Best way for AC to operate? Cycles a lot now.

10 years ago

Hello all,

I have a 2700 sq ft. 1.5 story house, with the master on the first floor. Our HVAC has 2 zones and a 4 ton 13 SEER Lennox unit. We never go upstairs, so we keep the thermostat up there at 85 degrees. For downstairs, which is 1800 sq ft, we keep it at 78 degrees. My girlfriend is a teacher and is off for the summer, so I don't set the thermostat back during the day.

Since we are almost just using a 4 ton ac unit for 1800 sq ft, it seems like our AC unit is cycling a lot, and doesn't stay on too long since only the first zone needs cooling and it cools the first floor very quick. Would it be better to get a new thermostat that allows a 2 degree differential, that way when it reaches 78 it can cool to 76, and cycle a lot less? Will longer run times and less start ups extend the life of the ac unit by quite a bit? My last question is what is the recommended set ahead for a thermostat in the summer? When my girlfriend starts working again, I plan on setting the thermostat ahead 5 degrees while we are gone. We live in NC by the way. Thanks for all of the help!

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Comments (6)

klem1
10 years ago
Economy and longivity will not be substantualy effected by what you perpose doing with the t-stat. I believe you would find greater differential between on and off temps to be uncomfortable. The only issue with short cycles is wherther enough moisture is remove to control humidity. This is one of several reasons I favor 2 units instead of 1 unit zoned.
tigerdunes
10 years ago
It would be interesting to know the RH for downstairs zone.

What are your typical run times? CPH?

I would lower the thermostat setting for upstairs zone to at least 80 degrees at least for a trial period. Any idea of RH for upstairs zone?

Any idea what your CFM setting is on blower speed? I would want to know.

Don't like the idea of large setbacks.

Too bad, you don't have a two stage condenser.

IMO
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mike_home
10 years ago
On some thermostats you can set the number of cycles per hour. I would recommend setting it to three cycles per hour and see how it goes.

The problem is your AC is over sized for your zoning set up. You are always going to have this problem if you never use the upstairs zone.
miata94
Original Author
10 years ago
Not sure of RH for upstairs or downstairs. Keeping downstairs at 78 makes it feel comfortable, so I don't think RH is high. Our thermostat does not allow us to set a CPH, but I actually don't think it runs more than 3 times per hour. Our house is newly built, and is built real tight, so I'd say with 85-90 degree outside temp, the ac runs for 5-10 minutes, then off for 20. This may be ideal, and I'm just being paranoid, but it still seems like its cycling quite a bit more than it should.

Any new thermostat you would recommend with a CPH setting? Ours is a bare bones programmable thermostat. I was thinking about getting the Honeywell Prestige or Nest for downstairs only, and keeping the old one for upstairs.

So tigerdunes, you would not recommend setting the thermostat to 83 when we leave for the day, then 78 when we return? What about 80 leave, 78 or 76 return? Thanks for the help everyone!
tigerdunes
10 years ago
That's a good suggestion by Mike to purchase a thermostat with CPH setting.

How do you heat?

I would check the Honeywell thermostat website. I like Mdl 8321 that also has humidity sensor for overcooling in AC mode.

IMO
mike_home
10 years ago
A 3-5 degree setback should be fine. You have plenty of extra capacity so the lower floor should cool down fairly quickly.

Best way for AC to operate? Cycles a lot now.

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klem1

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Best way for AC to operate? Cycles a lot now.

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