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From the AO Smith gas water heater instructions:

The Discharge Pipe:

• Shall not be smaller in size than the outlet pipe size of the valve, or

have any reducing couplings or other restrictions.

• Shall not be plugged or blocked.

• Shall be of material listed for hot water distribution.

• Shall be installed so as to allow complete drainage of both the

temperature-pressure relief valve, and the discharge pipe.

• Shall terminate at an adequate drain.

• Shall not have any valve between the relief valve and tank.

If the CPVC is rated to carry hot water, then barring any local code, I would say it's ok. If the house is plumbed with CPVC, it carrying the same temp water (give or take) that would be coming out of the t&p valve.

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It is a listing and labeling thing. Some manufacturer's don't want their product within 6 inches of the draft hood on top of a gas water heater. Since many plumbers use the top nipple for the T & P instead of the side nipple, that puts the CPVC within 6 inches of the draft hood.

It's probably not even a founded worry. If you check out the Plastic Pipe Institute website, you'll see that burst pressure for CPVC pipe at boiling temperature is over 2000 psi. However, rules is rules.

With PEX it is definitely a concern. Most PEX manufacturer's don't want it within 18 inches of the water heater.



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Originally posted by swarga

I have read that you cannot use CPVC on the TPR line within 12 inches of the valve on a gas water heater.

Where is that written?

I inspected a new house today and the city inspector did not call it out. I want to have my facts before I call it out.


This is probably too late for your purposes, but I think your question may be confusing two issues: plastic pipe on the hot supply side, and plastic pipe used for the TPR valve extension. It's PVC that's not allowed for the TPR extension (since it's not approved for hot water use). CPVC's OK. Re the hot supply, I've read a manufacturer's spec (I think US Brass) on PEX that says they don't want it within 18" of the WH. I've seen two PEX lines turning yellow right near the myself.

I've never heard or read anything that limits use re a gas, vs electric, WH. If anything, the proximity issue would only pertain to proximity to the vent connector, since that's the only substantial heat difference between the two kinds.

-David Leein VA

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