Pressure Release Valve location - Ariston Undersink Water Heater

23 Jan 2017
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United Kingdom
Hi all, my first post.

I am installing an undersink water heater in the garage loft.

The instructions say the pressure release valve must be on the cold feed and the diagram show it right next the heater, my question is can i install it below the floor. i.e. in the garage below (as drawn)

It also says the water feed must be 2.8m from the nearest cold water draw off, i assume that means the toilet looking at my diagram?

Thank you for any help.

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This might help you understand the 2.8m requirement. Also I believe anything from the PRV is meant to go into a tundish and also be visible and drain off to a specific place, but that might be only for certain size water heaters.

But I wouldn't go fitting the PRV under a floor where you wont see it and a flood could develop
Thanks for the diagram, it needs the pipe for expansion so even though my pipe tees before 2.8m i am going to assume i can keep counting the pipe after the tee and would have alot more than 2.8m to a draw off.

PRV would be under the floor but at ceiling height in the garage so i will see it/have access. I just cant find anywhere that says that is/isnt allowed or a really bad idea.
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I fitted one of those once. Well actually twice.:D
Fitted a pressure reducing valve then 16m of 15mm plastic piping. Lasted a year before it busted.
I thought the 2.8m minimum they talk about was the minimum run of plastic pipe to allow for expansion. ie.. the pipe flexing.
I had 16m so thought it would be OK.

The 6 bar pressure relief valve must have acted an ornament.
Went back with a new one anyway and fitted the expansion vessel. No problems since.
And what I learned since was that the 2.8m is to ensure the hot doesn't reach the cold tap T off and expansion goes back down the mains pipe.
Last edited:

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