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Adjusting bladder tank pressure while the system is hot

ZosoZoso Posts: 26Member
Everything I have always read or videos I have seen about bladder tanks discusses adjusting the cold fill pressure. I have a situation where I want to make an adjustment while hot. It would seem that there is nothing fundamentally wrong with this concept, but I just wanted to get some thoughts. I have never adjusted one while hot.

The situation is that a client was popping reliefs due to lack of expansion tank volume. The highest points in the system were actually in vacuum due to the low pressure. Although all of the components in the system were rated much higher, the boilers had 30 psig relief valves. I calculated that changing the reliefs to 50 psig would do the trick, but the owner was reluctant. So instead, we installed an additional bladder tank in parallel to the existing. This was over the summer, but they still ran the boiler for reheat.

The system is 50% propylene, which I have argued is too high, but it is what it is. The client was going to change the system glycol in the fall, and then my company (I am an engineer working for a contractor) would adjust pressures and start up the new tank. Unbeknownst to us, the client activated the system after changing the glycol, and now we are of course in the heating season.

When I ran the calculations, I recommended an 18 psig cold fill pressure, and figured that the system would reach around 25 psig when fully expanded at 180 deg. I was in the building the other day for another reason and asked about the system. They are having issues in the same remote location where they have always had a lack of flow. I will be taking a GPM reading, but I have my suspicions. The expansion tank pressure was only 20 psig while hot - not the 25 psig I calculated. The pipes that feed this remote unit are the highest in the building. I am wondering if the lines are air bound, etc.

I can't get a straight answer on the fill pressure they used, so I am wondering if I should add some air pressure to the tanks while hot. i will see if I can get a gauge pressure reading at the highest point also.

Thoughts?
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Comments

  • Jamie HallJamie Hall Posts: 11,458Member
    The main reason I know of for filling cold is that one knows what that pressure should be while filling hot involves some assumptions and odd variables. In your situation I'd say to give it a shot.

    I'd also say that the system as a whole is running much too close to safe margins. Your idea of going to higher pressure relief valves -- still within the rating of the boiler, of course, and raising the system pressure sounds good to me!
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • STEVEusaPASTEVEusaPA Posts: 3,491Member
    If you have the proper size expansion tank(s) the pressure shouldn't vary much at all.
    More important that you're pumping away from the expansion tank and the system has proper air elimination & the system is completely purged of air.
    What's the total height from boiler to top of highest radiation?
    What type of radiation.
    If the boiler is rated to 50 psi, the last thing I would want to try is 50 psi relief valves, not a lot of room for error if they fail closed.
    steve
  • Jolly BodgerJolly Bodger Posts: 146Member
    How High is the tallest piping. This is the static pressure need on the system. Tank pressure should be 2PSI below static pressure.
    25 PSI is equal to 57 ft.
    Going from 18 PSI to 25 PSI sounds like the tanks are undersized or water logged.
  • HomerJSmithHomerJSmith Posts: 718Member
    edited December 4
    I've thought about this for a long time. I have always removed the tank to adjust the pressure, but it is a lot of work especially with a glyco fill. A 50% glyco fill is pretty high and it may require a pump change to accommodate the reduction in flow because of increased viscosity. You did check the glyco with a Refractometer.

    My conclusion on a hot recharge: I use a automobile 120V tire compressor with a gauge. I would want to know the sys fill valve pressure and close the fill valve to off so no sys fill occurs. I would connect the compressor and note the gauge reading. I would put 2 psi of pressure in the X tank and observe the boiler tridicator pressure. If the tridicator pressure is too high I would open the boiler drain and let some water out until the tridicator pressure drops.

    Repeat this procedure until the X tank's pressure as read on the compressor gauge is the same as the tridicator pressure. You want to reach that point where more air added to the X tank won't change the tridicator pressure reading. Open the fill valve. The sys pressure should re-adjust.

    Just my thoughts.
  • Steve MinnichSteve Minnich Posts: 2,474Member
    edited December 4
    I didn't read every comment but here's my .02 -

    The tank, ideally, is isolated from the system with a ball valve and a boiler drain to remove whatever water pressure is at the tank. It's better than taking a shower while dressed.

