Pressure Relief Valve leaking? Service your bladder type expansion tank!

Old 02-21-12, 07:29 PM
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Pressure Relief Valve leaking? Service your bladder type expansion tank!

Is the pressure relief valve on your boiler leaking or spewing hot water all over the floor? Most of the time the problem is that the expansion tank has lost it's air charge causing uncontrolled pressure increase in the system.

Bladder or diaphragm expansion tanks should have the air checked and/or charged at LEAST every two years as they normally lose 1-2 PSI per year, just like car tires do!

Here is a step by step procedure for checking/charging that tank with air.

NOTE: This procedure does NOT apply to the large steel expansion/compression tanks which are strapped to the ceiling above the boiler, only to the type that look like propane gas grill tanks. There is a completely different procedure (not covered here) for servicing that type of expansion (compression) tank.

The so-called 'proper' way of mounting a diaphragm or bladder tank is HANGING from an 'air scoop'. If the tank is standing on it's head, or mounted sideways, the bit in the following procedure about water coming out the air valve may not not be a definitive indicator of the bladder condition, even if the bladder is broken! If you continue to have problems after following this procedure, the tank may be bad, or there could be one or two other less likely causes (not covered here).

WARNING! A failed expansion tank will likely be FULL OF WATER and will be MUCH HEAVIER THAN EXPECTED! BE PREPARED for this weight, as much as 40 pounds, when removing the old expansion tank! Have a helper ready to assist!

Please also read the following related information:


1. Shut off boiler and allow to cool to under 100F.

2. Shut off water supply line to boiler.

2a. If there are any 'automatic float type' air valves on the system, close the caps on top tightly to prevent them from sucking air IN to the system when you drop the pressure.

3. Drain only enough water from the boiler drain (note: this does not have to be THE boiler drain. It can be ANY drain valve on the system. Use the one that is most likely to be able to not leak when closed again!) to drop the system pressure to ZERO. REPEAT: DO NOT COMPLETELY DRAIN THE BOILER! ONLY ENOUGH TO DROP THE PRESSURE TO ZERO!

4. With an ACCURATE tire pressure gauge, check the air charge in the tank on the air valve opposite the end of the tank that's connected to the system. If ANY water comes out of the air valve, the bladder inside the tank is shot and the tank needs replaced. If no water comes out the air valve, and the pressure is less than 12-15 PSI, continue to step 5. If the pressure is OK, turn the water supply to the boiler back on and repressurize the system, turn the power back on to the boiler, no service is necessary.

5. Using a bicycle pump, or a small air compressor, add air to the tank until you have 12 PSI air charge.

6. Check the boiler pressure gauge again, and if it has risen off ZERO, drain some more water from the boiler drain until it is again at ZERO.

7. Check the air charge on the tank again. If it is below 12 PSI, add air to the tank until it is at 12 PSI.

8. Repeat steps 6 and 7 until the boiler stays at ZERO and the tank stays at 12 PSI. At this point, the tank is properly recharged and the water supply can be turned on to re-pressurize the system, loosen the caps on any air bleeders that you closed in 2a and turn the power on to boiler and return to service.


If you want to make it easier to service the tank in the future, you can add some optional valves when replacing in order to make it a simple 5 minute job to check the air charge. The procedure above need not be followed then:

What this will allow you to do is relieve the pressure on the tank in order to check and charge the air in the tank without having to drain anything from the boiler. It also allows tank change easily when necessary.

With these optional parts installed, simply shut the boiler off, close the new ball valve, put a bucket or a hose on the drain valve and let the pressure off the tank by opening the valve.

Then, leave the drain valve open and check or adjust the air charge on the tank as above.

When finished, close the drain valve, open the ball valve and you're done!

You can make up that optional assembly with black steel threaded fittings, no soldering needed.

Parts you will need :

2 short 1/2" steel nipples
1 1/2" threaded ball valve
1 1/2" tee fitting
1 1/2" boiler drain (1/4 turn type is best)
Some teflon tape or pipe dope.

In place of the above, one could use this valve:

40612 - Webstone 40612 - 1/2" Threaded Pro-Pal Ball Valve w/ Hose Drain

and eliminate most of the above parts. The tank can thread into the valve and then only one short nipple would be needed in addition.

Or a bit fancier, the Watts RBFF unit :

It's a good idea to 'exercise' the valve on this unit. They get 'sticky' in time if you don't. Run it all around a few times each year.

Last edited by NJT; 10-30-15 at 02:43 PM. Reason: keyword relief valve spewing boiler pressure

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