Fwd: [GBlist] Heat transfer, Setback Thermostats

[Corwyn <corwyn@midcoast.com>]

> On Friday, January 4, 2002, at 11:56 AM, Bettina & Steve wrote:
> > My assumption is that heating up a solid thermal mass is more energy expensive than heating the surrounding air
>
> Nope.  You are just replacing heat that is lost.  If you are losing heat at a rate of 20K BTU per hour, and the thermal mass of the air is  20K BTU / degree, (all numbers made up) then in the first hour your air has lost 1 degree.  If you have thermal mass in concrete of 100K  BTU / degree. it will give up heat to the air keeping it from dropping to a lower temp until the thermal mass has lost 100K BTU.  In other words after 6 hours, the air and the thermal mass will all have dropped one degree.  In order to get the temperature back up you need to put back all 120K BTU that left the building.  (1 thousand caveats left out).
>
> > , and that at some point in a 10hr. period, a lower (greater difference) setback temperature at night would end up using more energy than a higher setting  (Could a setting of 40F use more than 50F?) 
>
> No.  Where I said that the building was losing heat at a rate ot 20K BTU, well that is dependent on the difference in temperature between inside and outside.  So, if you set back to 40 degrees and it is 43 degrees outside, you will lose _no_ heat after the inside gets to 43 degrees.  Same thing to a lesser degree for less extreme examples.
>
> > I guess that's a question about the rate of heat transfer from a thermal mass to surrounding air--how much in 10hrs., and the substance of what you hinted at in the above snip.  Without going through an entire heating season trial and error, any idea as to where that point might be?  Would it be that, in a 14on/10off hour cycle, the thermal mass is always at a lower temperature than the surrounding air? (Forced hot air system). 
>
> The issue is how long it takes your furnace to produce the heat you have lost.  If you lose 20K BTU / hour and your furnace produce 50K BTU per hour, you need to start your furnace 2 hours before you need the heat at room temp for every 5 hours that the setback is in effect (in the worst case)  If you reach the setback temperature you will need to run you furnace more than that to keep the minimum temperature,  that energy is essentially a loss.
>
> > Comfort level is not really an issue--  the building is unoccupied during the daily setback period and most occupants would not really notice any effect of the radiant heat mentioned in your and another thread posts.  No water pipes to be concerned with either.
>
> If nothing cares if it gets cold then you can set your setback as low as it will go.
>
> >  Several responders noted that they saved no energy from setting back their nighttime temperatures  (in contradiction to your opener that it will almost always save energy).   Care to hazard a guess as to why?
>
> What _I_ meant was that _the equipment_ did nothing more than you could do by hand.   If they were already doing a good job regulating their furnace, then the setback thermostat gains them nothing.
>
> Or it is possible that the people who gain nothing are in a situation where their furnace is more closely matched to their heat loss.  If your furnace from the above example were only capable of producing 20K BTU / hour, then it would need to run continuously no matter the setback, in order to maintain the temperature for the next morning.
>
> Hope this didn't hopelessly cloud the issue,
>
> Corwyn
>
>
> -- 
> Corwyn
> corwyn@midcoast.com