Location of the Expansion Tank and Circulator

Normally the expansion tank is mounted on the discharge side of the boiler on the air purge valve and the circulator is mounted on the return side of the boiler.
But I read a artical that said you should mount the expansion tank on the suction side of the circulator so you are pumping away from it.

reading the mounting instructions of the expansion tank it shows the expansion tank on the discharge side of the boiler in the air purge module and the circulator mounted just pass the air purge valve pumping away from the expansion tank.

I always thought that the circulator should be on the return line because the water is cooler and more dense and the pump will pump better.

can anyone clear this up?

Thanks for your input.

The reason that most systems have the pumps on the return is..... is..... habit! Old habits die hard, and this one is a tough one to break. The reason they put the pumps on the return was because the water is cooler and the original pumps would not take the heat. All new pumps will take the heat.

I just read a book called "pumping away". It gives insight on just why the proper way is to pump away from the boiler. While it might seem that a pump pumps cooler water better, it is not better when that pump is part of a boiler system. If I had to do it over I'd pump away.

I'll leave space for others to give details.

I did some searching and finally found the artical, here is the link;
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Disclaimer: I'm not a plumber, just someone who read a few books.

If your pump is already on the return side, you can leave it there and put the expansion tank before the pump. Both will be on the return side.

You can do this as long as you have a traditional, 10-15 psi system. The reason you normally do not want to do this is that some systems operate at higher pressures and by pumping into the boiler it could increase the pressure to 30 psi or greater and cause the relief valve to pop!

Why would you run a system at more than 15 or 18 lbs. presure?
DON

According to the book, in a tall building you need a higher psi to lift the water to the top and pressurize it.

I can only assume he knows what he's talking about.

I could use Yanche right about now, he could probably explain it!

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Higher pressures within reason are desirable. Taller buildings need higher pressure but it is also needed to avoid pump cavitation. Cavitation occurs when the liquid's pressure drops below its vapor pressure. This can happen within a pumps impeller. It will destroy a pump. The higher pressure in effect pushes the vapor into the liquid.

Ol' rule of thumb: It takes five pounds of pressure to push water up each ten feet of pipe. If you have a two story house, it'll take 10 pounds of pressure just to get the water to the top floor. If you have a four story building it'll take 20 PSI.

I honestly don't know how they plumb 100 story buildings..... maybe that's why steam is popular in large buildings?