Lowering the thermostat during the day

[larrybud2002]

Here in Michigan, it's been bitterly cold for the last couple of days. I have an automatic thermostat which I lower the house to 65 during the day, and back up to 69-70 at 4:30 so it's warm by the time I get home from work.

The problem is, it took FOREVER to get back to that temp last night. When I first came home I thought my furnace wasn't working right (had the ignitor replaced last week), but it was running... It just wasn't getting warm.

I change the filter regularly, but I have a lot of windows in my house...

So the question is, is it worth it (energy wise) to lower the temp during the day, when it takes so long to get back up to temp? I'm thinking I'm buring as much gas to bring it back up than I might save during the day.

[Edwin Pawlowski]

If the time spent at the lower temperature is longer than the recovery time, yes, you save.

The greater the temperature differential between inside and outside, the faster the heat flows to the outside. Lowering the temperature 5 degrees lessens the heat flow. Since there is a greater loss rate at say, 0 deg. outside (70 degree differential) than 35 deg. (35 degree differential) it will take longer to recover.

[scott_z500]

In some cases that might be true. In my case I leave my heat off completely during the day and most of the night. My programmable timer turns it on for about an hour in the morning so I can step out of the shower into a warm room. When I get home from work it's in the 60's, even when it's been 17 degress outside all day, even with no heat on. And my house has no insulation in the walls and I have no weatherstripping on my exterior doors (I'm refinishing those areas right now). I live in the northeast US. I think the difference is that either your furnace is undersized, your house is poorly insulated, or you have a heat pump. Perhaps if you could give more details we might be able to help you better.

[Travis Jordan]

With fossil fuel furnaces (as opposed to heat pumps with heat strips) you save ANYTIME you set the thermostat back. The rate of heat loss is proportional to the temperature differential. The cooler your house is the slower it loses heat. As the house cools it loses less heat even if it never reaches the setback temperature.

[Stormin Mormon]

The ammount of heat your furnace produces is equal to the heat you lost.

When the house is colder, you are losing less heat. The temp difference across the wall is less. And the heated air leaking out is lower temp.

Yes, it's a saving. The long reheat time might be more a pain than the savings are worth.

[Dave Gower]

wrote

Energy wise, the amount of heat you lose is directly proportional to the temperature difference between the inside and outside of your house, so setbacks do work. If it's taking too long to heat up your house, that is a different issue. Either your furnace is too small or it needs fixing.

[Buck Turgidson]

I think Edwin's response makes the most sense. Remember that heating a house entails not just heating the air space, but the building materials themselves (bricks, floor, drywall). If the temperature swings are too extreme, there are diminishing, if not negative, returns.

Be cautious about turning your heat off completely as one suggested unless your pipes are sure to remain reasonably warm.

[Travis Jordan]

Buck Turgidson wrote: /snip/

Please cite source for your statement regarding 'negative returns'.

[scott_z500]

"Be cautious about turning your heat off completely as one suggested unless your pipes are sure to remain reasonably warm"

That was me who is doing that. Even with no heat and no insulation my house never got below 50 degrees even on really cold nights. With insulation it's obviously warmer. I have no idea why, just the way the house was built, or something. No need to worry about broken water pipes.

[nicksanspam]

No. If a house spends 1 hour at 67 F and returning to 68 takes 2, you save.

Nick

[trader4]

"That was me who is doing that. Even with no heat and no insulation my house never got below 50 degrees even on really cold nights."

I'd like to hear more about what kind of heat you have, what kind of house, how big, where it's located, etc. It's hard to believe that in the northeast in winter, like now with it 17 outside as you indicated, you could get by with the furnace on for only about an hour during the early AM, have the house remain in the 60's during the day and only go down to 50 overnight. Or that an hour in the AM could bring it from 50 back to 70 under those conditions.

I could see this in maybe a condo, where you have common shared walls on at least a couple of sides, but can't see how it could work in a single family house.

[Duane Bozarth]

Buck Turgidson wrote: ...

It would have to be awfully cold for heat to only be off for a portion of the day to lead to actual freezing of pipes solid enough to cause damage...

It is possible over a longer period such as vacation that pipes in an exterior wall, for example, may not receive sufficient waste heat and cause a problem even though the interior temperature is still well above freezing.

[Matt]

I've found that my furnace works the best if I don't move my thermostat around. It's about 5 off the floor, which is perfect for me. I tried lowering it to 2 feet, but that was inconveinient, and I found that sometimes mice (or the little tiny people that live in the fireplace, not sure which) would play with the thermostat, which was fun for awhile - but quickly just got plain old annoying. So then I tried raising it to 10 feet; but then I needed a ladder to get to it.

So, to answer your question - don't lower your thermostat. Your plumber put it where it is for a reason.

[raven]

No, there's ever increasing returns in terms of energy savings, but you may incurr sufficient PITA 'costs' to make those savings not seem worthwhile.

John

[Joseph Meehan]

You did not tell us what kind of energy you are using. If you have a heatpump then it depends, you may or may not save or may pay more. Heat pumps are less efficient when it is colder.

Gas and electric resistance don't really differ much in efficiency. So you will always save if you are using them and they don't have a time of day differential as electric sometimes does.

As your home cools less heat is lost so while you home cools it saves X amount of energy and when it reaches the lower time it continues to save say Y amount of energy. When you get home and turn it back up it will take X amount to re-heat it, but you will not have to pay back that Y amount.

If you reduce the temperature difference by 10% you will save 10% of the heating expense while your home is cooler.

[Percival P. Cassidy]

At least one thermostat (a Honeywell costing approx. $80 at Menards, but I don't recall the Model#) is claimed to "learn" how long it takes to get the house to the required temperature at a particular time and adjust its switch-on time accordingly. I had one in my possession for a couple of days but had to take it back because it doesn't work with

2-stage furnaces.

Perce

On 01/18/05 08:47 am [e-mail] tossed the following ingredients into the ever-growing pot of cybersoup:

[Travis Jordan]

All HVAC dealers carry 2-stage Honeywell stats, or you can buy one on the internet (and perhaps pay more...).

(its NOT really pink)

[Travis Jordan]

Just on oversight on the seller's web page, I would guess.

page 30-31

[Jeff Wisnia]

I'm not sure I follow that one Joseph. Are you saying that because the outside air is likely to be colder in the early evenings than it is during the days the reduced efficiency of a heat pump at lower outside temperatures will make each "raising" BTU cost more if you replace them in the evening than if you put them back continuously throughout the day? And maybe by enough more to turn the whole equation around and make your electric bill higher if you let the house temperature drop down during the day than if you don't?

Our home (Boston area) uses two heat pumps (two zones), and the auxillary resistance heaters have been kicking on a lot lately. It was 4 degrees F this morning, quite a bit colder than we usually see this time of year. Global warming? Bullshit!

Jeff

[Joseph Meehan]

Yes.