Some sort of drain (vacuum or pressure?) at the supply line to a gas water heater. What is this and how does it work? It drains to an opening in the water heater pan drain. Is that correct?
Isn’t that where the bleed opening is on the shut-off valve? Normally there is a screw-thread cap on that (instead of a compression fitting and a pipe) which can be removed to drain the pipe down when the water supply is turned off and one wants to do some plumbing, or I suppose drain down for the winter vacation. I wonder why they have done this? Possibly to bleed the pipe down but I think it would leak when the water was turned on. What is stopping it draining at the other end?
Please do not tell me the bleed cap was leaking/missing and they did this instead of buying/tightening the cap.
I might try and do the same to run my icemaker!
I am sure it is not correct, but I am not an HI. The pipe and fitting risk a leak. But I would think it is an easy fix. Turn the mains off, remove the white pipe and add a cap.
And is that red PEX for the COLD supply line? Curious.
Or is it in the wrong way round? If you put it in the wrong way the bleed port would be on the pressure side of the valve when it’s turned off. Look for the directional arrow on the body of the valve. The arrow should point towards the water heater end of the line.
I am just guessing now.
Yes, red pex throughout. (Painted white at the shut-off valves under sinks.) Thanks for your comments.
We could use some comments from a real plumber. Anybody?
I concur. Waiting before I send out the report. I did not un-tape the drain at the pan to see if anything was coming out.
Hi Joe,
Concerning the first pic…Which way does the water flow? left or right?
I’ve had valves with similar setups (though always copper tubing). Generally they are just taking water from that spot and feeding something (icemaker in garage, etc…)
Based on the 2nd picture (which I guess is sideways) did the tube exit the drain pan line and go somewhere? Or could there be a reason for flushing the drain line?
Sorry I’m not much help, but I’m trying visualize this.
Bruce
In the first picture, I believe the water is flowing to the right or in the direction of the handle toward the water heater.
The 2nd photo is looking down on the water heater drain pan. The drain tube is taped to an opening on the drain line for the drain pan.
That has the appearance of a full port stop and waste valve, but it is what’s called a full port “side tap ball valve”. These are used most often to feed ice makers. If the dwelling is on a well, that port is sometimes also used for chemical injection, such as with chlorine injection. You’d normally see a check installed right at the valve’s side port, in that case. When side tap valves are used in a gas application, the side tap port normally goes to a pressure gauge or similar monitoring equipment.
Then my guess is that water is constantly flowing out of that line. Could this be a crude attempt to reduce water hammer? Somebody knew you were coming!
I do not even know what water hammer is but my guess is that this would stop it! 
What you have there has the appearance of a full port stop and waste ball valve, but it is actually a ball relief valve. These valves normally utilized on water heaters.
Your specific valve appears to be a Watts Regulator model BRV.
http://www.watts.com/prod_images/hi-res/B-6080.jpg
See page 23 of this PDF.
My concern is that the relief line, as small diameter as it is, may “whip” out of the drain tee if it is ever pressed into service.
You’re good Marc. I’m now thinking it’s a case of ‘bought the wrong bit’ or ‘it’s the only one left’.
That’s why I’m not a plumber!
I’m lost. I still do not understand the purpose of hte relief line. For what reason whould it ever need to be “pressed into service”.
Follow the link I provided to the .PDF document, and it will become clear.
Heat-related pressure increase due to thermal expansion on the cold line. Sounds like global warming gone mad.
Nope, just regular operation of a water heater. After all, you’ve seen X-Trol tanks on water heaters? Traditional pressure relief valves? There is no check valve on the input nipple of a water heater, so any thermal expansion will be observed on the hot and cold sides of closed systems.
I do not see why any of this would not be dealt with by the ordinary pressure relief valve, which this is obviously not (I assume the ordinary one remains on the heater). Is it to regulate smaller (i.e. less dramatic/dangerous) pressure increases?
Bingo. The required TPR is for worst case. Any additional pressure relief is to maintain or enhance system performance. These optional valves are available in several more pressure settings, as low as 75 and 80 lbs. THey were required by at least one PEX manufacturer about 10 years ago, but I believe that requirement has evaporated now.
I normally install an expansion tank on the cold water supply line to tank for thermal expansion. So what your saying is that this valve does the same function as a expansion tank? If you were to install a new waterheater, can you just use this watts valve instead of a expansion tank? Am I understanding this or am I wrong?