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Is this proper reasoning for replacing my furnace?

Randy_7Randy_7 Member Posts: 23
My Rheem, propane fired, Classic 90 Plus furnace is 14 years old and I have lived in this home in far north Michigan for 2 years. For two days this week the unit fired up and ran through a heating cycle only about every 6th attempt at starting. The remainder of the time only the induced exhaust blower energized, and then cut out after about 45 seconds. During the many failed attempts to fire the panel light continually flashed 2 times, signifying an "open pressure switch."

The technician I have used for 6 years evaluated and worked on the furnace about 2.5 hours. Pressure switches & hoses, condensate drain, pvc intake & exhaust, firing components were all operating properly. He did determine that the furnace fired up and operated properly ONLY when the upper access panel was not in place, which allowed more air into the combustion area than could be supplied by the 2 inch pvc intake. He also evaluated the pvc intake & exhaust on the exterior of the home: The 90 degree elbows of the pvc intake & exhaust on the exterior were both pointing to the ground and the exhaust gas was immediately being suctioned into the intake pipe. The interior of the exhaust pipe is coated with an oily, black film. He removed the elbow from the exhaust pipe so that it now exits horizontally so that the exhaust would not readily mix with intake air. He also removed a plastic plug from the side of the combustion area of the furnace in order to allow additional combustion air into the chamber, from the basement. The furnace has been operating normally since his visit.

Recommendations of the technician: Seriously consider replacing the furnace. In all liklihood the heat exchanger is at least partially plugged due to the furnace using the exhaust gas for combustion. The pressure switch was sensing the exchanger problem and not allowing the furnace to ignite.

My Questions: I'm not familiar with scorched air as my previous homes have had hydronic heat (certainly my favorite).
Does the technician's reasoning for replacing the furnace sound legitimate? Any suggestions or recommendations would sure be appreciated.


  • Tim McElwainTim McElwain Member Posts: 4,357
    I would look into giving it a full service and maintenance before spending the money. It can be cleaned now that the air problem has been solved. My question is why did it take so long to act up. If you had products of combustion cross contaminating the intake air supply it would have failed a long time ago.

    You are definitely not getting sufficient air for combustion and opening up the sealed door area by removing the plug you mentioned is not a good idea.

    Was a combustion analysis done with an electronic analyzer? There is the potential for poor combustion with the air supply contaminated which means "CARBON MONOXIDE". Do you have CO detectors in the house?
  • IronmanIronman Member Posts: 5,560
    Did the tech check the pressure in the vent and exhaust lines with a manometer to see if the switch was in calibration? It may be the switch needs replacing.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • Randy_7Randy_7 Member Posts: 23
    Much appreciate the comments to date by Iron man and Tim. I definitely have CO detectors in my home plus I've noted the suggested Combustion Analysis and checking pressure in vent & exhaust with Manometer. I stayed fairly close to the tech (without looking over his shoulder) for "moral support" and to answer questions. He performed an electrical analysis to make certain the pressure switches were within specs, but no combustion analysis or pressure test. I do like the technician but have been around too long (retired contractor) .......... Anyway, my plan is to gather as much information as possible from here over the next few days and to then quietly get a 2nd opinion from another tech.

    One other thing - sometime back I read somewhere that 2" intake lines may not provide enough combustion air for some furnaces and that 3" diameter may be necessary. Obviously don't know if that is valid.
  • icesailoricesailor Member Posts: 7,265

    "" One other thing - sometime back I read somewhere that 2" intake lines may not provide enough combustion air for some furnaces and that 3" diameter may be necessary. Obviously don't know if that is valid. ""

    That sword is sharp on both sides of the blade.

    2" is fine if it isn't too long. If it is linger than the manufacturer or the listing says, there may be too much restriction in the 2: for proper flow. And that includes exhaust velocity. But, with 3?, you can end up with not enough velocity if it is too short because the fan doesn't overcome the lack of restriction or friction of the larger pipe. IMO, especially with modulating gas boilers on low fire. There's just not enough pressure in a larger vent to push the exhaust out of a bigger pipe.
  • IronmanIronman Member Posts: 5,560
    The only way to know if the pressure switch is operating within specs is to check it with a manometer. Using an ohm meter only tells if the contacts are opening and closing; it doesn't tell what pressure they're doing it at. The purpose of the switch is for it to close when a specified pressure is reached assuring proper venting/draft. The pressure value of the switch is usually printed on it.

    As far as vent sizes go: that's determined by the manufacture and depends upon the size of the furnace, the length of the venting, number of fittings, termination method, etc. Venting instructions and tables are included with the installation manual for the furnace.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • Randy_7Randy_7 Member Posts: 23
    Thanks to all. I'd like to pose one final question: Can you tell me what the normal service life is for a high efficiency furnace? The tech who worked on mine said that they normally last 12-15 years. Mine is 14 years old.
  • SWEISWEI Member Posts: 7,356
    I would expect a new 90+ furnace from a quality manufacturer to last 20 years if properly sized, installed, commissioned, and maintained. A slew of new designs hit the market in the first decade or two of high efficiency, and some of those did not turn out as well as hoped. Many (perhaps most, in some markets) were either installed or maintained poorly.

    We (as an industry) actually are getting better at this, but the skillset required of an installing contractor is growing every year -- and not all of them are stepping up to the plate. JMO.
  • BobCBobC Member Posts: 5,101
    I think the key to any properly installed heating systems longevity is that the manufacturers maintenance schedule be followed.

    Trying to save money on maintenance is a mistake.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
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