Size Expansion Tank

Joe_8

Paul, The best way is to go onto www.amtrol.com and follow the pages to the sizing and it will compute what tank you need. Generally, you need to know gallons, pressure, and maximum system temperature. Good luck,

Paul_28

I am told

that a DWH tank shouild have a expansion tank on the hot water out. How is that sized? I have a 40gal tank.

Dave Yates (PAH)

you need

the following information to properly size a DHW thermal expansion tank:

coldest incoming water temp seen each year and the final storage temp (I use 140 F). If you follow the Amtrol (or anyone else's charts), you'll find they base their sizing on a 50 F rise - not at all realistic in the real world IMHO.

If you then check out the Wessels web site, they have a very accurate chart listing multipliers for any given start/finish temp range. Multiply that times the tank volume & you'll get the exact expansion rate.

Next comes Boyle's law, which relates to pressure changes as the inverse of volume changes. Stated simply as V1 x P1 must equal V2 x P2. On average, you'll see about 1 gallon of thermal expansion in a 40-gallon tank when heated from 38 F to 140 F. If you have a steady state incoming pressure of 70 PSI and utilize the 2-gallon tank listed, Boyle's Law looks like this: 2 x 70 must equal 1 (the new tank volume) x 140. That's just ten PSI below the relief valve's setting.

But that's assuming two things: you pumped up the 40 PSI air charge to 70 PSI and the incoming water pressure never varies.

You need the larger tank, which has a 4.5 gallon volume. Boyle's law now looks like this: 4.5 x 70 must equal 3.5 x 90. 90 PSI is much more desirable that 140 PSI and the larger tank grants you leeway for pressure spikes and loss of air charge (said to be about 1-PSI per year).

You can also use Boyle's Law to determine volume loss due to acceptance volume reduction created by pressure spikes. Those numbers seriously whack the ability of a 2-gallon tank that's already riding a knife's edge. 99% of the failed tanks we see are the 2-gallon variety.

I'll be going over this in much greater detail in Baltimore for Oil Heat Cares and at ISH-NA.

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Jezz

What's the advantage

Of having a DHW expansion tank? I've never seen one
installed in any of the recent construction projects
I've been involved in. Is there that much air in the water supply to do damage to pipes? Just curious....

Jezz

Dave Yates (PAH)

Jezz

Thermal expansion tanks became necessary following the installation of backflow prevention devices on service lines, which traps the water inside the home's potable water system.

Given that water is essentially non-compressible and that it expands when heated, a properly sized thermal expansion tank gives the water a place to grow.

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Einsiedler_2

tank placement

correct me if I am wrong please, but should the tank not be installed on the cold supply to the tank rather than the hot outlet..

I've always installed them on the cold side.

EIN

Dave Yates (PAH)

yup

One reason is that the air charge will be lost more quickly (so I've been told) and the other is that warm water with potential stagnation in the tank offer a better breeding ground for bacteria. Another reason to boost the bladder's air pressure to match average system pressure.

Since water is not very compressible, the tank doesn't have to be adjacent to the water heater.

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