Boiler Pressure High-Not AutoFill or Expansion-need help

I have a question about my boiler that is confusing the heck out of me. I'm a new home-owner (less than 1 year), and I read everything I could about my problem on here first before posting, but I'm at a road block. Allow me to describe as much as I can (you guys always ask for lots of info) as best as I can:

I have a natural gas boiler (7 years) in the basement of my 2 story home, with radiators on both the first and second floors. The pressure on the system is getting up above 30psi, which is causing the PRV to leak water. I first tried unscrewing the Auto-Fill valve a bit and manually releasing water from the PRV until the pressure got down to 15psi. Few days later and back up above 30psi with a wet basement.

Next I tried the expansion tank. The tank is at about chest level in the basement, and has a schrader valve on the bottom of it. The top of the expansion tank, where it is hooked up to the water pipe, hass a valve (non schrader) that has a small black "cap" on top of it that can be unscrewed. Beneath this there is evidence of water leaking, though I have never seen or felt any evidence of this. Most likely it is an old problem. Checking the schrader valve (bottom) with a tire gauge revealed the pressure was exceedingly low (below 5psi, but no water came out) so with the system warm, I reduced the boiler pressure to 15-18psi and pumped the expansion tank back up to around 12psi. A few days later the system was leaking again, and when I checked the pressure on the schrader valve it registered nearly 25psi on the expansion tank!!! Again I released boiler pressure (through PRV) and also released the expansion tank pressure back down to 12psi.

I have only once heard water enter the system through the Auto-Fill, and that was when I let too much water out through the PRV. However, the pressure on the system keeps on getting way above normal, so my guess is that too much water is getting in. Could it be that (despite my inability to hear it) water is slowly trickling in through the Auto-Fill, and therefore I need a new one? Or is it possible that there is something wrong with the expansion tank? I would think if it were a ruptured bladder that I would get water out the bottom.

I have also noticed that the temperature gets quite low. I thought the boiler was supposed to go on at 140, off at 180, but the temp drops to 90 and 100 degrees when off for a long time. Don't know if that's important or not. As far as I can tell, I've tried about everything simple that you guys typically recommend, so I'm looking for some help as to the next step. Any help you can provide is appreciated!

 

Welcome to homeownership.

Some of your info confused me. You can only set the expansion tank pressure when the system pressure has been relieved. It sounds like an automatic feed that is overfeeding. Somehow too much pressure is getting in there. I would start by setting the pressure in the expansion tank to 12 psi woth the system pressure relieved and see how it reacts from there. If needed, you can replace the auto feed valve next.

Ken

 

One way to be sure the fill valve is the culprit is: After dropping the boiler pressure to zero & adjusting the air in the tank, you need to get the system pressure back up to +/- 12 psi. Next shut off the valve either ahead of or downstream of the fill valve (hopefully you have one or both). If the pressure then remains stable, re-open the valve(s) you closed. If the pressure now goes up, there is a very strong probability the fill valve is bad & weeping thru.

 

Perhaps I was setting the expansion tank pressure at the wrong time. When I checked it tonight after it had been running for about an hour, the pressure on the boiler was 30psi and the tank again registered over 25psi. I relieved the tank pressure back down to 12, which lowered the system back to 20 or so. I will keep monitoring both over the next few days and see what happens. Thanks for the quick reply, I'll check back in with an update soon!

 

As I just said to Ken, perhaps I had been setting the expansion tank pressure at the improper time. If the problem persists, I will follow your advise to verify the fill valve. Question regarding this, however, you mention dropping the boiler pressure to zero… would this require letting all of the water out of the system through the PRV? or is there another way to relieve pressure. No matter how much water I have relieved the boiler pressure has never dropped below 15psi, and when it does get that low the fill valve starts filling it up again (even with the top unscrewed completely). I'll be checking in again soon with an update, if you can advise on my few questions here that would be most appreciated. Thanks to you, and all of the generous people at this site, you really provide a great service to eager people like myself.

How hard would it be to replace the fill valve if I have to go that route? approximate cost for a new one? Thanks again!