    Once isolated and pressure relieved, then you can check the existing tank pressure. Hot or cold, ya gotta do it. Nobody is going to wait while the clock is running and someone has to pay for that time. It's just impractical.

    12 PSI is usually enough for any residential two story house.

    I agree Pumping Away is important and is as easy as putting the pump in the wrong location so why not do it the right way.

    Check boiler firing rate.

    Is your fill valve leaking water through?

    Is the tank waterlogged?

    Low Water Cut-Offs should be on every boiler in my opinion. If they were, you could easily close the water fill and see if that stops the pressure rise.
    PHC News Columnist
    Minnich Hydronic Consulting & Design
  • Jolly BodgerJolly Bodger Posts: 146Member
    @HomerJSmith I see what you did there. Clever method of adjusting the charge without draining the tank. My concern would be, could you tell if the tank is failed. I find many tanks that are not serviced regularly are failed.
  • hot_rodhot_rod Posts: 12,037Member
    > @HomerJSmith said:
    > I've thought about this for a long time. I have always removed the tank to adjust the pressure, but it is a lot of work especially with a glyco fill. A 50% glyco fill is pretty high and it may require a pump change to accommodate the reduction in flow because of increased viscosity. You did check the glyco with a Refractometer.
    >
    > My conclusion on a hot recharge: I use a automobile 120V tire compressor with a gauge. I would want to know the sys fill valve pressure and close the fill valve to off so no sys fill occurs. I would connect the compressor and note the gauge reading. I would put 2 psi of pressure in the X tank and observe the boiler tridicator pressure. If the tridicator pressure is too high I would open the boiler drain and let some water out until the tridicator pressure drops.
    >
    > Repeat this procedure until the X tank's pressure as read on the compressor gauge is the same as the tridicator pressure. You want to reach that point where more air added to the X tank won't change the tridicator pressure reading. Open the fill valve. The sys pressure should re-adjust.
    >
    > Just my thoughts.

    Would you have a fill valve on a glycol system?

    The first step I would suggest is confirming the tank size, especially with glycol systems. Amtrol and Wessel have free online sizers. If you use “commercial” you can put in actual system volume.

    Precharge should equal fill pressure.

    On solar often the precharge is a few lbs lower so some fluid is in the tank for cold weather contraction, else the pressure drops to zero on cold collector conditions. I think They refer to that as a safety seal
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • ZosoZoso Posts: 26Member
    The height of the highest point in the system is about 30' above the expansion tank connection. My concern is that the system was not filled to the point where the highest pipes had adequate fill pressure. 30' is roughly 13 psi, plus I thought a extra 3 psi at the top would be good. I mentioned 18 psi cold fill in my original post. Sorry, I meant 16 psig.

    I have 448 gallons of expansion tank. When I calculate the expansion, I need roughly 108 gallons of acceptance. When I run the pressure calcs, I should see a hot pressure around 25 psig. Since I am only seeing 20 psig hot, I suspect the system will not filled to the correct pressure.

    I suppose I could isolate the tank, relieve the pressure and check the air charge, but I would rather not lose and glycol in the process. That is why I was thinking of adjusting while hot.
  • STEVEusaPASTEVEusaPA Posts: 3,491Member
    edited December 4
    One of these would help if it would work for you. One time install, can check any time without losing really any glycol. Don't have to drain the tank.

    steve
  • mattmia2mattmia2 Posts: 401Member
    How would that work without draining the tank? Wouldn't you have to empty the tank while it was isolated to set the cold fill pressure?
  • ZosoZoso Posts: 26Member
    I would just set it to my calculated hot pressure of 25 psi. However , now that you say that, I actually have three tanks in parallel and they are different sizes. I would have to do some calculating on that but I would probably need to isolate all three and set them at three different pressures so that it all equalizes when I open the valves back up. Otherwise, one tank may have a lot more water than its supposed to or vice versa. My whole situation is based on an assumption that the tanks did not have the proper cold fill pressure to begin with. And to make matters worse I don't know if they were all at the same pressure. I know the right way to do this would be to isolate, relieve the pressure, and truly get a reading of the static charge in those tanks. I'm just trying to find another way
  • HomerJSmithHomerJSmith Posts: 718Member
    STEVEusaPA, or this Webstone valve that I use all the time.

