Thermostat setback efficiency

[doitmyself]

Assuming an "average" home with adequate insulation and properly sized furnace (oil or natural gas), is there a point where lowering your thermostat setting too low (away from home and/or when asleep) becomes less efficient?

In other words, if I lower my T-stat to 50 F instead of 60F, does the amount of fuel needed to get back to 68F make the lower setting less efficient in terms of fuel used?

 

[eljefino]

Never, because the delta between indoor and outdoor temperatures is your bill. Make an hourly graph of typical indoor vs outdoor temps, assuming setback and no setback. Area between the lines is your heat. I'm no good at calculus but it will show you the "heating degree days".

Even if your house is so efficient it never drifts down to your lower temperature in your eight hour window, you're still saving.

When the furnace runs, it runs. No WOT enrichment.

 

[Gary Allan]

Sometimes. It depends on the heat transmission medium. Steam works best if it cycles just above and just below boiling. The ideal temp, in terms of efficiency, will be one where it holds that narrow above and below status. The longer between energy pulses, the more energy is required to bring it up to the phase change to steam.

Keeping that efficiency level may result in 60F interior temp or 85F interior temp ..and it will use or more or less energy depending on how easy or hard it is to keep that narrow range of keeping the system fully energized.

 

[brandini]

It's a tough call, because you're not just heating the air in your home, you also have to heat up all the stuff in your home. For shorter periods of time you can drop it a few degrees. If you're going on a trip, keep it at the minimum required to keep your pipes from freezing. And of course there's a sliding scale in between. The annoying part is that it'll depend on each individual home and heating system.

 

[doitmyself]

For this argument, lets assume forced air heat.

Eljefino, I don't quite understand your answer.

Following the great brandini's answer.....yes, the entire mass of the house cools down. So, if I am away at work for 10 hours each day, is it less efficient to have to heat that entire mass from 50F to 68F compared to from 60F to 68F.

What am I not getting? There has to be some kind of "wax on, wax off" logic here. Energy in....energy out? Btu's are Btu's?

 

[Captain_Klink]

What I do, is I pretend I'm a native person. The human body will easily tolerate living in 32F continuously. So I keep my house at 60F all the time. that way there is no cycling up and down, and it never feels chilly, the human body has a way of adapting the metabolism to take into account the ambient temperature. Depending on who you are sharing the house with, you can even drop it to 55F, and wear a light sweater, although, even with no clothes on at all, your body WILL adapt. It takes about 3 days for your body to shift its metabolism. so that's what i do anyway. at night, get yourself some nice wool blankets from the thrift store - good used wool blankets go for peanuts now, not like when natives had to trade 30 beaver pelts for a single blanket back in the days of the fur trade. It is so nice under just 2 blankets, if they are woolen. plus buy some nice wool socks to keep your feet warm, and you should be all set. my heating bills are way low, even though i live in a very cold climate.

 

[Gary Allan]

Quote:
For this argument, lets assume forced air heat.


Fastest recovery time ..by far. It will require the least energy to restore the 68F condition. It has no residual heat mass when you open a door or whatever like baseboard hotwater or steam. It's a lower efficiency burner compared to fire tube hot water system.

It also won't over shoot the set point due to the basic disconnect between energy applied and heat seen by the thermostat with other systems.

Assuming a smart thermostat, the hot air should require less pre-time pulsing to restore the 68f temp ..while hot water ..and (forget it) steam would have it pulsing hours before hand. For steam it would be several at continuous (size of boiler will vary this).

 

[doitmyself]

Well, a google search brings up this answer at many sites:

"A common misconception associated with thermostats is that a furnace works harder than normal to warm the space back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. This misconception has been dispelled by years of research and numerous studies. The fuel required to reheat a building to a comfortable temperature is roughly equal to the fuel saved as the building drops to the lower temperature. You save fuel between the time that the temperature stabilizes at the lower level and the next time heat is needed. So, the longer your house remains at the lower temperature, the more energy you save."

 

[Quattro Pete]

I'm guessing this is very much dependent on the efficiency of your furnace and insulation and construction of your home, but here is what our local electric company has to say about this:
http://www.comed.com/sites/HomeSavings/Pages/lownocosttips.aspx

They're suggesting to set the thermostat 5-10 degrees lower while you're out. So it looks like they're thinking that if you let the house cool too much, it'll take a lot more energy/reheating to bring it back to normal, not to mention it may take a really long time to do it, and hence the savings won't be as big.

Here's another thread I found on the subject, although opinions are divided...

http://askville.amazon.com/turn-heat-everyday-leave-house/AnswerViewer.do?requestId=6692882

Once again, the only correct answer is "it depends" I'm afraid.

