boiler overfilling

Craig_8

I have a customer with 3 motorized valves on their steam boiler. The valves have not worked the entire time they have lived there so the heat was very uneven. I replaced all the actuators and installed 3 new t-stats. The system worked great for a few days and then they called and said it was banging.

I went there and found the boiler way overfilled. removed 3-5 gallon buckets of water and it once again worked fine.

My theory is that when only one zone is calling, water is collecting in the off zones, causing the feeder to come on. all of the wet returns have check valves on them that should stop the water from backing out into the off zones. I have no way to really test them.

The zone valves DO leak a little. I think thats where my problem lies. They are leaking steam into the piping on the off zones, which condenses and fills the mains with water. However there is no pressure to push this water back into the boiler because zone valve is closed. Then when the boiler shuts off and its pressure decreases, the water in the off zones returns by gravity.

Right now the boiler has a pressuretrol on it that is set to the minimum, but the boiler still gets up to 4 PSI, meaning the water will not return to the boiler by gravity from the off zones untill it is almost 8 feet above the water line. at this point the mains would be full.

My plan is to add a vaporstat and set it at the lowest pressure the system will work at. I hope that this will allow water to return from the off zones by gravity alone.

any thoughts?

Jamie Hall

Pressure

I'd start with the pressure. Definetly a vapourstat, and set it low -- say 12 oz. But while you're at it, make sure the pigtail to the pressuretrol now to be a vapourstat is clear; a pressuretrol should be able to regulate down to 1 or 2 psi. Better yet, use a new red brass pigtail...

If water is backing up on 4 psi, which it will -- no check valve is perfect -- and gets into the mains, it's going to sit there until the zone valve opens. Then it might drain. Might not, too -- just bang like crazy. One might ask: are the mains pitched so that any condensate which does get in there has somewhere to go when the zone valve is closed? If the pitch is to a drip at the far end, well and good. If they pitch back towards the zone valve, you should have a drip just downstream of the zone valve so that that condensate can drain out and not be held back by the zone valve.

I am not in favour of check valves on wet returns when working at low pressure. Much better to let gravity do the work. It's goof proof, doesn't leak, works every time...

Anyway, start with the pressure thing.

Craig_8

The mains have good pitch towards the ends where they are then vented and dripped into the wet return

Big Ed_4

Steam Zone Valve drips

There should be drips just after the zone valves to drain trapped condensate on a proper set up. Wet returns or traps on the return. Remove the flappers in the checks.