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Can I simply add a loop of pex from my water heater, under my flooring, and back into itself? Would I need some sort of pump? I haven't laid the tile yet and water heater is right on the outside wall of the bathroom I'm building. Would I need to put this between the backer and the tile?

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    An electric space heater with a fan would be more practical. – blacksmith37 Mar 5 at 16:42
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    You would need some sort of pump. – Hot Licks Mar 5 at 19:57
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Adding hydronic radiant floor heating is a lot more complicated than you could imagine. To do a system for a bathroom running off of a domestic water tank, in operation for potable water requires all potable water components. Potable water components are expensive.

This is the simplest picture I could find and it still doesn't technically apply to youenter image description herewww.floorheatsystems.com

The storage tank isn't being used for domestic water. It's only being used for floor heating.

Other things that you need to consider are legionnaires bacteria radiant floor temperatures are right at the bacteria's ideal growth temperature of around 115 degrees Fahrenheit.

The Solution Electric radiant floor heating is your best bet.

Resistive electric floor heating mats are available enter image description herewww.findanyfloor.com

This requires a little bit of electrical but is much simpler to install. It's also much thinner then pex.

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    On a side note, there are systems that do tankless water and radiant heat in one unit. From what I can tell these are called 'open-direct' systems. I don't think the OP is looking to replace but I thought it was worth mentioning as a relatively new option. – JimmyJames Mar 5 at 18:01
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    Yes and they are basically a fully functional boiler with a built-in heat exchanger requiring all the necessary boiler components. Colloquially called Combi units. I use Navien and oddly enough I'm actually working on one right now. To be honest electric resistive rating Heating is my arch nemesis is it cuts into my action. Sometimes it is the only practical option. – Joe Fala Mar 5 at 18:06
  • I'm curious about these since I'm thinking of going tankless and have forced water radiant. Potential downside is that you lose both heat and water if it goes out. But on the other hand, you don't have to worry so much about whether it will start up when it gets cold since it will get year-round use. – JimmyJames Mar 5 at 18:11
  • Power outages are a concern but in reality it's only a problem if you are showing with the lights off and you get hit with an unholy cold suddenly. – Joe Fala Mar 5 at 18:15
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    I was unclear, a power outage with a Combi unit will result in immediate cold water hitting you in the shower as opposed to a tank or the water will gradually cool – Joe Fala Mar 5 at 18:32
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A loop unto itself cannot flow.

Seriously. Imagine a hula-hoop full of water. Not much floor will get heated from that.

You need a suitable pump and thermostatic control, plus it may not be legal to tie heating into your potable water supply. Most floor and radiator heating systems are closed-loop and contain anti-freeze and anti-microbial treatments.

  • You can use convective flow. I've seen someone do it for a no-moving-parts computer cooling system, though that was more proof-of-concept than anything practical (and I can't picture scaling it up to something room-sized). – Mark Mar 5 at 22:39
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I tried heating my garage in the winter using my water heater and all copper piping connected to finned tube radiation. What a waste of my time and money. With the water heater temperature set at 130 degrees and a small bronze pump the finned tube radiation yielded almost no heating. If you want to heat the floor it can be done but you will need a lot of under the floor tubing and not just a few feet of it due to the relatively low water tank temperature. If you have a hot air furnace you can heat the basement fairly well if you do it correctly

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    Finned tube btu outputs are usually rated at 180 degrees fahrenheit. At 130 they will no output much heat at all. The rely on convection and need a significant temperature gradient to be effective – Joe Fala Mar 5 at 18:21
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    This answer is a great example of why you should do the math and try to predict effectiveness before spending money on something like it. – R.. Mar 5 at 18:47

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