POLL: A trick energy usage question

[welst10]

I heard a heating and cooling expert advising not to set your house temp too low when you're at work, because it takes a long time to heat it back up when you're home and ends up costing you more. He advises to leave it only a few degrees lower than your usual desired temp. But I'm puzzled. If you set the temp relatively high during the day, your furnace will run a lot while you're away. So overall, it may still cost you more than setting it low.

Who is right?

EDIT: assuming the house is heated by forced-air gas furnace, not heat pump.

 

[txxxx]

So leaving the heating on a fraction lower will save energy when reheating the room.... and what about the hours on end its active?

Set low.

 

[KB]

Did this expert work at best buy? Sure it takes longer to heat up later in the day, but you won't be paying to keep it higher while you are away.

 

[Kelemvor]

You save the oney by not having to keep the house warm while away. It has to heat back up regardless of how much you set it down. THe plain fact is the lower you set it, the more you save. Whoever told you that was a dumbass.

We keep ours set at 69 when we are home and 60 when we aren't. If we set it to 50 we'd save even more but we have pets and such and don't want them to get too cold.

 

[welst10]

Originally posted by: FrankyJunior
You save the oney by not having to keep the house warm while away. It has to heat back up regardless of how much you set it down. THe plain fact is the lower you set it, the more you save. Whoever told you that was a dumbass.

We keep ours set at 69 when we are home and 60 when we aren't. If we set it to 50 we'd save even more but we have pets and such and don't want them to get too cold.

So you're saying there are at least 2 dumbasses who voted this thread

 

[nwfsnake]

Not sure you guys are correct. You are not only heating the air, but the furniture, walls, cabinets, etc...
When you turn it down, assuming everything has achieved a steady state temp previously, everything in the house gets cooler. To heat it back up again, you have to heat everything. There is probably some temp delta, say 10 degrees or so, that is the most efficient. I think I will google and look for some more input.

 

[TranceNation]

you guys need to post data. I tend to agree with the expert.

 

[rh71]

I tend to agree with the guy too... look at it this way... you're cruising at 45mph... what would take more gas... going from 45 to 55 and maintaining it there (drivers know it doesn't take much) or 0 to 55 ? And bear in mind the heat has to get through the whole house, and not as quickly.

 

[Kelemvor]

The thermostat doesn't care what temperature the cabinets and walls and such are, it just cares about the air. The other things will keep the air from warming up quite as fast, but it's negligible. Most people set their back 5 - 10 degrees. Setting it back like 2-3 degrees isn't going to save you much of anything. Setting it back 30-40 degrees is overkill and could damage your interior things like pipes and such if it gets to freezing. Also the more you turn it back, the earlier before you come home you need to set it back up again....

If you leave at 8 and come home at 6 and set it back 10 degrees, you can probably turn the thing back up at 5 or 5:30 or so to get it warm by 6. If you set it back 20 degrees, you'd have to turn it back up possibly at 4 or 4:30 so you lose a bit of savings TIME but gain some back by not having to heat the house as much during the day....

For a happy medium, I'd say 10 degrees is good.

 

[Kelemvor]

Originally posted by: rh71
I tend to agree with the guy too... look at it this way... you're cruising at 45mph... what would take more gas... going from 45 to 55 repeatedly (drivers know it doesn't take much) or 0 to 55 ?

But if you're cruising at 45, you're using way more gas than if you were cruising at 0. You save the money by not using as much "maintenance heat" to do the mini cycles during the day. Yes you use more when it has to come back up, but not as much as you save....

Especially if you have a new high efficiency furnace....

Furnaces don't have gears. THe higher temperature they have to maintain during the day, the more energy they consume. Getting back up when it changes to the higher temperature uses less than keeping it at a higher temperature during the day...

But again, it's all based on how long you are away. The Longer and Lower you set it, the better. But if you set it too low, it takes longer to warm back up so you lose some of the savings...

 

[timswim78]

My experience has been that it is definitely less expensive to maintain the temperature rather than let it fluctuate. This is especially true when it is very cold or very hot outside,.

 

[bonkers325]

what type of heating system do you have? a forced air system, i would agree that keeping it lower is much better. its a lot easier to warm a house with forced air. if you use water/oil filled heaters, then it would be better to keep it a few degrees lower than normal when you arent at home.

 

[Kelemvor]

Originally posted by: timswim78
My experience has been that it is definitely less expensive to maintain the temperature rather than let it fluctuate. This is especially true when it is very cold or very hot outside,.

That just doesn't make any sense. The closer the outside and inside temperatures are, the less energy it takes to keep a house at that set temperature. So this means that fluxating the house would be even MORE efficient during really hot or really cold days.

