Let’s discuss “Short-Cycling”

[Yankeefarmer]

I commonly see contributors here in the HVAC forum refer to the evils of short-cycling caused by oversizing heating and/or cooling equipment. I am not convinced, though, that there is a common agreement on what constitutes “short-cycling.”

When I do a web search, the most common result returned defines it as the equipment shutting off before the set temperature is reached. Oversizing equipment is not likely to produce this result except possibly in cases of poor thermostat placement. One website defined short-cycling as the situation where the equipment runs for only one or two minutes before shutting off, Another site defined short-cycling as a situation where the equipment runs less than 7 minutes per cycle. I’d dispute this latter one, especially in cases where it is early in the heating or cooling season and the loads are very low.

Is there a common definition here on the GJ? (I’ll be in the kitchen making popcorn to munch on while reviewing responses. )

 

[dave*99]

Consider that you are asking for a definition of short cycling and that is different than asking what can cause short cycling.
A short cycle can be caused by a number of things. And how short is short may vary based on cause.

AC too big? House cools quickly, AC shuts off. BUT the normal dehumidification that occurs in a normal length cycle gets cut off. House is cool, but humid.

Forced air furnace vent stack plugged up? Furnace heat exchanger overheats and the high limit causes the unit to turn off. House is cold.

A short cycle is one that results in deficient performance compared to the a system operating properly.

I could go on.....

 

[FMB4]

Imo, 'short cycling' is often caused by HVAC thermostats that are factory programmed to maintain the set temp at plus or minus 0.5 degrees.

 

[Bert_]

I'd say a couple minutes is short cycling. You can get condensation in parts of the heater where it's not made to handle it.

For me it more of a comfort thing. A smaller heater runs longer, more even heating. A big heater gives you a short blast of heat then nothing.

 

[gmcgeo]

A smaller heater running longer = undersized. it puts stress on the system. Cutting a cycle short, or making it run too long, harms the system in a number of ways

 

[fitter30]

What type of system are u writing about. First is the room thermostat set up correctly. What type of control are u writing about? What type of system are u having problems with heating is the most problematic. Heating and cooling cycling more than three times a hour. Space who design the equipment, did they run a manual j load ( engineer go to program) have a blower test? Guess not!

 

[Dagny]

Here the perfectly sized system will never shut off on the coldest day of it's existence and still maintain temperature.

 

[PoorUB]

A smaller heater running longer = undersized. it puts stress on the system. Cutting a cycle short, or making it run too long, harms the system in a number of ways

I disagree. What stresses a system is the start and stops. One a properly sized and maintained unit is up and running the duration of time is runs has little effect on the longevity of the unit. It is like comparing start and stop driving to highway driving in a car. Two cars each have 100,000 miles on them, one never left the city, the other was all highway, which one would you buy?

Take a shop unit heater. It gets a call for heat from the thermostat, the burner lights and it runs for a few seconds warming up the heat exchanger then the fan turns on. The heat exchanger expands and moves around from the heat. Once the unit is running everything is stable. Then the unit shuts off, the heat exchanger cools and flexes and it shrinks. Also the gas valve goes through cycles, relays in the circuit board go through cycles. Components are happier on or off, the chance from on to off causes arcing in contacts, electronics in the board heat up and cool down. All these changes are whatsuck the life out of everything.


Now I have been one that has warned about short cycling. In my case I an referring to an oversized unit heater that starts up and runs for a couple minutes and shuts down. The big culprit here is condensation from the byproducts of combustion. Short cycle and the heat exchanger doesn't get fully warmed up to boil off the condensation so the heat exchanger is always wet inside, mostly in the lower portions and it starts to rust, eventually rotting out the heat exchanger.

Also consider when a pro sizes a heating unit he tries to shoot for the balance where the unit might run almost all the time in the coldest weather. Keep in mind this unit is already oversized for all but a week or two of the winter so the unit cycles in warmer outdoor temps already. Oversize it even more and it cycles shorter times and might cycle more times to keep a space warm where a smaller unit runs less cycles, but longer run times because it has to run longer than the larger unit to heat the space.

