FIGURING HEAT LOSS:

Below, we detail how to figure heat loss for any house or other small building. Manual calculations can be boring. If you want us to do it for you click here.

You can make a rough estimate of a building's heat loss by looking at a year’s fuel bill and converting it to BTUs. Then apply the data from the section on Cost of Fuels. Adjust as best you can for hot-water usage, air conditioning, heat input from lighting, etc. However, don't be satisfied with the results until you check against the method outlined below.

For the levels of insulation and fenestration that you are considering:
A.   What is the peak heating demand that the heater must meet?
B.   What is the total seasonal heating load?

 A.   Calculating Peak Load for the one-time Extreme:  To decide on the size of the heating unit that your home should have, calculate the heat that will be lost through all surfaces on a night almost equal to the coldest night on record for the area. To explain "almost:":  If there have been four occasions when the temperature dropped to ten degrees F below zero, and only one when it reached 15 below, then design for 10° below. The added cost of protecting against one chance in  hundreds is hard to justify.

Using the surface areas determined above, calculate the heat loss based on the difference between 65° and the "almost coldest” night. Starting with the example below, sketch each of the walls, plus ceiling.   Input the areas and appropriate U values. Multiply out and add.

If you wish, you can set up a simple spreadsheet in Works that speeds the process, and makes it easy to test different scenarios.

EXAMPLE:
Applying values to a 4" insulated wall at night, 8 feet high and 60 feet long, with six double-glazed windows, each 2.5’ by 4’.

Indoor/Outdoor temps, 70o/20°F.Net wall: (8x60)-(6x2.5x4) x U x  T = (480sf – 60sf) x.055 x 50° = 1,155 BTUs/Hr

Windows:  (6x2.5x4) x U x  T = 60sf x .251 x 50° = 753 BTUs/Hr

Total Heat loss of wall = 1908 BTUs/Hr when outside air is at 20°F.

Notes:
For houses on slabs, assume heat loss through the (4"?) slab for the outer two feet
all around the perimeter.

For houses on full foundation, assume heat loss through the exposed concrete, plus as far below grade as the frost line extends in your area.  The rest of the basement wall, and its floor, are considered to be fully insulated by the earth.

For the full-season calculation, if there is a wall that faces within 20-25 degrees of Solar South, and it meets the specs outlined in Design, remember not to include that wall in your heat-loss total.   Over a normal season, that south-facing wall will gain more than it loses. If the building is rather old, check the ceiling insulation thickness and condition to make sure that the nominal value can still be used.

B.   Calculating BTU load for a complete heating or cooling season:

The name of this game, in case the alligators in the swamp have distracted you, is to figure the cost of fuel using one level of insulation or fenestration, and then see what you would save by upgrading that, or by eliminating a skylight, or changing fuel.

Same calculations as above, except:

For the expression “DT”, use  the total seasonal degree-Day figure for your area times 24 hours. The result is the total BTU loss for the whole season. Use the Cost of Fuels equation to see what space heating should cost you next year.