# Heating a home.

Tell me if my logic is flawed on this. My thermostat will tell the furnace to kick in when the house temperature is 3 degrees below its setting. So if I have it set for 65 degrees it will call for heat at 62. When the temperature rises 3 degrees it cuts off. So, if I initially set the temp for 75, once the house is at that temp it will run through the same 3 degree on/off cycle, am I right? So all I’m doing is raising the temp. 3 degrees each time it drops. Then why am I always hearing that you should lower your thermostat as much as possible to save energy? After the initial runup it’s always only 3 degrees difference. Does it take more energy to raise it from 62 to 65 than it does to raise it from 72 to 75? Three degrees is three degrees. Straighten me out on this please.

Raindog, you out there man?

The greater the difference between the inside and outside temperatures, the faster your house will lose heat. If it’s colder outside, your furnce will run more. If it’s warmer inside, your furnace will run more. The furnace will turn on more frequently at the higher thermostat setting, because your house is losing heat to the outside faster.

It takes more energy to raise the temperature from whatever it is outside to 75 than to 65. And there will be more “heating cycles” needed (therefore more energy spent) if you set the thermostat at 75, as the temperature will lower those 3 degrees faster. The speed at which the house cools down is a function of its insulation, which you can’t change, and of the “temperature gradient”, ie the difference with the outside: greater difference, faster cooling, hence a need for more heating.

You’re right if you only consider the amount of energy needed to cause that 3 degree increase in temperature. However, the hotter your house is, the faster it loses heat to the (much colder) exterior. IIRC, the rate of heat loss is directly related to the temperature difference between the interior and exterior temperature (though it’s been a long time since I’ve had to work with the relevant equations). So if it’s 32 F outside, and you set the thermostat at 65, you’re talking a 33 degree difference. If you set it to 75, there’s a 43 degree difference, and a corresponding increase in the rate of heat loss from the house.

So if you set the thermostat to a higher temperature, it will kick on far more often, since it loses those 3 degrees much more rapidly.

Okay, so it’s all about faster heat loss. I don’t understand physics but I get the concept. Thanks for the explanations.

Raindog, you can take the rest of the day off now.

Sure you can. People upgrade their insulation all the time, especially in their roofs.

OK: “which you can’t change easily.” There are basic things that can be done to improve a house’s insulation quite cheaply and without involving construction trades, but even putting insulating tape on your windows takes more effort than setting the thermostat.

Unless you have a real, real complicated thermostat, of course.

PS: the roof thing doesn’t even apply to most homes in Spain…

Hey Peanuthead. Sorry I’m late. I was loitering in another forum.

I agree with all the posts so far, although I am a little curious about your stat. Is it a mercury bulb stat? Because a digital stat will provide much tighter control.

Current model stats (I am a Honeywell fan, although there are other good ones out there) will respond in less than one degree. I routinely have stats that are set at 70F (as an example) and the room temp is held right at 70F with little or no deviation.

If you have A) a predictable schedule (that makes programming easier) and B) large blocks of time when no one is home, I’d recommend a good digital programmable stat.

I would also recommend a digital thermostat. I bought one a few years ago for myself, and bought my parents one for Christmas when they wouldn’t get it themselves. (They love it now.)

The one I got was on sale for ~\$30, and makes it so it’s warm when I get out of bed or come home, and I don’t have to remember to turn if down when I leave for work or go to sleep. It displays the temperature and humidity in large numbers, how many minutes it was on today/this week, lights up when I push the buttons, and is one of the best devices I own that I like to take for granted. (If I moved to a place without one, I’d buy another one that week).

Digital thermostats should also overshoot the set point a bit, to keep the overall average temperature as close to the set point as possible. I think mine also has an adjustable setting for ramp-up temperature so it can hit 21 from 16.5 at as close to 7:30 as possible, but I’ve never bothered to adjust that.

Yeah, the thermostat is programable, but it’s not much use for me as I keep really strange hours. So right now it’s set at a constant 64F. I’ll probably have to bump that up soon as the weather is getting colder. Mrs. P is already complaining. (Filipinos don’t take too well to Chicago weather)
The furnace and AC are only 2 years old and working fine. I’m real pleased with the Trane xv90. I use top of the line pleated filters and change them regularly. No problems at all.
The reason for the OP was that although I believe that lowering the temperature saves energy my layman’s logic told me different and I knew I’d be enlightened here. Thanks again all.

Raindog. We’ll put you down for some overtime.

The temperature difference between turn on and turn off is called hysteresis and is necessary and it may not be too good if the range is too narrow. You do not want the furnace (or A/C for that matter) cycling on/off too often and some thermostats have a system to prevent that.

Sailor is correct the better stats have a delta T of 1.5 degrees. Best choice for most people is a 7 day programable.

Digital stats have “5 & 2” in which Mon-Fri are programmed together and Sat-Sun together, **“5,1 & 1”**where Mon-Fri are programmed together and Sat & Sun independent of each other, and “7 Day” in which you have the flexibility of programming each day independently.

The “7 day” gives you the most flexibility, but naturally is more expensive. If your schedule Monday through Friday remains the same day to day, your may choose a 5 & 2, or 5,1 & 1, and save a few bucks.

If your schedule varies Monday through Friday, the 7 Day is the better choice.

I don’t know why you use such a large drop to trigger the furnace. You can likely change to drop turn on temperature for your programmable furnace. Our two stage can be programed for as little as half a degree for the low burner setting, and the higher stage for one degree. The less extreme the fluctuations the lower you can leave the thermostat and be comfortable.