Someone also mentioned the larger unit firing up, putting out a hot blast for a short time then shutting off where a smaller unit runs longer putting less heat into the space for the same amount of time.

 

[Yankeefarmer]

When I started this thread, I was not trying to solve a short cycling problem. As noted in my first post, there appears to be disagreement among HVAC companies on how short constitutes a “Short cycle.” I presume that, like so many topics, people here don’t agree on the definition and this can confuse people inexperienced with HVAC equipment and practices. I was trying to gauge the degree of consensus.

Dagny and PoorUB nicely summed up my thoughts on short cycling, in that a system that is sized perfectly for the heating load on the coldest days of the heating season will be oversized for every other day, and “very oversized” for the mildest days requiring heat. I’m wondering what percentage of contributors here would define those short runs on mild days (or conversely, short cooling runs on days requiring a little A/C) as short cycling.

Thank you to everyone for your comments so far.

 

[Showkey]

Pretty much sums up the reason a two stage furnace in the home is perfect solution. Does not save a lot money on fuel……but …….comfort and even heat/ cooling with low noise are achieved no matter the outside temperature.

 

[gmcgeo]

I disagree. What stresses a system is the start and stops. One a properly sized and maintained unit is up and running the duration of time is runs has little effect on the longevity of the unit. It is like comparing start and stop driving to highway driving in a car. Two cars each have 100,000 miles on them, one never left the city, the other was all highway, which one would you buy?

Take a shop unit heater. It gets a call for heat from the thermostat, the burner lights and it runs for a few seconds warming up the heat exchanger then the fan turns on. The heat exchanger expands and moves around from the heat. Once the unit is running everything is stable. Then the unit shuts off, the heat exchanger cools and flexes and it shrinks. Also the gas valve goes through cycles, relays in the circuit board go through cycles. Components are happier on or off, the chance from on to off causes arcing in contacts, electronics in the board heat up and cool down. All these changes are whatsuck the life out of everything.


Now I have been one that has warned about short cycling. In my case I an referring to an oversized unit heater that starts up and runs for a couple minutes and shuts down. The big culprit here is condensation from the byproducts of combustion. Short cycle and the heat exchanger doesn't get fully warmed up to boil off the condensation so the heat exchanger is always wet inside, mostly in the lower portions and it starts to rust, eventually rotting out the heat exchanger.

Also consider when a pro sizes a heating unit he tries to shoot for the balance where the unit might run almost all the time in the coldest weather. Keep in mind this unit is already oversized for all but a week or two of the winter so the unit cycles in warmer outdoor temps already. Oversize it even more and it cycles shorter times and might cycle more times to keep a space warm where a smaller unit runs less cycles, but longer run times because it has to run longer than the larger unit to heat the space.

Someone also mentioned the larger unit firing up, putting out a hot blast for a short time then shutting off where a smaller unit runs longer putting less heat into the space for the same amount of time.

Thank you for taking the time to explain this. I understand fully about short cycling.

I do not understand the thinking of running smaller to run longer and to the point of it never shutting down.

However, I am not going continue with the subject.
This seemed to be a starting post about my comment of under sizing from a previous post.

You all have a great holiday.

 

[PoorUB]

Thank you for taking the time to explain this. I understand fully about short cycling.

My whole post was not aimed at you, just the first part, but was on a roll!

 

[Yankeefarmer]

Thank you for taking the time to explain this. I understand fully about short cycling.

I do not understand the thinking of running smaller to run longer and to the point of it never shutting down.

However, I am not going continue with the subject.
This seemed to be a starting post about my comment of under sizing from a previous post.

You all have a great holiday.

@gmcgeo, I don’t know if the comment in bold refers to me starting this thread or to PoorUB’s post. I want to assure you that I did not start this thread because of a post you might have made in a previous thread. It was started because I frequently see warning about oversizing and the evils of short-cycling, and I wonder if there’s even agreement about what constitutes short cycling. My web search suggests there’s not. It reminds me of the famous judge’s statement that he couldn’t define ***********, but he knew it when he saw it.

