Thermo expansion tank on water heaters

Paul LeGron

Paul LeGron

Thermo expansion tank on water heaters

I have a possible job of installing a RPZ backflow preventer in an existing 2" domestic water line. This building has (2)two 75 gal. 100,000 BTU water heaters piped in series. Do I have to install expansion tanks on both water heaters or just the cold water line on the first heater. If so what size of expansion tank would I have to install. Thanks to any one who has any input on this subject.
paullegron@yahoo.com

Dave Yates (PAH)

V1xP1 must equal V2xP2

That's Boyle's law of perfect gases in a nutshell.

Works like this:

P1 = incoming pressure & V1 = expansion tank volume, which can be the original tank volume if you add air pressure to match the incoming line pressure. The answer will be C, which is the constant that must remain the same.

You'll need the thermal expansion rate multiplier, which can be had using the Wessels chart, but at a 100 F rise, that's .01501. You're dealing with 150 gallons, so the acceptance volume will be 150 x .01501 = 2.2515

Therefore if V1 was 4.5 and P1 was 70, the constant would be 315

4.5 - 2.2515 = 2.2485 = V2

P2 must now equal 140.09 PSI in order for V2 x P2 to equal 315

Still under the T&P relief valve's limit, right?

Let's give our system an overnight pressure spike.

V1 = 4.5 & P1 = 70 & C= 315

P2 gets a 20 PSI boost to 90. What's V2? V2 now equals 3.5

We know the thermal acceptance must be 2.515, so V3 will be .985. What's P3?

You need a larger TXT!!!

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