How do I determine the optimal setting for my thermostat when I’m not home? I live alone and set the thermostat back to 55 when I leave in the morning. Instinct tells me that if I set it back too far I will waste more gas heating it to a proper level in the evening then i will save. But instinct has let me down before. Is there an optimal temp?
Your instinct is wrong. The lower you set the thermostat, the less fuel you’ll use. This is true even if you later raise the set point back to normal.
National Fuel, the company who provides our gas, instructed us to never set the thermostat below 60, even when leaving the house.
That’s reason enough for me to turn the thermostat to its lowest possible setting whenever possible.
If you could compare how long the furnace runs when you get home and move it up to how long it runs while you are gone you could calculate it as a function of time.
If it’s a burner sitting in an open field, you’d turn it off while you are gone. If you left a house for a year, you’d turn it completely off.
This comes up so many times on here. In fact, just 48 hours or so ago.
Within reason, the lower the temperature you set your thermostat to, the less energy you use. I say within reason, because household humidity issues could come into play, and you of course want to avoid risking freezing pipes.
There are some links to past more detailed threads in here.
http://boards.straightdope.com/sdmb/showthread.php?t=339135&highlight=thermostat
And for about $30 you can get an electronic thermostat that will do this for you. For a few dollars more you can get one that will start the furnace enough in advance to have it at set temperature by the time you arrive home.
We have one of these, and we LOVE it. It recognizes both weekdays (Mon-Fri) and weekends (Sat-Sun), so we can have separate settings. There are also four separate “times” for each day (although you can set the exact start time and end time for each part of the day):
Morning: This lets us set a decent temperature to wake up to (68F), so the house is warm when we get out of bed.
Day: On weekdays, we drop the temperature to about 60F. On weekends, we just let it ride at 68F.
Evening: We have the thermostat set to start warming up the house to 68F at 3, so by the time we get home at 3:30, the house is reasonably warm.
Night: We have the temperature drop to 65F at night. Any colder, and our kids don’t sleep.
You can also override it, so if we’re home on a weekday (like for Christmas break), we can keep it at 68 all day. If we know we are going to be out for most of the day on a Saturday, we can manually turn down the temperature.
Perhaps you can clarify something I read maybe 10 years ago. Someone wrote to a home improvement columnist about closing off the heater vent in a room and then keeping the door closed. The writer’s question: how cold can I allow the room to get? The columnist gave a surprise answer, claiming that if you lower the temperature too much in the sealed-off room, the heat in the heated portion of the house will actually radiate through the wall and offset much of your supposed gains, in some kind of thermal equilibrium process. The bottomline, he said, is that the heat will seek to warm that cold room, so keeping it too cold inside only works so much.
Reactions?
We have two rooms closed off for the winter. The windows are doubled-paned glass. I added that indoor plastic wrap (sealed up quite nicely with a hair dryer) around the windows in each room, and the heat registers (forced air) are closed and sealed tight as well.
It’s cold in each room! While there may be some heat loss into each room from the rest of the house, it’s not that much. Oh sure we’re not talking ice crystals forming or anything. It’s a toasty 62F just outside each room and about 10-15F cooler in each room. Still, for my money, that means I’m not heating these unused rooms and that’s good enough for me.
My initial reaction is that they’re not setting the right frame of reference. As far as I can tell, you have to look at it this way in the thought experiment: why does the room get cold and stay cold? It’s because it loses heat to the outside. Now if the house radiates some heat into the room, that process will not transfer heat as efficiently (at household temperatures) as the direct air heating from the vent. So there will be some heat lost from the rest of the house to the cold room, but in the grand scheme of things, you still have a room that is not losing heat as fast to the outside environment because the differential temperature is still much less than if it was heated.
In short, in my thought experiment, some heat is lost to the cold room from the rest of the house, but it’s still less than the cold room would have lost to the outside if the cold room had been kept at the same temperature. You might think of the cold room as really being an extra insulating air space.
Now, I guess one could think "OK, but there’s more surface area since 1 floor, 1 ceiling, and possibly 3 walls radiate into the cold room, as opposed to, say, 1 outside wall. But once again, I think the key is one has to ask why is the cold room cold in the first place?
It might be easier to draw a diagram with heat fluxes. Or else I’ve set my thought experiment up wrong. Maybe someone else has a good way to set it up and/or describe it. My belief is that the cold room issue is sort of a red herring, but I’m unsure of how to describe what I’m thinking of…
I agree with your reasoning here.
To extend your thought experiment, suppose we only heat one room and shut all the others off. The entire house will get cold except for one room, and it becomes obvious that you are not spending as much energy to heat the one room, even with heat loss to the surrounding rooms, as you would by heating the entire house and having even greater heat loss directly to the exterior walls.
I have this discussion every time my wife opens the kitchen cabinet that is against an external wall, and feels cold air come out. She tells me we need more insulation there so the cabinet won’t get cold, because we must be losing heat there. No! The cabinet is cold because there no heating vent inside it to warm it up! The cabinet itself acts like insulation. But she doesn’t get it.
Yes, the house would be warmer with the room than without.
Q=UAdT
Q = Heat Loss
U= 1/R . R is commonly known as the rating of insulation. An airspace provides an R factor for example.
A=the area having that R factor.
dT=Temperature difference between the outside and the house. If it’s 70 inside and 20 outside, dT is 50.
Anyway, if you hacked off the empty room, there would be more heat loss.
Yes, thanks. I have one.
Off Topic: I have had three different models of these, and a huge source of frustration is that the override is cancelled as soon as you hit the next one of those four “time zones”. Why would you design a device where the manual override is automatically overriden?
No no no! I just got home from not being in the house a year. I was glad to find that none of my pipes had burst and the plants hadn’t died.
That was an example of how much heat you would use while you are away, not Practical Plumbling Practice. 
Wait a minute. Maybe whoeer watered your plants also thawed the pipes. :dubious:
So you don’t have to remember to set it back to automatic!
That sounds reasonable doesn’t it? Remember, though, that you are around worrying about the heater running all that time to heat the house when you return. You are not around to notice the heater not running while the house cools down after you leave. The two just about cancel each other out.
Thats exactly the kind of override you want…most programmable thermostats also have a HOLD feature which will keep the temp at the set point forever, UNTIL you “run the schedule” again.
For example… here …
==> Temporary program override available by using WARMER and COOLER keys.
==> Use T8112 thermostat HOLD TEMP key, and then WARMER/COOLER keys, to indefinitely change a temperature.