Pex as heat exchange tube

[EricV]

I've seen some folks here talking about using pex as a heat exchange tube. I've also seen a figure thrown out there that you would need 3 times more pex than copper. Has anyone actually tried it?

I need a new heat exchanger for my DHW and I can get a 200' roll of 3/4" pex for $89 with free shipping. I had a 42 gallon galvanized steel tank that sat inside my large tank bit it crapped out.

My thought is to coil the pex around the top of the tank sitting on top of my copper coils coming from the Tarm.

This is meant to only temper the water before it goes to my indirect water heater. It helps considerably with the heavy DHW draw I have to have the water tempered before it hits the water heater.

Any thoughts?

Eric

 

[WES999]

See my post (#6).
https://www.hearth.com/econtent/index.php/forums/viewthread/49253/

The PEX works pretty well, the outlet temp in the PEX is about 5 °F less than the tank temp.
There is also some goog info here
builditsolar.com
http://www.builditsolar.com/Experimental/PEXColDHW/TankConstruction.htm

 

[WoodNotOil]

I noticed a while back that STSS has some coils that are part pex and part copper.
http://www.stsscoinc.com/Products_HeatExchangerCoil.aspx

 

[wood-engineer]

I have a 100 ft coil of 1/2" pex that is used to heat my hot tub. The coil sits in a 500 gallon unpressurized storage tank. When I refill my hot tub, that coil will heat up 450 gallons of 54 degree water to 100 degrees in about 8 hours. The storage tank is about 170 degrees, and fairly steady - as I run the Tarm boiler when filling/heating the hot tub.

You can do the math, and get a (watts/(sq meter * degree celcius)) - which is the heat transfer coeffiecient. That number should work with 3/4" pex - maybe reduce it a bit for the increased wall thickness of 3/4" vs 1/2".

Hope this helps...

 

[EricV]

Thanks for the info. I'm going to go ahead and order a roll and install it next week. I'll post my findings.

Eric