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Do high efficiency boilers make sense with older hydronic system?

MPanutich Member Posts: 3
We have a 4000 square foot home with a natural gas boiler (Weil McClane) sending heated water to three air handlers (First Co.) which provide forced hot air heat to four zones. The system is 20 years old and we are exploring replacement of the boiler. One contractor has told me that a high efficiency, modulating, condensing boiler would save significant gas consumption. Another contractor has told me that there won't be much savings because the air handlers require 140* water for maximum heating BTU, which works against the design of the high efficiency boiler. Don't know who to believe. Help!
Massachusetts home owner


  • Jamie Hall
    Jamie Hall Member Posts: 23,433
    In a sense, both of them are right. However, the person suggesting that the modulating/condensing boiler could produce significant energy savings needs to demonstrate to you that the air handlers will produce adequate amounts of hot enough air to avoid cool draughts and overall inadequate output when running at condensing temperatures -- significantly less than the 140 which the other person is quite correct in saying that they require for maximum output.

    With air handlers I'd be rather suspicious. No one really wants a nice cool breeze from your system on a cold winter day...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattmia2
    mattmia2 Member Posts: 9,741
    The cost of making it not drafty would probably be more than you'd save on fuel unless you were replacing the air handlers anyhow.
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,045
    edited March 2021
    No reason to get a mod/con boiler with air handlers. They won't condense at the high temperatures you need and longevity is way less that an atmospheric, cast iron boiler.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • SuperTech
    SuperTech Member Posts: 2,186
    Keep the cast iron boiler.  Its more reliable and will cost much less to service and maintain over its lifespan, which will be longer than a high efficiency mod con. As others have stated, a mod con is not the right choice for a hydro air application. If you are looking for fuel savings your money will be better spent on tightening the building envelope and adding insulation.  
    HVACNUT Member Posts: 5,863
    edited March 2021
    I see this a lot in my area. Granted they are newer homes (brand spanking to 15 years) with ECM motors in the air handlers and slightly over sized ducts and diffusers to keep things quiet. The 12 ft ceilings don't hurt either. 
    A good mod con set up correctly will condense with air handlers and hydro coils. The firing rate will modulate dependent on input from supply and return sensors. 
    It shouldn't be drafty. You shouldn't even know it's on. 
    That being said, nothing wrong with a simple cast iron boiler. 
  • Mjplumbing
    Mjplumbing Member Posts: 6
    edited March 2021
    Be careful installing a mod con boiler on an old cast iron system. Maintaining the water quality in the heating loop will be most important if you do. Definitely install a magnetic filter and IU would also recommend power flushing the system before installing a new boiler. I don't think its worth it to install a Mod con boiler on an old system with older cast iron loops. Those who do will usually spend a good amount of money on annual maintenance, either that or their unit most likely ends up failing prematurely.
  • hot_rod
    hot_rod Member Posts: 22,256
    To many installers the modulating feature of a mod con makes them worth considering even if they do not run in condensing mode all the time. You would want to look at the spec of the First Co handlers, see what the heat output is at different SWT, see how that matches the heat load. If the return runs below 130F you will get some condensing and higher efficiency.

    The ongoing maintenance cost is the bigger determining factor for homeowners. Modcons do require more maintenance, and major component failures can be $$.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream