wwhitney
In the Trades
All of your data so far is consistent with (a) a check valve (or PRV or the like) between the outside hose bib and the water heater and (b) pressure increase from inadequately addressed thermal expansion. And (b) could be either a non-functioning or undersized expansion tank, or the water heater going nuts and overheating the water, like a thermostat failed closed.
Of course, that doesn't explain the 3:00 a.m. timing. If it's just approximately 3:00 a.m., then it could just be consistency on when your last hot water draw is, and the excess pressure taking 6 hours or whatever to build up. But if it's exactly 3:00 a.m. then there must be some timed effect occurring.
John's theory of the municipal water pressure spiking at a consistent time in the night is promising. But if it only spikes to 90 psi (per the one night's reading at the outside hose bib), I don't see how that could directly trigger the T&P. It could raise the baseline pressure to 90 psi, and subsequent uncompensated thermal expansion could get you to 150 psi. If the timing is consistent, that's possible.
Cheers, Wayne
Of course, that doesn't explain the 3:00 a.m. timing. If it's just approximately 3:00 a.m., then it could just be consistency on when your last hot water draw is, and the excess pressure taking 6 hours or whatever to build up. But if it's exactly 3:00 a.m. then there must be some timed effect occurring.
John's theory of the municipal water pressure spiking at a consistent time in the night is promising. But if it only spikes to 90 psi (per the one night's reading at the outside hose bib), I don't see how that could directly trigger the T&P. It could raise the baseline pressure to 90 psi, and subsequent uncompensated thermal expansion could get you to 150 psi. If the timing is consistent, that's possible.
Cheers, Wayne