Bell & gossett hft-15 hydronic heating expansion tank 1bn326 – 2 gallons the series hft tank is designed to absorb the force of thermal expansion of heating water and to maintain proper pressurization in a closed hydronic system. The heavy duty butyl diaphragm separates system water from the air in the tank preventing water logging problems. features: residential and light commercial applications helps prevent excessive pressures in heating systems increases system performance reduces oxygen corrosion prevents waterlogging in-line and floor mount installation max. Operating temp 240f max. Working pressure of 100 psi