Another boiler pressure problem...

[ajaytuque]

I have read several posts about high boiler pressure, and I have the same problem. However, none of the solutions posted have worked for me. Here's my problem:
Three winters ago the PRV began leaking (slow trickle of water) but only when the boiler fired up to heat the water. Recently the amount of expelled water has increased when the boiler fires up (about 3 buckets a day compared to 1 bucket before). A couple of weeks ago I replaced the PRV and the auto-fill valve, and I installed a backflow preventer upstream from the auto-fill valve (the system did not have one before). The boiler pressure remains at a steady 13 psi when cool but rises to 30 psi when firing and causing the water to leak from the PRV. I have bled the system of air (top radiator in a 2-story house) and emptied the expansion tank (a steel drum, no bladder, hanging from the ceiling), which never have much water in it (usually about 1 cup). I have drained the system of water and refilled it. I have reduced the pressure on the auto-fill valve to 11 psi and increased it to 18 psi, but nothing seems to keep the pressure under 30 psi when the boiler fires.

What am I missing? Could the boiler thermostat be bad causing the boiler to over-heat the water? Could the expansion tank be bad and not allow sufficient water to enter to reduce the pressure? I need help! :wall:

[KField]

If you have a regular old expansion tank, I can assure you that you should get a lot more water than a cup out of it. It is probably totally waterlogged and you can't get air back in to let the water out. One way is to use a bucket and no hose. That way air can gurgle back in past the water coming out. Some tanks have a special fitting in them above the drain valve that has a small plug to remove to let air get up to the top of the tank. I'm pretty sure this is the cause of your problem. You can lift on the tnak and use your imagination as to what it should weigh if empty or full. Sometimes they are wedged up there but many times there are just 2 straps like a sling holding thim up. Be sure to isolate the tank from the rest of the system by closing the valve on the pipe leading to the tank before the drain process.

Ken

[ajaytuque]

Thank you very much, Ken. I found a plug on the valve of the expansion tank. When I unscrewed the plug, the water started pouring out of the valve. I drained the entire expansion tank of water and flushed out a bunch of sediment. The tank is now practically empty (it filled a little when I opened the line to it again). I will give the boiler another try and see what happens, but I am confident that this will solve the problem. Thanks again for the insight.

[KField]

Don't worry about the water that went back in. It is supposed to do just that to build an air cushion inthe top of the tank. Good job. If you didn't use any sealer on the small plug, you probably should next time you drain it. If air can leak past the threads on the plug, the tank will slowly fill with water. I'm sure you are OK for many months and possibly years.

Ken