If it's 30 degrees outside, it's cheaper/easier to keep the house at 50 than it is at 70 because it doesn't have to work as hard.

If this were not true, then no one would sell programmable thermostats....

 

[bonkers325]

Originally posted by: FrankyJunior

Originally posted by: timswim78
My experience has been that it is definitely less expensive to maintain the temperature rather than let it fluctuate. This is especially true when it is very cold or very hot outside,.

That just doesn't make any sense. The closer the outside and inside temperatures are, the less energy it takes to keep a house at that set temperature. So this means that fluxating the house would be even MORE efficient during really hot or really cold days.

If it's 30 degrees outside, it's cheaper/easier to keep the house at 50 than it is at 70 because it doesn't have to work as hard.

If this were not true, then no one would sell programmable thermostats....

there is a balance that should be maintained for indoor temperatures. if you allow the temp to drop too low,the heater will constantly engage and will end up costing u $$$. its a lot easier to keep a room warm when it is a few degrees below room temp, as opposed to 10-15 degrees lower tan room temp

 

[rh71]

Originally posted by: FrankyJunior

Originally posted by: rh71
I tend to agree with the guy too... look at it this way... you're cruising at 45mph... what would take more gas... going from 45 to 55 repeatedly (drivers know it doesn't take much) or 0 to 55 ?

But if you're cruising at 45, you're using way more gas than if you were cruising at 0. You save the money by not using as much "maintenance heat" to do the mini cycles during the day. Yes you use more when it has to come back up, but not as much as you save....

I should've thought through my numbers better. Take this... cruising at 45 hardly uses any gas especially if you're just maintaining it there after a 1-5mph drop ... if you have a digital readout on your car's fuel efficiency, you'll see that the mpg while simply maintaining speed is astronomical. As you hit the gas more, the mpg readout will lower since you're using more gas. That part isn't rocket science... but it is explaining how if you go from nothing to full blast until you reach your desired speed (and the heat has to cover the entire house, mind you), the gas it takes is likely more than it takes to maintain the speed at 45. Yes, it depends for how long... but it sure takes a long time for an entire house (or 1 floor) to get 15F warmer. That's heat full blast for that long.

 

[timswim78]

Originally posted by: FrankyJunior

Originally posted by: timswim78
My experience has been that it is definitely less expensive to maintain the temperature rather than let it fluctuate. This is especially true when it is very cold or very hot outside,.

That just doesn't make any sense. The closer the outside and inside temperatures are, the less energy it takes to keep a house at that set temperature. So this means that fluxating the house would be even MORE efficient during really hot or really cold days.

If it's 30 degrees outside, it's cheaper/easier to keep the house at 50 than it is at 70 because it doesn't have to work as hard.

If this were not true, then no one would sell programmable thermostats....

It works for me. I should add that I don't keep my house super warm. Maybe mid to high 60's in the winter.

 

[Looney]

Originally posted by: rh71
I tend to agree with the guy too... look at it this way... you're cruising at 45mph... what would take more gas... going from 45 to 55 and maintaining it there (drivers know it doesn't take much) or 0 to 55 ? And bear in mind the heat has to get through the whole house, and not as quickly.

WTF that doesn't make sense... a better analogy, and one that reflects it much more accurately, is if you're stuck in a snowstorm overnight. Will it be easier on gas to leave the engine and heater running all night, or to turn it off and then heat it in the morning. The answer is obvious.

Whether the heat is on or not, the amount of heat loss is the same. I am NOT a math wiz, but lets look at it this way. Lets say your house is losing 25% of it's heat every hour. In 4 hours, it will be completely gone if you don't have the heat on. But if you do have the heat on, you're STILL LOSING 25% of the heat every hour, and in 4 hours, sure, the house would be warm, but it cost just as much in those 4 hours as it would ahve cost to reheat the house when you came home... but in addition, you're paying 4 more hours of heating bills for the time you've been away (assuming you're only gone 8 hrs).

 

[0roo0roo]

my car idles 24/7...tis sweet

 

[Kelemvor]

Here's an example that will hopefully help clear things up.

If the outside temperature is 69 degrees and you have your furnace set to 70 degrees, it's not going to come on very often. This is because the difference in temerature is so small, it takes a long time for the outside temperature to cool the house off just one degree. I don't have the specifics (been out of school for a long time) but the rate of heat loss is determined by the difference between the two temperatures (as one of the factors). So if it's 0 outside, and 70 inside, it's going to start to cool off very quickly. If it's 68 outside and 70 inside, it's going to take much longer because the difference isn't as great.