 

[LS6 Tommy]

I will add that not reaching setpoint is not "short cycling". That is called "undershooting".

Tommy

 

[Yankeefarmer]

After spending even more time sorting through previous threads, I found the post that initially created the questions in my mind regarding short-cycling. In that thread, the poster, who was debating A/C since for a roughly 2000 sq. ft. building, mentioned that he’d decided to go with a 4 ton unit rather than a 5 ton unit because he was concerned the 5 ton would have a shortened life due to short cycling. In my mind, the size difference between the two units was unlikely to produce a significant difference in system life with all other factors being equal.

This raises another question, and here I am referring specifically to heat pump/AC units, where condensation of corrosive product of combustion compounds is not a factor, but electric motor starts is. Are inverter compressors far more tolerant of frequent starts due to their soft start capability?

 

[dave*99]

After spending even more time sorting through previous threads, I found the post that initially created the questions in my mind regarding short-cycling. In that thread, the poster, who was debating A/C since for a roughly 2000 sq. ft. building, mentioned that he’d decided to go with a 4 ton unit rather than a 5 ton unit because he was concerned the 5 ton would have a shortened life due to short cycling. In my mind, the size difference between the two units was unlikely to produce a significant difference in system life with all other factors being equal.

This raises another question, and here I am referring specifically to heat pump/AC units, where condensation of corrosive product of combustion compounds is not a factor, but electric motor starts is. Are inverter compressors far more tolerant of frequent starts due to their soft start capability?

Regarding the first paragraph - the equipment should be sized to heat/cool the space properly and efficiently. If that is done, the cycles per hour will be appropriate. Is the equipment is of good quality, it will tolerate the required number of stops and starts over its usable service life. Using start stop cycles as a selection criteria for sizing equipment is a bit backwards for a design process.

Regarding the question - Inverter compressors do have soft start. But they are designed to operate at variable speeds and therefore provide a variable output. They slow down to match the output to the load. So by design, they will run longer and have fewer stops and starts than a traditional compressor. Again - the equipment should be designed to heat and cool properly etc......

Always start a design process with meeting the need for functional operation (will it provide comfortable heating and cooling and perhaps even humidification and dehumidification) Follow with efficiency (energy required to operate the system) and then consider longevity (can the selected system provide good service life in its intended operation)

 

[yeldogt]

After spending even more time sorting through previous threads, I found the post that initially created the questions in my mind regarding short-cycling. In that thread, the poster, who was debating A/C since for a roughly 2000 sq. ft. building, mentioned that he’d decided to go with a 4 ton unit rather than a 5 ton unit because he was concerned the 5 ton would have a shortened life due to short cycling. In my mind, the size difference between the two units was unlikely to produce a significant difference in system life with all other factors being equal.

This raises another question, and here I am referring specifically to heat pump/AC units, where condensation of corrosive product of combustion compounds is not a factor, but electric motor starts is. Are inverter compressors far more tolerant of frequent starts due to their soft start capability?

Short cycling for the most part is about comfort.

In most areas where AC is needed there is also moisture in the air ...... the human body likes 75 degrees with low humidity vs 70 degrees with high humidly. Oversizing an AC system will have the space hitting the temp (let say 70) before it has run enough to remove the moisture. What happens in houses with oversized system --- people turn down the thermostat so the system runs enough to remove the humidity. It's relly a problem in areas where the humidity is high and it not overly hot outside. Ideally an AC system should be designed to run full time on the hottest day of the year. Most times what happens when HVAC people are designing a system for a house --- they factor for the worst case -- lots of people on the hottest day. This is a mistake ... in my area how often does it hit 102? Do I design my system so I can have a wedding on the hottest day we get in 10 years? And keep it at 70 ? No -- you understand that a system running with low humidity will be fin for a few hours even at 75. Picking the next higher sized equipment (often one full ton) locks you into an oversized unit all 99.9% of the time.

