Water – Need help choosing t&p replacement valve

I would replace the P-T valve regardless, as it sounds questionable. They're about $10-15, plus usually a pipe directing it to the floor or even a nearby floor drain (which you can probably re-use).

It's a safety device that prevents your tank from over-pressurizing (which in very extreme cases can actually cause an incredibly powerful explosion -- check youtube for some entertaining mythbusters clips). You also don't want it to trip accidentally, as it will pour a lot of very hot water out and will not stop until the P-T valve shuts itself off automatically (unlikely if it's broken), or someone shuts off an upstream valve. If your floor drain is clogged, or gets overwhelmed, you have a flooded basement/house.

You should make two phone calls. The first to your local building department, which should be able to answer all your code related questions. They may even have a handout or other documentation, that outlines the requirements of replacing a water heater.

The second call should be to a local Rheem dealer, or directly to Rheem. They will be able to answer any questions you have about the equipment.

Some things to watch out for:

• Make sure you drain the old tank completely, as water can be quite heavy.

• When draining the tank, make sure you open the hot valves at fixtures. This will allow all the water to drain back to the tank.

• When opening valves at fixtures, watch out for mixing valves. These can accidentally backflow cold water through the hot water pipes, which would lead to an undrainable tank.

• If you have to install an expansion tank, make sure you charge it properly.

• Make sure you know where to dispose of the old tank, as some municipal garbage collectors will not take water tanks.