Getting ready to plumb in my new boiler and have a question as to the best place to put the expansion tank. My old system was a single zone setup with the pump pushing through the boiler, through the expansion tank, and out to the loop. My Lochinvar installation manual shows the expansion tank before the circ pump. Does it matter where the expansion tank is placed? It has an air scoop and autofill valve. Old system was plumbed as follows: return line>----circ pump>---boiler>---expansion tank>---supply loop. New system: return line>----boiler>----expansion tank>---circ pump>---supply loop.
Thanks.

 

Yes it matters where it can be placed. It should be at or near where your air elimination sits. It can be on either loop of your system (primary or secondary) and either will work just as well for the most part.

The air elimination device, scoop or spirovent, should be placed just up from the pump(s), 18" no bends as a good example depending on pipe diameter. This could be before the system pump, but the boiler may already have an AAV on it in which case it would be better to do the tank on the heating loop, just never isolate it in situations where you might only want to have DHW piping active.

From there I would actually use a separate side tee to pipe to the expansion tank(s) and place it right after the spirovent so it helps keep crud from dropping on the rubber diaphram in the expansion tank. Piping the path by starting with a lateral tee also helps crud from ending up in the tank. I would also try and put maybe 6' or more pipe between the side T and the expansion tank - this could be even in 1/2". Mounting the tank lower in the system reduces its air charge capacity so try and go over and then down. This way the tank won;t be getting heated up and cooling off to the same degree. The tank will be a far steadier temperature.

 

You pump away from the expansion tank.

See Dan Holohan's books on hydronics:
Heating Help

 

You put the expansion tank and all the other parts of the system exactly where Lochinvar tells you. Their diagram is very clear and easy to follow.

 

xiphias NAILS it.

You are asking for all sorts of trouble down the road if you dont follow Manufacturers Piping.

You have no recourse, if there is a warranty issue in the future.

 

Pump AWAY from the boiler.
Applies to cast iron boilers, or low restriction mod-cons.
Pump into the high head mod-cons (you know who you are ), and some (if not all) copper tube boilers.
Follow what the manufacturer tells you, unless you have a VERY good reason not to.

Air elimination should be placed on the hottest, lowest pressure point in the system, and not all require 18" of straight pipe to work effectively.

Expansion tank must be at the suction side of the circulator to prevent cavitation issues with the pump if you try to pump into the expansion tank, remember what it is there to do.

Basic hydronics 101.