The vent nipple on the T&P valve should have a hose securely clamped to it, led to a safe place, and securely fastened along its path, for safety reasons.
Amen. If you are in the wrong place when the safety valve goes off, it can be very bad.
Looked like the actual volume increase for his circumstances, was in the 12 oz range. So, fill a steel tank full of water, then try to force 12 oz more water into it. No question why the 150psi relief was popping.
The same phenomenon should occur with engine heat, given the same starting pressure, and start/finish temperature. So, my hunch is, something is different, since the relief valve didn't trip. I hope he will verify his pump pressure and thermostat operation in both modes before he calls it done.
The expansion tank is very important for systems that start with a cold tank often (like when my oldest daughter comes to visit and turns the second floor into a steam room every evening). As
@jssailem pointed out, you could bleed the expansion pressure off at a faucet as it warms up. Then, once it's stable at the max temp, you're good till the next cool down/warm up cycle.
In systems without expansion tanks, the T&P valve basically becomes an operating control, bleeding off the expansion pressure at every cycle, unless usage, or leakage takes care of it. As the intent of the T&P is safety, from an impending catastrophe, I'm sure that's why code officials decided to mandate expansion tanks on residential systems.
Note also, that without the check valve, the accumulator and cold water piping (and dock hose, in his configuration), would have probably absorbed the expansion, preventing a T&P trip.
Thank you for your patience and participation
@jviss. You have a good handle on the situation. Enjoyed the discussion. I hope he can get his system up and running to his satisfaction soon.