Expansion tank

I am going to need a new water heater soon, so I have started to think about the technical bits.

The latest issue is whether I need an expansion tank. I do not have one on my existing water heater and have no problems.

I do not have a PRV and I do not think the system is closed (I cannot see a check valve and I think I remember seeing the meter go into reverse once when the plumber came round).

So do I need an expansion tank with a new installation?

No, an expansion tank is only needed in a closed system.

This would be a good time to check your water pressure to see if you should have a PRV.

Thank you. I will check my pressure to see if a PRV is required. If so, I will install one and an expansion tank.

Can I assume that up to 80 PSI is OK?

80 is the often used trigger level, but most fixtures are spec'ed at 60 pounds. If it is 80, then you should probably check it at various times of the day as it can vary quite a bit depending on the system and how many people happen to be doing wash at the same time.

A good way to double-check whether you have a closed system is to observe the static pressure, then partially empty your water heater (either run a tub faucet or two at full hot, or use the water heater drain). Then close all your taps and watch what happens to water pressure as your water heater warms up that large mass of cold water. If you have a closed system, you'll get a dramatic rise in pressure within minutes.