# How do I calculate how long a furnace has to work to compensate a given temp drop?

I am trying to understand the dynamics of heating a home after the furnace size was calculated. Say you have a 60000BTU furnace to heat a 2000 sqft home. At the beginning of the heating season you brought the in house temperature near the point set on the thermostat and since then the temperature has been fluctuating around that value as the house lost heat and that was compensated by the furnace

At this point I have two questions:
-how do you set how much the temperature can drop before the thermostat calls for heat?
-How can I calculate equivalent temperature rise of let's say 1000BTU/h delivered in an 1000 cubic feet volume?

The point with my last question is this: if it is possible to configure the temperature drop before the thermostat calls for heat I would like to know how long the furnace will have to work to bring the temperature back to the set point

The vast majority of home heating thermostats do not have a user configurable "temperature drop before the thermostat calls for heat" or hysteresis. It is what it is, and it's usually small.

To calculate the temperature rise, you need to know many things, some of which vary. In particular, the insulation (resistance to heat flow) of the structure, and the external temperature. The thermal mass of the building and contents also come into play, as does air leakage and/or deliberate ventilation; air leakage will generally vary with wind speed and direction outside the house.

The net effect in practice is that the furnace works longer as it gets colder (and/or windier) outside. If you monitor how much the furnace (burner) runs in an hour, you can infer the rate of heat delivery required to maintain temperature for that hour (If it runs for 30 minutes and is a 60,000 BTU/Hr furnace, the house needed 30,000 BTUs for that hour.) An ideally sized furnace will run 100% of the time on the coldest day you get (and maintain temperature in the house while doing that.)

• Thanks Ecnerwal I have already done the manual J calculation and I am now stuck with Manual D.I already have an old furnace in place and of course the air ducts are there but I have a weak air flow in the basement.I have already posted a couple of other questions about my calculations and about my design problems. Mainly I am now trying to determine if the things will change if I replace the furnace and I get a better blower. Since the basement was finished after the furnace was installed I want to know if I have a CFM problem or duct design problem. The air paths are clean no dumpers closed. Dec 27 '14 at 16:38
• This is all good advice. The only thing I would add is if your duct works are cold/hot or really really poorly designed (this would have to be super bad) then getting a more powerful furnace will help. For the most part though once a furnace fills the ducts with air, it should spit out decent air in even a poorly designed system given the ducts are conditioned. If your ducts are not conditioned or poorly designed a smaller furnace even though more efficient will cost you more because it will be filling up more not blowing out. Jan 26 '15 at 23:25

I think that this the answer to my question

http://www.engineeringtoolbox.com/cooling-heating-equations-d_747.html

Example - Heating Air An air flow of one cfm is heated from 32 to 52oF. Using (1) the sensible heat added to the air can be expressed as:

hs = 1.08 (1 cfm) ((52 oF) - (32 oF))

``````= 21.6 (Btu/hr)
``````