Home Heating Phsyics Question

I have three floors in my house. The ground floor has a radiator heating system. The second and third floors are on a single forced hot air system. The house has a central stairway that is wide open, so the heat goes right up to the third floor.

Now, my wife and I live mostly on the second floor and we like it cool, so we keep the forced hot air at about 68. My wife’s parents live mostly on the ground floor and keep it at about 72. If we were to raise the thermostat on the second floor to 70 or 72, would it save us money on the heating bill?

I think it would not, since the amount of heat lost stays basically the same regardless of the number of heaters running. It would split the load more between two heaters, but if anything I think that would increase the bill not decrease it. However, I am not a physics person, so I defer to the SD!



It depends on how much more expensive the first floor radiators are to run. Forced-Air systems distribute warm air more uniformly, and tend to have more efficient heating elements than radiators.

However it is unlikely that an adjustment of a couple of degrees on your floor will do much to change how often the radiators downstairs run. I’d be greatly surprised if you saw more than a slight decrease in total expense.

I see. Here’s a followup: if both systems were the same (either hot air or radiators) would using both change the bill?


I’ll throw in another follow-up question on FlyingCowOfDoom’s behalf: Would a ceiling fan at the top of the open stairway, and blowing down, help to keep the heat on the first floor? Obviously I don’t know whether that’s even feasible, but my first thought is that if it were feasible, it would help.

I think I’m misunderstanding your question. You want to know if turning your thermostat up will reduce your energy consumption? Why do you think increasing the temperature in your house would do anything but increase energy consumption?

It sure would change the bill. It would make it go up! The only point to adding heat to the top of the system is that the upstairs system is likely more energy efficient. Essentially you’re adding a buffer of warm air on the second floor to keep the warm air on the first floor from rising as fast, which is an incredibly inefficient way to insulate between the floors.

Incidentally, have your parents considered just closing the doors to the rooms they’re in? It sounds like you’re in an older house, and I doubt that the majority of your heat loss is through the stairwell. I expect it’s through the floors.

I don’t think so, because the air blowing down has to come from somewhere. It’s either going to suck in outside air, or also bring air up from downstairs.

I think it would change the bill, but not the way you want it to. Let’s assume for a second that the downstairs temperature remains the same, regardless of what you’re doing on the second floor. When we raise the temperature of the second floor, we’re going to increase the temperature gradient between the second floor and the outside, while the first floor’s is going to remain the same. I’m with zut on this one, I don’t see how it could possibly help, unless the second floor heater is significantly cheaper to run than the first floor radiators.

The OP mentions heat going right up to the third floor, presumably by airflow. There would be some return airflow path also. The ceiling fan would impede the already existing flow of air.

Yeah, you got it. My mother-in-law claims that the bill would be less if we turned our heat up. I can’t think of a logical way that her theory could be correct.


Or increase it.

In our house, there is a central stairway too, and we’re also interested in preventing the heat from going upstairs. My wife had the brilliant idea of just placing a curtain over the bottom doorway to the stairwell, and it works much better than I ever would have guessed. With the curtain open, everything is normal and the heat flows up. With the curtain closed, the upstairs is an icebox. You can even see that the curtain bulges into the stairwell due to the warm air pushing against it (there is still a small amount of flow around the edges).

Aha! I see; I really didn’t read your OP carefully enough. Your mother-in-law’s contention is that the warm air from the first floor is rising into the cooler upper floors, which means the first-floor heat is on more than it otherwise would be. Because the boiler heating the first floor is less efficient than the forced-air furnace heating the upper floors, reducing the load on the first-floor boiler would be more efficient, and that could be done by halting the airflow up the stairs, which could be done by increasing the temperature of the upper floors.

That’s sort-of logical, but I doubt it actually works that way. The heat loss from your house is directly proportional to the temperature difference between each part of your house and the outside. That means that increasing the temperature on the second floor will most certainly cause your house to lose more heat.

The only way this could be offset is if the boiler were substantially less efficient, and it were used substantially less often, so much so that you wound up using less fuel to create the same amount of heat.

Now that’s possible, but again I doubt it: if you have a reasonably modern furnace or boiler, the efficiencies are pretty good. (Caveat here: boilers tend to have a lower efficiency rating due to [I understand] the energy expended heating up the water. However, that heat is eventually leaked back out into your basement and thence into the rest of the house, so the effective efficiency is better than the rating, meaning boilers and furnaces are closer than the official numbers seem to indicate.) If your boiler is a 1920s variety, then the conclusion may be different here.

Also, the airflow up the staircase is unlikely (IMO) to pull a substantial amount of heat from the first floor to the second. If your in-laws have an experimental bent, you could have them record how often the boiler turns on and off, and how often the forced air goes on and off, to compare the relative duty cycle of each. Then turn up the heat for a couple of days and repeat the observation to see it it’s changed.

Experimentation? Empirical evidence? That’s just crazy talk.

Back to the OP. Personally I think it’s way to expensive to heat any place up to 68, much less the truly hedonistic 72. If your mother in law is truly wants to save money, tell her to turn down the thermostat, and put on a sweater. I keep my whole house at 64. It involves lots of blankets on the couch, wearing pajamas, and slippers, but it’s saved me about a hundred bucks this winter so far.