Furnace Question

I have a Weil Mclain boiler. About a week ago, I discovered a puddle under the relief valve drain. I've had these things fail in the past, so I removed it. The old valve had some chunks of rust inside, so I wasn't sure if the valve was actually bad, or if it opened and then couldn't fully close because of the rust.

I replaced the valve with a new one. I also removed the old plumbing from the furnace to the valve and replaced that with new galvanized pieces.

The new valve keeps opening. It drips a little with the furnace running, but occasionally opens about thirty seconds after the burner shuts off. It's a 30 psi valve. Not surprisingly, the pressure gauge reads about 32 psi when it opens.

I understand how relief valves work and why we need them. What I need to figure out is what's causing the overpressure situation.

It's a 12 year old hydro-air system.

Any suggestions?

Check the expansion tank. If (when) the diaphragm leaks and the tank fills with water, then it won't do it's job anymore and you can get pressure issues.

We had a situation where the coils in the water heater tank failed and house water pressure was making its way back into the boiler circuit. Not sure if you have a similar setup for household hot water?

Ian F wrote: Check the expansion tank. If (when) the diaphragm leaks and the tank fills with water, then it won't do it's job anymore and you can get pressure issues.

Ahhh...good suggestion. I hadn't thought of that.

Woody, report back on what you find.

I'm interested in boilers and what not.

Yup, expansion tank is waterlogged. Now I have to figure out if it has failed or just needs adjustment. I've got to get a pressure gauge first.

In reply to Woody:

There should be a tire-type air valve on the end of the tank. Press the core. If water shoots out, then the diaphragm is shot and the tank will need to be replaced. Fortunately, they're not that expensive - $50 or so from Home Depot depending on the size.

The schraeder valve on the tank was leaking. I replaced the valve and reset the pressure in the tank (it's holding), but I continue to get water out the relief valve.

Leaking air? If the air side of the bladder was compressed and you pushed air back into it, where does the displaced water go? You might have to drain down your system about half of the volume of the tank.

At least I think this is correct. You would then have to make sure you get the air out of the water side afterwards too but your air vent should do this.

(Just for my own reference, I was looking at this http://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2012/07/A01500I.pdf Maybe it'll help you too.) I use Bell and Gossett products all the time in my work. They have a lot of information on their website.

It was leaking air. I isolated and drained the tank, hanged the schrader valve, reset the pressure and so far, everything seems to be working fine.

First off it's a boiler not a furnace. Common practice is to change the relief valve, the pressure tank and the 9-11 combo fill valve pressure reducing valve. They all work in concert with each other. The fill valve makes sure there is always at least 14PSI of water in your boiler. The relieve valve makes sure there is never more than 30. The expansion tank gives the water a place to expand and contract as it heats and cools. If the relieve valve went off in the first place, something allowed the pressure to build to 30 PSI. In reality it was probably much higher when it first tripped. The relieve valve is spring loaded holding a rubber seal against a brass seat. It sits stuck in the spot until the pressure is great enough to unseat it. Usually the rubber doesn't separate cleanly. Some of it rips off and sticks to the seat. That means it will continue to drip after it trips. Since you are still getting pressures greater than 14 PSI, you are either getting excess water added to the system or there is no place for it to expand. Most likely it's the tank, so if you want to cheap out just buy a new tank and try that.($35 at the depot) If you continue to build pressure, then you will need to replace the fill valve/pressure reducing valve. If by some chance it continues to gain pressure, and you have a tankless water heater, then you are truely screwed. You have a pinhole in the water coil allowing higher pressure domestic water to leak into the lower pressure boiler water. Then it's time to remove the coil and replace it. Suck A**** job that most plumbers want no part off. One test you can do is shut off the water supply to the boiler fill and see if the pressure remains constant. If it continues to build pressure, its the coil.

Dan Holohan is the best boiler guy around. http://www.heatinghelp.com/Forum

he writes a lot of great articles and has written books too.

http://www.heatinghelp.com/article-categories/15/Dans-Archives