  • Jolly BodgerJolly Bodger Posts: 146Member
    All tanks the same pressure. They will accept what they are designed to and work together. The bigger tank will accept more volume than the little tank but will equal out to the same pressure. Otherwise your trying to put 10 gallons of something into a 5 gallon bucket. ;)
  • Jolly BodgerJolly Bodger Posts: 146Member
    I have these isolation/drain valves on all my expansion tanks. Charge them yearly. Almost every tank needs a little every year. Even new ones.
  • Jolly BodgerJolly Bodger Posts: 146Member
    Yes, you have to drain the tank to check the charge. For a large volume of glycol, drain it into a bucket or drum and pump it back in.
  • STEVEusaPASTEVEusaPA Posts: 3,491Member
    edited December 5
    You don’t have to drain the tank. Just isolate from the system, which the shut off valve does.

    12/5/19 Edit: Besides isolating the tank you also have to relieve the pressure to take an accurate reading, easily done with the webstone valve I posted.
    steve
  • STEVEusaPASTEVEusaPA Posts: 3,491Member
    edited December 5

    STEVEusaPA, or this Webstone valve that I use all the time.

    That valve doesn’t have a connection to hook up from the feed valve
    steve
  • mattmia2mattmia2 Posts: 401Member
    If you don't drain the tank you have some unknown volume of water in it. You need to find the "warm" pressure for that volume of fluid in the tank, not the cold fill pressure of the empty tank.

    It is really not much different than adding some air and draining some water from the system and seeing how it goes.
  • ZosoZoso Posts: 26Member
    Yeah but now even if I isolate the tank and take the pressure off, I do not want to fill it with my cold pressure because the system is hot. If I fill it to what I think the hot pressure should be while isolated it's not going to accurately represent the condition once I reopen the valve. The system will be at 20 psi ans the tank at 25 with no water pressure in the bladder so it will quickly equalize somewhere in the middle

    Having three different tanks is adding to my problem because I don't know if they had the same initial air charge or not. So I don't know if they have the proper accepted volume or if they are almost full because the air charge was not adequate to begin with.

    Maybe I'll just add a little air to each tank for now and make sure it's corrected whenever the system goes cold. I'll see if I can get a gauge at the high point and turn off the system pump and see what my static pressure is hot.
  • icy78icy78 Posts: 274Member
    Set your Extanks at 18. That's 5 psi over the highest point. The expansion available in the Extanks keeps it there, if sized correctly.
    If you have 25 psi now, and still have heating issues on the 3rd floor, it's not due to too little pressure.
    Flow issue...multitude of possibilites under that heading :)
    Pumping away?
  • GordyGordy Posts: 9,348Member
    edited December 5
    Pumping away from the point of no pressure change (the x tank) indeed. Add the circulator differential pressure to the system.
  • ZosoZoso Posts: 26Member
    Yes I am pumping away. I only have 20 psi at the tank now and I think I should have 25 which is why I started this post in the first place. I'm just not sure how to achieve that 25 now that it's hot, especially with different sized tanks in parallel
  • mattmia2mattmia2 Posts: 401Member
    If it is less than the system pressure then it is full of water. The bladder isn't some magical pressure translator, it is just a membrane, the pressure on both sides has to be the same unless the bladder is expanded all the way against the tank.
  • liveto99liveto99 Posts: 6Member
    The size of the tanks does not matter just the pressure.
    Hot or cold check the pressure they should all be the same they work independently but will all react the same way based on pressure.
    You probably have a air bubble put a vent at the very top.
    Or don’t have enough head pressure from the pump.
    If it has ever worked with the glycol In it you have an air bubble.
    If it is been a problem and did not work when It had just water in it do you have an air bubble.
    You can just take the pressure off of the system isolate your expansion tanks and cut that air vent in on the third floor you don’t have to drain the system.
    There is a thousand thing to look at, but without seeing them. Nine out of ten times your problem is air.

