Bladder Expansion Tank Set Pressure?

49chevyman said:

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

Click to expand...

Why not take it back to 7.5 and try it?

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?

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 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.

Sounds like you could use more expansion tank.

Do you have a pressure guage on the boiler?

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.

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

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.

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.

Try this:

Expansion calculator

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