ue. Our example has two outside walls—one
ten feet long and the other 15 feet long. To
make it easier, simply add the lengths together
and make it one big wall. (NOTE—with heating,
north, south, east or west facing walls are
not a factor as the room will be heated after
sundown when no solar gain is applied).
In this case, there are 25 linear feet of wall
times 9 feet high for a total area of 225 square
feet. Subtract the window area from that
total—225 minus 60 equals 165 square feet
of net wall area. This is the number to use in
calculations.
The insulation (Figure 5 on preceeding page)
in the wall will determine its U value—in this
case, it’s six-inches of fiberglass bat at an R-19.
R-values for the exterior sheathing, house wrap
and finish, as well as for the inside sheetrock
may be available and lead to a more accurate
calculation. For simplicity and to provide a
safety margin, simply use the insulation Rvalue
for the calculations. In this case, divide 1
by 19 to find the wall U-value of .05.
165 x 70 x .05 equals 578 Btuh lost through
the walls.
Outside doors, if there were any, would
be calculated in the same way and their area
would also be deducted from the net wall area.
The ceilings and floors also need to be
evaluated, but only if they are below or above
unheated space. If the area above the ceiling is
heated, there would be no heat loss through the
ceiling, and if the floor is above a heated area,
there would be no heat loss through the floor.
For the ceiling: Multiply the length times
the width times the DTD times the U-value.
For simplicity, this example will use insulation
values. With R-38 in the ceiling, U-value
would be 0.02.
10 x 15 x 70 x .02 and there is a loss of 210
Btuh.
24 National Oilheat Research Alliance
The floors have R-19 insulation, so calculate
a U-value of 0.05. Then multiply 10 x 15 x 70
x .05 for a floor loss of 525 Btuh.
Finally, add them all together:
Infiltration 1,701
Windows 1,512
Walls 578
Ceiling 210
Floors 525
Total 4,526
The total heat loss for the room is 4,526 Btuh
This procedure would be repeated for each
room of the building to determine the terminal
unit capacity needed. The total of heat loss for
all the rooms determines the heat loss for the
entire building.
Assume the complete heat loss for the building
is 70,500 Btuh. A boiler of suitable output
must be selected.
Depending on the manufacturer’s terminology,
choose a boiler based on DOE capacity,
the NET IBR RATING or the output Btu/hr. In
this case, the proper boiler is the EK-1 model.