Boiler pressure constantly rises to/past 30psi relief valve trigger
Homeowner here. We have an older Buderus Logana boiler used for one heating zone and one indirect water heater.
Everything seemed to be operating fine besides one annoyance. There was constantly air at one of the radiators which had an old taco automatic bleed valve that I always kept closed and bled manually when it got bad enough that this radiator wouldn’t heat. Between that and the screw to bleed being stripped, I had enough and decided to change the valve.
After changing the valve and bleeding all the air from all the radiators, everything seemed way better. No more noises and radiators felt warmer.
But later that day, I heard a pop from the basement and went to find the pressure relief valve had dumped water on the floor.
Since then the pressure would ‘t stay stable. Sometimes it took longer to rise and sometimes it rose quickly.
Researching pointed me to the issue likely being the pressure reducing valve for the feed or the expansion tank. I picked up both parts plus a new pressure relief valve which I read should be changed too after triggering.
Started with the expansion tank. It was indeed water-logged. Installed the new one and the problem still occurred. Seems it was a symptom and not the cause.
Then I changed the reducing valve. Again, no change.
I did bleed all air out of the system as needed following any work.
Next I tried isolating the indirect water heater on the off chance there was a leak inside there. Didn’t help either.
I’m completely out of ideas now. There’s no leaks and no air to bleed, and the reducing valve and expansion tank are new.
It seems like somehow once it was running better without air in the system it started building pressure. Or that is just coincidence, I don’t know.
I never monitored pressure before this, so I don’t know what normal was for it before. But it never triggered the pressure relief valve so I assume it was staying below max.
The reducing valve came preset to 15psi, which I left. The expansion tank came precharged to 12psi so I increased it to 15psi before installing. I read that 12psi is good for residential but there’s no gauge by the reducer so I wasn’t going to mess with trying to lower it. Also, in the 6 years we’ve been here that was never an issue.
Any ideas? It’s cold in here!
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What is the size of your new expansion tank?
So with the prior situation, that radiator was acting as a large expansion tank.
What is the pressure of your system when everything is cold?
How many stories is the home? Do you know vertical ft difference between top radiator and pressure reducing valve/ exp tank?
Are you able to reduce your pressure reducing valve to 10-12psi (and exp tank accordingly) or valve it off completely?
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Expansion tank is 4.4 gallons. It’s a single floor home, and the distance between the top radiator and expansion tank is only around 4 ft or so.
I am able to valve the reducing valve completely. I tried that prior to replacing parts and it didn’t seem to help.
I didn’t try reducing the incoming pressure just because there is no gauge immediately after it so I feel kind of blind.
After leaving everything off all night, the pressure was sitting at 15psi on the boiler, so it seems the new reducing valve/expansion tank are working as expected, right?
Not sure if it is relevant, but the expansion tank does not have an air vent above it. Doesn’t seem relevant since I bled the air out and new air isn’t coming in. Not sure about how it is mounted also. I did tee a pressure gauge between the shut-off valve at the tank and the tank itself to help gove some visibiliry. Not sure if that is good enough at that location to trust for reducing the feed to 12psi.
Attached a pic. The pressure reducing valve is just a bit to the left on the rear high pipe by the wall that leads to the tank.
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It could be that the expansion tank is simply too small for your water volume and delta T. As john said, if you routinely had air in the radiators before, that extra air volume was also acting as additional expansion volume.
Now that you've eliminated all air, there's only your relatively small expansion tank to accept the increased volume. And if you have a large water volume as is typically in gravity conversion systems with big old cast iron radiators, that affects the needed tank size. Also, it depends on your water delta T. The bigger your delta T, the more expansion volume you need.
Here's an online sizing calculator that you can play around with. Your biggest unknown will be water volume. For comparison, in our 100-yr-old house in the Boston area with 2400 sq ft and 10 cast iron radiators on 2 floors, I figure we have around 100-150 gallons of water volume.
