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Old 05-04-2012, 01:29 PM   #1
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water heater supply lines

Hello, I'm going to be installing a new water heater in about a week. The old water heater had a 1/2 going into and coming out of the tank for supply lines.

Do I need to increase these sizes or can I just put in a reducer from 3/4 inch to 1/2 before I install the flex hoses? What are the code requirements in the International Plumbing Code? and am I required to install a thermal expansion tank...is there a required tank size I should install?

Any recommendations would be very appreciated? Thank you


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Old 05-04-2012, 02:16 PM   #2
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When it comes to pipe sizing, a water heater is no more then another a piece of pipe. If your 1/2" lines are sized correctly, then thats all you need at the heater.

As for the expansion tank- I'd put one on. Any location on your cold line will do as long as its not isolated by a valve.

I can't speak for IPC codes though. Besides, a lot of cities, counties, states, etc. have their own amendments that over ride the general code requirements. Best to confirm with your local authority having jurisdiction. (AHJ)


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Old 05-04-2012, 03:14 PM   #3
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Originally Posted by kingbuggy View Post
... and am I required to install a thermal expansion tank...is there a required tank size I should install?
A thermal expansion tank should always be installed if there is a backflow preventer check valve installed between the water heater and the water source. A backflow preventer could have been installed by the city, or for a sprinkler or irrigation system. Even if no backflow preventer is installed and the source pressure is high (like city pressure of 110 psi for instance), a thermal expansion tank can prolong the life of faucet and toilet seals and even water heater life by not having constant cycling of normal and high pressures.

See this link for sizing of the expansion tank:

After entering the values in the blue fields, click the gold field to see the results. Click the "Show expansion tank chart" button to see the expansion tank sizes and specs. Note that varying the supply pressure has a large effect of the size of tank needed.

A calculation that I did for a 40 gallon heater, 80 psi system pressure (80 psi is a max pressure. 60 psi is a recommended pressure.), 150 psi relief valve and 140 degree max temperature requires a 2.1 gallon expansion tank. --- An 80 gallon heater with the same values requires a 4.5 gallon tank.

Note that a "pressure reducing valve" (aka pressure regulator) should be installed if the source water pressure is higher than 80 psi since 80 psi is the maximum working pressure for expansion tanks (Watts expansion tanks anyway). Recommend that your system pressure be set at 60 psi.


Last edited by Homerepairguy; 05-04-2012 at 03:18 PM.
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Old 05-04-2012, 03:46 PM   #4
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Thermal Expansion Tank


Plumbing Code normally requires a Thermal Expansion Tank in any closed water supply system (Backflow Preventor, Pressure Reducing Valve, etc).

In addition to preventing potentially unsafe pressures, the expansion tank will reduce pressure fluctuations during the heating cycle- this is one of the leading causes of premature water heater failure. Check your water heater installation manual and it will probably reference the expansion tank.

Sizing is fairly straightforward if you know the size of the heater and approximate city pressure.


Glad to assist if you have additional questions regarding tank selection or installation.

Ken C.
Amtrol Inc.
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