From 60 degrees to 125 degrees gets you 65 degrees of temp difference. Say there's 50 gal of water being heated. You gain 1.495 gal. of water. That's gonna cause a pretty big pressure rise without a working expansion tank. Prv or no. It's possible to calculate but very difficult without a lot of specific info. He needs at least a two gallon expansion tank if there's 50 gal of water in this system. I'm guessing there's more.