How to size a Unit Heater

1.  Calculate the area to be heated in cubic feet.

Example, 40ft x 40ft x 14ft = 22,400 cubic feet

2.  Select the type of building from Table 1 (based on 0° F minimum outdoor design temperature)

Example, Warehouse 65° inside, One Story with skylights in the roof = 5.8 Heat Loss Factor

Table 1

Type of Building / Major Qualifications / Minor Qualifications / Heat Loss Factor
Factory or Warehouse
65° F inside temperature desired / One Story / Skylights in roof
No skylights in roof / 5.8
5.3
No Skylights / Two story
Three story
Four story
Five story
Six story / 4.3
4.0
3.8
3.6
3.4
Public Garage 60° F Inside temperature desired / All Walls Exposed / Skylights in roof
No skylights in roof
Heated space above / 5.5
5.1
4.0
Warm Party Walls on Both Long Sides / Skylights in roof
No skylights in roof
Heated space above / 4.7
4.4
3.0
One Long Warm Party Wall / Skylights in roof
No skylights in roof
Heated space above / 5.0
4.9
3.4
Store
70° F inside temperature desired / All Walls Exposed / Flat roof
Heated space above / 6.9
5.2
Warm Party Walls on Both Long Sides / Flat roof
Heated space above / 5.8
4.1
One Long Warm Party Wall / Flat roof
Heated space above / 6.3
4.7

3. Multiply cubic feet by the Heat Loss Factor

Example, 22,400 x 5.8 = 129,920 BTU’s of Heat Loss

4.  Adjust for warmer minimum outdoor temperatures by using the correction factor in Table 2

Example, 40° minimum outdoor design temperature = 0.43 correction factor

Table 2

Correction Factor
Minimum Outdoor
Temperature / Correction Factor
50°
40°
30°
20°
10°
0°
-10°
-20°
-30° / 0.29
0.43
0.57
0.72
0.86
1.00
1.14
1.28
1.43

5.  Multiply the BTU’s of heat loss by the correction factor to equal the heat loss at a 40° minimum outdoor temperature

Example, 129,920 x 0.43 = 55,866 BTU’s of heat loss

6.  Select the correct Unit Heater in the specification table according to the Unit Heaters BTU output.

Example, F75E will deliver 60,000 BTU’s output

NOTE: Make sure to select the Unit Heater by its heating output not input. Choose a larger heater than the buildings BTU heat loss. Multiple Unit Heaters can be used for a more uniform pattern of heating or if more BTU’s are required than a single large unit can produce.