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need some advice on how to size a new heating system in a remodel

epmiller Member Posts: 17
I am remodeling the house I am living in, here's some background.  The house is frame, 120 to 150 years old and currently has electric baseboard heat installed in the late 60s.  I have good double glazed replacement windows in 3/4 of the house but the insulation is blown in the walls and has occasional voids, I'm guessing R13 where good, also no insulation between the first floor and the basement.  I am taking a room at a time and insulating the exterior walls with rigid foam board right on the old plaster then lath and drywall.  It adds 2-3/4" to the window jambs and gives a real interior vapor barrier.  2nd floor ceilings also get 1/2" foam board as a vapor barrier then lath and drywall.  I'm figuring R20 walls and low air infiltration when finished so there will be a significant change in heat loss. Since I have to tear up most of the 2nd flooring to level/straighten and fix squeaks, I am figuring on in-floor radiant heating probably with some type of mod/con boiler.

Here are my questions.

1) How should I go about sizing a boiler if I am starting now with 2 rooms and a bath of approx 350 sq ft but eventually I will be heating almost 2000 sq ft? It possibly could

expand to 2800 if I add the inlaw quarters which needs a new heat system pretty soon and is a good candidate for a staple up radiant installation. Currently it is a 50+ year old Thermopride oil furnace, still works great but the ducts are terrible.  In fact, that may be the next zone added.  It will be several years until I get this job done at the current rate of weekends and vacation time but I would like to have heat in some of these rooms before I'm finished with the whole job, I am in Pennsylvania after all. 

Oh yes, and I am doing an indirect fired domestic water heater when I install the new system so that will be some load.

2) Because of the way we live, we like to set back certain rooms because they are not used much, anywhere from once a week to once every couple of months (we have out of town kids).  How small can I make in-floor radiant zones?  I could have a couple of bedroom zones that are 120 to 150 sq ft.  Bathrooms are even smaller.  I have this idea that I could use an indirect domestic hot water heater and pump the small zones out of that and use its water jacket as one zone on the boiler.  That way I don’t end up with several bathroom zones of only 150 feet of pipe each, and I could zone each room individually like it is now with the electric baseboard.  Is that viable?  I know what short-cycling does, I service HVAC systems, mostly forced air and I don't want to abuse a nice system.

It almost looks like 2 boilers would be the best idea, one now and one later, but the expense of that is a bit daunting!  Of course I could twin them when I’m finished. :-)


  • Ironman
    Ironman Member Posts: 7,379
    edited March 2014
    2 Boilers

    I think you're on the right track.

    Everything starts with an accurate heat loss calculation. SlantFin has a program that is easy to use and is free to download.

    Sorry, but I don't have a link for it even though it's on my IPad. A simple search will find it for you.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Paul48
    Paul48 Member Posts: 4,469
  • SWEI
    SWEI Member Posts: 7,356
    Radiant floor

    really shines with a mod/con.  Buy the smallest size you can get -- it will quite likely heat the whole place (especially if you add some envelope upgrades to your remodel.)