How long should the average thermostat run?

Answer 1

Equipment run time is not determined by the thermostat but rather the the size of the heating system in relation to the heat loss rate of the home. In other words a system that is over sized will cycle on for short periods several times each hour vs a properly sized system which will run more (longer on times & less cycling) when the outdoor temp is at design for your area. I`ll give an example: Last May I replaced the furnace and ac unit that was in the house when I bought it. I live in NJ and the outdoor winter design dry bulb temp here is 0*F. I previously had a 100,000 btu input, 80% efficient gas furnace which would cycle on and off no matter how cold it got outside. That told me it was oversized. So when selecting equipment I opted for a 75,000/52,500 btu input 2 stage furnace also 80% efficient. Now the furnace is running longer and only in low fire (52,500 btu). The only time it runs in hi fire (75,000 btu) is during recovery or when it is coming out of set back from the unoccuppied to occuppied setting. Longer run times are less hard on the equipment than constant cycling is and longer run times are also more efficient than short cycling or heating the furnace up, cooling down heating up etc. The same amout of gas is used to replace the heat that has been lost from the home since that is still a constant (until the improvements in the envelope are made) but I am no longer wasting the fuel that it takes to warm up a cold furnace every 15 minutes or so like I had been. So I am seeing an improvement in my heating fuel consumption.
If you have a digital programmable thermostat there is most likely a parameter referred to as CPH or cycles per hour, try decreasing that setting to say 3 or so if you are cycling a lot. Doing that may increase temperature swings in the home a little bit but it`s ok to experiment and find a happy medium. Lastly, the same is true with central ac if you have it, oversized equipment will cycle rapidly which is not good for it and it will keep the home cool but humid. AC needs long steady run times for proper dehumidifcation. When sizing heating and cooling equipment, a proper heat load calculation should be done and the equipment sized accordingly.