 

You may not be making the problems worse but you sure aren't making much headway in nailing them down. First of all, you don't have to take ALL the water out of the system to take all the pressure off. As you noticed, you won't be taking every pound off of the system unless you isolate all the vertical piping above the boiler. That usually isn't practical. That is one reason I think a 15 psi precharge is appropriate. It accounts for the vertical height of a 3 story building and that is the most we usually see in a residential installation. There are exceptions but we are going for general knowledge here. The sequence would be to turn off the incoming water, take the pressure off of the boiler by closing any automatic air vents near the boiler and tripping the relief valve until the flow almost stops. Then pressurize the expansion tank to 15psi. Open the air vents and the auto-feed. If the pressure rises and the relief valve opens again tomorrow, replace the auto-feed valve.

Don't mess with the pressure in the expansion tank when the system is under pressure or you will be chasing your tail.

Ken

 

I will give this a shot either tonight or tomorrow night. Thanks for all the explaining ... Do you know where I would find the air vents you mention? To my knowledge I haven't seen any air vents, but I may not be looking in the right place.

I had also been modifying the expansion tank with the system hot, so that probably hasn't been helping. oops!

 

Hot isn't the problem, pressurized is the problem. The air vents are small cylindrical things usually mounted on top of a pipe or air scoop and usually have a schrader fitting (like a tire has) and it should have a cap sitting loosely on top. The cap is there so you can close it if it leaks water or when you don't want to admit air to the system on a drainout.

Ken

 

Thanks again for the updates. Here's what I tried for now:

- Shut off manual feed valve.
- Released all the pressure from system and pumped expansion tank up to 12-15psi.
- Slowly opened manual feed valve letting in water until pressure reached 15psi. Shut manual feed.

I have left it like this for a few days, trying to see if any leaks occur, and so far so good. The pressure is somewhat frequently (after long periods of downtime) getting below 5-10psi, is this normal? I also noticed that even when the pressure was up to 15psi+, if I opened the manual feed the autofill was still letting the water through, even with the top completely screwed in tight, or with it completely unscrewwed... is this a sign of a broken autofill?

Thanks again for the help!

 

It sounds like you have it set right at this point. Your auto-fill valve is probably part of the problem. You would want to unscrew the top to reduce the setpoint pressure. The system pressure may drop when the water is at its coolest. There is a possibility that the expansion tank is undersized for the water volume too but that usually shows up soon after installation. You should probably keep the auto-fill closed off and replace it to be safe sometime when it is convenient. Otherwise, keep an eye on everything and post back if you have any more questions..

Ken

 

So everything seems to be working fine, manual feed is shut off and the system is topping out at 18-20psi. I think I figured out why the pressure was rising though, and I'm not sure it's a faulty Auto-Fill… here's my theory:

I have an automatic thermostat, so the boiler is off for extended periods at night, and while at work. During these long downtimes, the pressure drops well below 10psi and even to near zero. Well when the auto-fill senses this, it does its job, and adds water to keep the system above 12-15psi. Then when it kicks into high gear, there's too much water=too much pressure. Does this sound accurate? Is there anything else I can/should do to keep the system stabalized other than keeping the manual feed shut off? Is the pressure NOT supposed to get below 5psi? and why doesn't this happen to everyone?

Just some last questions, trying to fully understand the problem. Thanks again for all your advice, I've already been able to help someone else at work with their boiler problem thanks to my new found knowledge!

Thanks again,
Mike

 

You are right about the feed valve adding water. The other part of the pressure control team is the expansion tank. If yours is undersized or not functioning, that is the cause of the pressure change. Water expands and needs some place to go. The expansion tank is where it goes. And the air pressure in the tank pushes back on the water. When the water cools, the expansion tank pushes that water right back into the system. That way the auto feed doesn't kick in to try to help correct the pressure drop. If the air pocket in the tank is too small, the amount of water it can accept is reduced and although it looks normal, it can only take a small amount of water before increasing its own air pressure to the point where the relief valve opens. Then when the system cools, the expansion tank can only give back as much water as it has air to push it. That may be only a small amount and then pressure drops and the feeder kicks in. Hopefully you can visualize this and see why it is often difficult to know which one is at fault. Sometimes feeders overfeed and sometimes expansion tanks lose their air. I'm sure your problem is one or the other and I am leaning toward the expansion tank at this point.

Ken