Gas Furnace Firing Rate

How do you know the rate at which your furnace is actually firing? You really can’t assume that it is firing at the manufacturer’s published rate just because the manifold pressure is at 3.5”, even if that is what is specified on the nameplate. To determine the Btuh firing rate of your furnace you must first determine the cubic feet of gas per hour being burned (CFH) and multiply that times the heat content of the gas. To determine the CFH, you need to clock the gas meter. First, you want to turn off all other gas appliances. Then clock the number of seconds it takes for the smallest dial on the gas meter to make one revolution. For more accurate results, clock the time for two or three revolutions. The CFH can be calculated by dividing the time clocked into 3600 (the number of seconds in an hour). Multiply this answer times the value of the dial. If you clocked for multiple revolutions, multiply by the number of revolutions. The formula looks something like this

(3600/time clocked) x dial value x revolutions = CFH
So if the a 1/2 ft3 dial took 90 seconds to make three revolutions
(3600/90) x 1/2 x 3 = 40 x 1/2 x 3 = 60 CFH

Next, you need to know the heat value of your gas. You get this from your local gas supplier. If you can’t get that you can get a good estimate from the EIA. The US Energy Information Administration (EIA) has a table online that shows historical values for every state. GAS HEATING VALUES.

In our example, if the furnace is in Mississippi in 2011, the gas heat content is somewhere around 1010 Btu/ft3. So our firing rate would be 60 ft3/hr x 1010 BTU/ft3 = 60,600 BTU/hr

The same results in West Virginia in 2011 would mean a firing rate of 60 ft3/hr x 1083 BTU/ft3 = 64,980 BTU/hr

If the furnace in West Virginia is rated for 60,000 Btuh, you would need to reduce the manifold pressure slightly to reduce the firing rate. Another complication is altitude. Furnaces need to be de-rated approximately 4% for every 1,000 ft of altitude. In West Virginia, it is quite possible to be 3,000 ft above sea level, necessitating a 12% reduction in firing rate. So the new firing rate should be 60,000 Btuh x 88% = 52,800 Btuh. Now you are looking at firing a furnace at 64,980 Btuh which should only be fired at 52,800 Btuh. You most likely will need to change orifices as well as adjust the manifold pressure. After making any changes to the manifold pressure or orifices, make certain to clock the meter again and recalculate the gas firing rate to insure your adjustments brought the firing rate into line. Over firing a furnace can increase the production of CO in the flue gas. In extreme examples, it can cause soot formation.