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Heated concrete garden seats question? Answered

Next year I want to use the garden a lot more, so this winder I'm building a gas bottle wood burner like btop & maybe a BBQ like Babe-BQ from abitdifferent (have access to a few bottles of various sizes) and some concrete (maybe papercrete, if i can gather enough old paper for the whole job) bucket style seats, as I have lots of left over cement from a previous project where the inlaws over estimate (by a large amount!)

So i can use it longer through the year, I'd like to have them heated with some pipes going through them being heated off the burner. I'd have a copper coil at the burner end, but ideally to keep costs down, i'd like to use PEX in the seats. 

Will this transfer enough (if any) heat outside the pipe? and also will the concrete absorb the heat? 

I was thinking of placing the pipe 10/15mm deep and having it coil back and forth from front of the seat  by the knees to the back by the shoulders and back again, 2 or 3 times per seat. It will be a modular design, for smaller molds and adjustability, with each seat connected to the last.

I've added a quick drawing i made. Hope it makes sense.

Was also thinking that I could also set up a solar heater as a pre heater to heat them in the to just take the chill off, but thats a different project for another time

Any suggestions or advice greatly appreciated.



Can I suggest, instead of using water, you build a "rocket mass heater" They're usually made using clay cob, but I don't see why you couldn't make benches in concrete instead.

Check out heated concrete floors. They use a stiff plastic hose that runs warm water. It doesn't take extremely hot temps to heat the floor. I would maybe use the burner to heat water in an exchanger and then pump it through the seats. That requires a little more equipment but it's a proven solution.

A friend of mine heats his house that way.
In fact he has 4 separate zones that are individually temp controlled.

Yeah, I just helped my parents install it in their new house. Their using it to heat as well so there are a bunch of zones.