When your house was built, it probably had the old-style steel compression tank with no diaphragm. Those have to be roughly half-full of water and half-full of air to work right. So only half the tank volume is free to accept expanding water, and you can only use a small fraction of that free volume for expansion before you start hitting the 30 psi relief valve pressure. So the old steel compression tanks had to be fairly large.
The newer diaphragm tanks are more difficult and more expensive to make, so they make them as small as possible. Also, since the diaphragm tank is designed to be empty of water when the system is cold, in theory it can be roughly half the size of the old steel tank. So it would be interesting to know the size of your original steel compression tank. I'm guessing your new diaphragm tanks have been smaller than half of the original steel tank size.
So you may just need to go up one tank size.
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Thanks! You guys are good.
It really does sound like I need a bigger or second tank. Guess I should see if I can manage to do the math as best I can.
I suppose I should also try to reduce the incoming 15psi to 12 psi. Best I can do is try using the measurement on the gauge that I tee’d in by the tank. Do you think that is accurate enough?
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That gauge is good enough. Try to reduce the water side even to 10, since you only have one floor above. Then reduce the air side to 10. That should buy you some margin.
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maybe check the precharge in the tank again. i wouldn't worry about reducing it when you reduce the system pressure. it won't change the amount of expansion it can accommodate significantly, it will just increase the pressure where it starts a bit.
what type of emitters do you have? is it copper fin tube? cast iron radiators? convectors? radiant floor?
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Sometimes the bladder is stuck to the bottom port on a new tank and won't allow expansion. You can remove the tank and push on it with a wooden dowel to see.
Could also be one of your pressure gauges is off and either the boiler feed is set higher or the air fill is incorrect. I would cross check both with a 2nd gauge set.
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Thanks all. I just removed the expansion tank and plugged the tee it was connected to (see my pic a few posts above) and after purging a little water and letting it refill the gauge I have there said 11psi. The gauge on the boiler said 15 but also would have been slightly warm (but not warm enough to register above the 60 degree minimum reading). The reducing valve was supposed to be preset to 15psi. Could it really be losing 4psi in like 6 ft of 1/2” copper and 4 90 degree turns, or is the gauge off?
In any case, I wanted to get the feed pressure to 12psi so I decided to aim for 8psi on that gauge since the supposed 15psi feed from the reducing valve registered at 11psi there. Is that logical? Got it there with about 1-3/4 turns.
Then I dropped the expansion tank to 12psi before reinstalling.
I left the heat off and turned the hot water level down a bit on the indirect water heater then let the boiler run to heat the water, hoping to be able to take showers at least.
After getting to temp (it stopped running at 160) , boiler pressure shows 20 and the gauge by the expansion tank shows 15. Ran the hit water until it came back on and it remained the same. So I think I’m ok for hit water at least.
I probably won’t attempt heat tonight. I have a feeling that will still push it too high.
I’m thinking Monday I’ll get a bigger expansion tank, though I can only probably fit the ex60 which apparently only absorbs 0.2 gallons more than the ex30 so I’m doubting that will cut it (haven’t attempted any math yet). But at the moment I haven’t found a reliable hvac person to repipe anything and I unfortunately have no experience sweating a pipe.
One additional question is what should normal temp range be when running full load? My boiler seems to hit highs of around 220, which seems like a lot. Wondering if I can/should lower it to also help keep pressure down while I look into getting this all done properly? If I can, how is that controlled? I could not find any info on my old seemingly uncommon Buderus model.
Thanks again for all the info and advice!
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Hi, This isn't answering the present question, but reinforces what @Kaos said. Stephen Minnich's book has this fun gauge, showing about six inches of water column even though he's holding it in his hand. Hmmm 🤔
Yours, Larry
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gauges don't agree. if there is no flow and it is a closed system there is no pressure change
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try taking the pressure off the air side of the tank and see if you can push the bladder
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I just had the thought that I probably got some air in the system while testing the incremental reduction to 12psi on the feed plus new very cold water in the system. So maybe that is why it seems to be doing ok with water heating only. The rest ofnthe system is cold plus there’s air. I’m thinking the pressure might climb quickly once I start up the heat and bleed all the air.
