Tuesday, October 29, 2013

Natural Gas Furnace Firing Rate

Do you know the heat content of the natural gas supplied to your area? Although we often use the nominal value of 1000 Btuh per cubic foot for natural gas, the heating value is really a bit higher in most places. A look at a table by the U.S. Energy Information Administration, EIA, shows that only a few states have natural gas with a heating value close to 1000.Gas Heating Values In 2011, average gas heating values ranged from 1004 in Nebraska to 1076 in West Virginia. Furnace manufacturers must choose a heating value when they select the orifices to put in the furnace. Typically, they use a value on the higher end of the range, 1075 BTUs/ft3. A furnace with the factory supplied orifices set up at the same manifold pressure in each of these states would deliver a different amount of heat. Assuming they were all installed at sea level in their respective states, they would burn the same volume of gas because they would have the same manifold pressure and the same orifice size. A furnace that burns 100,000 Btuh in West Virginia would only be firing at 93,000 Btuh in Nebraska. Small adjustments in the firing rate can be made by adjusting the manifold pressure. However, it is often necessary to change the burner orifices to get the manufacturer’s listed firing rate.

Higher altitudes can make de-rating a furnace necessary because the lower pressure, less dense air just does not have enough oxygen in it to support the full capacity of the unit. In general, furnaces are de-rated 4% for each 1,000 ft of altitude. Some manufacturers provide tables showing what orifices and manifold pressure should be used depending upon the gas heating value and altitude. The point is that setting up a furnace for the correct firing rate involves a little more than simply adjusting the manifold pressure to 3.5" wc using the manufacturer supplied orifices. You may need to change the orifices and/or adjust the manifold pressure to something other than 3.5" wc. Below is an example from one manufacturer. Note that this is an example - it does NOT apply to all furnaces.

Gas Heating Value
Sea level - 2000
2001 - 3000
3001 - 4000
4001 - 5000
Orifice
Man press
Orifice
Man press
Orifice
Man press
Orifice
Man press
975
44
3.3” wc
44
2.8” wc
44
2.6” wc
47
3.5” wc
1000
44
3.2” wc
44
2.7” wc
44
2.5” wc
47
3.3” wc
1050
44
2.9” wc
44
2.5” wc
48
3.7” wc
48
3.4” wc
1100
46
3.3” wc
48
3.7” wc
48
3.4” wc
48
3.7” wc

23 comments:

  1. Most utilities here in the Northeast use 1050 BTU per cubic foot. The reason being you will be slightly undergassing the equipment rather than over firing. You can typically get a good idea what the local utility is sending out by looking at the gas bill it will list so many CCF at say 1023, the 1023 was the average BTU sent out for that month from that utility.

    It is not practical many times to change orifices as you would have to order them and then wait for there arrival. There are other methods for a temporary solution until new orifices can be obtained. If the orifices need enlarged they can just be drilled out by using an orifice kit from a company in Colorado called Anderson Forrester. If the orifices have to made smaller you can peen over the existing orifice and redrill or solder the orifice covering the existing hole and redrill.

    All determinations of correct input should be confirmed by clocking the gas meter on natural gas or checking the orifice size on LP and matching it against BTU in charts found in the National Fuel Gas Code NFPA 54.

    Finally a combustion test should be done using and electronic analyzer and firing the equipment to its maximum designed input based on combustion readings.

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