What is the formula for figuring btu's neccessary to heat and or cool a house?

Question

What is the formula for figuring btu's neccessary to heat and or cool a house?

Answer 1

I dont know about a formula but this is what you would do.

First assume an external temperature.

Second decide on an internal design temperature.

You would then calculate the heat loss in BTUs/hr through each wall, windows, doors, roof, floor.

There are tables of heat transferance values for each material available.

You would decide on the number of air changes per hour and the heat required to heat that air. There are tables available for each type of room use for this also.

You would calculate how much hot water, its temperature, coupled with an assumed initial temperature and thus the heat required for that.

You would add all these heat requirements together and add

say 30% for extreme conditions and to ensure a rapid achievement of design temperature from cold.

(You would of course have thermostats to stop the heat generation at the design temperature until it dropped sufficiently as to restart)

You would then chose an adequately sized boiler to provide the number of BTUs/ hr which you calculated allowing for the fact that the boiler will not be 100% efficient - it might be only 80% - and will deteriorate over its lifetime of say 15years.

Hope I helped.

Answer 2

The first answer is right, you have to make all those calculations in order to heat and or cool a house, just that he only did it for heating. If you want to cool the house, you get the btu/hr you want to remove and together with the air changes you get to dimension the evaporator of the cooling system.

I would recomend you use a program for this if you are not familiar with the equations of heat transfer because it could be a lot of work to do the calculations by hand or if you want to get the heat flow rate for the 24 hr of a specific day. Be careful to calculate for the position the house has in the world because this will affect the amount of heat comming from the sun. I mean, it's not the same to calculate a cooling system for a house in the bahamas  than a system for a house in canada.

you have to calculate too how much humidity you actually want in your house and you have to investigate the speed of the wind in your country. Because all this affects the calculations.

HOpe this helps.

Answer 3

Most contractors just go by square feet, but here's a good explanation:

http://www.air-conditioner-selection.com...

Here's another:

http://www.energystar.gov/index.cfm?fuse...

And two more...

http://answers.yahoo.com/question/index?...