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[ajcookman]

has anyone ever used a coil of pex inside of a homemade wood boiler for domestic hot water ?

[juddspaintballs]

PEX doesn't transfer heat very well.  It would raise the temperature inside of the pipe after sitting for a long while, but when you start flowing water through it you'll see very little temperature rise in the domestic water.  Even using copper, which transfers heat very well, the coil inside the boiler would need to either be pretty long or have fins on it to help transfer the heat faster to keep up with domestic hot water demands.

This is what the typical domestic coil inside of a boiler looks like.  They're made of copper and cost about $200, but they work very well. 

[ajcookman]

thanks for the reply juddspaintballs i am building a garn style wood boiler water tank is 4 feet in dia. 1000 gal. was thinking of placing 100 foot roll of 1/2 pex in tank for domestic water just seems cheaper than copper and may last longer

[juddspaintballs]

100' roll still might not transfer enough heat when the water is flowing a lot, like during a shower.  As for durability, the PEX might not last all that well inside the boiler, especially if the thing ever over heats or you get a hot spot inside the water jacket that the PEX would run through.  I think that if you want to try it out, it wouldn't be too hard to fix if it didn't work out for you.  All you simply would need to do is cut a hole on a flat face for a plate that the coil could run in and out of and seal that plate up with a gasket between the boiler and the plate.  If the PEX coil doesn't heat enough water or fails, you'd simply shut down your boiler and drain off enough water to reach the level of that plate, then pull the plate and the PEX coil out through the hole.  If you did a standardized hole, you could then insert a pre-made copper coil in and bolt it up for a very quick fix.  You could also make your own domestic coil with copper tubing too (if the PEX coil failed).  I say give it a whirl but leave yourself a way to easily remove it if there is a problem.  100' coil of PEX is cheap enough.

BTW, I love GARN's.  If I didn't already have a boiler I'd look long and hard at getting one and setting it up in a Garn Barn. 

[Sprinter]

Not a good idea, copper is your best material for heat transfer. I did see a customer use the CSST gas pipe for a coil. Said he has endless hot water. Its stainless, so no corrosion factor. It's ribbed for her pleasure, or I mean for more surface area per foot to transfer btu. Its flexible and can easily be made to fit. 
I use trac pipe brand myself and you can't kink that stuff, then there is ward flex and gas title I think is sold at lowes, but these will kink and have thinner wall material. I'm not sure about the fittings these seal with either, but I do know the tracpipe brand has a flare type connection can be used more than once and I've used it for pressures upto 212psi, air and liquid. Right now I join my compressor tanks with it at 145-150.
I guess you could try it, this customer been running it for years. I'll have to look up his invoice and see if I can call and ask. Its in a big storage tank. Pulled out of Rochester college we did.

[ITO]

 This is what I use for DHW, not inside the boiler but inside the house, will heat through the HX and use it with either the OWB or propane boiler (summer).
http://www.htproducts.com/superstor-ultra-waterheater.html
 I bring this up because there is a picture and description of their copper (alloy) coil on the link, this has worked great for me but I always thought someone could make a DIY version of this tank with a little ingenuity.