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Single stage, two-stage and multi-stage, which one

Jack Member Posts: 1,045
This is a follow-up to the "Condensing Furnace" thread. I've been selling modulating heating since '91, and have done so very successfully, but it was with Rinnai's Wall Furnaces. Net to the space in other words. Tim's preference for single stage and his reasoning make sense in the scenario he describes, but what, on a clean sheet of paper design, would you prefer? On a retro-fit of 2 stage or Variable speed, what problems have you encountered and how did you fix them?


  • Tim McElwain
    Tim McElwain Member Posts: 4,482
    Here is what I posted

    in the other posting:

    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. I will wait to see what others have to say. I must say this however that the same poor installation practices of many installers also shows up with warm air systems.[u][color=#0066cc]Reply[/color][/u] [u][color=#0066cc]Edit[/color][/u]
  • SWEI
    SWEI Member Posts: 7,356
    Very interested in learning more here

    I have seen quieter and more comfortable results from two-stage FWA systems by far. 

    When I did some research for a customer last fall, I found out that Rheem eliminated their full modulation residential FWA furnaces.  They still have fully-modulating fans in their highest end models, but those now have two-stage burners.

    I'm willing to be educated here, but how can a single stage burner and single speed fan deliver real comfort?
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,307
    edited February 2014
    Return Size........

    It's funny you should mention return sizing.  I was back over to that same house to install a water softener, and the winning bidder installed a York two stage condensing furnace.  The 24x24 return plenum is panned and a single 12" round goes to the furnace and is panned to the 24x16 filter.  WOW talk about a shoddy install.  The HO asked me what I thought, and I told him to get the installer (local gas supplier) back to clean up their abomination.    People simply do not have any understanding of how important the return is.  There is plenty of ductwork in this old house with multiple 12" rounds into the 24x24. 

    WOW!  I should have done the work, if only I had time. 


    EDIT: I should add that the vent has a low spot and I could hear the condensate gurgling upon combustion blower startup and run.  The whole thing looks like Wonkie Piping installed it.  I really need to get a picture of this.

    He called the installer about the small return and they said that's what they usually do if there is enough air flow ................
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • John Mills_5
    John Mills_5 Member Posts: 935
    2 or more!

    We've sold 2 stage furnaces for 30 years and modulating since Rheem came out with theirs. Customers love them. Never heard customers rave about a furnace til the Rheem mod on the matching stat. It is NOT gone now, just not available in the downflow and horizontal at this time. The new line they haven't brought out the multiposition mod yet. AFUE rating issues I hear. We put all of our 2 stage on 2 stage stats.

    Only thing I'd worry about with 2 stage and mods is ceiling vents. Since almost all of ours are floor, even with downflow equipment, we haven't had complaints about comfort on low fire. As these new junker homes with ceiling vents need new furnaces, it could become an issue.

    As for blowing cold air, since most furnaces start blowing as soon as a hint of warmth builds up in the HX, that likely will be an issue with any high efficiency furnace. A properly sized 2 stage with 2 stage stat will have long run times on low in bitter weather eliminating a lot of the cycling from the older, likely oversized single stage furnace.