Setback thermostats

[jdp1152]

what do you mean?

Entirely too many variables to say with certainty that X degree setback is detrimental or beneficial to everyone. It's going to depend on how efficiently you maintain temps ( insulation, air leakage, solar gain, type of structure retaining thermal mass , etc) as well as how efficiently you can rebuild those temps. Your house and your heating system will determine what level, if any, you should set back to save, if any, money. There is no one size that fits all and any HVAC person who states otherwise should know better.

 

[fbelec]

Entirely too many variables to say with certainty that X degree setback is detrimental or beneficial to everyone. It's going to depend on how efficiently you maintain temps ( insulation, air leakage, solar gain, type of structure retaining thermal mass , etc) as well as how efficiently you can rebuild those temps. Your house and your heating system will determine what level, if any, you should set back to save, if any, money. There is no one size that fits all and any HVAC person who states otherwise should know better.

but it does take a certain amount of energy to heat the air and the walls and the furniture and a cold wood stove. yep it's a percentage. depending on what yer heating. how many square feet. no one said any dollar amount but the fact is it does and should be a certain percentage which might be slightly different money wise but with the way the majority of heating systems are designed the thermostat and boiler companys will give you the number in degrees which is what we're dealing with that is what you can gauge yer set back point if you find that your house is only 4 thats great. if you find your house saves more at ten great. it's just a gauge to go by. lots of people shut their thermostat down manually or with a setback but look at how hard it has to work to bring that temp back.

just passing on info from what i received from those companys

 

[firebroad]

In my experience with heating oil, I found that setting the therm back in the morning before I left (5:30) to 62, then having it come back to 67 when I returned at 5:00 then shutting it back to 62 when I went to bed until 4:30 or so did save me money; I have a reasonably insulated home; and I had a programable stat that ramps up slowly as it learns my habits.
This part is key. Even now that I ditched oil and got a heat pump, I still use the same programming, and have found that the aux heat doesn't usually come on unless it's pretty cold outside. The stove, of course, takes up most of the slack, so my heating system is basically the back up for my stove, and not the other way round.

 

[Adios Pantalones]

"Building back heat" in walls etc, is not a net loss. If you had kept the temp turned up- the walls would still be losing heat (at a higher rate, as it relates to the DIFFERENCE in temp inside and out), but it would be replaced constantly by your heating system.

Your house does not magically stop losing heat just because your boiler is running.

Temp swings may be bad for some stuff in your house, and may overwork a boiler (I guess), but the thermodynamics of this are pretty clear.

 

[semipro]

I think there may be some confusion on why setting back may save energy.
Heat loss from your house to the outdoors occurs faster when the temp difference between outside and inside is higher.
Therefore, lower thermostat settings result in less heat loss when its colder outside.
However, overall setback energy savings are predicated on the fact that your heating system operates at the same (or greater) efficiency when maintaining:

1. +/- 5 degrees on longer cycles (setback mode). when compared to
   
2. +/- 1 degrees on short cycles (normal thermostat mode)

(This is a conceptual comparison for simplification. During setback your thermostat will still be cycling to maintain +/- 1 degrees though at a lower set temp).

I can't see where solar gain, thermal mass, other factors mentioned would impact one these modes more than the other. Solar gain would seem to occur independent of setback. Thermal mass may create a time lag between setback time and HVAC operation but the operating time would be the same just delayed with higher thermal mass.

Additional setback energy savings occur because most HVAC systems operate more efficiently at a steady state rather than cycling on and off often. The same start/stop inefficiencies that occur in other motors also apply to compressor and fan motors in HVAC systems.

ORNL has done some interesting research on setting back. They even went so far as to build different houses and then compared the energy usage using setback and not.

 

[firebroad]

This is where I got my information:
http://energy.gov/energysaver/articles/thermostats

 

[semipro]

This is where I got my information:
http://energy.gov/energysaver/articles/thermostats

Great site. Thanks for posting.
There's a very direct and simple explanation of why setting back works there.
It goes into how even heat pump and boiler based system users can benefit from setting back with the right thermostats.

 

[1750]

It depends on the heating system and it's recovery time. High output systems like oil and gas furnaces can see a significant savings with a setback thermostat as long as the set back isn't too radical. 5 degrees is usually plenty.

Can you help me understand why this same thing wouldn't be true if you turn back the thermostat more substantially (e.g., 10 or more degrees)? It seems like the same variables would work in your favor, but I admittedly don't know much about this.

This is where I got my information:
http://energy.gov/energysaver/articles/thermostats

Thanks for posting this.

"...By turning your thermostat back 10° to 15° for 8 hours, you can save 5% to 15% a year on your heating bill -- a savings of as much as 1% for each degree if the setback period is eight hours long."

 

[semipro]

begreen said: ↑ It depends on the heating system and it's recovery time. High output systems like oil and gas furnaces can see a significant savings with a setback thermostat as long as the set back isn't too radical. 5 degrees is usually plenty.

1750 said: Can you help me understand why this same thing wouldn't be true if you turn back the thermostat more substantially (e.g., 10 or more degrees)? It seems like the same variables would work in your favor, but I admittedly don't know much about this.

I suspect that the limit to how far you can set back is dependent upon how fast your HVAC can recover. If you were to set back too far (and/or too long) your system might not be capable of bringing temps up to desired levels fast enough.

 

[dmmoss51]

Boiler's might be different because you have the water to keep hot anyway. I think there can be a decent amount of savings with forced air as you are reducing the amount of times it cycles during the day when no one is home, and also subracts from the time that that fan runs on the electric side too. The problem is if you set it down too many degrees it runs for one long cycle when it goes back up that is longer than the missed cycles combined.

Also its a great combonation with wood heating as our furnace hardly ever runs anyway, but won't call for heat if it cools off a little bit before I get home and rekindle the fire. Then My goal is to get the stove heating the house before the thermostat calls for heat.

 

[btuser]

"

Temp swings may be bad for some stuff in your house, and may overwork a boiler (I guess), but the thermodynamics of this are pretty clear.

Yep. A btu unburned is a btu earned.

(hey, that would make a good t-shirt)

 

[fbelec]

don't forget when the sun is out giving solar heating it might not actually heat your house but the outside wall thinks it a little warmer and doesn't lose it's heat as fast. then the sun goes in the temp of the wall drops and at 5 or 6 o'clock when the heat comes on it has to over come the cool air in the house the cool wall and the temps outside. then when the air is heated the heater is not done. you'll notice it come back on sooner than normal because it takes more time to heat the walls. the hardest thermostat to control is the one that puts the wood in the stove. sometimes it's just one log to many or one to short and the back has to pay for that one

 

[firebroad]

Boiler's might be different because you have the water to keep hot anyway. I think there can be a decent amount of savings with forced air as you are reducing the amount of times it cycles during the day when no one is home, and also subracts from the time that that fan runs on the electric side too. The problem is if you set it down too many degrees it runs for one long cycle when it goes back up that is longer than the missed cycles combined.

Actually, I find that my HP fan runs so much while maintaining heat that that isn't really true.


"Also its a great combonation with wood heating as our furnace hardly ever runs anyway, but won't call for heat if it cools off a little bit before I get home and rekindle the fire. Then My goal is to get the stove heating the house before the thermostat calls for heat"

I do the same thing! Soon as I get home I race for the stove to get it going before the system kicks in. (Sometimes I cheat and turn it down.)