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Can Radiant Work as a Primary Heat Source in My House?

Hey All,

I've been digging deep on this site. Amazing resource. This is my first post.

I live in a wonderful but poorly insulated post and beam barn that was turned into a house in the 1930s. I mostly heat with a wood stove, but have forced air as backup, run by an oil furnace. The furnace is in dire need of replacement, and I've always hated the forced air, so I was thinking of going with radiant, likely at first run on propane or oil (I already have both). Then in the future, add a gasifying wood boiler, and use the propane or oil as backup.

BUT, I'm not sure if the radiant can supply the BTU's needed to get the job done in this old house. I did a quick heat loss calculation at USBoiler and at BuilditSolar, and came back with 99-115,000 BTU's needed. My home has only 1500 SF of floor. Assuming the radiant can pump out 35 BTU/SF, that's only about 52,000 BTU's. Am I missing something? Am I even thinking about this right?

As background: The radiant would be mounted between joists on 6" pine floor boards. No subfloor. Likely in aluminum transfer plates, and then insulated below.

I'd love to be able to heat that way, but I don't want to go through the expense and hassle of doing it, only to find it doesn't work.

Love to hear your advice.


  • Parsnip
    Parsnip Member Posts: 3
    Also, I know the "right" thing to do is to work on tightening and insulation. BUT, the home is built in a way that would make that not very feasible without rebuilding major components that are beyond scope right now (removing all siding, adding rigid insulation, residing; rebuilding roof entirely, etc.).
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    You're going to need a bigger boat... You could set up a 2 stage consideration, whereby radiant floors does 1st stage, and hot water baseboard does second stage, or forced air using fan coil does 2nd stage with first stage always running on any call for heat.

    In my experience, the heat loss calculations are overkill by about 50%, so theres a really good chance that you can carry the load with the RFH system for the majority of the time, but will need the back up when it gets really cold outside.

    The "wild card" is going to be the infiltration factor. Might want to have a blower door test done, and at least do some crack sealing. Also a great way to eliminate the one wild card that could wreck your house of radiant... Absolutely need insulation below the heat floors.

    You could also do radiant ceilings, but will cost more than forced error. A radiant sandwich if you will. A human microwave oven. :smile:

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Gordy
    Gordy Member Posts: 9,546
    As Mark says you need to increase emitter sf, or alternate second stage emitter.

    Had a home with rooms that had both ceilings, and floors. Nice
  • DZoro
    DZoro Member Posts: 1,048
    You have the forced air now, keep it for 2nd stage, during the time of need. As you fill the holes in the boat you maybe able to get rid of the forced air.
  • Zman
    Zman Member Posts: 7,569
    I suspect your heat loss software is over estimating. Can you back check it using past utility bills?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Mark Eatherton
  • Parsnip
    Parsnip Member Posts: 3
    Hey All, This is so useful. Thank you. Given your advice, I redid the heat loss calc. room by room. What I found was really interesting to me. The smaller rooms should be okay. They need less than 35btu/sf. It is the one big main room with the cathedral ceilings, windows, front door, etc. that are where we are way undergunned.

    A few quick follow up questions:
    1. Who do I look to to do a blower test? Is that a DIY thing, or something I hire someone for, or something a contractor does to show where work is needed?
    2. Right now the forced thermostat is set up at 45-55. We basically lean on it to keep pipes from freezing when gone, and when the wood stove can't keep up, or we are sleeping and too lazy to get out of bed to restock it. This means the small rooms are frigid, but we mostly hang in the big one around the wood stove. My hunch is that if we set the radiant a bit higher than that (say to 60 or 65) we'd be much more comfortable than we are now, and the wood stove would be able to pick up the difference in the big main room where we are overgunned. Does this sound right?
    3. I like the idea of leaving in the forced air and put a water to air heat exchanger in the plenum for further backup when it gets really cold. Like now. But that means we are leaving up the duct work, which could mean tying up a few of the bays where the radiant would otherwise go. Is that a problem? Do we need to lower and rework the duct? Or just skip those zones?

    Thanks again all!
  • djc2232
    djc2232 Member Posts: 136
    edited January 2018
    Sure it can. I'm no expert but it's what I have in my house.

    1620 sq ft single store ranch with a crawl space. I live in northern NY and the temp this morning is -16. Was -26 last week. I have a pellet stove for backup but rarely use it. I run only two zones. It used to be fed by an outdoor wood boiler. Got rid of that stupid thing and had a contractor install a Weil McLain eco 70 boiler. My heat loss isn't a lot. Our design day here is -15 and Im probably right around 35-40k btu.

    In floor radiant is very slow reacting. Set it at a temp you want and leave it. The way to have it is in slab or in warm board. Mine is staple up which is not the most efficient.

    I know a handful of people around my area that have it as their primary and love it, but theirs is In slab.

    Love the radiant. Would I do it again? Maybe not. Insulate very very well. Use heat plates with good thick insulation. No carpet either on floor above.