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Why does lowering the thermostat save fuel?

Well, a difference of zero degrees between desired indoor and actual outdoor temperature means that the heating system will need to do no work. If it's 50 degrees outside, and I'm looking for 50 degrees inside (and I'm not heating up after any overnight heat loss) then I wouldn't expect the burner to kick on. And if I wanted it 51 degrees, I'd expect the burner to run about twice as often as if I had set the thermostat to 52 degrees.

Given the number of responses, I think my question was more interesting than I had thought. It's clear that I had underestimated how much faster heat is lost from a typical house as the differnce between outdoor temp and desired indoor temp increases. There must be tables or computer programs to help with this estimating, and allow me to compare, say, the effect of adding insulation to the effect of dropping the thermostat.

Thanks to everyone who responded - I'm no much smarter!


  • Why does lowering the thermostat save fuel?

    This seemed obvious until I started thinking about it...

    The way our thermostat works, when the temperature drops, say, 3 degrees below the set temp, it kicks the boiler on. Boiler runs (w/in pressure settings, of course) until the temperature reaches the setting and the boiler is switched off.

    If in my house it takes 45 minutes to bring the air around the thermostat back up three degrees, why should it matter if it's 65-68 instead of 68-71 degrees?

    According to the DOE, for every 1 degree you lower the thermostat, you save 3% off your fuel usage. Is the additional heat loss from the greater delta between inside and outside temp enough to make a difference?


  • R. Kalia_9
    R. Kalia_9 Member Posts: 5

    The cooler the house is, the slower the temperature drops. So even if your boiler kicks in after 3 degrees, it kicks in less often.

    But 3% per degree setback is complete nonsense. This would only be true on a 33 degree-day day if the house were cooler by 1 degree ALL-DAY. If it's colder outside, you save less in % because you are dividing by a larger number to calculate %. If it is warmer, you save more in %. Although both ways you save the same in $$.
  • will smith_4
    will smith_4 Member Posts: 259
    Problem with water vs air-

    Don't know if the DOE considers statistics in regards to both. My opinion is that they spend 1 million in hidden defense spending for every 1 dollar spent in consumption savings research. In other words, consider the source. I don't think they really spend a lot of time worrying about the actual cost of fuel in relation to heating your home using water or air, cause they really could care less. But here's my two cents. I can set my stat for my upstairs bedrooms to 60 during the day. Bring my forced air furnace on at 9:30 so that the temp is 68 when I hit the hay at 10:00, and it's there. If I set my basement and first floor hydronics to 60, leave it off all day, there is no way I can expect it to recover to 68 within a half hour-too much volume of water. What is the comparison of fuel? I don't know, but I could guess. Long story short, I can see the sense in setting back a furnace which has a quick recovery. With hydronics, I think you have to be thinking in terms of maintainig more of a set consistency, with thoughts of outdoor temp as well.
  • Tom R.
    Tom R. Member Posts: 139
    Answer: Thermodynamics

    Two of the first laws of thermodynamics are that (1)heat flows from a warm object to a cooler object and (2) the larger the delta T, the faster the flow. It follows then that the closer the inside temperature is to the OSA, the slower the heat loss and the less need to replace the heat (run boilers/furnaces, etc.) The practical limit is in how fast the heating system can recover from the setback temperature. In any event, setback works, especially in conjunction with controls that utilize outside air temperature input to optimize the reset time. This also works to initiate the setback time early.
  • soot_seeker_2
    soot_seeker_2 Member Posts: 228
    HVAC for Dummies vesrion - Leaky Houses

    Every house leaks heat when it is cold out.

    The more heat you try and store in the house to maintain a certain temperature above the outside temperature, the more heat that ends up leaking to the outside.
  • Weezbo
    Weezbo Member Posts: 6,232
    the sooner the t stat is satisfied...

