Is your closed loop hydronic system expansion tank tied to the correct point in your system? In many more cases than you might think, we find it is not. Expansion tanks tied to the incorrect point in the system don’t perform as they should.

Like many answers you hear in the field of engineering, the answer is…It depends. What does it depend upon you might ask? First of all, it depends upon what type of expansion tank you have. Nearly all expansion tanks we have seen installed in recent years have been either bladder or diaphragm tanks. In both cases the concept is similar in that there is a butyl membrane between the air side and the fluid side of the tank. The air side is the compressible cushion that the incompressible fluid expands against as the fluid warms up. As the fluid cools and shrinks, the air expands and keeps the fluid pressure relatively constant in the system. With either one of these tanks, they should be tied into the side of the main line downstream of the air separator and upstream of the pump suction. You don’t want air or sediment getting into the branch line connected to the bladder or diaphragm type expansion tanks.

On the other hand, if you have an older style compression tank, which does not have a bladder or diaphragm, they need to be tied into the top side of the air separator in the system. They are intended to store the air above the water line in the tank and use it as the cushion against the expanding fluid.

A common fault is finding either the bladder or diaphragm tank connected like the older style compression tanks were to be piped, into the top of the air separator. In some cases, this is the fault of the designer for using an old detail for a compression tank, but specifying or scheduling around a bladder or diaphragm tank. In other cases, it’s the contractors piping the tank the way they learned how to pipe them 30 years ago when the compression tanks were the norm. The good news is, it is a very easy thing to check during design and construction once you know what type of tank you have.