FYI, in our house, we keep it at 70F when we're there, and let it drop to 62F when we're not. However, this is our first winter here, so I have no point of reference.

 

[Gary Allan]

The question was of efficiency, not necessarily total energy consumed.

 

[doitmyself]

Originally Posted By: Gary Allan
The question was of efficiency, not necessarily total energy consumed.


What's the difference? When I compare "fuel efficiency" of a car, I would prefer one that gets 30mpg vs. 20mpg. Similar with a house, I want to use the least amount of fuel to keep a given temperature. More "efficient" = less fuel used for a given set of temperatures selected.

I was wondering if managing the thermostat one way vs. another would be more efficient = less fuel used.

Is my logic or understanding of "efficient" flawed.

 

[itchrelief]

Originally Posted By: doitmyself


In other words, if I lower my T-stat to 50 F instead of 60F, does the amount of fuel needed to get back to 68F make the lower setting less efficient in terms of fuel used?


The other posters in this thread know a lot more about the specifics of furnace efficiency than I do. All I know is that eventually, all heaters are at least 100% efficient because it all becomes thermal anyhow... Maybe there are problems with control or with maintenance that dictate a certain type of usage over another

However, assuming your heating system does not have any particular nuances that affect its operation, the amount of fuel needed to get back to 68F froom 60F should be equal to or less than the amount of fuel needed to maintain 68F, because as the temperature difference rises between your hot and cold areas, the faster the heat transfers. The lower the temperature in your house becomes, the less energy you lose to the surroundings.

You would already have been using the same amount of energy (or more) to maintain 68F, due to the greater delta T, than to regain 68F. This is, of course, assuming your heater doesn't have any quirks that would make sustained, high output heating somehow disadvantageous.

 

[Quattro Pete]

Originally Posted By: itchrelief
all heaters are at least 100% efficient

So, they could be more than 100% efficient? I'm not sure I follow your logic here...

 

[duaneb9729]

the packages that the honeywell set back thermostats came in, used to say someting to the effect of a 6 to 8% savings in the heating mode, and 12 to 14% for air conditioning.

after you install so many of these things you sort of quit reading the hype on the outside of the box.

 

[mechtech2]

For the long term, setting it real low is best.
For up to to 2 days, it's a tough call.

 

[mrsilv04]

I just looked into this myself.

I was interested in how to properly set a programmable thermostat for the most savings.

Energy Star states for best savings to set thermostats back a minimum of eight degrees, for a minimum of eight to ten hours.

http://www.energystar.gov/index.cfm?c=thermostats.pr_thermostats_guidelines

 

[Gary Allan]

Quote:
Is my logic or understanding of "efficient" flawed.



in some cases, yes. Like in my steam example. If allowed to cool too far, the energy to return to the boiling point will take a long time. While that in itself will probably work out to the same deal on the total energy balance, the time I'm applying energy until I'm realizing it (having it transmitted in the form of steam) will be protracted. I will then run continuously for up to an hour at above 212 (pressurized) with the radiators putting out great heat ..and wait even longer until the thermostat sees it. This will then overshoot the set point and, again, idle the boiler for a prolonged period of time and you start the cycle up all over again.

Very inefficient. OTOH, if I just pulsed the boiler at a preset cycle. I would/could maintain the more/most efficient state at variable temperature. It's the liability of having a fixed output boiler. I can't vary the air:fuel delivery to vary it between simmer and full rolling boil.

It's somewhat different with hot water. With hot air, I'd turn it down as far as you please. Exchanging the air content is easy/quick.

 

[rjundi]

I find my hot water boiler runs a long time trying to catch up with large temp swings. I think is due to 8 old school radiators hooked into steam piping. I let it swing about 4F max overnight as sleeping is better for us.

 

[eljefino]

Originally Posted By: Gary Allan

I I will then run continuously for up to an hour at above 212 (pressurized) with the radiators putting out great heat ..and wait even longer until the thermostat sees it. This will then overshoot the set point...


Perfect! Even if you get home from work and the air is the perfect temperature the furniture, carpets etc will still be holding in some cold. The overshoot will make the house feel habitable until its furnishings stabilize.

 

[benjamming]

Efficiency concept is lost on the wife when her orchid dies b/c I set the thermostat too low while we were away on vacation. A new orchid ate up all my savings on that one. Hypothetical situation of course.

It is also lost when it takes hours to get the temperature back to "comfortable" (too low of a setting), & she complains the whole evening that the house is cold.