Replacing steel expansion tank with diaphragm expansion tank. Is location critical?

Question

This morning I noticed a very old, closed, steel expansion tank in the boiler loop is leaking. Looks like the leak is coming from the side of the tank and not from the plumbing. Not sure if that can be repaired or replaced easily so was wondering if a more easily found diaphragm expansion tank could be used instead.

I have a Weil McLain CG series gas hot water boiler system. The manual shows two different setups for when using a closed tank or a diaphragm tank.

My system isn't set up with the expansion tank in either of those configurations. What I have looks like this.

There's no pressure regulator on the main incoming water line and therefor the hot water heater doesn't have an expansion tank on it but at some point I'd like to add both. I do have an automatic air vent in place on top of the boiler.

Can I just replace the steel expansion tank with a diaphragm expansion where it is? How important is the placement of the tank in the heating loop? Getting it connected under the circulator as shown in the diagram would be a real pain. I was planning on shortening the piping run to the old tank and placing it along that run.

Answer 1

The old tanks had to be pressurized to provide an air space the newer bladder based systems are better in my opinion and will work fine as long as rated for the temp/pressure.

Answer 2

The placement differences are partially explained in the manual. The closed expansion tank functions as an air eliminator, which is why it must be mounted above the boiler. That is the tank must be located so that any gases which come out of solution inside the boiler migrate to the tank.

The diaphragm tank requires air to be removed through an automatic vent. In that case the entire system is designed to run without air in it, which is why the system is plumbed to pump out of the tank. To put it simply, the tank is a point of no pressure change. Gory details here.

Basically the circulator creates a change in pressure while it's running. If it discharges into the tank the pressure is subtracted, because the point of no pressure change is at the discharge. The vacuum caused by the pump can then lower the water level enough to allow flashing to steam or drawing air in through vents.

Conversely, if the circulator draws from the tank the pump suction will remain constant whether the pump is running or not. When the pump is running the discharge pressure will increase causing a gradient of increased pressure in the direction of flow around the loop, which is a favorable condition.

There are practical engineering limits at play. Obviously the "relative badness" of improperly locating the tank varies with the size of the pump and pressure of the system. I really have no idea how critical the issue is in your case. Then again, the tank need not be physically located below the circulator to be plumbed in before it. Excluding ludicrous scenarios such as putting it forty stories above the boiler you can locate it someplace convenient and pipe over to it.

Do keep in mind that I'm not a licensed plumber or HVAC tech so I wouldn't consider this coming from a "credible and/or official source." Do feel free to double check with a qualified professional.