Heat flow out of a house by conduction is like current flow through a wire. And the equations are similar. I’ll start with the electrical which everyone should be familiar with.
I (amps) = V (voltage) / R (resistance)…12 amps = 120 V /10 R
Current (electron) flow (I) is driven by the voltage…The voltage may be looked at as the driving force or presssure that forces the electrons to move along the wire (Direct current). Double the voltage (V) and you double the current flow; halve the voltage and you halve the flow.
Current flow is reduced by the resistance ®. Double the R and halve the flow.
In a building, the heat flow, U, (current) is in btu’s per hour per sq ft per degree temperature difference. The driving force (voltage) is temperature difference between outdoors and indoors (heat flows from higher to lower temps)…called Delta T (DT) in scientific terms. The higher the DT from inside to outside, the higher the heat flow. .
Heat flow U = TD / R If we double or halve the DT, we double or halve the heat flow. If we double the R, we halve the heat flow.
When we set the temperature back in a building, we reduce the temperature difference (DT) between inside-outside and thus the heat flow outwards is reduced due to a smaller driving force. Therefore heat is saved!! We have to be careful how far we set back due things like freezing of pipes and potential condensation from cooler air temps.
A little aside here: In testing materials for resistance to heat flow, they first find the heat flow through a material in btu’s per hour per sq ft per degree temperature difference (btu’s/hr/sq ft/deg). The R value we are familiar with is simply the reciprocal or inverse of the heat flow. If the heat flow was 0.1 btu’s /hr/sq ft/deg, the R value would be 10. But the R we are familiar with does have units…we just don’t see them on insulation bags or charts:
note-dots are to keep units aligned.
R = hr-sq ft-deg …the inverse of the flow …btu
…btu… hr-sq ft-deg
For the example, the outside temp is kept constant at 30 deg
**Inside temp 70: DT = 40 deg; **
U= 40 deg / 10 hr-sq ft-deg
Thus heat flow= 4 btu/hr/sq ft
setback to 60, DT now = 30:
U= 30 deg / 10 hr-sq ft-deg
Thus the heat flow= 3 btu/hr/sq ft
Therefore, the more we can safely set back, the more we will save.
Yes, when the t’stat calls for 70 degrees again, it will take longer to heat the house up than we see in a regular heating cycle…But when we first setback the temp, the walls and objects of the house will release stored heat (1,000’s of btu’s) to the air as its temperature drops. The heat source will not come on for quite a while until the air temp falls below the set back of 60 deg. So we have saved running time here…this saved time should be equal to the time required to heat the house back up. This then is a balanced situation. Thus the savings during the setback period are real savings.
Fuel companies don’t want us to set back and save. They use the “long running time” and “takes a lot more heat to warm back up” false arguments to make more $$$$. Isn’t is “All about the money”?