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Hi Co from furnace.

tom_49 Member Posts: 269
I got a call where the gas co. red tagged a furnace ( 92%)  for cracked HX. Upon my checking the system, i found really high CO in flue( out  of my meters range). checked the gas pressure at valve 7.85" wc. manifold pressure was 4.5". dialed it in to the manufactures recommended setting of 3.5". Still REAL high CO. and 1% oxygen.it pegged my meter in 5 seconds.

So i dialed the gas back to where i could get a decent oxygen reading (7%). now i am getting a 30ppm Co reading. Looks good! double check my manifold pressure and its reading 1.1".???

My question is: Why do I have to reduce the gas pressure this much to get good combustion numbers? i have tested TONS of these units before and always dial the gas pressure in to or very near to factory specs.

i found no evidence of a cracked HX. Once I  dialed it in, the O2 and Co readings were very stable.



  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    Sounds like it is overgassed

    did you check the orifice sizes on the burners or did you clock the burner to see if it is burning BTU equal to the rating plate? It is not a good idea to run with the gas pressure that low unless this is a negative pressure gas valve? Does this have a secondary heat exchanger?

    Give us some valve numbers and other info so we can get some direction on what is going on.
  • tom_49
    tom_49 Member Posts: 269
    Hi Co from furnace.

    Hi Tim,

    Thanks for the reply. After talking to the manufacturer, we have determined it has a blocked secondary HX. will be going out today to replace. I havent seen this before and based on my pressure readings, was thinking it was a restriction somewhere between the gas valve and exhaust pipe ( HX).

    Thanks for the reply.

  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    That tells me it may

    not have been serviced for a while. The entire exhaust system has to be flushed at least once a year and typically no longer than 18 months without doing so. When this is done the blower needs to be removed and the under part of the secondary exchanger checked to see if it is blocked. The entire condensate line also need to be flushed. Most secondary heat exchangers are much like an AC condensing coil and you can treat them the same way.
  • VictoriaEnergy
    VictoriaEnergy Member Posts: 126
    Failing high eff furnaces

    I recently looked at a furnace where a condensate trap line was full of crud from the secondary hx, on tearing it apart, it turned out the secondary was fine and it was the primary hx was failing and rust was going through the secondary hx.  The furnace was not getting enough temp rise on low heat although the fan was on it's lowest speed. 

    The issue is aggravated by the use of a single stage setback stat.  The furnace runs 15 min on low heat before kicking up to high so the first cycle in the am, when RA temps are really low.

    One of the  manufacturers (no names; they all have skeletons in their closets)that uses laminated steel secondary heat exchangers tend to corrode and slowly choke off the flow through it.  You'll see some ,but not all of: rusty crud in the condensate trap, high CO in the exhaust, flame roll out switch tripping due to too much heat at the burners (but flame is still being drawn in to the primary), air proving switch faults, and condensate leakage in the blower compartment.

    Another company used to use nice AL29-4C stainless heat exchangers, that had a plastic connector transition on the inlet that fell apart causing flame roll outs when the fan fired up.

    I find them all very troubling since the burner is always running starved for air and hence very high in CO levels when things are starting to fall apart.
    Home Owners Please Note:

    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
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