am I short cycling or not?". Here's a snapshot of the past 24 hours of my system's 2-stage compressor operation. It's been getting to the mid 80's and as low as mid 60's where I am (Long Island, NY). You'll see I have short cycles, long cycles and no cycles within a 24 hour period. I currently have my stat (only 1 zone) set to a maximum of 3 cycles per hour.

I'm guessing that my unit is running long enough to get oil to the proper places as described above. Plus since the current outside temps are less then 10 degrees above what I have my stat set out I would see significantly longer run times if it was 90 plus degrees outside.

Short cycling could also be caused by the thermostat and/or its placement relative to a supply.

I understand exactly what you’re talking about. Let me explain a different way.

Austin is correct when he said, “sized' AC system is
'oversized' 90% of the time it runs. There will be some 'short cycles”.

When sizing your unit, it needs to be able to successfully
operate normally and cool your house under the MOST extreme heat, humidity and dew
points for your specific area of the country.

I’m in the same boat as stickman42 in New York. We’ve had a couple REALLY hot humid days, but
not a record setting heat breaking summer.

You mentioned your temp was 83 degrees and 60% humidity,
that is a cake walk for an AC (that is clean, serviced and working properly) to
keep your inside temp at 77 degrees. If
it was 100 degrees out, with 90% humidity, the AC would run much longer.

Basically what Austin is saying, is that the unit needs to
be sized for the MOST EXTREME heat for your area. But 90% of the time, you won’t see the
extreme heat. You’ll see average summer
temperatures for your area. Which is why
the unit is only “sized perfectly” when you see the extreme hot days.

So far, this summer hasn’t been bad in Ohio where I live. My central AC cycles 3 times and hour and ran
for 6 - 7 minutes each time. Four years
ago we were over 100 degrees with high humidity for 5 days in a row and my AC
ran about 45 - 50 minutes an hour to maintain my indoor temp.

Now, my neighbors AC for example, is 35 years old, not serviced
and the outside coils have 4 inches of dryer lint, dust & dirt clogging
them. I’m sure their air filter is
clogged too. Their AC runs 24/7, no
matter what the temp is outside and it cannot maintain their desired inside
temperature. As the hours go by, the
hotter it gets inside so it runs all night too, trying to put the temp back to
what they selected on the thermostat.
(It’s a rental, obviously the owner and tenants don’t care so I’m not
saying a word.)

I shared your same concern a few years ago and asked my tech
if I should manually turn the AC off and on myself and let it cycle once an
hour, so the compressor would run a good long time. He said not to waste my time. Some summers are just cooler than others.

He did change a setting in the thermostat for 2 up and 2
down. So if the thermostat is set to 70,
the unit won’t engage until its 72, or 68 if heating. He also changed a setting for 3 cycles an
hour instead of 4 and he changed the blower motor fan speed to low or extra low allowing the
unit to run longer.

When the unit was new and the fan was set on high, it would
cool my house in 4 minutes. But it never
got the humidity out. But that was 15
years ago and I bought the cheapest model.
I believe the fan speeds may be automatically controlled by the unit
now.

I do see what you’re saying, I wouldn’t worry though. Maybe August will be hot as blazes, or next
summer, then you’ll see higher run times.

Stickman42, I give you the HNL (hole nother level) award for compressor cycles.

Seriously, 37 cycles or there about in a 24 hour period is nothing to worry about. I would worry more about ants getting into the unit stopping it from coming on, but that's just me.

If you have 10 or more minutes of off time with close to run time of additional 10 minutes that means 18-20 min cycle or 3-4 cycle per hour.

If your system does this in extreme heat (temps of 95 OAT or higher) then it is probably grossly oversized. That is where problems of mold come into play especially if you are in a high humid climate, like mine in Katy, Texas

Austin - why thank you! I'd like to thank the Academy, my family and friends, and the Good Lord... sorry - just goofin.

I no longer worry about whether I'm over/undersized or not. I had that a-ha moment when I realized that load changes as the outside conditions do. JeCrigler explained it in very clear terms above that your system is sized perfectly ONLY on extreme days. My system (GSHP - so no worries about ants thankfully) cycles as it should IMHO, so I believe I'm right-sized. The logging data does confirm what the installers manual J said I needed.

I hope flyin_lowe finds some comfort in all the responses here.

Now, my neighbors AC for example, is 35 years old, not serviced
and the outside coils have 4 inches of dryer lint, dust & dirt clogging
them. I’m sure their air filter is
clogged too. Their AC runs 24/7, no
matter what the temp is outside and it cannot maintain their desired inside
temperature. As the hours go by, the
hotter it gets inside so it runs all night too, trying to put the temp back to
what they selected on the thermostat.
(It’s a rental, obviously the owner and tenants don’t care so I’m not
saying a word.)

Wow. Just... wow. Such ignorance is painful to see and hear about.

Vith, not to change the subject, but what angers me the most is that all the houses in my development have the condenser unit placed 4 inches in front of the dryer vent. Ridiculous.