 

[yeldogt]

Heaters that go one and off many times in an hour are not as comfortable as those able to run at lower output ..... think of your own house. When are you feeling warm and when not ? Why are old school radiators so comfortable? Constant heat.

The same is true with a shop ..... when are you happy and when the shop is cold. When it's running. People put big heaters in shop for quick heat ups .... it heats the air ... then the shop gets cold. Often what happens is we put the thermostat up to keep the heater running --

 

[Steve W.]

Short cycling for the most part is about comfort. ...... the human body likes 75 degrees with low humidity vs 70 degrees with high humidly.

Another factor here is the calibration of the thermostat. We moved into our current house 27 years ago. When it got cool a few months later, we bumped the thermostat up to 70-ish and were soon ROASTING. Took a while of playing with the thermostat to learn that COMFORTABLE temperatures for average use were found when the thermostat showed 62-63°. No drama or anything like that, just a mental note to "set HERE for comfort". We knock it back a few degrees at night. Might actually leave it there, if nobody will be home the next day.

Will soon be having 'fun' learning a new thermostat, along with a new HVAC system, but will adopt the same "set HERE for comfort" philosophy.

.

 

[fitter30]

Variable refrigerant flow heatpumps mini splits and whole house units load has to very small till they shut off.

 

[finn]

The last conventional boiler I bought, maybe fifteen years ago, had two modes: on or off.

The last two are modulating condensing boilers with a 10:1 turndown ratio. They are much more suitable for heating my shop. One can set a low outlet temperature for maximum condensing and heat extract from the burned propane.

modern home forced air furnaces are the same, but the technology hasn’t worked down to the cheap hanging furnaces typically used in outbuildings, you have to be more carful when sizing these since they are much like that boiler I bought fifteen years ago: on or off. They will cycle frequently until all of the wall mass, tools, etc stabilize near your temperature set point if you have an oversized system. The rapid or frequent cycling induces thermal stresses int the heat exchanger

 

[danski0224]

I'd define "short cycling" as:

Call for heat (or cooling).

Equipment comes on and operates for "a few" minutes. Literally a few (like 3 or less) minutes, no longer.

Set point is reached.*

Equipment cycles off.

Cycle repeats within a few minutes, on a day where normal operation** is required. This is key. It comes on, then shuts off relatively quickly. This cycle repeats. The conditioned space is never really "comfortable". Air movement is required to even out the temperature, the fan is never on long enough in "automatic" mode if the equipment is oversized and is short cycling.

**: "Normal operation" is defined as the window where the equipment is needed to maintain indoor conditions outside of either extreme (NOT when the desired indoor temperature is within just a few degrees of the outside temperature OR the equipment needs to be on most of the time).

*: A grossly oversized furnace may (and usually does) "short cycle" and "run on the limit switch" rather than the thermostat due to poor airflow (lack of ductwork and/or air filter). The thermostat is continually calling for heat during this event, but the furnace is coming on and turning off- the desired set point may not be reached. In my experience, all forced air furnaces with 1600 CFM and 2000 CFM blowers are not installed properly.

Two stage equipment (heating and cooling) is a huge help for those installations that are in between equipment sizing loads. So is a variable speed fan.

Edited to add some clarity (I hope).

 

[Hobby_Man22]

I commonly see contributors here in the HVAC forum refer to the evils of short-cycling caused by oversizing heating and/or cooling equipment. I am not convinced, though, that there is a common agreement on what constitutes “short-cycling.”

When I do a web search, the most common result returned defines it as the equipment shutting off before the set temperature is reached. Oversizing equipment is not likely to produce this result except possibly in cases of poor thermostat placement. One website defined short-cycling as the situation where the equipment runs for only one or two minutes before shutting off, Another site defined short-cycling as a situation where the equipment runs less than 7 minutes per cycle. I’d dispute this latter one, especially in cases where it is early in the heating or cooling season and the loads are very low.

