Boiler pressure creeping up - any ideas?

amin1992

Hey guys, I appreciate your help here. I know it's a long shot but just wanted to see if you had any other ideas. I've got a 1976 Crown cast iron oil fired boiler, with two zones of hydronic baseboard heat via copper pipe.

The boiler heats up to 180F, and drops as low as 145F, but pressure stays pretty solid at 15 to 16 PSI.

Anyway, I was in the utility closet earlier this week and noticed that the pressure was much higher, at 21 PSI. Thought that quite odd. I drained about a gallon out of the system, pressure dropped back to 15 PSI.

Next morning, it's back up at 20 PSI... I drain another 2 gallons out, back down to 15 PSI.

I check it 2 days later and it's at about 19 PSI... just keeps creeping up. I figure 3 times is not just some fluke. I drained out 2 gallons, got it down to 15 PSI, and so far over the last 24 hours it's stayed at 15-16 PSI...

It's a simple setup, so I figure there's only so many ways the pressure could go up. Here's what I've checked...

1. It's got an old original bladderless expansion tank. I know a tech drained and refilled it maybe a year ago, and when knocking on it I can hear the difference in pitch between top and bottom, so I really don't think it's that.

2. The fill valve fills from the domestic hot water into the hydronic system, and it's just an old gate valve. I opened and closed that a few times to make sure it is properly seated. Doesn't sound like water is moving/leaking through that to slowly add water to the system...

3. It does have a domestic internal coil. I hope and pray that that doesn't have a pinhole leak as I think that'd be the end of this boiler. I ran a test though - me and the wife didn't use water for almost 3 hours, and the water meter didn't budge. The way this has been over pressuring every day or two, I would have thought that we'd see some movement if it was in fact an internal coil leak...

4. My only other thought is the fact that, for whatever reason, we are constantly purging the zones because air gets in. I've since given up with trying to eliminate whatever the air source is and just live with the gurgles here and there. However, it's been over a year since we last purged it and we usually purge it twice a year. Is it possible air in the system could cause jumps in pressure? The thing is, the air isn't causing any blockages, as I can feel the hot water moving from beginning to end in both zones.

I'm stumped at this point. Part of me wishes this just goes away but I wont' be able to sleep until I figure it out - hate just waiting for it to fail. Appreciate the input you guys may have.

EdTheHeaterMan

This is a classic example of air migration from the expansion tank to the system. That will get you thinking... How does the air go from the top of the expansion tank DOWN the pipe at the bottom and show up in the baseboards, or radiators, or connectors? Well, it might be witchcraft or magic, but it is actually physics. You would have to read up on Henry's laws and Boyle's laws on the physics of water to completely understand the process. The basics are this: Air molecules are trapped in the water, just like water molecules are trapped in the air. We know the latter as relative humidity. The reason we call it RELATIVE is that it changes with air temperature and pressure. Thats how the weather goes.

So let's call the same thing happening in water "RELATIVE DRYITITY". or not...
Anyway as the water temperature and pressure change the ability for the water to hold air in suspension changes, and the way your system is set up (as many other systems are ) is bass backward. The design is absorbing the air from the expansion tank, traveling thru the pipes, and depositing the air in the heating elements. That is why you need to purge the air occasionally. When you purge the air you are replacing the missing air with water.

So why the pressure rise? With less air in the expansion tank, the normal water pressure at the heating system is 12 PSI That happens when everything is off. no hot water usage and no thermostats are calling for heat. After several hours of this condition, you get 12 PSI. If you get less than 12 PSI the autofill will add water to get to 12 PSI. This happens over hours of time in very small increments of water. You probably checked your water meter for 3 hours when the system was heating up. You would need to check it when the system is cooling down for several hours.

As the home heats up during the colder months the water expands and the pressure rises. With less air in the expansion than, there is less room for expansion, so the water pressure gets higher. No big deal, You can add more air to the expansion tank and the system will go back to normal. You can do this yourself. Check out this post from last year. https://forum.heatinghelp.com/discussion/comment/1620527#Comment_1620527

Jamie Hall

"an old gate valve". Right. That's the first place I'd look. It may feel seated, but... it wouldn't take much of a leak through that valve to produce what you are seeing, and I doubt very much that you'd hear it. And old gate valves rarely really seal tight. Do you have any other valve on that line?

If the old compression tank has air in it, it's working. That's one of the beauties of those things. However, the air which gets into your system should be going into that tank. Has anything been done to the plumbing and fittings in the vicinity of that tank?

EdTheHeaterMan

Jamie Hall said:

If the old compression tank has air in it, it's working. That's one of the beauties of those things. However, the air which gets into your system should be going into that tank. Has anything been done to the plumbing and fittings in the vicinity of that tank?

Agree with you, Jamie. In a perfect world, all the boiler installers would design this way. You would not believe how many are not. With the "regular" purging of air from the loops, I bet you that air is coming from the expansion tank.

I could be wrong.

Naa, forget that last sentence.

delta T

Assuming this started all of a sudden, and has been fine up until now, my money is on the gate valve. We don't know if the tank is piped correctly or not, but the fact that this is a new problem points to the gate valve in my opinion.

amin1992

Thank you all for the awesome explanations and suggestions. Makes me happy knowing it probably isn't the coil as I figured that would be the death blow to this boiler.

I tried quickly opening and closing the gate valve to reseat it again. Will keep an eye on the system in the mean time. I don't have an auto fill actually - just the gate valve to let water into the boiler.