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@person56 said:
"One additional question is what should normal temp range be when running full load? My boiler seems to hit highs of around 220, which seems like a lot. "
Say what? You do know that water boils at 212 degrees at atmospheric pressure, right? 220 is dangerously close to becoming a steam boiler bomb. The only reason your boiler water hasn't turned to steam already is the slightly higher pressure.
So I think we found your problem, or at least one of them. Your water temperature is way too high. Your aquastat should be set for 180 max as a safety. And you can set it lower if you still get adequate heat at lower temps. The lower the max temp, the less expansion volume you need.
On the subject of expansion tank sizing, what matters more than acceptance volume is the free air volume at max acceptance. The EX-30 has only about 2 gallons of free air volume at max acceptance, while the EX-60 has 4 gallons of free air volume at roughly the same max acceptance. The doubled free air volume means the max pressure in the EX-60 will be much lower for the same acceptance.
But fix your aquastat setting.
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One old cast iron radiator and the rest are copper fin.
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Pardon my ignorance here, but do you mean an aquastat on the boiler? There’s a temp control knob on the indirect water heater (not temp markings, just blue to res) but nothing on the boiler, which is why I asked how that would be adjusted.
Also, I don’t recall when I last saw that 220 temp and if it was frequent, but right now it’s at 195 after turning the heat back on and bleeding. Pressure is at 24 at the boiler and showing 18 at the expansion tank.
Better than before but I’m not getting my hopes up because there were times it seemed ok before too. I won’t be leaving it on overnight. I expect to need more changes than the slight reduction in feed pressure. Sounds like temp adjustment is next. And I will try the ex60 tank this coming week.
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@person56 yes, there's typically an aquastat on the boiler for safety to limit the max water temperature. I'm not a heating pro, but if you can post a pic of your boiler, one of the pros here will tell you where the aquastat should go and how to set it up.
But until you can get the temperature problem solved, a bigger tank should stop the relief valve popping.
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I just checked the temp now after running the heat a bit and it’s at almost 200. Still not sure how to limit it though. The dial on the water indirect water heater is less than halfway. There is no obvious adjustment on the boiler. The only digital display it has is a basic one line display that shows run times and any faults, which is only on when it is firing.
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Take a picture of the boiler and post it. @EdTheHeaterMan will know if/where the aquastat is, or where to put one. At the very simplest, you could add a strap-on adjustable aquastat to the supply pipe coming out of the boiler that would cut power when the supply water temp hits the adjustable setpoint. But I would assume the boiler itself must have provision for an aquastat to be mounted on it somewhere.
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See attached pics.
Thanks again to all for such valuable input.
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Hi, That grey box that has the upside down Honeywell… could you get a photo of that, with the cover removed? 😊
Yours, Larry
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You have to check the expansion tank pressure when the tank has no water pressure on the bladder. Shut the isolation valve and remove the gauge to do this set the tank at 15psi.
Fill and bleed the system and leave the MU water shut off for now until things settle down. Thake the cover off the aquastat and looks for a dial marled " high limit" set it to 180 or 190 degrees or low enough so the thermometer stays below 190.
If the boiler doesn't shut off at that temp the aqustat is bad most likely or the probe is not pushed into the well.
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How did I not think to just relieve pressure by removing the gauge! I did the extra work of removing the tank to adjust it yesterday. Never again!
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I think you guys found the problem!
This thing is filthy but after cleaning as best I can it looks like it is set properly but not functioning properly.
What’s involved in installing a new one? Obviously the wiring. But how does it attach with the probe to the boiler? What prep is needed if it has to be removed and replaced?
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I'm not the expert, so don't do anything without further advice from the actual experts. But I think you have this aquastat:
https://www.supplyhouse.com/Resideo-L8124A1007-High-Limit-Protection-Low-Limit-Circulator-Triple-Aquastat-Relay-High-10-DegreeF-Low-Limit-10-25-Degree-Adj-Differential
If you open that link and click on the third thumbnail pic in the lower left, you'll see it has the same innards as yours, as far as I can see. Your boiler will have an immersion well behind the existing aquastat that the new aquastat's temperature probe will fit into.
