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Designing a radiant heating system with condensing gas boilers.

bob eck
bob eck Member Posts: 930
I will be looking at a new construction house that will have radiant heat system and they are looking at having a snow melting system. The house is 10,000 SQ FT plus.
There will be two high efficiency condensing gas boilers. To keep the boilers from short cycling and to get them to run longer cycles would it be best to use a large storage tank and pump out of that tank to the different zones (radiant headers).
Plus with a snow melting system we can run a zone off the tank to a plate heat exchanger with the snow melting zone separated from the water in the boiler and radiant heating system.
Does Dan have a book on designing this type of system? If not where is the best place to go to get a book on how to do this type of system.


  • Ironman
    Ironman Member Posts: 7,377
    You may not need the buffer tank unless you create micro zones.

    I wouldn't design the SIM to be connected to the buffer tank since the tank temp should be controlled by ODR.

    Depending upon the size of the SIM area, a separate boiler may be needed to supply enough horsepower without over-sizing the boilers for space heating.

    Siggy's book is probably your best text and Dan has it in the online store.

    Rehau, Uponor and Viega all offer design services if you use their product.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    Bob Bona_4
  • Rich_49
    Rich_49 Member Posts: 2,766
    If you're using 2 boilers Bob and have done the math there should not be a shortcycling problem .

    A 10,000 sf home can have a hell of a driveway . Are you sizing the boilers to also do SIM during a weather event ?
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Zman
    Zman Member Posts: 7,572
    This project works well.

    The snow melt tubing and distribution is on a separate page. P6 is set up for injection mixing.

    It was an existing home with a bunch of microloads and existing radiant. The addition has panel rads, more radiant and snow melt.

    The DHW has priority over Heat which has priority over Snow Melt.If all 3 are calling and the boilers are keeping up, they all run, if not loads are shed.

    Each mixing valve matches a separate curve for it's type of emitter.The boilers fire to meet the maximum temp requirement for the zones calling.

    One could critique the DHW going through the buffer tank. It is nice when the towel warmers come on in the summer after a dhw call and the boiler does not have to fire because they can use the stored energy in the buffer.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Agree on the seperate boilers needed for the snowmelt, last I recall you'd be looking at around 140 BTU per Sqft. Might want to have that whole deal seperate with all the fun associated with the amount of glycol needed and future upkeep as well. Guess I'd need to see the area of the snowmelt to make a better decision.
  • Zman
    Zman Member Posts: 7,572
    Sometimes a separate snowmelt boiler is the trick. Sometimes not.
    I would look at all the loads and make a decision from there.
    Particularly in a residential application, Load prioritizing and shedding makes a lot of sense.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein