How do the expansion tank and automatic fill / reducing valve work together in a hot water heating system? It seems to me that if the expansion tank is charged too high the autofill valve will never be able to autofill the system, whereas if it is charged too lowr, the reducing valve will determine the system pressure. Is that how it's supposed to work? The expansion tank does not determine system pressure, but it should accept the pressure determined by the reducing valve? I don’t really understand how they are meant to work as a system or how to test and maintain it in place.

Note: this was a multi-part question that was edited to focus on the one received answer.

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    Get a helper. Hand him a cell phone or walkie talkie. Give her directions on what to do. Walk up the stairs. Communicate "ready". Wait for him to acknowledge. Do your "upstairs stuff". Communicate "done". Wait for her to acknowledge & do downstairs stuff. Lather, rinse, repeat. Save 6 trips up/down stairs. Alternative - at least you're getting your steps in by doing it all yourself...
    – FreeMan
    Mar 15, 2021 at 13:06

1 Answer 1


The expansion tank (if intact) has a bladder or diaphragm keeping a bubble of air (which acts like a spring) separate from the system water (so the air does not dissolve into it and end up elsewhere, needing to be bled out and rusting iron/steel parts.)

The reducing valve (what I call an autofill valve) adds water to the boiler loop when boiler pressure falls below its setpoint. It does not remove water when its setpoint is exceeded - it only puts water in. For a typical 3 story building, pressure should be 18-25 PSI (1 PSI = 2.3 feet of head of water, roughly. So 18 PSI should vent up to 41 feet above. If your gauge is accurate, you're closer to 53 feet up at 23 PSI. 25 psi will push to 57.5 feet.)

Say your valve is set at 20 PSI, and the boiler is cold, and dropped to 18 PSI. The valve adds water until it's 20 PSI. Now the boiler fires up and becomes hot, and the pressure rises, because water expands as it heats (unless you are very close to freezing, where water is "interesting".) The bubble of air in the expansion tank compresses, so rather than the boiler and pipes quickly rising above 30 PSI and engaging the overpressure release valve, the air in compressed as the water expands and the pressure rises to some value less than 30, if the system is designed and sized correctly. Your boiler runs for a while, and then shuts down and starts to cool - the air in the expansion tank expands as the water in the boiler and pipes cools & shrinks, and if there are no leaks, the system ends up back at 20 PSI (at the boiler - less, the higher you are in the building) and no new water needs to be added.

The "Pre-charge" pressure on the expansion tank is typically just a little bit less than you expect the lowest acceptable boiler pressure to be, so that there's a little bit of water in it at lowest pressure, but room for more as the system heats and pressure rises. If the "Pre-charge" pressure is too high, no water is stored at lower pressures, and the system pressure will drop below fill pressure when the tank is fully empty but system pressure continues to fall as it cools. If the "Pre-charge" pressure is too low, lots of water is stored at low pressures, leaving less room for adding water and compressing air at higher pressures, effectively reducing the size of the tank, and possibly leading to operation of the relief valve as pressure rises too high when heating and expanding system water.

  • The last paragraph of this answer has cleared up a misconception I had, that the expansion tank can be "pumped" to control system pressure. That's quite wrong, and in fact the pumping of the tank I believe must occur when not connected to the system in a way that allows the tank to accommodate the expected range of pressures that the system will present. Good learning.
    – jay613
    Mar 17, 2021 at 12:41

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