When I moved in, I never thought about it. But my tech showed me all the dryer lint that had piled up.

They are the old 1986 Rheem's so you really can't see inside because of the casing, and they're very short so you have to get on your knees and look up to see the coils.

Logically I understand why the condenser is placed so close to the dryer vent, at least in Ohio most all of us have basements. So the AC & furnace are in the basement and the washer dryer hookup is usually very close. So they try to run everything outside within the same vicinity. I don't understand why they can't just raise the dryer exhaust vent outside above the level of the condenser and point it up so the lint blows upwards.

I'm not a developer and don't know the laws, so maybe it's illegal. Just makes more sense to me.

Stickman, I did the same thing a few years ago. I had everything on an excel spreadsheet and showed my tech. I knew mine wasn't running long enough, but the house was cool so I wasn't complaining.

Mines only a 1.5 ton, which I still feel is oversized 95% of the time. We've only hit 93 degrees once this summer. All in the mid 80's.

However it does run longer when it's 88 degrees outside & I'm baking in the oven or using the crockpot or pressure cooker.

I feel sorry for my Mom's AC in Phoenix. Actually all of them. They hit 120 degrees a couple weeks ago. She has 2 AC's and said they didn't shut off all day.

I've watched my system a little over the past day. Run-time varies from 30+ mins during heat of the day to 10-ish mins in early morn hrs. 5 tons in 2,350 sq ft with 9' and 10' ceilings. Setpoint 77°F to 78°F. Outdoor varies 92+°F daytime, mid 70°Fs to low 80°Fs night. Killer humidity.

"They hit 120 degrees a couple weeks ago. She has 2 AC's and said they didn't shut off all day."

"Run-time varies from 30+ mins during heat of the day to 10-ish mins in early morn hrs"

This varying equipment behavior is to be expected, isn't it? I would believe that a lot of people "set it and forget it" at their thermostat. I do. 73F from May to September, no setbacks programmed. With this in mind, how can one expect their system to do anything else but have cycle times of very different lengths? When does the thread topic change from "it's short cycling!" to "it runs 24/7!"? After all, isn't the purpose of HVAC to provide comfort? I'm comfortable at 73F. If I were to have a bigger droop set in the stat, or agree to raise or lower the set point as outside conditions change, I guess I would have uniform, perfectly long cycles. No thanks, I'll leave it as is. Comfort does come at a price sometimes.

When I moved in, I never thought about it. But my tech showed me all the dryer lint that had piled up.

Install a dryer duct lint trap, catch it before it goes outside. Clean this out each use along with your dryer trap.

That seems to be the best reviewed one I found. Other traps seem to have problems, this one the only reoccurring bad review was its made of plastic.

Other than that, condensers collect stuff. Just the way it goes. Mine had a bunch of cottonwood cotton stuck on it, since I have cleaned it off couple years ago it hasnt built up yet. Required me to take off the housing to get at it however, need to be handy to do that (and put it back together).

That looks interesting! Never heard of those.

I've heard about the cottonwood & the mess they make. Don't have any here. Thank goodness.

Thanks for all the responses. When I had to replace my system this spring I was going nuts. I had 4 people come out and give me quotes and all of them had different ideas and were wanted to go completely different ways to fix my problems. I ended up going with a a friend of mine who has been in the business for over 20 years. So far I am pleased with the work. I am curious to see what this winter brings. My old system ( way undersized) would literally run 24 hours a day for several days if the temps got close to zero and it would never keep the house to the temp on the stat. I know my new system will cost more to run but hopefully it won't be running near as much so it might even out a little. Our comfort will be worth it assuming the increase is not too much.

30 mins turns longer, of course, when the outdoor temp gets to the upper 90°Fs and above.

Movie theater where I work, the four systems (lobby/office/projection, three auditoriums) may run for hours depending on the load ... which includes factors of outdoor temperature and varying number of bodies. All are two-stage, and may cycle the 2nd stage as needed with the 1st stage running continuously for the duration of the business day ... which is as it should be.

You really can't compare commercial systems to that of residential. It's just not a fair fight, because commercial systems are more likely to be multi staged and then additional equipment can be staged above that depending on the goals of the structure owner and the cooling / heating needs of the building.

While you can do this (staging) on a residence it is not common... due to the cost of implementation.

With systems that run in 1st stage continuously all day, this is a early form of 'load matching' as time moves forward and we get more into inverter based systems this will become more and more common.

The guy that installed my units called me last weekend out of the blue. He indicated he wanted to come check my system, specifically refrigerant levels. Since it was cold out when they were installed he wanted to check them while the temps were near 80 and the AC unit was running. He had to make a slight adjustment on one unit but everything was working properly. He checked the run times, he also checked the temp differences between the duct and the returns. He also had a meter that tested one more reading, I can't remember what it was called but it checked temps, humidity, etc. He had a name for it. Needless to say so far I am very pleased with the system.

sling psychrometer

Sounds good to me lol.