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.

Applying the Temperature Rise Airflow Formula

Many techs are familiar with the temperature rise formula for checking airflow. It is derived from the specific heat formula:

BTU = weight x ΔT x Specific Heat
(Note ΔT is simply shorthand for the change)

After rearranging the formula to solve for weight, changing the weight of air to a volume, and reconciling BTUs per hour with Cubic feet per minute you end up with

CFM = Btuh/(1.08 x ΔT)  

For heat pumps we get the Btuh by measuring both the voltage and current and multiplying them by 3.41. The formula becomes

CFM = (volts x amps x 3.41)/(1.08 x ΔT)

For furnaces we measure the firing rate in Btuh and multiply it by the furnace combustion efficiency. The formula becomes

CFM = (Btuh input x %Efficiency)/(1.08 x ΔT)    
 (Note %Efficiency is stated as a decimal in this formula.)

Did you ever wonder where the 1.08 comes from? The "magic number" 1.08 is a convenience constant. It is basically a bunch of math combined into one factor as a short cut. When you multiply the airflow by 60 to get airflow per hour, multiply by the density of air 0.075 pounds per cubic foot to convert volume to weight, and multiply by the specific heat of air 0.24, you end up with 1.08. The factor is often rounded to 1.1 because it makes the math easier.

The number is not really constant because the volume of the air varies with altitude, temperature, and humidity. A change in any of these variables changes the density of air, which in turn changes the "magic number." The factor 1.08 in this formula is only accurate for dry air at 70°F at sea level. For example, 1.08 really does not work with flue gas or airflow in freezers because the air volume has changed, which changes the convenience factor. Similarly, 1.08 does not work in Denver because the altitude changes the air pressure, changing the density. Even the relative humidity changes the factor. The ubiquitous 1.08 is for dry air at 0% relative humidity – a condition that is never seen in Georgia. Changing the relative humidity to 50% changes the air volume, which changes the factor.

Lets look at some examples. 0°F air at sea level has a density of 0.086 pounds per cubic foot, while 300°F air at sea level has a density of 0.052 pounds per cubic foot. Instead of the commonly quoted 1.08, these densities produce factors of 1.24 for 0°F air and 0.746 for 300°F air. If the air is 70°F but at 5000 feet elevation, the factor becomes 0.9 because the air density has changed due to the increased elevation. Even changing from 0% relative humidity to 50% relative humidity changes the density to 0.0741 instead of 0.0745 (the 0.075 for 70°F air is rounded). This changes our convenience factor to 1.07.

If you would like to play with different scenarios, there is an online air density calculator that takes all three factors into account at http://www.denysschen.com/catalogue/density.aspx
Just multiply the density times 14.4 to get your new magic number. What is 14.4? Oh, you get that by multiplying 0.24 times 60.

For more details on checking airflow using the temperature rise I recommend a great article by Norm Christopherson on the nuts and bolts of measuring airflow using temperature rise. You can find it on docstoc by clicking HERE.

2013 RSES Annual Conference and Technology Exo

Are you planning to attend the 2013 RSES Annual Conference and Technology Exo? 

It takes place October 23 – 26, 2013 at the Sheraton Station Square Hotel in Pittsburgh, Pennsylvania. The Refrigeration Service Engineer’s Society is dedicated to advancing the knowledge and skills of the HVACR professional.  And professional is definitely the correct word to describe the people you will meet there. The conference is a wonderful opportunity to network with industry professionals. You will also meet many instructors and trainers. After all, RSES is all about advancing skills and knowledge, which is what teachers do. There are more educational sessions than you can attend – so you have choices of which ones to see. Here is a link to the educational sessions. 

When you go, be sure to see David Skaves, my writing partner and co-author of “Fundamentals of HVACR” published by Pearson. He will be discussing how the emphasis on STEM education can benefit the HVACR industry. In case you are not familiar with the acronym STEM, it stands for Science Technology Engineering and Math. HVACR fits in quite nicely. (No, David will not be giving a two hour lecture on valve stems). 

There is also a great Technology Expo where companies display their latest and greatest and you have an opportunity to see all their great products. It is really fun to go to, wander around and talk with people. Of course, David will be there at the Pearson booth. Stop by the booth and say hello. David is a great guy with a wealth of interesting experiences. Truthfully, that is typical of the people there: professionals with a wealth of experience in the HVACR industry. 

Want to find out exactly how much you know about HVACR? You can take a whole range of professional exams at the RSES conference. I believe the  RSES CM and CMS exams are the most difficult professional exams I have ever taken - they will challenge you. They are very reasonably priced - so you can afford to take several if you like. 

Do yourself a favor – take in the 2013 RSES Annual Conference and Technology Exo and see what RSES has to offer. You belong.