    the sooner the burn cycle shuts down. if btu's arent being redistributed into the field the burner becomes Satisfied. with the t stat satisfied and the burner satisfied the fuel doesnt burn .Your comfort may be somewhat sketchey buh if you locate a t stat far from the point of distribution say in another building...it is possible the t stat will never become satisfied even if it is set at a lower temp. think of this one ...i have a boiler in one building with 1700sq ft of radiant and that t stat is controling the temp in an adjacent building of3600 sq feet of radiant...tell me how do you think i am managing to do that? the key word is in this writing... if you guessed Comfort then you got the right word. in the one building i have the t stat and flow rates dialed back alongside the boiler the main home next door has all the zones wide open fairly equally balanced similar construction slightly varing heat loss and useage... this is a temp lash up while construction is taking place.....soon it is time to recommence the work on my end of the deal. the comfort level of the other building varies a bit the one with the t stat stays pretty much okie dokie :) as it is basically The controll over the entire 11 zones.
    the situation is somewhat answer to your question,the heat loss isnt located specifically in the area of the Greatest heat loss nor is the thermostat located in the room of the least heat loss so what is happening is there is sort of a constant tug of war going on everytime the btus rail out along the passageways highways and byways if i dial the t stat down will it show up as cooler temps else where? sure enough, although, it will be time dependent based on thermal mass and heat loss of various rooms so will the t stat being dialed to 60 for a week gradually cause everything to reach anew equilibrium will dialing it up or down save fuel ?it has got to be apparent that the colder i make the required t stat temp the more easily it will be satisfied and the quicker it will turn off the burner....if the burner isnt consuming oil it has Got to be saving a certain amount of fuel simply because it isnt being redistributed as btu's to the environment.it is staying in the fueloil tank as Available btu's.

    Think about what that means and the picture becomes clear:) Meditate on Oil as "Available" he who meditates on oil as Avilable reaches as far as oil reaches He who meditates on oil as Available*~/:)
  • Myth Busters...

    I too have often wondered about this 3% per degree F setting reduction plan that the DOE touts. Only one way to find out for sure, and that is thru good old hard RESEARCH. I have the tools necessary.

    I have a gas meter on my boiler that will give a pulse closure for every cubic foot of gas that goes through it.

    I have a HOBO recorder that will record each pulse in real time from the meter.

    I have a 4 channel HOBO temeperature recorder.

    I have a Honeywell programable AIR type thermostat.

    And lastly but most imprtantly, I have COLD WEATHER.

    So, I figure I will lock the set back out for the first part of the test, keeping the home at 68 degrees F, record relevant degree day data and gas consumed. We'll let it run for say 5 days to get good average exposure.

    Then, I will release the program and allow it to do it's THANG, again recording the degree day data and CF of consumption. This will tell us if MY Home receives any significant benefits from programaable set back controls.

    As they are prone to say at the car shop, your milage may very...

    Nonetheless, should make for some interesting graphing.

    Now, if your home has an infiltration rate that is real high, the benefits should be obvious. You know you have a problem when you can see the drapes moving everytime there is a breeze outside... TO me, it would indicate the need for a good caulking job to seal up the home. I feel that would save more energy than a deep set back. I suspect the DOE's numbers are based on an extremely leaky home.

    JMHO. Time and facts will tell all.

    Comments? Suggestions?

    Here's a graph I did of R values on a wall of my home...

  • R. Kalia_8
    R. Kalia_8 Member Posts: 54

    "Why does lowering the thermostat save fuel?" Well, suppose it doesn't save fuel. You lower it 3F, same fuel consumption. Lower it another 3F, no change. Finally you are down to outdoor temperature, yet your boiler is chugging away, using the same fuel even if no heat is needed.

    Does this sound reasonable to you? Discuss.

  • I look forward to seeing your results!
  • R. Kalia_8
    R. Kalia_8 Member Posts: 54

    > And if I wanted it

    > 51 degrees, I'd expect the burner to run about

    > twice as often as if I had set the thermostat to

    > 52 degrees.

    Actually it will only run half as often (*). Heat loss is proportional to the temperature difference between a hot body (the house) and a cold reservoir (the outside). This is NOT a law of thermodynamics, just typical behavior of heat conduction in most cases.

    (*) The proportionality rule actually does not apply to small differences like 1F or 2F. It turns out that there is a small correction of about 5F. An average house at 70F loses heat based not on the difference between 70F and the outside temp, but based on the difference between 65F and the outside temperature. And that's an average house; every house is different.
  • Harold
    Harold Member Posts: 249
    Infrared thermometer

    Excellent discussion. I am looking forward to the test results.

    Could you use an inexpensive infrared thermometer to do the measurements? Fast response and reasonable directivity. Probably cheaper than the thermocouple equipment if anyone else wants to test and does not already own one (like us homeowners).

    Off this topic, I have thought of trying IR to find out if there is tubing in a section of floor. Let things run for a bit and then map the area of interest point-by-point with the IR thermometer. I need some holes in the slab and the original contractor will not give me pictures, a layout, or lengths of the installed piping and my current contractor does not have IR scanning equipment. Any other ideas on determning this that do not involve high priced equipment.
  • leo g_13
    leo g_13 Member Posts: 435
    I have

    done that with a fair amount of accuracy with a hand held IR temp sensor. Let your slab cool a bit, then bang up the heat. after about 15 minutes, start scanning. Not exact, but still better then nothing.

