Expansion Tanks (Water Heating)

Thermal expansion tanks for potable water systems, sized 0–5 gallons for residential and up to 60 gallons for larger installations, with mounting brackets.

These exist because of the check valve or backflow preventer on the incoming service. Once that device is fitted, the plumbing becomes a closed system: water heated in the tank expands and has nowhere to go, so pressure climbs until the T&P relief valve weeps. A weeping relief valve on a recently re-plumbed house is almost always this, not a faulty valve.

A potable tank is not a hydronic tank. It must be rated for drinking water, with a polymer-lined or stainless connection instead of the bare steel used on heating models, and it carries a different certification.

Pre-charge the tank to match your static water pressure before installing it, with no water in it. Charging afterwards reads the water side, not the air, and leaves a tank that is effectively full and doing nothing.

Connections run 1/2" through 1-1/4", MIP and FIP. Brands include Amtrol, Flexcon, Flamco and Zilmet.