SERVICING
19
HEATING PERFORMANCE TEST
Before attempting to diagnose an operating fault, run a heating performance test and apply the results to the
Service
Problem Analysis Guide.
To conduct a heating performance test, the BTU input to the furnace must be calculated.
After the heating cycle has been in operation for at least fifteen minutes and with all other gas appliances turned off,
the gas meter should be clocked.
To find the BTU input, multiply the number of cubic feet of gas consumed per hour by the heating value of the gas being
used. (The calorific value of the gas being used is found by contacting your local utility.)
EXAMPLE:
It is found by the gas meter, that it takes forty (40) seconds for the hand on the cubic foot dial to make one
complete revolution, with all appliances off, except the furnace. Take this information and locate it on the gas rate chart.
Observe the forty (40) seconds, locate and read across to the one (1) cubic foot dial column. There we find the number
90, which shows that ninety (90) cubic feet of gas will be consumed in one (1) hour.
Let's assume the local gas utility has stated that the calorific value of the gas is 1,025 BTU per cubic foot.
Multiplying the ninety (90) cubic feet by 1,025 BTU per cubic foot gives us an input of 92,250 BTUH.
Checking the BTU input on the rating plate of the furnace being tested.
EXAMPLE:
INPUT:
92,000 BTU/HR
OUTPUT CAP:
84,000
Should the figure you calculated not fall within five (5) percent of the nameplate rating of the unit, adjust the gas valve
pressure regulator or resize orifices.
