Abstract: A hydro-pneumatic expansion tank is provided in a water pipe circuit. The hydro-pneumatic pressure vessel is in the form of a water/air expansion tank communicating with the pipe circuit by means of a conduit and a float valve is provided in the conduit. The expansion tank includes air under pressure and the water. A barrier liquid is provided as a barrier layer between the water and the air, the barrier liquid being made of a clear polyalphaolefin fluid which is impermeable to water and does not readily absorb oxygen. The float valve has a float member having a specific gravity less than the specific gravity of water but greater than the specific gravity of the barrier liquid such that when the barrier liquid is being drained from the expansion tank, the float member in the valve will sink onto the valve seat when the barrier liquid is in the valve chamber thus closing the valve.