But don't order anything until someone who is a heating pro tells you it's the right (or wrong) aquastat.
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I would try turning the high limit down a bit to see if it just has an offset. This way you can have heat while waiting on the new part.
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I thought exactly that, figured it can’t hurt to try and shouldn’t make it worse if it’s not functioning anyhow. I think I will try it.
So far with running it a couple of hours last night and then a few hours today, it seems it averages around 190 when working a bit and maybe hit max of like 193. Sometimes it’s down lower too. But it’s not working as hard as it was last week because it’s warmed up a bit around here. Next few days are supposed to hit 40’s-50’s too, so I should be able to limp by as needed.
Pressure has stuck around 24psi even at 193, so I’m thinking I might not need to bother with the bigger expansion tank, though maybe I’ll do that later for some extra piece of mind.
Unfortunately, I was so focused on pressure that I have no memory of when or how often it was hitting those more extreme temps of like 220. So it’s hard to know how much babysitting I need to do if I’m using it. I don’t want to hit a high pressure situation and blow up my new expansion tank or relief valve and have to rebuy them.
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Yeah that looks like it could be it. It doesn’t match the layout 100% but it’s very close so I’d guess it’s just a slightly updated revision.
But damn, it’s more expensive than what was coming up as I was looking before!
Edit: Seems like there are at least a few models that look like that. I’m not sure what the differences are. I suppose there must be some specs that matter and those are the ones to confirm are good on any replacement?
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UPDATE:
Seem that the aquastat does work, but is inaccurate. I can feel the click if I move it enough in wither direction, and it does trigger start/shutdown.
But I can’t seem to figure out if the inaccuracy is consistent. When I first tried it, I had to turn it down around 40 degrees to get it to cut off.
But since then, I’ve been trying to dial it in for a resulting temp of 180 at the boiler gauge, and it seems to be acting as if it’s more in the order of 10-12 degrees off.
All that said, it had hit that 220 number at least once during the problem period, so that would indicate it being off a lot.
Is it possible the issue is the probe? And if so, is that replaceable on its own? Or is it better to say this whole unit is very old and it’s best to just replace and be done?
Last question is would you be comfortable running this thing with the dial turned down about 10 degrees from when this all started? I’ve been running it all day and before I even touched the dial it was not going past ~191. But I know it went to around 200 just after midnight last night because I have a picture from just before I shut it down for the night.
I’d love to just leave it on overnight if it is safe. Have to try to get back to normal life. I can work from home tomorrow to check it throughout the day again.
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I would run it 25 degrees lower than it was set to initially. And keep an eye on where that leaves you. Then dial it up from there if needed.
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So, earlier on it was looking like the dial is about 20 degrees off. But in my last few cycles, I had it set at 165 and this what what the boiler gauge shows when it cuts off (seems 10 degrees off from the setting):
I’m leaving it running as is for tonight and tomorrow I will try to inch it up and see if it will consistently cut at 180.
Pressure wise, I think it’s going to be at 23psi when running at 180 degrees. Is that ok or should I still go for the ex60 for that bit of extra relief?
And finally, if the tweaking the aquastat to reach 180 tomorrow seems to hold through many cycles throughout the day, amd maybe a couple of checks on subsequent days, do I then say ok all good I’m done, or should I be concerned this thing might be on it’s way out and consider replacing it now anyway? There was that one time I saw the temp hit close to 220 in the midst of the issue, and earlier today when I made my first adjustment, it seemed further off than now.
One final thought is that none of these parts have been touched or adjusted in the 6 yrs I’ve owned the house. But until now, I’ve never been aware of temps and pressures it was running at, so I have no baseline for any of this.
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To change the aqustat look behind it there should be a screw between the backplate of the control and the boiler jacket that mounts the control to the sensing well. You do not have to drain the boiler. Loosen the clamp and pull the control and sensing well out.