Is there a common definition here on the GJ? (I’ll be in the kitchen making popcorn to munch on while reviewing responses. )

If you oversized the cooling abilities, you also oversized the humidity control. As long as it reaches the set temp and keeps the humidity under control its fine.

 

[Hobby_Man22]

I'd also buy a two stage system. I do believe the thermostats make it run a minimum amount of time also, because I've noticed sometimes my heat pump will way overshoot the set temp if it kicks on on a day where the temp isn't that cold. Like say 65 or 70 and for some reason it say I had it off for a few days and came home and turned it on.

 

[yeldogt]

If you oversized the cooling abilities, you also oversized the humidity control. As long as it reaches the set temp and keeps the humidity under control its fine.

Humidity is controlled by moving air over the coils under the dew point ... oversized equipment reduce the humidity control.

Lowering the temp w/o removing the water from the air -- that's what happens.

 

[Hobby_Man22]

Mine is oversized in the shop but will still get the humidity down to 45 percent running 8-10 minute cycles. The house runs a much longer cycle, yet still stays around 58 percent humitidity.

 

[danski0224]

Mine is oversized in the shop but will still get the humidity down to 45 percent running 8-10 minute cycles. The house runs a much longer cycle, yet still stays around 58 percent humitidity.

There are ductwork issues that can contribute to humidity issues, plus building construction issues and there may be equipment issues.

Just because it appears to be making cold air doesn't mean that it's functioning correctly.

 

[Hobby_Man22]

There are ductwork issues that can contribute to humidity issues, plus building construction issues and there may be equipment issues.

Just because it appears to be making cold air doesn't mean that it's functioning correctly.

I'm not really bothered by it. The coil is probably dirty in the house. That's my guess.

 

[Showkey]

I would never consider 58% humidity an acceptable home comfort value.

 

[Yankeefarmer]

In reading and re-rereading responses here, it seems like there is a fair degree of agreement in defining short cycling in terms of comfort, rather than strictly on a cycle time basis. If so, if the owner/occupants are comfortable, short cycling is not a real concern.

A very Merry day-after Christmas to everyone here. Gonna be having family here today to continue our celebration. We’ll be comfortable.

 

[danski0224]

In reading and re-rereading responses here, it seems like there is a fair degree of agreement in defining short cycling in terms of comfort, rather than strictly on a cycle time basis. If so, if the owner/occupants are comfortable, short cycling is not a real concern.

Given that the equipment has a designed cycle life, short cycling will shorten equipment life from more on/off cycles.

It also costs more to start up the fan and compressor motors more frequently compared to staying on longer once started.

There can be damage to the heat exchanger and flue because the equipment does not run long enough to deal with condensation (parts do not reach operating temperature).

An oversized AC unit can have liquid coming back to the compressor, shortening its life.

These things can take years to show up, but by the time it is properly diagnosed, the equipment has been murdered by a crappy installation.

Poor indoor comfort is just the most obvious side effect (note: there can be other causes for this). The other stuff will cost money to fix, provided that it is fixed correctly.

 

[yeldogt]

In reading and re-rereading responses here, it seems like there is a fair degree of agreement in defining short cycling in terms of comfort, rather than strictly on a cycle time basis. If so, if the owner/occupants are comfortable, short cycling is not a real concern.

A very Merry day-after Christmas to everyone here. Gonna be having family here today to continue our celebration. We’ll be comfortable.

Where you "NOTICE" short cycling is in an older house with less insulation/ leaking windows etc. How about that bump out addition you did in the kitchen where you extended the existing ductwork -- The added windows and square feet increase has the room feel a bit drafty/ colder?

What is happening? -- the room is heating and cooling. You end up heating the air -- the warm air trips the thermostat ... off goes the heater. But -- the room never really reached the temp ..... just the the air in the room. It takes time to heat things. So depending where you are in the room the on off cycle will be more pronounced.

