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Anyone have any experience using tankless gas water heater w/ radiant floor? in a conditioned crawlspace?

GBA Editor | Posted in GBA Pro Help on

Doing major remodel and plann to add to kitchen/new bath w/ tile floors over crawlspace. Crawlspace to be conditioned space – sealed and insulated at perimeter ala BSC specs. Was planning to do radiant floor heating because done’t want/ don’t like forced air over tile floors. Has anyone used gas tankless water heater as radiant floor heat and domestic hot water source (combo)?


  1. Riversong | | #1

    I have no personal experience with that system, but that's because I don't trust the long-term efficiency of tankless heaters, don't like the minimum flow rates required, and know that even the biggest tankless units can not keep up with more than two simultaneous showers let alone heat and hot water.

    Hard water mineral deposits significantly reduce the thermal transfer effectiveness of instantaneous heaters, and they have to have burners twice the size of a tank heater in order to compensate for having no reserve.

    Direct-vent tank heaters are highly efficient and well insulated. The only time that they present a significant inefficiency is if installed in unconditioned space or within conditioned space that is air conditioned in summer.

    The most reliable and efficient combination hydronic system is a modulating condensing direct-vent boiler with indirect tank or the new combination boilers that have a built-in mini-tank water heater such as the Triangle Tube Excellence.


    We do it all the time. Here's how basically it's using the condensing demand water heater as a side-arm boiler on a small storage tank.

    BUT there are a zillion little things that can go wrong, and chasing them down can be maddening, the cost of propane per BTU is very high compared to the cost of Natural Gas and even electricity (for now...) So it seems like it should be easier and more economical than it really is esp. if your on an LP tank. Roberts solution is undoubtedly simpler and more reliable but I can get a good condensing demand water heater for $1,300 to 1,600 and supply all the hot water and radiant floor heat I need off of it.

    But for passive solar homes where you have lots of mass for thermal inertia adding some radiant as a complement to passive solar can be a great thing. I like doing about 25 to 30% of the floor area as radiant slab so you can draw enough BTUs from the slab to actually make it feel warm without over heating the house. I also like doing a hot spot in front of the fire and under the dining room table and mud room for spot conditioning.

  3. jklingel | | #3

    Not sure if this is exactly what you are after, but you may want to read here, too.

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