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Over fired gas furnace

egansenegansen Posts: 20Member
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?

Thanks!

Comments

  • Alan WelchAlan Welch Posts: 189Member
    I am not familiar with gas, but I would be concerned about the burner shutting off before the blower comes on.
  • JUGHNEJUGHNE Posts: 6,438Member
    20 years ago I installed an oversized furnace at the owners insistence, he wanted to be sure he was warm.
    Not enough duct work caused the high limit to cycle the burner, never noticed until the high limit failed to reset.
    Using the better pleated filters was enough restriction to limit air flow, the real problem was a humidifier added that bypassed the hot air into the return air.
    But the outcome in your case would be the heat exchanger being over heated.....maybe the high limit is opening and closing and going unnoticed.
    You are overfired and sending more heat up the flue pipe than needed. IMO
  • FredFred Posts: 8,201Member
    The fan should run for a while after the burner shuts down. It shuts off after the temp drops to a cut-out setting. I would think think the service company screwed up and it needs to be set at the 7.5 inches. Too hot and it can crack the heat exchanger.
  • HVACNUTHVACNUT Posts: 2,761Member
    Manifold pressure should definitely be set to manufactures specs.
    After it's set and the temperature rise (stated on the label with model, serial numbers, etc.) is still not within range, he can increase the fan speed.
    And make sure they do a combustion analysis and leave you a printed copy.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 6,306Member
    Never seen an LP unit name plate that called for 7.5".

    They are usually 10-11"
    But if that s what the nameplate says then that's what It should be adjusted to. Lennox units are sometimes a little weird

    One of the biggest problems with warm air is undersized ductwork and registers....It's wrong ....but not unusual
  • egansenegansen Posts: 20Member
    edited January 11
    Thanks for confirming what I am thinking. Just to clarify the furnace is operating correctly. It calls for heat, purges, fires, the blower starts, call for heat ends, blower continues to run and shuts off. It does not hit the high limit. I have added several additional supply duct runs as I've redone done some of the rooms in the house. I have also added returns as well. It's not the greatest but it's better than it was when I bought the house.
  • ch4manch4man Posts: 179Member
    call them back and have them set it back to the mfg specs and prove to you with a combustion analyzer they are not poisoning you and your family with carbon monoxide.
    overfired gas equipment can and many times cause the CO production to go from acceptable to dangerous. there is absolutly no way to know with out testing and without knowledge of how to use an analyzer they're just guessing and risking your health
  • egansenegansen Posts: 20Member
    edited January 20
    Just though I would give an update. They did send a tech out and he adjusted it back to manufacturer's specs. He was surprised that they called for a different manifold pressure than the typical 10" WC for a single stage furnace. He took pictures of the label and the installation manual to show his service manager to prove that it is different and that I was correct with my concerns. He didn't do a combustion test and I'm not sure he could since there isn't a tap provided anywhere to do it.

    I also have several CO detectors throughout the house.

    Thanks again for your help.
  • HVACNUTHVACNUT Posts: 2,761Member
    He'll need to drill, tap, and plug the vent pipe to do a combustion test.

    IMO, it should be mandatory that all manufacturers of high efficiency equipment provide a test port. Most boilers have it. Furnaces need to get on the ball.
  • Tim McElwainTim McElwain Posts: 4,309Member
    Those Lennox G26 furnaces were to run on 7.5" W.C. because of the special design. They were Pulse Combustion designed units and gas pressure setting is critical to proper operation. I would insist on a combustion analysis being done on this unit.
  • HVACNUTHVACNUT Posts: 2,761Member
    > @Tim McElwain said:
    > Those Lennox G26 furnaces were to run on 7.5" W.C. because of the special design. They were Pulse Combustion designed units and gas pressure setting is critical to proper operation. I would insist on a combustion analysis being done on this unit.

    I didn't realize we were dealing with a Pulse. I didn't think there were any left.
  • SuperTechSuperTech Posts: 999Member
    > @HVACNUT said:
    > > @Tim McElwain said:
    > > Those Lennox G26 furnaces were to run on 7.5" W.C. because of the special design. They were Pulse Combustion designed units and gas pressure setting is critical to proper operation. I would insist on a combustion analysis being done on this unit.
    >
    > I didn't realize we were dealing with a Pulse. I didn't think there were any left.

    G26 isn't a Pulse furnace. G14 and G16 were. Pulse furnaces are still around unfortunately, I see a few every year.
  • Tim McElwainTim McElwain Posts: 4,309Member
    edited January 20
    I stand corrected, I am getting old and should not rely on my memory so much. The G26 is not a pulse combustion unit. It still however requires LP gas Pressure of 7" W.C. Go to manual found at https://www.questargas.com/ForEmployees/qgcOperationsTraining/Furnaces/Lennox_ G26-SERVICE.pdf for the charts for natural gas and LP Figure 22 is for LP found on page 17.
  • ch4manch4man Posts: 179Member
    edited January 21
    no....., Pulses were G14 & G21. G16 had the mickey mouse door
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