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Thread: radiant heat question

  1. #1

    Default radiant heat question

    My son is renovating the basement of his 1940's era house. He's having the big old iron hot-water radiator pipes that hang below the ceiling replaced with flexible black Pex(?) tubing and put up into the ceiling. A plumber mentioned to him that some people are choosing to dispose of the radiators and instead just loop the pex tubing all around inside the walls behind the drywall - like radiant heat.
    My questions are: does anyone have an opinion about this? Will it cost more to heat a room like this instead of having a radiator in it? Are there any other pros or cons?
    Thanks,
    Donk

  2. #2
    In the trades Dana's Avatar
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    Radiant floors are "cruisin' in yer socks" cushy, and can usually operate as lower temperatures than old-skool radiators. For every 10F that you can lower the water temp you reap about a ~3% savings in fuel use. If you can heat the place using the floors as radiators with 120F water instead of 180F, that's a 15-20% savings.

    If you're doing just one room with low-temp radiation and still need high temp everywhere else it makes for a more complicated system design, but it can be done. If it can all be done with low temp water, further big savings can be gotten out of a condensing boiler. Running one small zone of "staple up" retrofit radiant with low temp water usually requires a buffer tank to keep the boiler from short-cycling and losing efficiency, but properly buffered it's more efficient than high temp hydronic.

    Retrofitting radiant floor is expensive, and won't pay back in fuel savings alone, but in creature-comfort it does big-time. It may never pay back ever on fuel savings alone, but radiant floors are nice, and a value adder at resale time.

    It's a big subject- hire a pro who stands by their work, and understands your efficiency, cost, & comfort goals.

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    water temperatures for staple up radiant will run much higher than in floor will, and there is a good chance that you will still need supplimental baseboard because it is very difficult to get enough heat output from a staple up job. that said, comfort is sometines worth paying for. Incidentally, the operation cost of radiant is not usually signifigantly lower than traditional methods. If it tankes 100,000 btu/hr to heat the envelope that is what you need to deliver regardless of how it gets there. Granted, most people run the thermostat lower with radiant because it makes them feel more comfortable at lower temperatures. Very often, the cost of the installation and equipment far out weighs any cost pay back ratio, especially when you add in the cost of a 95% plus high efficiency boiler.

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    In the trades Dana's Avatar
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    The 95% boiler certainly isn't a requirement, and the payback on it will be much slower with staple-up than slab due to the higher water temps.

    But even an 80% AFUE boiler can run radiant floors, and if you can deliver the heat at 130-150F, instead of 170-200F the savings are still ~3%/10F.

    But it definitely falls under the heating of "luxury upgrade", and definitely not "cost-effective efficiency upgrade", in most retrofits. It's more efficient, yes, but not so much that you'll make it up in fuel savings. The same efficiency can be had by increasing the size of your radiation by 50-75% and running lower temps. (Running extra baseboard it ridiculously cheap compared to retrofit radiant floor.)

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