Is my expansion tank bad?

[gadolphus32]

I have a hot-water system. The boiler was installed in 1949 and there is a metallic expansion tank hanging close to it in between rafters in the basement. The tank is also old but does not appear to be as old as the boiler.

Lately the boiler has been releasing a lot of water (about a gallon an hour) from the pressure relief valve when the system runs (and only when it runs). The expansion tank does not get warm at all while the system is running, and the tank appears to be filled with water at all times. (Water definitely always comes out when I open the valve on the tank, and while I can't say for sure that the tank is totally full of water, it sounds that way if I bang on it.)

Is this a sign of a problem with the expansion tank? If so, can I fix it simply by draining the tank of water while the system is shut off, then refilling the system? Or do I need to replace the tank?

I've read various things elsewhere that suggest the expansion tank is the issue, but some of the information I've seen conflicts, so I'm hoping to get better advice here.

 

[Jadnashua]

There are at least a couple of types of expansion tanks. IT sounds like the one you have does not have an internal bladder. That type does need to be drained periodically if it's what I think it is. From a long-term situation, the air you put into that tank by draining it ends up slowly getting absorbed by the water in your system. WHen that happens, the oxygen creates rust throughout the system. IMHO, this is one good reason to decommission that one and install a bladder type which separates the air from the water. They don't tend to last as long, but they do work better.

 

[Sponsor]

 

[gadolphus32]

Thanks for your reply. I think some of my confusion about what I was reading online was the result of not realizing there are two types of tanks. I'll try draining some water from the tank and see if it solves the problem.

A few follow-up questions:

1. Should I drain the water from the tank while the system is running or off?
2. There is a valve that separates the tank from the rest of the system. Should that be open or closed while I drain the tank?
3. The water supply to the whole system is currently shut off. Should I turn that back on (and open the valve that controls flow into the tank) after the tank has been drained? (I gather that I should, but wouldn't that just cause the system to fill the tank back up?)

 

[Jadnashua]

WIth a bladder tank, there's a waterproof, flexible bladder that separates the water and air. With an old style expansion tank, it's just a big tank with air trapped in it (with some additional mechanical things involved so the air doesn't directly circulate). Simplistically, in that type of tank, as the water is heated, it compresses the air, but is in contact with it, so the air becomes saturated, and some gets dissolved into the water. Over time, that dissolved air, at least the oxygen content, creates rust or corrosion throughout the system, and the air gets used up, making the expansion tank provide less space for when things cycle hot again. This may help

You need to shut the system down and isolate the tank so you can drain it. Personally, I'd replace it with a properly sized bladder tank, but just draining it should help unless the tank has a leak in it.

 

[gadolphus32]

Thanks for your help. I drained the tank and the leaking seems to have stopped. When the weather warms up I'll drain the system and replace the tank with a bladder tank. Thanks again for taking the time to provide me with some pointers.