This calculator gives a ROUGH estimate of the heat loss for a home. It is easy to use, but because of its simple approach, is subject to a number of potential errors:
All of the conduction/convection
heat losses are calculated with the standard one dimensional heat flow
equations -- e.g. Heat Loss = (Area) ( Tinside - Toutside )
/ Rvalue. These equations are very commonly used, but it should be
remembered that they are just approximations of the true heat flow.
The calculator estimates
infiltration heat loss by having you input a number of air changes per hour.
This is a difficult thing to estimate without at least a blower door test.
Infiltration also changes markedly with wind speed. So again, this is an
The yearly heat loss is estimated
using the number of heating degree-days for your area. This does not
take into account the actual sequence of weather conditions the home will be
subjected to (as a simulation would). Also, the number of degree-days
varies significantly from year to year, so any given year may deviate from the
heat loss predicted on the average number of degree-days considerably.
The calculator does not account for
passive heat gains through south facing windows. With any luck (or good
design on your part) this will result in the calculator predicting a
significantly larger heat loss than actually happens.
The effect of thermal mass is not
considered. Homes with high thermal mass will have smaller day/night
temperature swings, and usually require less heat.
Your habits can have a significant
effect on heat loss (thermostat setting, ...).
The internal gains for the home (warm bodies, lights, appliances, ...) are only approximately accounted for. They may in fact be significantly smaller or larger than the value shown.
And, I am sure there are quite a few more sources of error that are not listed above. Still, this tool is a lot better than a guess, and many design decisions are made using similar tools.
One way to get a better estimate, and also get a much better feel for how your home will perform in various types of weather is to use home thermal simulation software. This software does a year long simulation of your homes heat gains and losses on an hour by hour simulation using a weather file for your area. One such software package that is both easy to use and free is HEED.
I take no responsibility whatever for the accuracy or correctness of this software.
I take no responsibility whatever for any grief that errors, omissions, or just plain bad design in this software may cause you.
You use this software at your own risk.
If you can't accept these condition of use, then please do not use the calculator.
Gary 2/3/07, 5/5/07