Thermo Pride CMA4-50D36N, Installation And Service Manual, Page: 49 / 66

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C. ADJUSTMENT OF BTU INPUT RATE:
The orifice for this furnace was sized: 1) for natural gas having a heating value of
1025 BTU per cubic foot and a specific gravity of .65, or 2) for liquefied propane
gas with a heating value of 2500 BTU per cubic foot and a specific gravity of
1.55. The information plate inside the furnace vestibule will specify which gas
your furnace is orificed for. If the furnace is installed at an altitude that is more
than 2,000 feet above sea level, it is mandatory that the input to the burner be
reduced 4.0% for every 1,000 feet that it is above sea level. If the furnace is
installed at an elevation of 5,000 feet, its input must be reduced 20.0%. Example:
a furnace rated at 75,000 BTU at sea level must be reduced to a firing rate of
60,000 (75,000 x .80 = 60,000) at an elevation of 5,000 feet. If the furnace is
installed at an elevation of 2,000 feet or less, no reduction in input is required.
Your gas supplier or Thermo Products will supply you with the correct orifice
sizing information.
To check the input of your natural gas furnace, allow the unit to operate for 10 to
15 minutes and proceed as follows:
1. Call your gas supplier and ask for the BTU content (heating value) of one
cubic foot of the gas, supplied to the installation area. An alternate
approach is to assume a value of 1025 BTU/Cu Ft which is the national
average.
2. With all other gas appliances turned off and using a stopwatch, clock the
time required for the (small) dial on the gas meter to make one full
revolution. The meter dial will state the cubic feet of flow for one
revolution usually one, two or five.
FORMULA: BTU/Cu Ft x Number of Cu Ft x 3600 Seconds = Input BTU/hr
Seconds for one revolution
EXAMPLE:
1025 BTU/Cu Ft x 2 Cu Ft x 3600 = 73,505 BTU Input
100.40 Seconds
Check the model number of the furnace, the input rate, the type of gas,
and the manifold pressure on the rating label located in the burner
vestibule. If using the above example, the furnace is a CMA2-75N model
and the input rate is 73,505 BTU/hr. The input rate would be acceptable
because it was 2% of the listed input of 75,000.
3. Make sure that the gas supply pressure to the furnace is within the
allowable range of 5.5" to 14" WC pressure on natural gas and 11.0" to
14.0" WC on propane gas. The pressure to the furnace must be checked
while the furnace burner and any other gas appliances on the same
supply system are operating, using the 1/8 in. NPT manifold pressure tap.