  • STEVEusaPASTEVEusaPA Posts: 3,491Member
    edited December 5
    liveto99 said:

    The size of the tanks does not matter just the pressure....

    Since when?
    Pressure could be correct, but if the tank(s) are undersized for the volume of water in the system, the pressure will still rise.

    steve
  • Jolly BodgerJolly Bodger Posts: 146Member
    @STEVEusaPA how can you get an accurate air charge in the expansion tank if you don't release the pressure on the water side? If you don't, the bladder will be compressed to a smaller volume. The precharge won't be accurate. A portion of your acceptance (perhaps all of it) will already be used up.
  • icy78icy78 Posts: 274Member
    Why do you need 25 psi? You say you have 30 feet between the expansion tank connection and the top of the system so with a 5 PSI buffer that puts you at 18 PSI.

    Like a lot of people have said, isolate the tank , bleed the water pressure off, verify the bladder pressure, set it at 18 and then start troubleshooting. Are you sure the pump can generate enough head to overcome system resistance?
  • ZosoZoso Posts: 26Member
    I appreciate all the comments and help, but I do feel like discussion is going in circles.

    My calculated cold fill pressure is 16 PSI. My calculated hot pressure is 25 psi. That is based on 2500 gallons of 50% glycol being filled at 70° and heated up to 180 degrees and 448 gallons of expansion tank.

    I was not involved when the system was filled and only recently saw it while hot. Pressure gauge only read 20 psi, so I am suspecting it was not filled with proper pressure. It might even have air at the highest point. So I think that should be my first move to figure that out.

    I'm just not sure how to correctly adjust the tank air charge while hot. If I isolate them and set them to 25 psi, once I open the valves it won't be 25 anymore. If I leave the tanks connected and start adding air, I may put in way too much air because my system Maybe under-filled. Plus as I add are in one tank it will further compress fluid into the other tanks thereby raising they're pressures. But the pressure in those other tanks will be raised because there's too much fluid in there and not because there's the proper amount of air. So in other words they will start becoming water logged.

    I want each tank to have 25 psi air pressure while the proper amount of hot water is in the bladder. Right now, I don't have 25 psi air pressure, and I don't have the proper amount of water in the bladders. So I'm scratching my head on how to get those two circumstances to come in line with one another.

    So I'm thinking my best approach is to isolate the tanks, drain the tanks, adjust air pressure to 16 PSI, connect the tanks back into the system, and add glycol solution as necessary until the gauges read 25 psi. Thoughts?
  • STEVEusaPASTEVEusaPA Posts: 3,491Member
    edited December 5

    @STEVEusaPA how can you get an accurate air charge in the expansion tank if you don't release the pressure on the water side? If you don't, the bladder will be compressed to a smaller volume. The precharge won't be accurate. A portion of your acceptance (perhaps all of it) will already be used up.

    When you isolate the expansion tank, via the Webstone valve I mentioned, and open the hose bib cap and valve, you relieve the pressure. The tank doesn't drain. And then you take a reading. You don't have to remove the tank and dump out all the contents for an accurate reading.
    I edited my previous post to avoid some possible confusion.
    steve
  • mattmia2mattmia2 Posts: 401Member
    Why would the tank not drain? The pressure from the pre-charge should push the liquid out, unless it has lost all its pre-charge.

    If you have the system cold, you should be able to drain the tank and set it to the cold fill while isolated, there in theory shouldn't be any fluid in the tank when it is cold anyhow.

    If you want to get all the tanks balanced you will have to isolate and drain them, You could probably just add a bit to each until you get to 25 psig in the system then see if you can bleed any air you couldn't before, then if you do get air, add some more to the tanks to bring the pressure back up. You will probably need to remove some fluid and add more air to the tanks to get the right system volume and tank pressure.

    Or you can just isolate, drain, set the cold fill and pump in some more fluid if you have to.
  • Jolly BodgerJolly Bodger Posts: 146Member

    @STEVEusaPA how can you get an accurate air charge in the expansion tank if you don't release the pressure on the water side? If you don't, the bladder will be compressed to a smaller volume. The precharge won't be accurate. A portion of your acceptance (perhaps all of it) will already be used up.