When you put the new sensing well in make sure it hits bottom of the well.
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The 10 degree difference between the aquastat setting and the gauge is close enough. If you wanted to make a science project out of it, you could wrap some black electrical tape around the supply pipe where it comes out of the boiler, and shoot the black tape with a cheap IR thermometer to see what the actual supply water temp is, but it's likely close enough to the 165-175 range to not worry about.
There's no magic in the standard 180 degree limit. It simply gives adequate margin against boiling the water inside the boiler. But as you've learned, the higher your high limit, the higher your max pressure. So if you want to keep your pressure low, try keeping the aquastat setting where it is at 165 and live with it for a while. If it heats the house adequately, there's no reason to raise it. The only reason you'd need it higher is if you don't have enough baseboard radiator length to heat the house during super-cold weather, which means you'd need higher water temperature to get a higher BTU/hr output from the baseboards.
Then as long as your max pressure stays around 23, that's probably enough margin that you don't need to worry about upsizing the tank.
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Thanks, this is helpful.
It got much warmer around here starting yesterday and it looks like we won’t be seeing the lows we were experiencing prior. Given the time of year, I’m not sure if I’ll get another opportunity to see how adequate the current boiler temp is.
I’m thinking I will just tweak it closer to 180 and call it done, then check it a bit through the next few says to make sure it is staying consistent.
The one thing I’m still confused about it how the aquastat allowed it to run up to nearly 220 before I got everything in order. It was originally set to 185. So I’d expect or to have cut off at 195-200 based on current experience. However, at the time I didn’t know what to look for and wasn’t thinking about temp cut-offs or much about temp at all, so could it be possible it did cut-off and the temp continued to rise a bit after the cut-off in a high pressure situation? I don’t recall if it was actually still running when I observed that temp.
Lastly, I think I will leave the tank for now but consider stepping up later. I would like to have the extra leeway if I run into a future high pressure issue. But at this point I want a break from all this “fun”. When I do the bigger tank I’ll need to work out some support for it and it’s a tight space and hard to reach so that part won’t be fun.
Couldn’t I also potentially just reduce the feed pressure a bit more to add leeway? It’s a single floor home, so it’d probably do fine with less. Could even do that and the bigger tank for a really warm and fuzzy feeling I guess. But doing nothing at this point might be more likely…
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The contacts in the electricals may have gotten oxidized and stopped working. Then when you twiddled the dials, you may have broken through the oxidation and re-made the electrical contact.
Yes, you can also reduce the water pressure. For only a 4 foot lift you can get away with under 10 psi, but the less air pressure you start with, the less you can afford to lose by normal leakage. So check it at least annually and add air as needed.
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I think the reducing valve has a min of 10, so I guess that’d be my low limit if I adjust.
Forgot that I also noticed a “pro” version of the extrol tanks when I was looking for cost/availability of the larger size. The acceptance volume seems to be the same on the 30 pro as the 60 standard. But I know you said above somewhere that the free air volume matters more. So I guess that’s not worth swapping just to save a few bucks and some time to set up a support for it.
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Just wanted to add…at this point I am just kind of thinking out loud and letting my curiosity run wild, making this thread carry on longer than it probably needed to. Hopefully not wearing out my welcome.
It seems like I already hit a point where I can declare the problem solved even though there is room for future improvement if desired. And for that I owe you all a big thanks!
THANKS SO MUCH to everybody that posted such great info and advice! I feel like I learned a ton and saved a lot of money by taking it on myself, which I could not have been confident doing without your help.
Equally important to fixing this issue, I’m no longer completely ignorant about how this system works. That’s a gift that will keep on giving.
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Just FYI, the "pro" EX's have the same volumes as the non-pros of the same number. So a regular 30 has the same volume and acceptance as the 30 pro. The diffference is better materials for longer life.
Then the 60's have 2x the free air volume of the 30's at max acceptance, but the same max acceptance. Yes, it's confusing. Once you figure out the sizing math, you get another star. 😉
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