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

Checking Gas Pressure

Heating season is upon us and it is time to get out your gas pressure measuring instruments. What do you use to check gas pressure on natural gas furnaces? A popular tool that I see a lot is probably the least accurate available. Many techs use the diaphragm gauge gas pressure test kit which comes in the blue plastic box. But these can’t read anything below 2” wc, making them useless for the low fire stage of a two stage furnace. Even 3.5” is in the first part of the scale, not in the sweet spot. If you are testing natural gas and feel that you must use one of the diaphragm pressure gauge gas test kits, at least get the model that puts 3.5” wc more towards the center of the scale. Then you can actually read a pressure of 1” wc. The reason many people don’t choose the gauge which is more appropriate for natural gas is that it does not read high enough for propane. So they buy the gauge that was really designed to read propane gas pressure and use it for both. That sounds reasonable, until you try to read gas pressure on a two stage natural gas furnace using one of those gauges and cannot get a reading.

For an inexpensive gas manometer, an actual water column manometer is a far better choice. I like the straight tube water manometers that Yellow Jacket sells. They are actually cheaper than their gauge style gas pressure test kits and more accurate. No, they are not quite as easy to use – you have to put water in them and learn to read the bubble in the center tube, but that is not too difficult. For a little more money you can buy a U-tube manometer. If you want both convenience and accuracy, a digital manometer is the thing to get. They are considerably more expensive, but also considerably more accurate and convenient. An added bonus is that you can also use most digital manometers to read duct pressures – so the tool can serve more than one function.

Why should you worry about how accurately your manometer reads the gas pressure? Because you cannot insure the furnace is operating safely at the correct firing rate and efficiency if you don’t really know what the manifold pressure is. Checking furnace operation by just seeing blue flames is similar to checking the charge of an air conditioning system by just feeling to see that the suction line is cold. You can see that it is operating, but you really don’t know it is operating as designed. Incorrect gas manifold pressure can lower system operating efficiency and increase the operating cost. Under fired furnaces will lose capacity, over-fired furnaces can be a safety risk due to increased CO output. So do yourself and your customers a favor and get a tool that will allow you to take an accurate manifold pressure, even on low fire.