Why PEX-AL-PEX?

PEX-AL-PEX pipe is a PEX pipe with an aluminum layer sandwiched between an inner and outer layer of PEX.

I believe that its a better choice for collectors than standard PEX for the following reasons:

It is easily bent in to tight turns AND it retains its shape without spring back. This is a very nice feature.
It has higher working and burst pressures than ordinary PEX.
The PEX-AL-PEX I used (Mr. PEX) is rated for 160 psi at 200F.
Standard PEX is rated at 80 psi at 200F, and 160 psi at 73F.
It has better thermal conductivity that regular PEX -- this should result in somewhat better heat transfer from the aluminum fins through the PEX to the water.
PEX-AL-PEX 3.1 BTU/h-ft F for half inch tubing
PEX 2.6 BTU/h-ft-F for half inch tubing
It has a much lower coefficient of thermal expansion than PEX, and is more compatible with the expansion rate of the metal fins.
It may be somewhat more tolerant of freezing. It will probably withstand several freeze thaw cycles, BUT will NOT withstand continual freeze thaw cycles.

Just as an indication of the ability of PEX to withstand these conditions, one paper on Down Hole Heat Exchangers stated that a Swedish lab has had a sample of ordinary PEX under pressure at 153 psi and at a temperature of 203F since 1973!

On the negative side, it costs more than standard PEX. Its about 50 cents per ft, and a 4X8ft collector requires a bit over 50 ft. Another negative is that it appears to be more subject to damage from water freezing in the tubing, but it should still be much better than copper for this. And, a 3rd disadvantage to the material I used is that it does not appear to work with SharkBite fittings, which can be handy for quick hookups -- it appears that the inner diameter is slightly less, and the part of the SharkBite that is supposed to fit inside the tube does not -- maybe someone knows a good alternative to SharkBites for quick hookups?

Gary March 23, 2008, September 14, 2008