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Hot water coil in furnace duct

metrosilo Member Posts: 15
I'm looking to install a boiler in my home to replace my furnace heat. I still want to keep the duct work so I can utilize my air conditioning. My plan is to have some infloor heat but I also wanted to install a hot water heating coil in my main supply duct to provide heat for my home. 
My question is, will this coil provide the same btu output that a furnace heat exchanger can produce? Am I going to have the boiler set to 180 just to keep up?

I want to have the boiler turn down as much as possible and modulate as necessary. 


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,390
    Size the coil to operate on the water temperature you want to run
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  • Jon_blaney
    Jon_blaney Member Posts: 144
    If properly design and installed, a furnace will have a smaller duct system than a hydronic air system. This is due to a lower air temperature from a hydronic system. You need to know the btu requirement for the space to be heated and then evaluate the duct system to see if it is capable of delivering that required amount of btus. If it can, then size the coil accordingly. As a quick quess, how long does the furnace run on a coldest day. If it runs almost constantly, you need to give this some serious thought. The shorter the furnace run time, the more feasible the idea becomes.
    HVACNUT Member Posts: 4,214
    You'll have to do a heat loss calculation to determine the coil size while taking into account duct size, water temperature and fan speed.
    What type of boiler setup? Mod cons, outdoor reset, ECM motors, and setback thermostats don't always play nice together. 
    JUGHNE Member Posts: 9,001
    What is the efficiency of your existing furnace?
    What is the efficiency of you new boiler if you have to run high supply water temps?

    You could consider a control set up like a dual fuel HP/furnace.
    Hot water coil to heat as primary and then at certain set point switch back to gas furnace.
    This scenario may allow you to run lower water temps and settle on a smaller water coil, knowing that it may not provide enough heat on the coldest days.

    Just an idea.
  • mattyork
    mattyork Member Posts: 5
    Might consider that in a dual fuel arrangement the stat disables the heat pump when the fossil fuel system is called for. Considering he should have no problem allowing the boiler to run simeltaneosly with the furnace, install a 2 stage thermostat, with the furnace being stage 2. On a duel fuel, typically the delay for the fossil fuel is 8 minutes or so. On a stage 2 stat its much sooner. So if you go with a 2 stage stat, put in a simple time delay to mimic that 8 minutes. That way, both the furnace and boiler can run together in bitter weather. My bigger concern is the additional pressure drop associated with a coil in the air stream. The blower isn't going to be engineered for the additional resistance. Something to check.
  • mattyork
    mattyork Member Posts: 5
    And I agree with JUGHNE; if you're considering warm air hyronic heat, why are you considering scrapping the furnace? What is it's efficiency compared to the boiler? Unless someone botched the furnce install, I'd be hard pressed to believe it won't heat the structure. It might be appropriate to ask, "What are we hoping to accomplish?"