Expansion tank location

MikeDurigon

Installing two Weil McLain 480 cast iron hot water boilers. It’s a two pipe system, 2 1/2” supply top connection, 2 1/2 return bottom rear of boilers. Supplying heat to five large unit heaters in a maintenance garage. There will be two Grundfos circulators on the return line set up to alternate as required. The existing system doesn’t have an air scoop installed and the expansion tank connection was installed between the existing pump and boilers. Not sure if this is the correct location for the expansion tank. Thanks

hot_rod

Why the circs on the return side?

Jamie Hall

"the expansion tank connection was installed between the existing pump and boilers. Not sure if this is the correct location for the expansion tank"

It's not. Presumably the pump is pumping into the boiler? The expansion tank should be connected on the INLET side of the pumps. Not right at the inlet, but reasonably close. The idea is that the expansion tank will then be at the lowest pressure point in the system and be set for the desired minimum static pressure in the system, thus keeping the whole thing correctly pressurized regardless of pump operation.

hot_rod

Parallel piping is the simplest. One or both boilers can run without flow through the other, A staging, telmar for example could be used.

Reverse return, primary secondary, hydraulic sep are a few other piping options.

I'm a bit concerned about return protection, the boiler running below 130F return for more than 10 minutes, with all that large steel piping.

Any idea how the heaters are sized to total boiler output? If you have more boiler than connected load, that would help prevent extended cold run.
If that is a concern primary/ secondary, or a hydro sep could be used with a 3rd system pump.

Avoid series piping, as it will have flow through an un fired boiler at times, if you stage them.

MikeDurigon

This is an existing system. My guess is the building is 50 years old or more. Engineer has shown to install pumps as per original installation. Re: pumps on the return. Two boilers will be set up to alternate thru a tekmar controller. 

hot_rod

The key, regardless of the piping choice, is to have the expansion tank on the inlet to the circulators. So the pump differential shows up as positive pressure in the boiler and system.

Air purger wants to be on the hottest point in the system, at the boiler outlets.

Be nice to have a dirt/ magnetic sep with all that steel piping. The engineer should show that also. At the very least Y strainers.

MikeDurigon

Thank you guys for your information. I wondered about where the lowest pressure point would be. On new installations not existing replacement jobs pumps are shown on the supply side. Usually with an air separator c/w connection point for the expansion tank. Simple follow the drawings and run. 😂

Jamie Hall

It really doesn't matter all that much whether the pumps are on the supply, going away from the boiler, or on return, going to the boiler, so long as the expansion tank is on the inlet side of the pumps.

jumper

High point is lowest pressure. That's how pumpless could work on taller buildings.