Furnace cycle times - which is worse?

Problems related to residential installations.

Furnace cycle times - which is worse?

Postby T-Bob » Sun Dec 18, 2011 2:22 pm

Hi all,
Question about cycle times, which is worse for a furnace- short frequent cycles or longer infrequent cycles?

Asking because the thermostat was just moved to a room where the temperature was more constant from where it used to be located however it was noticed that the furnace is now cycling every 40 minutes at 8 minutes per cycle when it's 20 degrees outside (going to hate to see what it does when it gets colder). Anywho, is that to frequent of a cycle time for the furnace or an American Standard (Freedom 90) mfd 2003?
Also the thermostat is a White Rogers model from 2003 (1F80-261).

Honestly I'd like get a thermostat that would limit the cycling times to no more than once an hour.

TIA
T-Bob
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- Furnace cycle times - which is worse?

Postby Freon » Sun Dec 18, 2011 3:15 pm

You always want the longest cycle you can have. It's more efficient and far less wear and tear on the equipment. But since a heating system, if perfectly designed, would run 100% of the time on the coldest (design) day, you will always have some cycling on days that are not the design day.

One way to increase run time is to set the temperature differential to a higher value. Most fixed thermostats have a set differential of 0.5 degrees. This means if you set the thermostat to 68 degrees, it will turn on the furnace at 67.5 degrees and turn it off at 70. Now set the differential to 2 degrees and it will turn on the furnace at 66 degrees and turn off at 68 degrees meaning a longer run time but also bigger swings in house temperature. Some people may mind the temperature swings.

Some thermostats may have a differential setting or simply a cycle time (per hour) setting. Cycle time settings can be a bit more tricky because you won't know the final temperature until the cycle completes.
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- Furnace cycle times - which is worse?

Postby T-Bob » Mon Dec 19, 2011 12:07 pm

Sounds like a new theromstat is needed as the white rogers won't let the user tinker alot with the differential, all it gives is a fast or slow response option.
Thanks for the posting your response.
T-Bob
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- Furnace cycle times - which is worse?

Postby Freon » Mon Dec 19, 2011 2:04 pm

I suggest a thermostat that uses temperature differential (look for 2 degrees) and not cycles per hour.
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