I will keep an eye on this and post more as I gain more data. Does it seem more realistic that the gate valve is leaking, rather than the domestic coil?

Ed, I am a chemist by trade so I enjoyed all the science and Rel Humidity talk haha!

HVACNUT

Drain the expansion tank. 

EdTheHeaterMan

With no auto feed... You might be looking at the domestic hot water coil leak. The test for that is to set the pressure and close the valve to the domestic hot water coil. This means you won't have hot water for the duration of the test. so you may want to ask your wife if that is OK. She may want to stay at a romantic hotel for the weekend

You can stay with her too (if she will have you) because you don't need to watch the boiler for the entire test. Leaving the home for a half-hour won't contaminate the test results.

Mr.Ed

amin1992

Thanks guys for more tips. This morning I noticed the pressure was about 4 psi higher than it had been. Took 3 days to get that way though with me checking 3 times a day... Sadly can't go without water that long to test this with the coil. 

I drained a gallon and got it back to normal pressure. 

I'd like to call my boiler guy in to take a look. However, not sure what he can do more than I am doing. What do you guys think? 

MaxMercy

Since it's such a slow symptom, sometimes you just have to throw parts at it (which I hate to do). What I would do is replace the gate valve with a feed/fill valve like a Taco 329. Even if that doesn't fix it, I don't see that as a waste of money and it's not that expensive anyway. If you don't want to go to an autofill, replace your gate valve with a ball valve.

If that doesn't fix it, that seems to leave the coil unless someone else has another idea.

Jamie Hall

That is so slow that I would definitely put money into a new ball valve in place of that gate valve.

amin1992

Thanks guys, I will look into doing that first and go from there. Appreciate the help and will post back when I have updates

Bronxtech

If you do put in an auto fill valve, give it a cool drink. Hot water supply is for steam, not hydronic systems. Repipe to cold water.
Ball valves before and after , saves on svce $ in future. Check valve or backflow preventer would be nice as well. Drain compression tank as well. Good luck

Jamie Hall

Backflow preventer -- often reduced pressure zone backflow preventer -- is required by most codes.

amin1992

Thanks guys for the tips. Just got off the phone with my boiler guy and he is coming tomorrow morning. Plan is to replace the old gate valve with a Watts auto feed, and to drain and refill the expansion tank.

Either way, I suppose that I'll have some answers within the week. If 5+ days go by after this repair and boiler pressure stops rising, I can assume one of them was the original cause. If it continues, it's the coil, though I will add that my boiler guy seriously doubted it. He said in his experience, domestic coil leak would result in a more rapid pressure increase (ie, in hours) in most cases, and if it were a pinhole leak, would still over pressure within a day or two. My boiler takes 3 days to go from 15 to 20 PSI give or take, so I assume that would mean an overpressure would take at least 9 or 10 days. Guess we'll see!

MaxMercy

amin1992 said:

Thanks guys for the tips. Just got off the phone with my boiler guy and he is coming tomorrow morning. Plan is to replace the old gate valve with a Watts auto feed, and to drain and refill the expansion tank.

I would also add a ball valve in front of the Watts valve.

amin1992

Thanks for the tip. Not sure if it matters but there is already a ball valve on either side of the coil, which is what feeds the gate vlaave currently 

Bronxtech

Isolation valves before and after that way you don't have to drop the system pressure to svce feed valve and backflow in future .
Savings for homeowner , easy work for tech 👍👍🤗🤗

amin1992

Thanks all for your help so far. Wanted to give an update.

Boiler guy came today and installed a Watts auto feed valve along with the valves on either side to isolate it. He also noted that the expansion tank sounded pretty full of water, just a little bit of air inside, so he drained that and refilled it - it only had a plug so he installed an actual valve on it instead to make draining it next time easier. Also replaced the pressure gauge and purged system while in there.

My guy is great but the only dilemma I have is he likes shark bites. Both ball valves on either side of the auto fed are press fit ball valves. Seem solid and no leaks.

He has soldered stuff for me in the past so I have a feeling he used the shark bites in this case because the piping at the boiler is really tight, maybe 4" of clearance between the boiler and pipe, and on other side 4" of clearance between pipe and the exterior wall.

Shark bite says it's good to 200F, and my boiler tops out at 180F so I suppose it's fine. If it were a brand new boiler I'd worry about longterm longevity but this bad boy is already 45 years old so Id bet it dies before a shark bite fails.

Will give it 3 to 4 days and give an update then. Here's to hoping it's fixed!

Bronxtech

If you really wanna have a stroke ,grab the ball valve and twist it around in a circle. !!!
No leaks, it spins !!!
Just sayin,. Hope it's all good now 👍👍

amin1992

Ha the first time I used one of these silly things it twisted on me and freaked me out! Knew nothing about plumbing at the time haha.

Thanks pal, will post back here in the next few days with an update either way. Here's to hoping

toeknee

I once watched a small test done on the various connections among copper, pex tubing, and cpvc where each were fitted with shark bite and their respective conventional connection, then put in the freezer.

The shark bite held well on the pex and cpvc, but it blew off of the copper. You could see where the teeth scraped along the wall of the copper

this is unrelated to your setup and no cause for concern, but thought it was something interesting to bring up while we're all eating popcorn and patiently waiting for the results

@Bronxtech what is the reason for piping the autofill to the cold water rather than hot water? I'm learning

amin1992

I apologize all, I never posted an update. Seems like the boiler guy fixed this problem