Exspansion Tank Question

Brad White_203

Personally, I prefer dry nitrogen rather than air, to minimize oxidation of the bladder/diaphragm. Of course, air is 78% N2 but that pesky 20.8% O2 is nice to eliminate... Hey, that is what I do on commercial/institutional work.

Most folks do not have a B-tank of nitrogen sitting around, so yes, you can use air and no one but us will know...

I know what you are thinking, "Brad, I have a tank of Nitrous Oxide left over from a party I held in 1978, can I use that?"

No. At least not in your heating system. :)

That said, it is an excellent time to take stock of fill pressures and tank pressures. But you have to do this with the system cold to have any validity.

Let the system cool to ambient, room temperature. Fill (or drain) the system to the required cold-fill pressure, say 12 PSIG for a one to two story house and 18 PSIG for a three story house. If taller, let me know.

The expansion tank cold-fill pressure should match that of the system. Too high and the system will follow it. Too low and there will not be the air cushion you seek to absorb the increased volume.

Done.

Edit: See Norm's post and response, to charge the tank starting with no system-side pressure if you can. This would allow more available expansion room in the tank for calibration purposes.

joeba

I have a 30lb expansion take on my hydro-air, system. Simple question, as part of my boiler's annual maintenance, should I check the pressure of the tank and fill it if needed? Any special techniques or do I just simply check the pressure with my trusty tire pressure guage and fill with my trust bicycle tire air pump?

Norm Harvey

hey Brad,

Can you check the tank pressure with the water side pressurized? I allways thought the system needed to be at zero pressure in order to check the air pressure.

- Norm

Brad White_203

Hey Norm! You make a very good point and distinction

You are correct. I like your way better, but I had to think about this a bit.

To check the air pressure itself, yes, the tank should be vented or disconnected on the system side. But once connected, the pressures on each side of the tank will be equal. As you note though, testing and charging the tank under pressure lessens the available acceptance volume, so you make a very good point.

I always specify the "under pressure method" because our tanks are sometimes not isolated by a valve from the system and there is no simple way to do a "zero-base" charge and check. It seems I have been specifying larger tanks than you are.

I was always taught that the tanks of either type should start from half filled on each side, but that is probably arbitrary and a hold-over from the "captive air above water" tanks, so this is good to get me thinking about that.

But your way would allow me to use a smaller tank.

I learned something new today, as any day. You got me thinking again, Norm. Thanks!

Brad

Jim Erhardt_3

No system pressure...

The typical bladder type tank should be checked once a year for correct air charge as air will gradually migrate through the diaphragm. However, as the spec sheets call for, the tank must be isolated from system pressure when doing so. Unfortunately, most expansion tanks are piped into the system in such a way as to make this simple procedure much more difficult.

Al Roethlisberger

What about captive air in the larger vertical bottom fill tanks?


Here's something I've wondered about: Does air get trapped, even initially during installation between(above) the water and the rubber bladder in the larger vertical tanks that have a bottom connection?

I've often wondered if air is initially, or permanently trapped in that little gap that surely must exist down around the fitting or especially if the bladder doesn't completely fill the tank when empty of water. Perhaps any air trapped is tiny and inconsequential, but I just wondered.

OK, another question. How long are the larger bladder based tanks good for?

I've got an older steel air-cushion expansion tank that I think is probably just about worn out from rust and was never really installed correctly anyway, so I was looking at a SX-90V tank as a replacement. The vertical placement of this tank would be convenient, among other things, but I wondered how long the bladder was good for given that old-syle expansion tanks are good for many decades.

Thanks,
Al

Norm Harvey

As to your first question Al, Yes I think a little air does get trapped in there initially. I cant see a real negative effect as a result however.

Second, when replacing your expantion tank, there are a number of programs where you input the water volume of your system (pipes, radiators, boiler)and then it spits out the proper model tank for your needs. I use the on in Seigenthallers software, but I think amtrol may have some simmilar program on their site. If not them another manufacturer.

I so wish that all of the steel compression tanks were equipped with a gauge glass (like on steam boilers) so that you can see the water level of the tank at a glance. I have seen the glass on larger commercial compression tanks but I wish they were standard for all. Man would that make draining them so much easier (through the vent on the valve)

But alas, a man can dream

Derheatmeister

2 PSI less... HM

Hey Brad, I have been installing 2 psi less on the Bladder side then the system fill pressure so the Bladder does not Rub against the inlet fitting and create an hole in it.

So i read or was told an while ago,I do not remember?

But also my tanks seem to last (even when filled with reg.air)

What do you think? Richard

Brad White_203

What an interesting point, Richard

I love it when things make sense and not big difference in performance.

Truthfully, air will work vs. nitrogen, but a lot of our systems are in public buildings where maintenance is, well, done by rumor. So we tend to design for longevity.

I did have a 120 gallon bladder tank go on me once. Huge air bubble throughout the system, the noise was sort of like the building had a huge bowl of chili for lunch.