Recently replaced old electric water heater with a 40-gallon SuperStor
running off oil furnace. It now overpressures and vents several times a
week. Feed pressure is about 30 PSI. A cheap max-indicating pressure
gauge confirms that pressure jumps whenever a lot of hot water is used,
though it doesn't always reach the 150 PSI needed to vent.

I've tried to convince the installers that 40 gallons of water expanding
from 55F to 140F is going to strain the pipes in this small house, and
that some kind of expansion tank (like the one in the heating system,
right next to the SuperStor) is needed. They think I'm nuts.

Can anyone here offer relevant experience or practice?

Thx-

Theo