Short-cycles or normal?

[RobTech]

Hi All, I recently purchased a new A/C Unit for my condo, and it seems to have a short on-cycle; here is some info:

Location: South Florida
3/2 Condo: 1220 sq. ft.
Unit: 2.5 ton 16 seer Rheem 14ajm series, Split System, Btuh: 28600
EER Rating (cooling) 13

I'm not sure if this is considered short-cycling, but it seems to only turn on for only about 4 minutes at a time (and between 2 and 4 minutes at night). I read that new air conditioners are designed to run for long periods of time, and that short cycles can cause unnecessary wear-and-tear. I have read through a lot of posts, and my unit seems to be on the borderline of running too frequently?

Note: My old thermostat also allowed the temp to reach one full degree above the setting, and would then cool to one full degree below the setting (which was perfect for us). The new one seems like it's set to 0.5 degree difference... I checked, and the new thermostat they installed is non-programmable, while the old one was.

Can I do anything to slow down the air output, and would that possibly resolve the quick on times? If not, should I consider a thermostat where I can adjust the allowed range before the air turns on/off? (Or very possibly, this is all normal - and I should just let it be).

Here are some cycle times if it helps: (CPH 3)
Starting Inside Temp: 79, A/C setting: 77, Humidity 70%

On: 14 minutes
Off: 10 minutes
On: 4 minutes
Off: 16 minutes
On: 4 minutes
Off: 20 minutes
On: 4 minutes
Off: 19 minutes
On: 5 minutes

Thank you very much for any help!

[skippedover]

You're getting zero dehumidification when the system runs for only a few minutes each on cycle. I'd say you have an issues of some sort. What that sort is I can say not but it could be t-stat, pressure switch or equipment size issues. Was a load analysis done on the home prior to the new equipment being installed? I'd call the installing company out to check the system operation. You didn't state outdoor temps so maybe it's just not warm enough yet.

[RobTech]

Thanks for the quick reply skippedover. To answer your questions:
The first tech that came out asked the sq. footage, looked at the old unit (2.5 ton also) and that was about it.
The outside temp while recording the cycle times was about 80 degrees.
I actually did speak to the installing company (they called to follow up a few days after install) and mentioned the cycle times... The guy said to keep a cycle-time log, but this time for a longer duration. Same results with that; once the condo cools to the set temp, it only clicks on for 4 minutes every 10 to 15 minutes to maintain it.

Ideally, should a unit run for three 10-minute on/off cycles when CPH is set to 3?

[wahoo]

We don't use thermostats that are able to adjust cycles per hour, but if you have one, then it seems your unit is cycling about 3 cycles per hour. The amount or length of the on cycle will vary based on outside temp. as the on times will lengthen with higher temps outside (I'd assume). I'm sure there are some on this site with more CPH experience. I'd agree that you are not getting much dehumidification with that performance.:.02:

[RobTech]

Thanks wahoo - either way it sounds like something is off.
I was wondering if these units have different fan/blower speed options that a tech could adjust... (I really am surprised how strong the air is compared to the last unit). Maybe if it didn't cool down so quickly the on-times would be closer to normal.

[udarrell]

Hi All, I recently purchased a new A/C Unit for my condo, and it seems to have a short on-cycle; here is some info:

Location: South Florida
3/2 Condo: 1220 sq. ft.
Unit: 2.5 ton 16 seer Rheem 14ajm series, Split System, Btuh: 28600
EER Rating (cooling) 13

I'm not sure if this is considered short-cycling, but it seems to only turn on for only about 4 minutes at a time (and between 2 and 4 minutes at night). I read that new air conditioners are designed to run for long periods of time, and that short cycles can cause unnecessary wear-and-tear. I have read through a lot of posts, and my unit seems to be on the borderline of running too frequently?

Note: My old thermostat also allowed the temp to reach one full degree above the setting, and would then cool to one full degree below the setting (which was perfect for us). The new one seems like it's set to 0.5 degree difference... I checked, and the new thermostat they installed is non-programmable, while the old one was.

Can I do anything to slow down the air output, and would that possibly resolve the quick on times? If not, should I consider a thermostat where I can adjust the allowed range before the air turns on/off? (Or very possibly, this is all normal - and I should just let it be).

Here are some cycle times if it helps: (CPH 3)
Starting Inside Temp: 79, A/C setting: 77, Humidity 70%

On: 14 minutes
Off: 10 minutes
On: 4 minutes [Real bad]
Off: 16 minutes
On: 4 minutes
Off: 20 minutes [would be a better on time]
On: 4 minutes [horrible]
Off: 19 minutes
On: 5 minutes

Thank you very much for any help!

Where is the room stat located in relation to supply diffusers. etc.

The run time-on-cycles are way too short, it takes around 7 minutes for the A/C to get to a steady-state condition.

I'd set it for 2 cycles per hour & record the temp when it comes on & when it cycles off along with the on & off lengths of time.

Set the room stat to cycle on at say 78-F you should feel comfortable with a 3-F drop which would be 75-F, then the time it takes to go back up to 78-F is based on how well your home is weatherized & the conditions outdoors.

Get a humidity gauge at a hardware store if the humidity is above 55% crank the A/C down until it comes down to where you're comfortable; then return it to say 78-F & set # of cycles so it begins cycling off at say 75-F.

If you're not comfortable at 78-F try using a low amp-draw floor type fan to keep the air moving, or a ceiling fan.

