# Cautions on Using the Heat Loss Calculator

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 approximation.

• 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.