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[Canuck87]

So I can see a wood heated hot tub in the future for me and i'm slowly wrapping my mind around a design. I am going to make a mostly square tub with a poured concrete floor (on 4" of eps foam) and the sides probably the same, with a protective coating on the inside. I want a salt water tub so a water to water heat exchange would have to be titanium and probably cost $500. In the long run I think it wouldn't be that bad to spend the money on the heat exchanger but I was also thinking of putting pex loops in the floor and possibly sides.
Anyone have any guesses on whether or not it would work?
I am pretty sure it would slowly heat up and maintain temp but would it keep the temperature up when using it. Say on a really cold winter night would it keep up to temp.
Would the concrete get uncomfortably hot?
Would it have way to long of a delay before it could start to warm the water back up?

I plan on having it insulated well with a homemade (high R-value) cover. I am picturing a large tub size with room for 10 people give or take.

[willieG]

how about making your own plastic heat exchanger? i think you can buy plastic in a coil and put it inside a larger pipe...furnace water in larger pipe and tub water in coil? im am not certain how fast plastic gives up heat but i remember lots of folks using black plastic to heat their swimming pool
just a thought

[muffin]

Seems if you insulate properly, pex in the concrete would heat just fine and you wouldn't even need the exchanger.  I would give it a try.  You can always add the exchanger later if it doesn't work out.  Might want to just but the loops in the bottom though, heat rises.

[RSI]

Prices on heat exchangers has really come down a lot. A 55K btu titanium shell in tube is about $300.

[willieG]

im surely no expert on radiant floor heating (like you would be doing in your tub) but i know in a regular home floor they don't like to see the water temps too high. if you do decide to go with the pex in the concrete i would recomend (like i said i am not expert) oxygen barrier pex (i think that is what it is) it is the stuff with the aluminum in it. The oxygen barrier with the aluminum expands way way less than the regular pex at high heats. this may give your pex running hotter water a lot longer life span?

i think i read somewhere on the net that regular pex at 180 expands about 6 feet per 100 feet of length and oxygen barries only grows about 1 foot per hundred. seems to me there would be much less strain on the pipe that is trying to grow the least

[willieG]

Seems if you insulate properly, pex in the concrete would heat just fine and you wouldn't even need the exchanger.  I would give it a try.  You can always add the exchanger later if it doesn't work out.  Might want to just but the loops in the bottom though, heat rises.

think different in radiant heat.....heat rises? not always...heat is attracted to cold no matter where that cold is.. try puttting pex in a concrete floor with no insulation under it and see how hard it is to heat your room.

you dont insulate under the floor to just keep the cold out, but also to keep the heat in

heat and cold are attracted to each other...they are always trying to equalize. we insulate to slow that process down. no matter how well you insulate there will be heat loss. the rate of that loss can be slowed but never stopped

[RSI]

How hot do you need the water in the hot tub? Usually in floors they don't run much over 120 water. Running way hotter you risk cracking the concrete. I am not sure if that would apply to a tank though.
Pex doesn't expand when in concrete. 1" pex when in open air expands about 1" per 100' per 10 degrees.
Oxygen barrier pex has the same expansion properties. Pex-al-pex doesn't expand as much.

Anyway, it would probably be worth trying to put the pex in the concrete. If it doesn't keep up, it would at least allow you to use a smaller heat exchanger.