Bladder Expansion Tank Set Pressure?

[49chevyman]

Getting ready for first fire of the season as temperatures are getting down into the 30's now. I had a leak this summer, well, not quite a leak, but an inadvertent discharge from the closed system due to a valve being left open on an open line. Anyway, I measured the air pressure at the schrader valve on the top of my Well-X-Trol Model WX-350 bladder type expansion tank and it read only 3.0 psig. My circulator at the boiler was running early this morning when I went to the barn due to the low temperature and a thermal snap switch I installed last year. It did not sound good, sounded like it was cavitating. After shutting off the circulator, I opened a bleed on top of the boiler and it did not gush out under pressure as it has in the past. Last year when I measured the pressure at the expansion tank it was 7.5 psig with the system cold.

I inherited this system a few years ago, and have only been able to improve and operate it due to the valuable shared knowledge on this site. The elevation of the expansion tank is about 2 feet above the top of the 1000 gallon storage tank.. The expansion tank is on the suction/inlet side of the boiler circulator pump P1. The tank is 119 gallong total with approximately 46 gallons acceptance. I was worried about it being designed for wells due to the high temperatures inherent with the boiler, but it is rated for 200F, which it should not see on the inlet side to the boiler. I have not had any issues with my P&T valves opening during the last few seasons, so it must have adequate volume for the expansion of the 1100 gallons or so in my system.

I have opened the fill valve at my storage tank and refilled the system. The pressure regulator on the fill line is set at 12 psig. My relief valves (one at the boiler and one at the storage tank are set at 25 psig. My question is what should I set the air side pressure to in the expansion tank? Below is a schematic of my system.

Thanks for any and all help!

Kevin


Link Removed

 

[brenndatomu]

Last year when I measured the pressure at the expansion tank it was 7.5 psig with the system cold.

Why not take it back to 7.5 and try it?

 

[maple1]

Set the expansion air pressure the same as your cold system pressure. But it has to be done with no water in it and while isolated from the system.

EDIT: Without knowing how it is piped exactly and what you have for drain and iso valves, that could have been accomplished by adding air while your system had very low water pressure in it, since adding air would have pushed any water that was in it back into the system. So you could have aired it up to 12 (13 likely better) then watered your system up to 12. As long as they were connected and un-isolated.

EDIT AGAIN: But then the question would be, what happened to the air that was in it previously?

 

[jebatty]

My opinion is that for most purposes, I agree with this statement: "The water pressure in a hydronic boiler shouldn't be over 12- to 15-psi. It should have only enough pressure to raise the water a few feet above the top of the highest point in the piping. A 12-psi setting will lift water 28' above the fill valve. A 15-psi setting will lift the water to 34'."

air pressure in hydronic systems - Google Search

 

[49chevyman]

I like your advice Maple. I do not have any isolation valves for the expansion tank, so I vented the water side pressure off, then pressurized the air side to 13 psig, burped the system and let a little bit more water in, bringing the water side up to the same pressure. I do not believe my inlet water pressure regulator is working as it built up to 20 psig before I cut it off. Had to bleed some water back out of the system to get it back to 13 psig.

I have built my first fire of the season now, waiting for the system to come up to temperature. When the boiler circulator kicked on at 150F, I measured 14 psig on the air side of the expansion tank. I will keep an eye on it as it comes up to temperature.

I made a SST nozzle insert for the EKO based on advice from others on here. My firebrick had eroded pretty bad and was letting a lot of unburnt coals drop through last year. I really did not want to chip out all of the old nozzel and replace it based on the experiences of others I have read about on here. Seems to be gasifying good with a nice blue flame coming through. My patched up U shaped fire bricks in the bottom are still holding up.

Jim - the highest point in my system is the water/air coil in my attic that heats the second floor. It is about 24' above the circulator in the basement.

Thanks guys!

 

[49chevyman]

1st burn underway! Storage is coming up to temperature, with the top of it around 140F, which is plenty hot enough to switch over from my heat pumps. Bottom is still cold as there is not much mixing going on in my 1000 gallon horizontal storage tank. Air pressure on the expansion tank has risen to 16 psig. I will wait and see where it settles out, and then maybe lower it to 15 psig as Jim suggested.

New SST nozzle appears to be working well, good gasification and very little stuff dropping through to the bottom chamber.