Set the cycles per hour to get as many minutes of Continuous ON runtime as you can without feeling uncomfortable. that will increase both ON & OFF times each cycle. May need to change settings a little when it gets real hot outside. Where do you live?

Also, for better humidity control reduce blower speed to a measured 350-CFM per ton of cooling; 2.5-Ton X's 350-cfm per ton is 875-CFM.

A 16-SEER would (should) have a TXV metering device (it keeps the E-Coil colder in milder weather than fixed orifice metering devices) & the lower airflow will help increase the length of the runtimes.

[RobTech]

Hi Udarrell, thanks a lot for the info..

The thermostat is located near two vents; The first is on the same wall as the thermostat, and is about five feet to the side (and about 5 feet above of course). The second is on the adjacent wall, about 10 feet away when measuring in a straight line.

I did try setting the CPH at 2 under similar weather conditions, and it didn't seem to change much... Actually it still seemed to cycle three within an hour (after the first on cycle, which took longer because it started at 79 degrees, with the stat set at 77). I guess I will try it set to CPH 1.

I'm in south Florida, so the temps do tend to get up there, along with considerable humidity on some days...

The install company didn't seem to think anything was wrong, but again that was during the courtesy follow-up call... I'm not sure if the person I spoke to was a tech.

I will pick up a humidity gauge, and also try the CPH setting at 1. If that doesn't work I'll see if they can send someone out to lower the airflow.

Thanks again!

[beenthere]

A CPH of 2, and the blower slowed down will help.

On your hottest days, did the old A/C run 24/7, or did it shut off.

[udarrell]

Hi Udarrell, thanks a lot for the info..

The thermostat is located near two vents; The first is on the same wall as the thermostat, and is about five feet to the side (and about 5 feet above of course). The second is on the adjacent wall, about 10 feet away when measuring in a straight line.

I did try setting the CPH at 2 under similar weather conditions, and it didn't seem to change much... Actually it still seemed to cycle three within an hour (after the first on cycle, which took longer because it started at 79 degrees, with the stat set at 77). I guess I will try it set to CPH 1.

I'm in south Florida, so the temps do tend to get up there, along with considerable humidity on some days...

The install company didn't seem to think anything was wrong, but again that was during the courtesy follow-up call... I'm not sure if the person I spoke to was a tech.

I will pick up a humidity gauge, and also try the CPH setting at 1. If that doesn't work I'll see if they can send someone out to lower the airflow.
Thanks again!
I am guessing the room stat needs to be moved where it won't be influenced by supply air or other factors.

Try one cycle per hour setting; those variable settings should make a real difference in temp-drop difference, runtime & also longer off-time.

Your CPH settings do Not seem to be working at all.(!)

I have a SWING Cycle adjustment RM-stat & it makes a difference at every setting from 1 to 9.

Something else appears to be influencing your RM-stat cycling frequencies.


Beenthere: On your hottest days, did the old A/C run 24/7, or did it shut off.
Answer Beenthere's question; it will tell us something about equipment sizing, providing the old system was delivering close to its Rated capacity.

[RobTech]

Hi beenthere - I tested for a few hours with the CPH set to 2, and it didn't seem to make any difference.. It still turned on/off 3 times in just over an hour - not sure if that indicates a bad thermostat, or if other factors might be overriding the CPH... Hopefully the blower can be adjusted like you said.
Surprisingly the older unit (also a 2.5 ton, but a Goodman 13 seer I think) ran very consistently (until it finally just died a few weeks ago). On the hottest days, it would turn on a bit more frequently, but wouldn't run shorter than 10 minutes, or longer than 15 minutes before reaching the target temp. My complex was built in 1996 and it was the original AC; not sure if that helps in terms of technology. Thanks!

udarrell - your right, I tried the CPH set to 2 for a few hours, then 1... and the cycle times were still the same. I'm hoping to avoid moving the thermostat as the budget is getting tight. During the whole time I was testing, it never allowed the room temp to go lower than the target, even by one degree.

[beenthere]

Hi beenthere - I tested for a few hours with the CPH set to 2, and it didn't seem to make any difference.. It still turned on/off 3 times in just over an hour - not sure if that indicates a bad thermostat, or if other factors might be overriding the CPH... Hopefully the blower can be adjusted like you said.
Surprisingly the older unit (also a 2.5 ton, but a Goodman 13 seer I think) ran very consistently (until it finally just died a few weeks ago). On the hottest days, it would turn on a bit more frequently, but wouldn't run shorter than 10 minutes, or longer than 15 minutes before reaching the target temp. My complex was built in 1996 and it was the original AC; not sure if that helps in terms of technology. Thanks!

udarrell - your right, I tried the CPH set to 2 for a few hours, then 1... and the cycle times were still the same. I'm hoping to avoid moving the thermostat as the budget is getting tight. During the whole time I was testing, it never allowed the room temp to go lower than the target, even by one degree.

After changing the CPH. You nee to give it time to adjust. 2 to 3 days time.

[beenthere]

DNMechanical, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

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[jpsmith1cm]

mdm0001

This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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[mark beiser]

A lot of the issue could be that there is just enough load to "need" some cooling, but not enough to cause the system to run for very long at a time.

I just looked at the usage report for my thermostat and that is about how mine has been running with it set to 78º.
My system is sized for when it is 105º outside though, and it has been 80ish, so there isn't much heat load on the house.
My indoor humidity is a good 20% lower though...

My longest run time in the last week has been 20 minutes, when the system was overcooling by 2º at a low blower speed to dehumidify.

[beenthere]

CPH is used for both heating and cooling.