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What is this device on my heater water intake?

  • It have a black plastic cap on top;

  • It have markings around its body such as:

    2" Hg

  • It connects after the shutoff valve of the heater, and just before the water entry of the heater.

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I have not found an exact match for it, but I'm fairly sure it's a backflow preventer / siphon breaker / vacuum breaker.

Nothing else makes sense with that low of a pressure marking, and I have found similar examples, just not this exact model.

The intent would be to prevent water siphoning back out of the water heater into the cold water line in the event of a pressure loss on the cold water line. Or perhaps preventing siphoning to the hot water outlets is the intent, per this section of the UPC:

UPC 608.7 Vacuum Relief Valves. Where a hot-water storage tank or an indirect water heater is located at an elevation above the fixture outlets in the hot-water system, a vacuum relief valve that is in accordance with CSA Z21.22 shall be installed on the storage tank or heater.

The IPC also wants them:

504.1 Antisiphon Devices

An approved means, such as a cold water "dip" tube with a hole at the top or a vacuum relief valve installed in the cold water supply line above the top of the heater or tank, shall be provided to prevent siphoning of any storage water heater or tank.

504.2 Vacuum Relief Valve

Bottom fed water heaters and bottom fed tanks connected to water heaters shall have a vacuum relief valve installed. The vacuum relief valve shall comply with ANSI Z21.22.

Among the various reasons for that would be preventing the water level from falling low enough to allow the heating means to be exposed above waterline, resulting in damage to the electric element, or tank itself in the case of fuel-fired heaters.

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  • Thanks a lot! It seem to make a lot of sense. Looking at how it is installed (screwed into a socket), do you know if it would work the same way, when replacing the copper pipes with PEX pipes, using a Tee + 3/4 Female adapter? – Alexandre Willame Jan 31 at 2:51
  • Should work, or you might be able to find a fitting that does that directly (not sure, haven't shopped for one.) Check carefully what your local codes say - one of the changes to IPC plumbing code in my area is that all piping within 3 feet (pipe length, not proximity) of a water heat must be metallic, so replacing it with PEX would be a violation in my area. [i.e. they adopt IPC plumbing code generally, but list a few changes to it for the local code] What problem are you having with the copper pipes that you are replacing them? – Ecnerwal Jan 31 at 3:02
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    @AlexandreWillame it needs to be above the heater to be effective. I'm uncertain if there's an orientation requirement but I'd be wary of long term clogging in any other orientation. It seems easier to keep it in the right place by leaving that part of the copper there. – Olivier Jan 31 at 3:11

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