Help me plan my DIY wood stove water heater

Mukiwa · 01-14-18, 11:44 PM

We are building a house with an insulated and hydronicly heated concrete slab as our floor and heat source. It will be heated with a propane boiler but I want to be able to hook it up to my woodstove in the future. Unfortunately the woodstove is not located beside the mechanical room so I have to run water line through the slab from the woodstove to the mechanical room. I will not be hooking up right away but I have to have the pipe in the slab before because once we pour the concrete I won’t have access to install a pipe later (obviously). What type of pipe should be used in the slab?

A few options:

Pex - I’m worried pex will melt with the heat of the woodstove,
Steel - I don’t really want to have threaded fittings where I can’t access them if they start leaking or something
Copper - $$$$$$

What would you use? And what size should the pipe be?

oil pan 4 · 01-15-18, 04:41 AM

Pex. I think it's rated to something like 150°F at 100psi.
You could always make the first 5 or 10 feet out of copper incase of a rush of really hot water.

Mukiwa · 01-15-18, 10:47 PM

I wasn’t worried about the water temp but more the steel a loop running thru the woodstove conducting heat to the pex connection. How far away from the woodstove would the pipe get hot enough to melt pex... i am looking at the Hilkoil retrofit coil if anyone on here has any experience with this type of water heater.

AC_Hacker · 01-17-18, 01:08 AM

Quote:

Originally Posted by Mukiwa

Pex - I’m worried pex will melt with the heat of the wood stove,

I think your concerns are well founded regarding PEX and your wood stove. PEX has a maximum temperature rating of 180 F. After that it's bursting strength drops. I have seen photos of burst PEX that was used for solar collectors.

Your gas boiler probably has built-in precautions to make sure the PEX temp doesn't get to the danger zone. I have no information about what water temperature you will get from your wood stove. But you should be aware of possible problems. In my experience, the heat from a wood stove can vary quite a bit.

Quote:

Originally Posted by Mukiwa

Steel - I don’t really want to have threaded fittings where I can’t access them if they start leaking or something

Steel buried in concrete will eventually have problems. History is on your side!

Quote:

Originally Posted by Mukiwa

Copper - $$$$$$

Copper buried in concrete is not good either because of corrosion.

Best of luck,

-AC

Mukiwa · 01-17-18, 04:30 PM

So all my ideas are a no go lol.. Maybe some conduit buried under the slab with copper pipe inside... I would probably be ok with a copper-pex connection under the slab after 10-15’ if it was in a 3” conduit that I could pull out and fix if there were any issues.. I’m thinking 7/8 or 1” Copper depending on what size it come in. I don’t know if pex goes that big so I may even have to use polly

Mukiwa · 01-17-18, 04:34 PM

I’m pretty sure you only need 15-20 PSI for hydronic so I think over heating and bursting would be less of an issue but no one wants issues. I also don’t have endless funds and large diameter Copper isn’t cheap.. I probably have to go 25-30’ from the woodstove to the mech. room which isn’t ideal but it is what it is.

WillyP · 10-14-20, 09:06 PM

I am curious how this turned out. Did you go with copper?
I just want to throw out there. I live in a seventy year old house with copper pipes inside the slab (for radiators) there has never been a problem with corrosion.

nibs · 10-26-20, 09:23 PM

Put in a header tank on the hot side of the fire place & split the supply side line with a tee into the hot line with a water temperature controller. But the big danger is boiling the water in the firebox, that could run away and really hurt someone.

01-14-18, 11:44 PM	#1
Mukiwa Lurking Renovator Join Date: Sep 2015 Location: BC Posts: 16 Thanks: 2 Thanked 0 Times in 0 Posts	Help me plan my DIY wood stove water heater We are building a house with an insulated and hydronicly heated concrete slab as our floor and heat source. It will be heated with a propane boiler but I want to be able to hook it up to my woodstove in the future. Unfortunately the woodstove is not located beside the mechanical room so I have to run water line through the slab from the woodstove to the mechanical room. I will not be hooking up right away but I have to have the pipe in the slab before because once we pour the concrete I won’t have access to install a pipe later (obviously). What type of pipe should be used in the slab? A few options: Pex - I’m worried pex will melt with the heat of the woodstove, Steel - I don’t really want to have threaded fittings where I can’t access them if they start leaking or something Copper - $$$$$$ What would you use? And what size should the pipe be?

01-15-18, 04:41 AM	#2
oil pan 4 Land owner Join Date: Nov 2012 Location: NM Posts: 1,026 Thanks: 12 Thanked 127 Times in 107 Posts	Pex. I think it's rated to something like 150°F at 100psi. You could always make the first 5 or 10 feet out of copper incase of a rush of really hot water.

01-15-18, 10:47 PM	#3
Mukiwa Lurking Renovator Join Date: Sep 2015 Location: BC Posts: 16 Thanks: 2 Thanked 0 Times in 0 Posts	I wasn’t worried about the water temp but more the steel a loop running thru the woodstove conducting heat to the pex connection. How far away from the woodstove would the pipe get hot enough to melt pex... i am looking at the Hilkoil retrofit coil if anyone on here has any experience with this type of water heater.

01-17-18, 04:30 PM	#5
Mukiwa Lurking Renovator Join Date: Sep 2015 Location: BC Posts: 16 Thanks: 2 Thanked 0 Times in 0 Posts	So all my ideas are a no go lol.. Maybe some conduit buried under the slab with copper pipe inside... I would probably be ok with a copper-pex connection under the slab after 10-15’ if it was in a 3” conduit that I could pull out and fix if there were any issues.. I’m thinking 7/8 or 1” Copper depending on what size it come in. I don’t know if pex goes that big so I may even have to use polly

01-17-18, 04:34 PM	#6
Mukiwa Lurking Renovator Join Date: Sep 2015 Location: BC Posts: 16 Thanks: 2 Thanked 0 Times in 0 Posts	I’m pretty sure you only need 15-20 PSI for hydronic so I think over heating and bursting would be less of an issue but no one wants issues. I also don’t have endless funds and large diameter Copper isn’t cheap.. I probably have to go 25-30’ from the woodstove to the mech. room which isn’t ideal but it is what it is.

10-14-20, 09:06 PM	#7
WillyP Helper EcoRenovator Join Date: Jul 2020 Location: Virginia Posts: 30 Thanks: 8 Thanked 3 Times in 3 Posts	I am curious how this turned out. Did you go with copper? I just want to throw out there. I live in a seventy year old house with copper pipes inside the slab (for radiators) there has never been a problem with corrosion.

10-26-20, 09:23 PM	#8
nibs Apprentice EcoRenovator Join Date: Jun 2010 Posts: 206 Thanks: 1 Thanked 18 Times in 17 Posts	Put in a header tank on the hot side of the fire place & split the supply side line with a tee into the hot line with a water temperature controller. But the big danger is boiling the water in the firebox, that could run away and really hurt someone.