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Question on BTU need

A friend of mine recently built a house on a slab. He put radiant tubing in the slab (2,300 sq ft) and in the garage (1,000 sq ft). The garage has R19 wall and ceiling insulation, the home has R19 walls and R50 ceiling. He is having a tough time getting the system to even keep the house warm, let alone the garage. They used a navien nhb-080 boiler (74,000 heating BTUs) for the floor heat and to heat domestic water via a heat exchanger tank. Per the radiant plan. they are saying he needs 17btu/sq ft to heat the space. That seems really low and it may be proving it with the poor performance. I know a heat loss calculation would give us an exact number, but is 17btu/sq ft even in the ballpark? The home is in central Iowa, climate zone 5.


  • Jamie Hall
    Jamie Hall Member Posts: 23,453
    Well, 17 BTUh per square foot is in the ballpark -- for what a radiant slab can produce. What you don't know, is what the heat loss of the building is. Has anyone done a Manual J heat loss calculation for the building? Without that, you really don't know.

    Further, since this is on a slab, how much of that heat is going down into the earth rather than up into the house? Is there any insulation under the slab (should be 6 plus inches) or around the perimeter of the structure?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Dave H_2
    Dave H_2 Member Posts: 556
    Slab losses can be huge. The house above sounds well insulated, but what about the slab insulation underneath and along the perimeter. One way to know about edge insulation is there any snow around the house or does it look melted all around the house. That will increase the the heat loss of the house.

    Another thing to ask is when was the system started? A slab that is cold and just turned on can take a long time to come up to temperature.

    Third thing to check out is what water temp is being supplied to the radiant system and/or check the reset curve and programming to make sure it matches.

    Whats the delta t of the radiant? The supply water temp vs the return water temp tells alot about the system.

    Dave H.
    Dave H
  • bphillips921
    bphillips921 Member Posts: 22
    The slab (supposedly) has 2 inch foam board underneath. There is nothing separating the slab from the foundation walls.

    They haven't gotten the supply temps correct. It was feeding 140 degrees yesterday with a 70 degree delta. Today they did some more work on it to get it flowing better and now it's supplying 90 degrees with a 20 degree delta. But it was feeding 120 degrees earlier in the day, which is what the designer wanted it to feed.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,886
    17btu/sqft should be more than enough to heat a well built new home: now if they built the house poorly (insulation can be installed poorly even with high R values and air sealing matters a lot), all bets are off.

    A radiant floor usually uses a 10 degree delta T, so that is the aim. The Delta T will swing over the course of a heating call however.
  • Dave H_2
    Dave H_2 Member Posts: 556
    So if this system was just started from cold start, then yes it will take a few days to warm up. That 70 degree delta is not unusual.

    Concrete slab can suck every last BTU out of the pipe very quickly.

    It sounds like things are progressing in the right direction.
    Patience, it'll get there
    Dave H