 

[maple1]

Air pressure should be the same as system (water) pressure. Dont let any air out or you will lose expansion room. If pressures get too high, let water out. You might not have enough expansion tank to maintain the system pressures you want over the total temperature range your system will see. I would likely aim for 11 or 12 or so, at the minimum temp your system would see during winter operation, when storage is fully depleted. That would mean it would drop below that, to less than desirable, if it goes stone cold say through the summer. But you aren't using your system then so should be ok, and should come back up to desirable once you start up again and warm it to a usable temp again.

 

[49chevyman]

I did have to let a bunch of water out after the system got hot. Once the storage reached 170F top and bottom, the pressure was 22 psig. This is too close to the inside relief valve setting of 25. I let it down to 13 while hot.

 

[maple1]

Sounds like you could use more expansion tank.

 

[49chevyman]

Sounds like you could use more expansion tank.

Maybe! It is rated for 46 gallons acceptance. This is pretty close to the 4% total system volume rule of thumb for hydronic systems.

Having problem now with water disappearing! I have only had the one fire in it this past Sunday. On Tuesday morning my wife noticed a weird sound coming from the Taco 13 pump at the EKO in the barn. I was at work. Told her to kill the breaker to the system which stopped the noise. Got home to find the low water switch tripped at the top of the EKO. Expansion tank pressure was at 12 psig, system was cold at about 70F outside, but storage was at around 150F Pressure on storage about eh same as expansion tank. I opened vent valve on top of EKO and it sucked air in like it was under vacuum. I connected a water hose to the drain at the bottom of the EKO and filled it up until it ran out the vent valve on top. Closed everything up and test ran the boiler circulator, it sounded fine. This morning it was right at freezing outisde. I have a snap switch set to run the boiler circulator if it gets low temperature in the barn, and it was running, but noisy like ti had air in it. The low water switch was on again. I killed the power to it and went to work. This afternoon I refilled the EKO, expansion tank pressure still good at 12 psig. I built a fire as storage had got down to 100F. It seems to be burning normal and bringing storage up. The vent valve has good pressure at it when I crack it slightly. I have looked everywhere for any leaks and have none. Only place I can't visually check is the underground lines between the barn and basement, which run 125 feet each way, but no wet spots in the yard. See picture below. I am not sure what that circled piece is, but assumed it was some kind of automatic air vent. Could it be letting air in as the water in the EKO cools off? It does not seem to be piped up correctly by the original owner. Why would the expansion tank not keep it under pressure? There is the Danfoss valve between the expansion tank and the circulator pump/EKO inlet.

 

[maple1]

Do you have a pressure guage on the boiler?

 

[maple1]

Still think you need more expansion. 4% sounds low.

If you let water out when it was hot, dropping pressure from 22 to 13 after starting at 12, that is a 10 psi rise cold to hot. So going hot to cold would now be a 10 psi drop from 13, meaning really low pressure when cold.

Still not sure on system layout and vertical separations, but you will get pressure differences at various spots equal to about 0.4 psi per foot. So example 10 psi at the bottom of your system means almost zero psi 24 feet above that.

 

[maple1]

Could also be possible your bladder sprung a leak, looking back at the mention of only 3psi in post 1. How old is it?

 

[49chevyman]

No pressure gauge at boiler unfortunately. I could install one on a tee off the vent valve and have thought of doing that. I have been measuring the pressure at the expansion tank and storage tank while it came up to temperature today. Raised from 12 cold to about 13.5 hot. This seems acceptable. Not sure why the big change in pressure before unless the air chamber in the expansion tank had been reduced in volume quite a bit, then expansion of water would have made a bigger change in air pressure.

What is that round thing I circled? Going to try to clean it and see if it has any markings/labels. I have ran this system the last three years and not had any of this trouble. I did cut into the system to replace a water/air coil in the plenum of my downstairs air handler this past summer. That allowed some air to enter, plus I had an open valve that allowed some water to escape and leet most of the air pressure out of the expansion tank.

Elevation wise, my barn floor is about 4 feet above my basement floor. The vent/air relief in the barn at the EKO is about 6 feet above the top of my storage tank which is bout 4 feet above my basement floor. It is a horizontal tank. The highest point in my system is the water/air coil in the upstairs air handler which is about 24 feet above the storage tank. The circulators for the downstairs and upstairs zones sound good with no signs of cavitation. Pressure in storage matches the pressure inthe bladder expansion tank.

