Boiler Expansion Tank Problems: Waterlogged or Failed

Read the signs

The relief valve. Weil-McLain’s EG and PEG user manual lists frequent water from the relief valve with two tank causes, too small or flooded, and says call a qualified technician for both. Amtrol says a relief valve that drips after the boiler is restarted may mean the tank is wrongly charged or too small.

The water going in. Peerless lists four causes of a hot water boiler overfilling or taking too much fresh water. They are leaks, the tank’s charge pressure not right, a waterlogged tank or a failed diaphragm or bladder, and a fill valve not working or set too high, which makes the relief valve weep. Weil-McLain’s CGa manual says an undersized tank loses water through the relief valve while the fill valve adds more, and that boiler section failure can result.

The gauge. Bell and Gossett says a tank that is too small generally gives a higher pressure than desired once the system is hot. Reading it: boiler pressure.

How the tank is checked

Off the system and empty. Amtrol says check the charge with the tank isolated and empty of water, and Watts says a tire gauge only reads its tank’s air correctly once the tank is disconnected. Amtrol adds that checking lets a little air out, so the charge is restored each time.

What it should read. Amtrol says the charge matches the boiler fill valve’s setting, and Bell and Gossett says adjust it to equal the system’s fill pressure. Taco’s 007e sheet says that above 5,000 feet the fill pressure is at least 20 psi, with the tank’s charge raised to match.

What else is checked. Weil-McLain’s Aqua Plus manual adds failed gaskets or bladders, or a faulty Schrader valve, the air valve on the tank. Its technician’s test is to turn the boiler limit up and run the system hotter: if the relief valve lifts or the pressure climbs a lot, the tank is flooded or too small. Watts also checks the gauge itself, a fill valve letting water through, and water getting in from a leaking tankless coil.

Too small, or a plain tank losing its air

Too small. Bell and Gossett calls a tank that leaves the pressure high when hot a Summer/Winter installation, with the boiler always at temperature, and says the tank should be tagged to show it. Burnham’s K2 manual says antifreeze expands about 1.2 times as much as water, so a tank on an antifreeze system should be at least 1.2 times bigger. Weil-McLain’s Aqua Plus manual says the boiler water inside an Aqua Plus must be counted when sizing the tank.

A closed type tank. Weil-McLain’s Ultra manual says never fit automatic air vents on a closed type tank system: the air must stay in the system and return to the tank as its cushion, and an automatic vent lets it out and waterlogs the tank.

A leak, rust or a discolored cap

Every tank leaks in the end. Bell and Gossett says this tank, as all expansion tanks, will eventually leak, so it goes where a leak will not cause damage. Amtrol says a drip pan to a drain is needed wherever a leak could cause damage.

The InSight cap. Amtrol says the cap tip stays clear or white in normal use and turns discolored when the tank needs replacing. If it is discolored, contact the installer or a plumbing professional, and Amtrol says not to rely on the cap alone.

Replace, do not adjust. Peerless’s fix for a waterlogged tank or a failed diaphragm or bladder is a new tank, and Amtrol says replace a corroded or damaged tank rather than adjust its charge. Bell and Gossett says a damaged tank must be replaced.

The other expansion tank, on the cold water

A different tank. Amtrol says its Extrol is for closed heating systems only, never domestic water, where it uses a Therm-X-Trol instead.

When one is needed. Weil-McLain’s Aqua Plus manual says a backflow preventer, check valve or pressure reducing valve on the cold water into a water heater means an expansion tank must be fitted on the cold supply. Without it, normal expansion keeps forcing open the T and P relief valve. Above 70 psi supply pressure, it says a pressure reducing valve and an expansion tank are needed.

The overnight weep. The Aqua Plus manual says the relief valve may weep from thermal expansion during long spells with no hot water used, such as overnight, and Utica says repeated discharge can be thermal expansion or a faulty valve. More in a water heater relief valve leaking.