whats this on the water heater?

I cant figure what the thing is on the water heater with the black cap --can someone please help me out?

My only thought, is some kind of water hammer arrestor.

yeah your right - that is what i seems like - but i hate to assume.



Water Service Vacuum Relief Valves


1/2, 3/4 in. (15, 20mm)


Series N36 Water Service Vacuum Relief Valves are used in water heater/tank applications to automatically allow air to enter into the piping system to prevent vacuum conditions that could siphon the water from the system and damage water heater/tank equipment. It consists of a brass body construction with NPT male inlet connection, and a protective cap. Series N36 has a low profile design, is tested and rated to ANSI Z21.22, and is CSA certified. It is suitable for low pressure steam and water service, and is ideal for use in domestic water heaters and supply tanks, table top heaters, jacketed steam kettles, unit heaters, low pressure steam systems, and steam coil heaters. Maximum Working Pressure: 200psi (14 bar), Maximum Steam Pressure: 15psi (103.4 kPa).


Have you ever seen one hooked up to a HWH before?
It looks like a radiator knob,so it’s funny that it can handle steam.

No Bob but I knew it was a Watts from its shape.

It’s part of their water heater safety protection line.

I just found an excerpt from the Illinois water heater code that states they are to be used when the fixtures are below the tank.

Joe was the HWH in the attic or top floor.?

Here is the excerpt…

f) Vacuum Relief Valve. Where a hot water storage tank or water heater is located at an elevation above the fixture outlets in the hot water system, or if the storage tank or water heater is bottom fed, a vacuum relief valve as listed in Appendix A, Table A (Approved Standards for Plumbing Appliances/Appurtenances/Devices) shall be installed on the storage tank or heater.

Link to the page…

Off subject ,but an interesting tie in for Illinois Inspectors regarding the many posts on relief valves.

Any piping used for discharge from the relief valve shall be of metallic material and conform with the requirements of Appendix A, Table A (Approved Materials for Water Distribution Pipe) for potable water piping and shall drain continuously downward to the outlet.

Yeah…and dishwashers can’t drain into the garbage disposal. But how many times do you not write that up…


On my very last Inspection as a matter of fact Jeff.
I used to install all that stuff.

I am very a-nl on my reports.

The disposal has potential for cross contamination to the dishwasher.

In the city ,plastic water line is a no no.

I’ve never seem one like that here Bob.

Learn something everyday Joe…I have never seen one on a water heater configured like that.

If you mean the vacum breaker ,I never noticed one while Inspecting.

How Larson comes up with these pictures ,I will never know.
Guessing he has a picture recognition software or he is Vulcan.

Most of the time we will see the attachment to the disposal. If the hose is attached well above the bottom of the sink, I will rarely call it out.


You could ask Mike Meeker he should know

It was neither bottom fed, nor located above a fixture outlet ( except for the normal laundry tub and a washer hookup)
it was in the basement, just like i always see-

:slight_smile: isn’t the Internet amazing?? Im ashamed i didnt know where to look. Now i learned something, and i thank you Mike.

Ask him what,and why defer to his genius?


A vacuum relief is only used on any area where cross connection could happen. If I were to find this in Chicago area I would flag it. You have a T&P and a ball valve to shut down the water supply. Is this water heater in a munti unit property. Even if it was the unit would have an isolation valve. Contact me if you need help in the future. Im a licensed plumber LIC# Chicago pl194988

Plumbers are rich,thanks to Union Control
What are you doing Home Inspections for?