While researching ways to heat a small bathroom (28sq. ft. total), I came across a post that someone put some narrow piping above a bathroom subfloor and cemented it in under the tiling. They were using a tankless recirculation loop as a form of hydronic radiant floor heating.

I'm actually installing a tankless, though I planned to do so without a recirculation loop. I also planned to put electric radiant heating in the non-shower parts of the bathroom (~15W/sq. ft.). The post got me thinking about whether a recirculation pump really helps at all, whether it can be used for radiant heating like that, and how quickly it will heat the floor, especially compared to the electric heating. I'm thinking I might go with radiant in the whole bathroom, only under the shower where there's no electric, or nowhere.

For reference, I'm putting the radiant heating in because I don't expect the mini-split to adequately handle the bathroom. The shower is probably 10ft. of plumbing from the tankless, and the farthest faucet is maybe 25ft.

  • In some areas it is not legal to use the same water heater for a heating loop and potable water. have you considered an electric mat under the flooring? these are not difficult to install but do need a circuit to power them. I like hydronic heat but in a small room like a bath I find the electric easier and less expensive in the long run. – Ed Beal Jan 12 '17 at 14:25
  • I would think anywhere. Yes, the only way you can combine water heatrs i – Harper Jan 12 '17 at 15:01
  • I would think anywhere. Yes, the only way you can combine water heaters is with a water/water heat exchanger. Anyway walk into a seemingly cold bathroom, you want a quick blast of on-demand heat. – Harper Jan 12 '17 at 15:17
  • It's actually common and normal to use one water heater for both jobs, but it does require using potable water PEX and stainless or bronze pumps. See: radiantcompany.com/system/opensystem for an example. I bought some tubing from them once, not otherwise affiliated. – Ecnerwal Jan 12 '17 at 15:40
  • FWIW, a recirculation pump is allowed on potable lines (and there's no ethylene or propylene glycol in the mix). All lines will be copper too. Also, as per the question, all areas except under the shower are planned to have electric radiant heating. – Hari Ganti Jan 13 '17 at 7:21

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