Is my AC short cycling?

8 years ago

last modified: 8 years ago

A little background. I moved out of a 2600sqft house with mostly sun-exposed southwest facing windows that had a new 4.0 ton Trane XL16i/Hyperion XL AC with a Honeywell RTH6580WF thermostat. The 4.0 ton system was recommended after doing a heat load workup - it had lots of almost floor to ceiling windows. System was great - silent, consistent temps and ice cold even on the hottest days. On hot days it ran pretty much continuously and on warm days kicked on maybe 2-3x per hour with excellent humidity control.

I moved into a into a new house that has 3500sqft of conditioned space with a central AC unit installed in 2003. This is a colonial with some sun light in morning and early afternoon but it's shaded after 3PM. It too has a 4.0 ton system. Had my AC guy come out to clean and check everything and he said it was in good shape and the temperature differentials looked good. I took the Honeywell t-stat with me and had him hook it up.

Fast forward a few months and we've had weird weather in central MA. Very mild temps with highs in the high 70's and low 80's and a couple days to 85 but some very humid nights so we've had the AC on a lot. On a mid 80's day the AC is cycling on for 15-20 mins and then off for 10 mins and then back on for 15-20 mins. At night when the outside temps are closer to the inside temp but we have the AC on for humidity control it's on for 8-10 mins and off 10 mins.

Air temps are stable and humidity control even on the shorter cycles is excellent. My t-stat does not have a cycles per hour for setting for the AC and it is preprogrammed to tolerate on a 1 F temp difference before turning on and this too doesn't seem adjustable. Is this considered short cycling? I believe part of my issue is the AC unit in the new house is much noisier and obviously not a variable speed unit like the Trane so it's a loud and abrupt on/off compared to the silent and gentle ramp up and down of the Trane. I also believe it maybe the t-stat since it will kick on with even a 1 F increase in room temp. Would getting a new t-stat that allows control of CPH and how many degrees the temperature must increase to turn on helpful?

Featured Answer

tigerdunes

8 years ago

I don't understand the beef. If system is able to dehumidify and keep RH in the 40s even on mild days, TD is 15 or better, I would say your system is working as it should. I would not spend any serious money on a system 10+ yrs old including the thermostat. The old adage applies here. If it's not broke, why fix it?

IMO

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Comments (9)

idmd
Original Author
8 years ago
Thanks tigerdunes,
As I said I just think I'm more aware of it turning on and off. I also put a $16K AC system into my old house 2 years before moving and I have no desire to replace this system any time soon - hence I want to maximize the longevity of the current system and just be sure it's not cycling too much and putting unnecessary wear and tear on the unit.

Been waiting for a 90F + degree day to see how it does under some greater load but that hasn't happened even once this year.....

Thanks for your comment....
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tigerdunes
8 years ago
You are going to get some short cycles on mild days when system is not fully loaded. That's completely normal. Save your money and put it toward routine PMs. I assume your electric bills are not out of line.

IMO
idmd thanked tigerdunes
mike_home
8 years ago
What temperature are you setting on the thermostat? In what room is the thermostat located? What humidity level are you setting on the thermostat? Does the room with the thermostat get a lot of direct sunlight or have equipment that generates heat?

Buy a inexpensive digital thermometer and humidity gauge. There are many choices on Amazon. Monitor the temperature near the thermostat. If there is a vent near the thermostat try closing to see if that improves the cycling duration.

I can understand the AC may come on frequently during the day when the sun is shining. But the night time cycling would bother me. The house temperature and humidity is fairly constant at night. The AC does not need to cycle so frequently.
idmd
Original Author
8 years ago
last modified: 8 years ago
The t-stat is in the master bedroom on the wall opposite where the ceiling supply vent is in the room (~8 feet apart) and directly beneath a small return vent. Additionally, it's next to the bedroom door going into the hallway where the main return vent is located. I'm not sure what the CFM is on the air handler but when the unit turns on there is a very noticeable "breeze" that can be felt around most vents and it's particularly noticeable in around the t-stat. We set the temp at 68F and leave it alone and there's not humidity control.

The t-stat in the old system (where I didn't notice frequent cycling with this t-stat) was located in a hallway under a large return vent and with no supply vent anywhere near it - supply vents were in bedrooms only. The air felt the least conditioned here I guess because of the large return vent above the area.

I feel like the nighttime cycling has to do with the t-stat's 1F tolerance combined with it's close proximity to a supply vent. In other words, outside air temp is 70F meaning not much cooling is needed but with the 1F temp tolerance it's enough to kick the system on only to produce a cold draft and very quickly drops the temp to 68F and kicks it off. It kicks on at again at 69F and off at 68F. That's why I was wondering if a t-stat that would allow me to adjust that to say 3F would be better? If set the t-stat at 68F it would take additional time for the temp to rise to 71F before the system turned on again.
mike_home
8 years ago
last modified: 8 years ago
A thermostat that allows you to increase the temperature tolerance would help.

I get the chills thinking you set the temperature to 68 degrees. I feel cold if is 75 degrees at 40% humidity. It is much harder to keep the house at 68 compared to 75. I can understand why the AC is coming on so frequently.

Is it 68 degrees in all rooms of the house? Are you setting it to 68 in the master bedroom in order to compensate for other rooms being too warm?
idmd
Original Author
8 years ago
The temperature is pretty uniform throughout the house especially upstairs. Downstairs is supplemented by a through-the-wall 25000 BTU unit because there are half as many supply vents down stairs and some rooms have no supply vents at all - including a room with cathedral ceilings and 4 skylights but that 25K BTU unit can cool the entire downstairs easily. So 68F is just where I like it. Everyone at work knows to wear a sweater in the summer because I love my AC.
mike_home
8 years ago
I was wondering how you were able to cool a 2600 sq. foot house down to 68 degrees with a 4 ton condenser. You have an additional 2 tons on the first floor!
idmd
Original Author
8 years ago
Even in the old house (ranch with southwest facing windows and 2600sqft of conditioned space - 1000sqft in finished basement didn't have AC) the 4-ton Trane unit (seemingly) easily kept the house at 68F even on 95F days - although it ran 100% of the time.

Is my AC short cycling?

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