What Size Expansion Tank?

[emesine]

According to my calculatons, my system has the following volumes:

Boiler: 80 gallons

Main Line: 10 gallons

In-floor PEX: 64 gallons

The in-floor PEX will fluctuate from about 70F to 100 F during operation of the system.

The Boiler and main line will fluctuate between about 50F and 180F during operation of the system.


What size expansion tank should I use?

Thanks!

Andrew

 

[RobC]

I think the rule of thumb is 10%. Water expands at 4% and your suppose to have a 2 times safety margin. So that gets you to 8 or 10 %.
You might be able to get better calc's if you search expansion for storage. There have been previous posts.

Rob

 

[joecool85]

I'd get a 20 gallon expansion tank, should more than take care of you.

 

[DaveBP]

Andrew, there is a common confusion in dealing with expansion tank sizes between the overall volume of the tank and the amount of expanding water it can take. I've seen products listings that give them both ways. That "acceptance volume" is the amount of water that the tank will absorb and then give back to the system without the pressure building too high. That's usually about half the volume of the tank that the bladder (assuming you're talking about closed system tanks) is contained in. This is what Rob meant by the "doubling" factor.
The larger the expansion tank in a given system the less the pressure will rise as the system heats up. Most of the calculators that I've seen aim at preventing the pressure from rising more double at maximum.

Here's one calculator;

http://www.watts.com/pro/divisions/watersafety_flowcontrol/support/support_ETsizing.asp

And on page 8 here;

http://www.amtrol.com/pdf/extrolbrochure.pdf