    Leo G

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  • rox
    rox Member Posts: 23

    From a practical standpoint, its not just the heat escaping out, its the cold getting in. Everytime you open the door cold air comes in. Stand near an old poorly insulated poorly weather stripped window you will feel cold air coming in. If you set the thermostat at 65 and the cold air that gets in is 30 you need to use enough energy to raise that amount of air 35 degrees. If you have the thermostat at 70, you will use more energy since you have to raise it 40 degrees.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Isn't that 3% per degree figure regarding general setback, e.g. keeping thermostat at 67° instead of 68°?

  • Tom R.
    Tom R. Member Posts: 139

    You are right. The first law is that of conservation of matter. However, the second law says that heat will not, of itself, flow from low to high temperatures. Therefore, the inverse is true. One of the GENERAL PRINCIPALS of thermodynamics is that the rate of flow is directly proportionate to the difference in temperature. ANY measureable difference in temperature will result in heat flow, however slight, because all of nature strives to be in equilibrium. If a glass of 65 deg. water is left in a 70 deg. room, it eventually will become 70 deg., having absorbed heat from the room (assuming the room is kept at 70 deg.) If your last statement were true, this would not occur. Law, principal, or general acceptance, all of thermodynamics is much more reliable than electricity, which is still referred to as the electrical THEORY.
  • hr
    hr Member Posts: 6,106
    How close Harold?

    I can slide my hand across a radiant floor and get almost as close as a point and shoot thermometer :)

    If you have an exact location for a hole and it is close to what your hand or the thermomerter tells you...

    An infared camersa will pinpoint to a fraction of an inch. The cost to have this done could be a lot less than some drilled tube repairs :) Pretty hard to repair a slab to look like the original pour.

    Just depends on how flexable you hole location is, I suppose.

    hot rod

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  • Tom R.
    Tom R. Member Posts: 139
    Nice work

    Fine looking job, Leo. And you installed instumentation!
  • Gordy
    Gordy Member Posts: 9,546

    Actually its not always the cold air coming in that one thinks they are feeling when standing in front of a window. That is not to say a poorly "weather stripped" window will not let cold air in. But standing in front of a poorly "insulated" window will cause the body to give up heat, making you think there is a draft. "Cold 70" or what ever your taste in room setpoint temp maybe.

  • leo g_13
    leo g_13 Member Posts: 435

    all this time I have been quoting the above thumb rule as "for every 3 degrees you can lower the boiler temp, you save 1% of the energy use". I was sure that the Viessman instructor said this to help explain why the Germans made it mandatory that their boilers can only hit 167*F.

    Leo G

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  • rox
    rox Member Posts: 23

    Excellent point, thats why I wanted to be clear I was talking about air infiltration (i.e. poor weatherstripping).
  • S Ebels
    S Ebels Member Posts: 2,322
    That is a fact

    I believe it goes like this IIRC. For every 3 degrees of boiler water temp BELOW 140*, your energy use will drop 1%. Above 140* it doesn't make as much difference. Therein lies the key to energy savings due to low water temps.

    I was just talking with my son in law in regards to how their new home was heating. We insulated the 5,400 sq ft house in the best manner we could think of when we built it. The heating system consists of an 8-32 Vitodens driving in slab radiant, Climate Panel and Vasco radiators which were sized to deliver design heat output at 150* water temp. I don't think we will see 150* water in the system unles it hits -40* this winter. He has been cranking the curves down all winter and has settled on a .5 for the Climate Panel and .8 for the rads. This is significantly lower than what we had anticipated. Long story short......He has 35% remaining in his 500 gallon propane tank which was filled October 12th. The little Vito rains condensate continuously and the modulating burner cycles at temps as low as 25* outside. Kinda got off the subject there but it illustrates the value and efficiency of systems that are designed for low temp operation.
  • will smith_4
    will smith_4 Member Posts: 259
    Dontcha remember Dan's tip from one of The Books-

    About using a wet mop on the floor and watching how it dries? I know it wouldn't work with carpeting, but neither would a base model IR t-meter. Can't get any more cost effective than that!
  • Tom R.
    Tom R. Member Posts: 139

    It would be interesting to see your results. I can remember that claim from long, long ago, when houses had 4" exterior walls and rock wool broadcast in the attic. I'm sure they meant turn down the stat and LEAVE it there. And "heating costs" could include the power to run the burner and ciculator or fan.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Sorry if I wasn't clear. That's EXACTLY what I meant with "general" setback. Down and LEFT DOWN!
This discussion has been closed.