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So about mid October I had a heating company come out and do a tune up on my Lennox G26 series 100,000 BTU furnace on LP that was installed in 2002. Sadly I've lived here for 4 years and this is the first time I've had a tune up done but the furnace has always ran fine. When the tech checked the gas pressure he found it to be set at about 7.5 inches of water column and then proceeded to adjust the pressure to 10" which I understand to be the typical pressure on LP. At that point the delta T across the furnace was higher than allowed so he adjusted it to 9" and proceeded to cut a supply register into the plenum and a grill into to the return in an effort to lower the delta T and got it to the top end of the range for delta T. The tech claimed my furnace is oversized or I don't have enough registers and returns because of these issues. The A/C coil was checked and it was clean so it was not restricting airflow. This is in a 2700 sf 1910 drafty farm house.
As winter progressed I noticed that there has some ticking in the ductwork after the burner shuts off but before the fan turns off. I assumed it was because of the hotter supply air. I was waiting for the furnace to shut off so I could change the filter and discovered the noises I'm hearing are coming from the furnace not the duct work. I got curious and started looking at the furnace and the sticker inside states that the manifold pressure is to be set at 7.5" of water column for LP.
So if I'm thinking correctly this would mean that my furnace is overfired. I also noticed that I have gone through way more fuel than usual even with the mild winter we have had so far in Iowa.
I called the company today and they didn't seem to concerned that the pressure is set higher that the manufacturer suggests. The installation instructions for the furnace that I found online also states the manifold pressure is set at 7.5 inches of water column for LP. It even states in a big warning box in the manual not to exceed 7.5 inches of water column on LP but doesn't state what the ramifications of doing it are. I am concerned that my furnace may have been damaged because of this.
I am waiting for a call from the service manager on Monday to see what they are going to do. I do plan on replacing the furnace at some time in the future because of its age but also because I have been tightening up the house so the furnace will become oversized as improvements are made to the house. I also don't want to run all of the construction dust from my improvements through a new furnace.
Am I correct in thinking that the furnace is being over fired? What damage could be done as a result of this?