Fluids—whether water or propylene glycol used in SWH systems—expand when heated, which can cause enough pressure to trigger relief valves, causing fluid loss. Expansion tanks (used in antifreeze systems) prevent this by giving the water an air space into which it can expand—but they can fail, too.
In an expansion tank, a rubber bladder separates the fluid from the air. The bladder assists in resisting corrosion of the expansion tank if the fluid contains oxygenated water, such as in a hydronic heating system. Antifreeze-based SWH systems are closed loops that contain little or no oxygenated fluid once the air has been expelled by an air relief valve, making them much less likely to corrode. But they still require an expansion tank.
Manufacturers recommend that the expansion tank air be charged to the same pressure as the liquid system. Many experienced installers charge the air side slightly less (2 to 3 psi) than the liquid, thinking that always having a little liquid in the tank will decrease the wear on the bladder touching the inside of the pipe fitting. As the liquid in the system gets hot, it fills more of the expansion tank, pushing the bladder up. A tank that’s too small or severely undercharged relative to the liquid pressure upon it will show much higher readings on the pressure gauge when the liquid gets hot.
Bladder expansion tanks have a pipe fitting at one end and a Schrader valve at the other for charging with air. An expansion tank is best installed with the bladder up and the inlet down, so if the bladder leaks the air will not be introduced into the SWH system, which can stop fluid circulation and cause a pressure-relief valve to actuate, resulting in a loss of collector-loop fluid. This can require recharging the system.
In expansion tanks that are installed pipe-fitting up or sideways, failed bladders are detected by bleeding the Schrader valve with a thumbnail or small screwdriver; any liquid coming out indicates a failed bladder. A failure in an expansion tank may not be noticeable if the tank is installed with the pipe fitting on the bottom of the tank, since air is still present in the space above the water. A tank installed in this orientation is a good reason to slightly undercharge the air side of the tank.
New expansion tanks have the factory charge pressure listed on the box, typically 12 to 40 psi. Don’t trust this listing when installing or replacing a tank—check the pressure with a tire gauge. A severely undercharged or undersized tank is essentially the same thing—not enough room for the system pressure changes. Installers usually charge the antifreeze liquid pressure at 15 to 25 psi. An expansion tank with 12 psi of air is good for a SWH system charged to 15 psi if you desire a slight undercharge. Expansion tanks charged with 40 psi are typically set up for use on potable water systems with backflow preventers. The preventers are a double check-valve vented to the atmosphere that protect municipal water systems from contamination from branch lines to homes and buildings. If you’re changing out an expansion tank, have a five-gallon bucket handy, since the old tank might be full of fluid.
Watch for part three of this SWH troubleshooting series in an upcoming issue.
Chuck Marken is a Home Power contributing editor and a licensed electrician, plumber/gas-fitter, and HVAC contractor who has been installing, repairing, and servicing SWH and pool systems since 1979. He has taught SWH classes and workshops throughout the United States for Sandia National Laboratories, Solar Energy International, and many other schools and nonprofit organizations.