This is not going "feel" the same with an oversized boiler ---- or an old school oil burner. The boiler being oversized is just going to waste fuel -- starting and stoping all day long being inefficient with the way it burns fuel. Old school oil furnaces had so much mass that they gave off heat for quite a while after the burner stopped -- the fan still delivering hot air to the house from the hot heat exchanger -- it's still wasting fuel.

As houses get more efficient the effect of marginally oversized equipment is less noticeable -- the downside temp effect of windows and leaks being lower. It's still there and still wasting fuel. Another thing that is common -- ductwork is expensive and requires some thought ....... lower end heating equipment comes in fewer sizes and is all about the same price regardless of BTU output. What does this do ---- The vast majority of installs have ductwork that is undersized and heating equipment too big. The ductwork goes in by guesswork and the heater is picked so it;s sure to be big enough ..... this normally has a big fan trying to push through small duct work. Most times getting a proper load done reduces the equipment size and it's now quieter and more comfortable .... longer run times and no noise trying to push too much air through small dust work

 

[yeldogt]

Mine is oversized in the shop but will still get the humidity down to 45 percent running 8-10 minute cycles. The house runs a much longer cycle, yet still stays around 58 percent humitidity.

It's all about the humidity in the surrounding air. In humid areas of the country proper sizing is very important .... That's why multi stage cooling equipment or full VS equipment in a humid part of the country can really transform the comfort of a house. With an oversized system it's not uncommon for people to set the house at 70 to keep the humidity down. I see this at the beach all the time ... My equipment is set at 73 w/ a humidity set point. The system can ramp down and run very slow just keeping the humidity down to that target humidity ... it's rare that it will have to overcool under set point. 73 with low humidity is much more comfortable than 70 and high

 

[Yankeefarmer]

Given that the equipment has a designed cycle life, short cycling will shorten equipment life from more on/off cycles.

It also costs more to start up the fan and compressor motors more frequently compared to staying on longer once started.

There can be damage to the heat exchanger and flue because the equipment does not run long enough to deal with condensation (parts do not reach operating temperature).

An oversized AC unit can have liquid coming back to the compressor, shortening its life.

These things can take years to show up, but by the time it is properly diagnosed, the equipment has been murdered by a crappy installation.

Poor indoor comfort is just the most obvious side effect (note: there can be other causes for this). The other stuff will cost money to fix, provided that it is fixed correctly.

Back in 2019, GJ member @Mabalzach wrote in a thread that manufacturers “spec“ (I assume he meant “design for”) 2 to 5 cycles per hour. How do you and others here feel about that?

 

[danski0224]

Back in 2019, GJ member @Mabalzach wrote in a thread that manufacturers “spec“ (I assume he meant “design for”) 2 to 5 cycles per hour. How do you and others here feel about that?

Some thermostats have an adjustable cycles per hour setting.

As long as there is a call for heat or cooling, and the equipment run time avoids "short cycling", the I wouldn't be looking strictly at a cycles per hour number.

I would assume that "short cycling" would quickly eclipse 5 cycles per hour anyway.

 

[PoorUB]

In reading and re-rereading responses here, it seems like there is a fair degree of agreement in defining short cycling in terms of comfort, rather than strictly on a cycle time basis. If so, if the owner/occupants are comfortable, short cycling is not a real concern.

A very Merry day-after Christmas to everyone here. Gonna be having family here today to continue our celebration. We’ll be comfortable.

No!

Short cycling shortens the life of the equipment.

Like I said earlier, highway versus city driving.

 

[PoorUB]

Mine is oversized in the shop but will still get the humidity down to 45 percent running 8-10 minute cycles. The house runs a much longer cycle, yet still stays around 58 percent humitidity.

Your other posts claimed the AC would not keep up!

What changed?

 

[Hobby_Man22]

Only in the summer when it's 110 outside

 

[Hobby_Man22]

I did add that radiant barrier stuff to the two bay doors.

 

[Hobby_Man22]

I think it needs more than r30 on the roof.