I did search on several variations but couldn't find anything on non-bladder expansion tanks. My Dad's house, built around 1930 and originally coal heated was converted to natural gas sometime in the 50's (Bryant boiler, hot water cast iron rads). It recently began occasionally spitting some water out of the relief valve (boiler pressure shows indicated 15 psi). The expansion tank is just a cylindrical tank mounted on it's side, maybe 5 gallon capacity. The hookup is one 1/2 pipe coming from boiler to bottom of tank (w/shut off valve), a drain at other end on bottom with drain valve and a 3rd smaller valve connected to a pipe going in through the bottom and going part way up inside the tank(?). The tank is mounted between joists below floor (in basement) and as such is slightly lower than the bottom of radiators on floor above. The procedure to drain the tank says: shut off line from boiler, open small vent valve (3rd described above) and then open drain valve. We did this and tank proceeds to drain about 2 gallons before the vent valve starts peeing out water. Put the valves back to run mode and boiler gage reads 7 psi. Next day my Dad see more water from expansion valve and tank sounds full again (knock, knock). So a new expansion valve may be in order but the tank got me thinking: what good does it do if it is below the radiators? It would seem gravity would just fill it up for the most part (or does it rely on compressing a volume of air somehow regulated by draining to a certain level?). Would it be a good idea to replace with a bladder tank?

 

Probably not getting all the water out of the tank. Need to drain fully.

Pics will help.

Also the psi is too low. Should be 12-15 psi...... Possibly need to adjust or replace fill valve.

 

It is easy to be fooled by these tanks when draining them. If the water coming out isn't replaced with air, then the stream of water will slow down. You can easily measure the tank and calculate how many gallons it would hold at maximum. That will help you get an idea about what is happening.

 

They won't fill up right away because of the air pocket at the top but when they are FULLY drained they work perfectly fine. I suggest dumping the tank once a year.

 

Thanks guys. I'll try draining it again and get a couple of pics.

 

There is nothing wrong with these tanks. They are designed as never to need draining if properly installed. Drain the tank completely, remove or close all automatic air vents and sit back and never worry about it again.
Moe info here
Draining Your Expansion Tank
and here
Expansion Tank Information