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A heat load calculation of my home says it requires ~36K BTU/hr of heat at a normal winter temperature (40°F), and ~52K BTU/hr at the design temperature (24°F).

An HVAC contractor has proposed installing a 95% efficiency two stage furnace that operates at either 60K BTU or 39K BTU. This makes me wonder what the optimal duty cycle is for a furnace. It seems that on a 24°F day, my furnace will be running 82% of the time. I'm unclear how a two stage furnace would split operation between high and low, so I don't know how much it would run on a 40°F outdoor temperature.

How much of an hour should be a furnace be running for optimal comfort or optimal efficiency, if they're different?

  • Longer run time is better. By the way, gas furnaces are measured in fuel BTU/hr input. A 95% 60K BTU/hr unit would be expected to deliver 57K BTU/hr heat. At 39K BTU/hr, it should deliver 37K BTU/hr heat. These are both good matches to the design criteria you mentioned. – user39367 Oct 23 '15 at 2:10
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100%. Won't happen with most systems (some inverter-drive heat pumps may come close), but the optimal situation would be for the heat delivery to exactly match the load, so the furnace ran all the time, and there were no standby losses.

The furnace your contractor has chosen seems like a reasonable choice IF the heat loss calculations are accurate.

As to why this is the case, one is the standby (non-operating time losses) such as heat going up the flue of a gas furnace (much less so with power-vented furnaces without a "stack") and the other is a comfort issue, best envisioned where you have a vastly oversized system that might run 5% of the time - so 5% of the time it's blowing uncomfortably hot air, and 95% of the time the house is cooling and areas near outside walls may get uncomfortably cold. With a less-oversized system that runs more of the time, heat is delivered more gently and more of the time.

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  • @Encerwal: The fact that it runs more of the time, but at less energy, makes it sound less efficient. My head thinks of it as "shut of the car instead of letting it idle" (to eliminate the consumption of fuel). Is this scenario more like a "shut off the car", or more like a "flourescent lightbulb", where there's the initial low-efficiency startup cost?, and if so, why? – JJ Zabkar Oct 23 '15 at 17:00
  • It's more like how driving slowly and smoothly is more fuel efficient than stomping on the gas and brake. If the car is just idling, it's not doing anything useful. If the furnace is puttering along only as fast as it needs to, it's at it's most efficient, and being useful. There are some startup inefficiencies due to a cold furnace needing to be brought to operating temperature (though the comparison you use of florescent bulbs is often overblown/exaggerated - it's more that they have a limited number of starts in them, so rapid cycling burns them out faster.) – Ecnerwal Oct 23 '15 at 19:15

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