Operating Your Furnace
16
Type of Gas
Manifold Pressure, In W.C.
Natural 4
L.P. 10
The rated input will be obtained on 2,500 Btu of propane at 10
inches manifold pressure with factory-sized orifices. If L.P. Gas
having a different value is supplied, orifices must be changed
by a qualified installer before the furnace is operated.
Check the Manifold Gas Pressure
A tapped opening is provided in the gas valve to facilitate
measuring the manifold gas pressure. A “U tube” manometer
having a scale range from 0 to 12-inches water column should
be used for this measurement. The manifold pressure must be
measured with the burner and pilot operating. Any major
changes in the flow must be made by changing the size of the
burner orifice. Check with your local gas company for the
proper orifice size.
Check the Gas Input (Natural Gas Only)
Under firing could cause inadequate heat, excessive
condensation or ignition problems. Over firing could cause
shooting flame impingement or overheating of the combustion
chamber. Before starting the natural gas input check, obtain
the heating valve of the gas (BTU per cubic foot) at standard
conditions from your local supplier. This factor is used in the
"Check the Gas lnput" section and procedure.
To measure the input using the gas meter, proceed as follows:
1. Turn off the gas supply to all other appliances except the
furnace.
2. With the furnace operating, time the smallest dial on the
meter for one complete revolution. If this is a 2 cubic foot
dial, divide the seconds by 2; if it is a 1 cubic foot dial, use
the time in seconds as is. (3,600 = Sec. Per Hr.) This gives
the seconds per cubic foot of gas being delivered to the
furnace.
3. Assuming natural gas with a heating valve of 1,000 Btu per
cubic foot and 34 seconds per cubic foot as determined by
step 2 above, then:
Input: 1,000 x 3,600÷34 = 106,000 Btu/hr.
This measured input must not be greater than the input
indicated on the rating plate of the furnace.
4. Relight all other appliances turned off in step 1 above. Be
sure all pilot burners are operating.
WARNING:
Natural gas heating valve (Btu per
cubic foot) can vary significantly; therefore, it is
the installer's responsibility to see that Btu input
to the furnace is adjusted properly. Failure to do
so could cause combustion chamber failure,
asphyxiation, fire or explosion, resulting in
damage, bodily injury or death. Refer to the
Natural Fuel Gas Code (NFPA-54) to be sure the
furnace is burning fuel at the proper rate.
Check Pilot Burner
The pilot flame should surround the generator tip 3/8-inch to
1/2-inch. Pilot gas may need adjustment depending on the inlet
pressure. Increase or decrease the pilot flame to obtain proper
setting.
FIGURE 15
FIGURE 16
Burner Flame Characteristics
Start the furnace and let it operate at least 10 minutes. Open
the access door to view the burner flame. Limit your
movements near the furnace a few more minutes before
making your final observation. The flame may look yellow due
to dust particles in the room air. The flame should change to a
nice blue color with firm inner and secondary cones. An
occasional flash of orange might be seen as dust particles burn
in the flame. This is normal. No burner adjustment is provided,
or is necessary. (Figure 16).
Normal Appearance
Natural Gas:
1. Inner cone- blue color - 1/2 to 3/4-inch above ports.
2. Secondary inner cone - light blue - 1 to 2-inches above
ports.
3. Total flame - from blue to nearly invisible - approximately 6-
inches above ports.
L.P Gas:
1. Inner cone - blue color - 1/2 to 3/4-inch above ports.
2. Secondary inner cone - light blue - 1 to 2-inches above
ports.
3. Total flame - from blue to nearly invisible - approximately 6-
inches above ports.