Bladder tank is 13 years old. I see no signs of water at the schrader valve. Any other way to check it?

 

[49chevyman]

Just checked it again. Storage is up to about 150 top and 110F bottom. Expansion tanks has increased from 12 cold to 15.5 psig. Cracked vent valve at EKO and have good pressure, no air came out. Will check it in the morning before going to work. Boiler should be cold, but circulator at boiler may not be running since temp is only supposed to get to 45F tonight. Storage will still be hot.

 

[maple1]

Yes that thing is a vent. They can have issues. I had to replace one last year because it started seeping. I added a ball valve under it to isolate it for future changes.

Thinking new problems could be one of 2 things. A lot of air in system from the summer fix, or bladder leak. Should be able to check for bladder leak by letting all the air out of the expansion tank and seeing if the air coming out turns to water coming out. If you do that and get to nothing at all coming out, that should rule out bladder leak.

 

[49chevyman]

Ok. I will order a new vent and plan on adding a isolation valve under it. Once the system cools down tomorrow, I will vent all the air pressure out of the expansion yank and see if I get any water.

Thanks Maple!

 

[maple1]

The problem with checking your bladder air pressure all the time, is it isn't going to necessarily say what the boiler pressure is, all the time. If it is below the boiler, even more so. For example, if you air it up to 12 beforehand (which I think you did), then open it to the system and get that stabilized at 12 also, the air pressure should not ever drop below that start point. (As long as your bladder is tight). Even if all the water leaves your system. That means you would see 12 of air even if there was zero of water. Plus, every time you check it you lose a tiny bit of air - that part could be negligible.

Also some quick pressure numbers from what I think you said your elevation differences were. All told, you have 28' from bottom to top of system. That is quite a bit of height (mine is about 20', two story with baseboards), and equates to 11 psi. Not wanting less than a couple psi at the top, that would mean you should not see less than 13 psi at the bottom. That is when cold to the point you stop circulating for heat. Translating that to top of boiler (10 feet up), gets you to 9 psi there. So you would not see that pressure at the boiler, by measuring air pressure, since that should not go below 12, and which I think is also 4 feet below top of boiler?

You should likely have more expansion, if you want to make full use of your storage. If you stay within a relatively narrow temp range, you might get by, but that would be wasted storage potential. I have been through lots of expansion changes. Currently have one HFT-90V B&G tank, that specs 34 gallons acceptance. Not near enough for my 660 gallons storage (plus whatever is in system & storage) by itself, Also have 2, 25 gallon old-school cushion tanks, left over stuff from my old system, mounted up high. That does things OK, but I have to watch my high temps and usually don't finish a second lap of storage heating - giving me something like 180 at the top and between 160 & 180 at the bottom. I think it might be OK if I went higher, but I don't anyway. And I let it fully deplete. Also have a (much cheaper) 30v tank I got, but not hooked up yet, that I was hoping would allow me to do away with the cushion tanks. Another unfinished project.

 

[49chevyman]

The top of the expansion tank is pretty close to the top of the boiler. Vent valve and air vent are only about 6 inches above the top of the expansion tank, so no head differences there to fool with.

Volume of water in the system is about 1200 gallons or so, which is 9980 pounds at density of 8.3172 lb/gallon at 80F. Volume change going from 80F to 180F is about 32 gallons (specific volume at 80 is 0.12023 gallons/lb, and at 180F is 0.12348 gallons/lb. Mass does not change (unless I am losing water), but volume does.

 

[maple1]

Try this:

Expansion calculator

There might be others out there, first one I found with a google search.

 

[49chevyman]

Try this:

Expansion calculator

There might be others out there, first one I found with a google search.

Thanks - good site! It says I need a 130 gallon tank, but does not tell the acceptance volume of said tank in the brochure. My current one is 119 gallons with 46 gallon acceptance. Expansion tank reading today when I got home was 15.5 psig. Boiler temperature was 100F, storage at 155F. Circulator not running. Boiler pressure was OK (cracked vent valve and had good pressure). I will install a pressure gauge on a tee off the vent valve to monitor the boilers pressure. I had expected it to be the same as the expansion tank since they are so close, but the Danfoss valve may prevent the boiler from seeing it when the system is cold.