The ET size calculators will tell you how much volume increase there will be under various conditions. The physics of it is solid...heat water, it expands. In comparison to say the copper pipes, there's no comparison...the water expansion radically overshadows the copper. So, that larger volume has to go somewhere or the pressure WILL almost instantly rise high enough to open the T&P valve-water isn't really compressible. So, where can it go? Could be lots of places. Easiest one is if you have an open system...it goes back into the supply system. Because under some unusual circumstances, you might pollute the water, water companies have been installing check valves when they upgrade their systems to prevent that possibility, making your system now closed. If you have a closed system, there are still some choices:
- if your checkvalve leaks
- if your prv leaks
- if your prv has an internal relief valve then it will only rise to the level of the supply on the other side
- if your hoses (washing machine, toilets, faucets, etc.) balloon
- if any valve leaks. Note, a common one is a toilet fill valve...some of those can leak, but you'd never probably notice as that excess water will just raise the tank level a little, or just end up going down the overflow. MOst of them are quiet about it, too. If that, it can shorten the life of the washer or gasket, though. Plumbing fixtures for homes are designed with a safety margin where the highest normal pressure is 80psi. For example, a water heater is designed with about a 100% safety margin because of the setting on the T&P valve (typically, 150psi). Testing the elasticity of hoses is risky...eventually, they can weaken and fail.
As an easy test, shut the valves to your toilets, then take a nice long shower or a load of clothes using hot. Wait a little bit and see if you get a discharge from the T&P valve of your WH...if you do, you've found your weakest point - your toilet fill valves are leaking, preventing the pressure from rising too high.