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Condensing Furnace questions

Solid_Fuel_Man Member Posts: 2,646
Hi all it's been awhile.  Being out of my field (scorched air) I have some questions concerning a customer of mine. 

I was there and he had expressed interest in converting to LP from oil.  He has just purchaced this house and the oil tank is ruptured.  They are doing a Reno and out are coming the Bock, and Airco FHA oil fired units. 

He has had three quotes for the furnaces (I am there installing a Rinnai Tankless.....tried to talk himout of it).  Two quotes were for York 120Mbtu single stage, and one was for a two stage.  I thought that seemed way oversized and did a heatloss, came out to 74Mbtu and some change.  I told him based on my expierence to get nothing more than an 80Mbtu unit.  I am not sure if he should get a modulating unit or the two stage would be sufficient.  FHA is not my specialty, hate it actually, I am a wethead through and through. 

Are there good ones, and are the modulating ones worth the extra $2k? 

Thank you for the collective wisdom.

Serving Northern Maine HVAC & Controls. I burn wood, it smells good!


  • unclejohn
    unclejohn Member Posts: 1,833
    I would stay away

    From the modulating. Only slightly more eff. and way mare costly. Also at least with The Carrier brand and I'm sure most other brands work the same way. You can't actually read the input signal to the valve, your meter will just jump around and that means you have a signal to the valve or that it and the board are talking. No meter jump and they aren't talking. Unless of course you go to the truck an get your oscilloscope
  • Tim McElwain
    Tim McElwain Member Posts: 4,621
    Keep it simple go with the single stage

    with a heat loss of 74,000 the output of the furnace wants to match the heat loss so a 100,000 BTU is not too large.
  • Jack
    Jack Member Posts: 1,047
    First things first

    You should pressure test the ductwork. Google "doe duct leakage" and read for a while. Once the duct is tight I would always go for a two stage at least, modulating if you can. You end up with longer run times generally and with the blower at lower speeds you have a better comfort level and better uniformity of heat, again, if the duct work is delivering the air to the space. You know how so many say they hate warm air because they are either to hot or cold? That is because the furnace is typically way oversized and single stage causing overheating and then no air and drafty living. Then another cycle and it gets old.

    Keep in mind that the furnace is being sized for the coldest day/temp of the year. With the exception of this year that is a very small percentage of the year. I guess you can say we size our equipment for the 100 yr storm and then live with what it gives us the other 99 yrs.
  • John Mills_5
    John Mills_5 Member Posts: 951

    If heat loss is 74K and they go with a 100K 92-95% furnace, I'd call that oversized and sure wouldn't do it. Just my 2 cents worth. I'd find a furnace closer to the needs of the house. I would agree that the 80K is right. We sell mostly 2 stage and occasional Rheem modulating furnaces and our customers love them. Rare to sell a single stage except in rentals and lower budget situations.

    Today's 95% furnaces want to move a lot of air so oversizing can be noisy if the ductwork is designed for 100° temp rise of oil!
  • Tim McElwain
    Tim McElwain Member Posts: 4,621
    Jack and John I hear what you are saying.

    However here is the problem at least up here in the Northeast.

    Most warm air systems have insufficient return air, and in some cases it is drastically under sized. This does not bode well for two stage systems unless perhaps they are the type that starts with high fire and then if no hi fire demand cuts back to low fire. The complaint on almost all the applications I have seen recently is my old furnace did not blow cold air the new one does. That of course could also be because we had  temperature off temp on fan and limit on the old furnace and the new one is time on time off usually 30 or 60 seconds much too soon.

    The other issue and there are many more but I am busy and really do not have time to debate this subject. The installer never even does a heat loss and matches up to what ever was in the house to begin with. I have also had issues with motor failure on ECM motors with restricted air flow due to insufficient return air. then we have venting issues when side wall venting is not easily accessible and the list goes on.
  • Jack
    Jack Member Posts: 1,047
    Tim, thanks for that

    but again, this comes down to perpetuating the mistakes of the original low cost bidder. This one should have another thread on the single stage, two stage multi-stage discussion