This basically means that the closer together the outside and inside temperatures are, the slower it will cool off which also means the less energy it takes to keep at that temperature.

Therfor, for any given outside temperature that is less than the thermostat setting, it will take less energy to maintain a temperature that is closer to the outside temperature. So, if you set the thermostat at 68 vs 70, that's not much different if it's 30 outside so you won't save much. if you set it at 60 vs 70, that's a much greater difference and will take much less energy to maintain the lower temperature.

Now yes there is the time at the end when it has to heat back up, but moderm furnaces are extremely efficient and it doesn't cost more to do that final heat up than you saved by not having to have the furnace fire as often during the day.

All the main website (EPA, energystart, etc) state that you can say 2% on energy usage for every 1 degree you set back the temperature. If this was not true, then one would sell or buy programmable thermostats.....

NOTE: THere are other reasons why you don't want to set it back too far but they don't have anything to do with the furance not saving you more money. It's mainly things like appliances, wood, etc don't react well to temperatures going up and down all over the place and getting really warm then really cold etc. To save the most money you could literally turn the furnace off when you left the house and have it turn back on befor eyou come home, but that would cause damage to other thing sinside your house. That's the main reason most people say not to set it down past 55 or so... But you could do it if you wanted to.

 

[axnff]

In addition to FrankyJunior's reasoning (with which I agree), there is also the overhead of start/stop ineffeciencies. Furnaces are digital: they are on (full blast) or off. (There are probably exceptions, but I've never heard of one). If you leave it at a higher temperature, the furnace will have to cycle more frequently (since relative heat loss is greater with a greater delta-T). Like most machines, the start/stop time is very inefficient, so there are additional losses simply by turning it on at full blast for a few minutes to maintain a high temperature as opposed to making up the difference for a longer period of time.

 

[axnff]

Of course, having actually looked at the poll, I would not recommend dropping down to 45*; you want more of a 10 - 15* change that is still well within your heater's ability to warm air...

 

[Kelemvor]

Originally posted by: axnff
In addition to FrankyJunior's reasoning (with which I agree), there is also the overhead of start/stop ineffeciencies. Furnaces are digital: they are on (full blast) or off. (There are probably exceptions, but I've never heard of one). If you leave it at a higher temperature, the furnace will have to cycle more frequently (since relative heat loss is greater with a greater delta-T). Like most machines, the start/stop time is very inefficient, so there are additional losses simply by turning it on at full blast for a few minutes to maintain a high temperature as opposed to making up the difference for a longer period of time.

Actually that is only true of old furnances. All (most) current furnaces are called Two-Stage. This means that when it first comes on, it comes at "low fire" which is meant for the maintenance heating during the day/night. THen if it runs for a certain amount of time and doesn't get up to the right temperature they change to "High fire" mode which is the full force heat specifically for going up many degrees relatively quickly.

ALSO, I agre that the poll is very skewed. If "normal" temperature is 70, then a few degrees would be more like 67 for the "don't set it back much" crowd and the 60 would be more of the "set it back a lot" crowd. I don't think anyone should set it back to 45 because that's overkill and gets into the reasons in my previouus post about possibly damaging other things within your home.

 

[Dubb]

you guys are waaaay misunderestimating the principle of thermal mass. I'll see if I can dig up the proof with numbers - I know I have it around here somewhere.

unless your house has very low thermal mass, you are better off only kicking it back a few degrees at most.



edit: "misunderestimating"...it's a joke

second edit: what I have is for systems with a heat pump. looks like setting it back might be a good idea for some systems.

 

[TheLonelyPhoenix]

You shouldn't be backing your thermostat off to 45 anyway, as a pipe can freeze and burst. I believe 60 is a minimum

When your thermostat is up around normal, you only have to generate the heat to replace what you're losing through the windows etc... which is negligible compared to the space in the rest of your apartment. When moving up a degree, though, you have to heat ALL the air in your apartment up a degree, a much greater expenditure of energy.

I usually kick it down to 65 while I'm out for the day, back to 72-75 when I'm in.

 

[Colt45]

Originally posted by: TheLonelyPhoenix
You shouldn't be backing your thermostat off to 45 anyway, as a pipe can freeze and burst. I believe 60 is a minimum

When your thermostat is up around normal, you only have to generate the heat to replace what you're losing through the windows etc... which is negligible compared to the space in the rest of your apartment. When moving up a degree, though, you have to heat ALL the air in your apartment up a degree, a much greater expenditure of energy.

I usually kick it down to 65 while I'm out for the day, back to 72-75 when I'm in.

water doesnt freeze at 45.