Internet Arguments About Furnaces

[Written March 30, 2024]

Curious about how much energy my programmable thermostat might be saving, I looked up (what is actually called) “night setback,” and ended up finding a pretty basic disagreement.  Energy companies, HVAC people, and researchers said that a setback saves energy, while many forum posters (credentials not stated) insisted that the reheating phase negates all gains from the setback period, no matter what.

Since enlightenment was unavailable on forums, I dug deeper into the search results.  tl;dr: everything is fine with my 10-hour/5°F setback.

Researchers agree that any setback saves energy.  Returns diminish fairly rapidly, but even 9°F (5°C) saves more energy than 8°F over 8 hours.  Other than that, the only actual advice I saw was one HVAC person, who suggested “1° per 2 hours” (probably F???) as the upper limit for a practical setback.  Which is fair; that would recommend a 4°F setback for the 8-hour period studied, which is a majority of the savings for not too much of a temperature drop.

Eventually, I dug deep enough to find that in 2009, Energy Star certification for programmable thermostats had been revoked because they didn’t save enough energy.  The reasons were surprising, though.  About half of the owners weren’t running a program; they were no different than manual thermostats in practice.  Among the rest, the programmable model couldn’t save energy for people who had simply automated the schedule they had been manually performing before having a programmable model.  In the worst case, some people had heat pumps, which would kick on resistive backup heat during the recovery period, leading to slightly greater total energy usage for them.

Overall then, “programmable thermostats” save 1.1% or less across the entire installed base.  But these studies also support the “setback saves money” hypothesis!  A subset of users were able to save enough energy, by programming and using a setback, to bring the overall average up to 1%.

(The thing that really saves energy is to have occupancy sensing in a property that is only partially occupied, like a rental for the Illegal Hotel Apps.  That group can see a 25% savings in total energy usage.)

Back to our dudes (presumed) on the forums.  None of them are building a more detailed model than “heat lost must be replaced.”  There’s no attempt to experimentally verify it, or consider that the rate of heat loss depends on the temperature difference.  And on the other hand, if the research is all wrong and part of some big conspiracy, there are a lot of people involved in it.

[2024-04-06: This section has been rewritten and expanded.]

A different issue people on the forums had was with two-stage furnaces.  Some people think they are always a ripoff.  “The first stage has to run longer to provide the heat.”  “It lets installers get away with incorrect sizing.”  I didn’t have a prior opinion, but I couldn’t let it lie unexamined.

For peak efficiency, a furnace needs two things: to be fully warmed up internally, and to be able to circulate air throughout the system.  Two-stage furnaces typically offer ⅔ capacity as their first stage, rather than a smaller amount, for the latter reason.  Producing less heat requires less airflow.  Otherwise, too much heat is carried away from the furnace.  The ducts are fixed size, so the airflow can only be lowered by slowing the blower fan.  That soon reaches a point where the air is moving too slowly to get out of the system and into the main air of the building.

All that means that total energy consumption cannot simply be calculated as “cycle time.”  Yes, when in a steady-state, the first stage needs longer to produce the same amount of heat output, which consumes more electricity to run the blower.  However, outside the steady-state, the second stage may not warm up sufficiently to achieve its rated efficiency, and it may not be able to distribute that heat evenly on a short cycle time.

My own furnace might appear to be “oversized,” nearly always running on its first stage.  On the other hand, it was able to use the second stage to keep up with the blizzard of Christmas 2022, where low temperatures and high winds for days combined to turn a warm December into an average heating bill for the month.  Outside of that, there’s a daily event (recovery from setback) that fires up the second stage, so the furnace is regularly hitting its full design parameters.

There’s a deeper problem with the argument, though.  Considering that my first stage should be grossly oversized during the beginning/end of heating season (where it fires up about once every 50 minutes to maintain a 20°F differential) but doesn’t cause any issues with comfort, it seems like an installer can “get away with” a hugely oversized single-stage furnace, and most people would be none the wiser.  If a two-stage furnace saves money for the occupants, then it probably saves money regardless of the precise size that was installed.

In fact, when I bought my house, the home inspector detected a CO leak in the old furnace.  The heating contractor measured everything, asked about insulation, then installed a new one with 65% of the net capacity.  He explained, “In the old days, they put in the biggest furnace they could fit down the stairs.”  I’ve had that same (smaller) capacity for the last 17 winters.  The old one was oversized… despite being single-stage.

In the end, I spent hours doing all of this reading, and the only conclusion I got was “everything is fine.”  The indoor temperature has been set to 64°F by day and 59° for 10 hours at night.  The house doesn’t cool down immediately, so it is timed to drop a degree or maybe two before I actually get into bed.  In the morning, it doesn’t come on until I actually get up, so that it doesn’t wake me up early.  The recovery period also gives the furnace a chance to come up to the second stage every day for an extended period of time.

I used to keep the house at 68°/62°F, until the furnace broke, and I decided to save electricity (not too expensive, but certainly not free) by maintaining a 60°F temperature for a day or two.  By the time the new heater had gotten it to 66°, it was too hot!  My body had adjusted to the cold already.  I decided that, if I were cold-adjusted, I might as well keep it that way, so the thermostat was gradually turned down from 66° to its current point.

The great downside here is that now, all stores are too hot in the winter, to complement being too cold in the summer.  I have to either dress down and take off my coat, or plan carefully to avoid spending too much time inside.