    When you isolate the expansion tank, via the Webstone valve I mentioned, and open the hose bib cap and valve, you relieve the pressure. The tank doesn't drain. And then you take a reading. You don't have to remove the tank and dump out all the contents for an accurate reading.
    I edited my previous post to avoid some possible confusion.
    @STEVEusaPA when you isolate the tank and open the hose valve, the tank will drain until the water side it equal to atmospheric pressure. That is what I meant by drain the tank. I did not mean you must remove all the water. If the tank is good and has a precharge, it will empty the tank in the process of equalizing.
  • Jolly BodgerJolly Bodger Posts: 146Member
    @Zoso I think you are correct. The conversation is circling. You " just not sure how to correctly adjust the tank air charge while hot." There is the CORRECT way to charge the tanks, and ways to do it hot. Both methods have been explained.

    This forum has also told you that the static pressure of the system is not the cause of the heating problem.
  • mattmia2mattmia2 Posts: 401Member
    The pressure in itself isn't the problem, but the precharge could be the problem, if there isn't enough precharge to push the water out of the tank and up to the top of the system when the system is cool, the fluid will stay in the tank and it may suck in air in an automatic vent somewhere at the top of the system to compensate for the contraction of the fluid.
  • Jolly BodgerJolly Bodger Posts: 146Member
    If the cold pressure is causing a problem, then setting the tanks correctly is the only way.

    I would think if an auto air vent would let air get sucked in, then it would let the air out. Unless it failed in a magical way?
  • mattmia2mattmia2 Posts: 401Member
    That air could be getting to someplace where it can't get back out the vent, especially if there is some flow of the cold fluid until it pushes the air pocket somewhere that stops flow.
  • Jolly BodgerJolly Bodger Posts: 146Member
    Then there is a venting problem, No?
  • hot_rodhot_rod Posts: 12,037Member
    If all the tanks are good, no ruptured diaphragm just put a hose in a low point, boiler drain, and lower the pressure. The diaphragm will push out any fluid in the tanks. Adjust pre charge to your intended fill pressure. Valve off any piping above to lessen chance if air locking the system

    Now you are back to square 1 with the system

    Add iso drain valves to make it easy to service or replace Tanks in the future
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • liveto99liveto99 Posts: 6Member

    liveto99 said:

    The size of the tanks does not matter just the pressure....

    Since when?
    Pressure could be correct, but if the tank(s) are undersized for the volume of water in the system, the pressure will still rise.

    liveto99 said:

    The size of the tanks does not matter just the pressure....

    Since when?
    Pressure could be correct, but if the tank(s) are undersized for the volume of water in the system, the pressure will still rise.

    liveto99 said:

    The size of the tanks does not matter just the pressure....

    Since when?
    Pressure could be correct, but if the tank(s) are undersized for the volume of water in the system, the pressure will still rise.

    The tank volume is almost 20 percent of the system, the tanks are not undersized. If there is 3 different size tanks there if there is the same air pressure in them they will all work fine. The first one is probably the correct size and 2 people have tried to fix this problem by adding tanks.
  • GordyGordy Posts: 9,348Member
    Looking at the original problem (popping relief valves. Is it possible that the auto fill was left on, and the feed piping is/was on the suction side of the circulators. If the pressure differential was such that the suction side was below system pressure it could trick the auto feeder into adding more water to the system.

    But you also said you are pumping away from the ponpc.
  • liveto99liveto99 Posts: 6Member
    Put in the vent get rid of the trapped air.
    Could be a worn or broken impella on the pump
    Could be a partially closed gate, valve could be a dropped gate on a valve. Could’ve been designed wrong.
    Check with someone who is been in the building from the beginning and find out whether the heat ever worked on the third floor.
    How can the air on the 3rd floor get out?
    When we type our answers here we are assuming that there is a heating company that Is working on this and you are a Enganeer. That is what I read. If it is a residential-based plumbing company it may be too large of a system and they are not used to it. You could try a commercial company and they could probably fix it. Better yet the old plumber at the supply house chewing an a cigar can tell you the answer in a half hour after he looks at the whole system. We get bits and pieces, it sounds like a school?
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