Gas Furnace Installation.
Are you doing it right?
About 20 years ago I had the opportunity to visit CSA labs (Canadian Standards Association), the old AGA (American Gas Association) labs in Cleveland Ohio. Walking through the facility, I was amazed at the equipment and processes. Passing through a product certification area I saw a lab technician unboxing brand new furnaces, removing the burners and orifices and replacing them with orifices from a box on his desk. I was quite befuddled and asked why the furnaces were not being tested as designed and shipped from the factory. This made no sense to me at all.
Now realize that I am 15 years into my career. I am a commercial service technician turned HVAC instructor, I have worked on and set up burners as large as 40-million Btu/h. I had commissioned 1000’s of furnaces and boilers. Like most technicians who get comfortable in their craft, I really thought I was an expert in the subject. I realized there, that I actually knew very little about gas and heat content. If my guide would have told me that they had to change the orifices because the CSA used metric gas, I might have believed him. Instead, what I received was an education.
The conversation following went something like this. The head of product testing asked me how gas furnaces are rated. “By Input” I said. He then asked, “How do you set the input?” I said, “By setting the manifold pressure to 3.5” of course.” He then walked me over to a calorimeter (an apparatus for measuring the amount of heat involved in a chemical reaction) and showed me the average heat content in real time for the lab, smiled a coy smile and said, “If you have not been clocking the gas meter on every gas furnace installation, it’s safe to say that you have never set up a furnace or a burner correctly in your entire career unless it happened by accident.” That was the start of my real education on gas furnaces and why setting the manifold pressure is not the same as setting up for correct input.
Here is the education that followed. First the gas meter is a utility grade meter, meaning for our purposes if the inlet pressure (typically 7”) is accurate then the meter will pass exactly one cubic foot of gas each time the dial passes the one cubic foot mark. The gas meter is clocked to determine the precise number of cubic feet of gas input to the appliance each hour. The cubic feet of gas are then multiplied by the average heat content to determine the average Btu’s of input into the appliance each hour.
The factory equipped orifices are simply a “best guess” from the factory. They are not the factory designed orifices; they are simply the orifices that were factory installed. The installing technician is ultimately the person responsible to select the proper orifice and the person required to set up the appliance for the input it was designed for. That input must typically be within 2% of the design.
This may be a surprise to you, but as Bryan Orr would say R.T.F.M. (Read the fantastic manual). Many technicians and even many OEM support personnel do not realize that there is an acceptable typical range of 3.2 – 3.8 in H2O for the manifold pressure. The reason for the range is threefold. First, the average heat content of natural gas varies by your location. Second, not every furnace is set up at sea-level and you may require adjustments to be made for elevation. Last, the factory orifice is simply the most common for heat content across the board (1000 Btu/h typical) or appropriate for the highest end of heat content the furnace might encounter typically rated for 1075 Btu/h. For natural gas in the United States the average heat content (depending on the state) can vary from 918-1081 Btu/Ft3. While there are wide variations in heat content by region, the average heat content remains reasonably constant.
Remember, the manifold pressure is adjusted to set the input, however setting manifold pressure to its nominal pressure (usually 3.5 inH2O) does not mean the input is correct for the furnace. The factory will often sacrifice efficiency and error on the side of safety leaving the appliance underfired. This is a primary reason that you may not be able to get the temperature rise to the center of the range and the reason that you will have very high excess air numbers on your combustion analyzer.
So why should you care? First, because the furnace will never run as designed without proper input. With a condensing appliance it may never condense. This means the 97% efficient furnace that you installed may be closer to 90% efficient because the latent heat is robbed by the excess air diluting the flue gasses. Second, the furnace may be underfired or overfired resulting in premature heat exchanger failure. Third, because the furnace may be completely unsafe due to high levels of CO produced by flame quenching, flame impingement, overfiring or simply an improper mixture of gas and air. Finally, your furnace may need to be derated simply due to altitude and the resulting decrease in air density due to elevation.
Setting the input is your job! It is also in every furnace installation manual. It is critical to proper furnace operation, that input must be set within 2%. It is simple to do if you follow the manufacturer’s instructions and even easier if you use measureQuick’s Gas Furnace workflows and follow the process step-by-step. measureQuick will even normalize all the calculations for you and tell you where to set the manifold pressure to achieve the desired input. measureQuick will even tell you if you need to upsize or downsize the orifice.
Setting the input is also key to getting good and stable combustion numbers. Set the input before combustion testing and I can guarantee that you will spend a lot less time chasing your tail trying to resolve combustion related problems. Input is simply the key to proper operation and long life of gas heating equipment.
– James Bergmann
This article was written by James Bergmann and was edited and posted by Joe Medosch and Valerie Buckles.
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