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!
Comments
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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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