Trish -
Just a side note. I'm under the impression that PEX-to-PEX fittings require a special crimping tool to make the connections. Shark Bite fittings are expensive, but they do not require a tool, and they are easily removable/reusable if you buy the little plastic remove-tool collar (sold separately, cheap). They also work on all kinds of rigid and semi-rigid pipe, including copper, PVC, and PEX.
I believe that the pipes and fittings in the TM are the standard household size, and many of our members have used them in the TM, so I'm not sure what the Home Depot guy was talking about. On the other hand, I don't know what PVX plumbing is.
As you said, it sounds like the pressure in the system is rising as the water heater comes on. Assuming you don't get to the boiling point (and you would know it if you did), water doesn't expand very much when it heats. On the other hand, the air in the head space of the heater will try ferociously to expand as it heats up. One thing you might do is blip the temperature/pressure release valve when the water gets hot, and see if the leaking stops. The t/p valve is intended to operate automatically to release excess pressure, so we are seldom even aware that it has done its job. It could be that the problem is a faulty t/p valve. They are not adjustable, but replacement is cheap and easy - or at least easier than what you are doing now.
Viewed from the outside, the t/p valve is located in the upper right corner of the heater. Just lift the handle a little bit, for just a moment, and drop it back. CAUTION - HOT WATER MAY SHOOT OUT - DON'T BURN YOURSELF!
Bill
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