Relief valve

Is this a relief valve on the cold water line? Can you install on cold water line? No other relief valve on heater casing. Any help would be appreciated. This was an “older” water heater.

Thanks in advance.

Yes and it is OK on the cold line as the pressure is the same.

However it is not a temperature sensing device so a TPR is still required.

Exactly what I was looking for, you da man. Thank you. Have a great weekend!

BTW, this is the oldest one I’ve seen still in operation:shock:

i see the warranty has expired…

no you dont put a relief valve on the cold water side it goes on the hot side because of the heated pressure

The pressure is the same on both sides.

Riddle me this then.
Why are expansion tanks only installed on the cold side ?

No because it is code or manufacturer instructions as an answer ,please.

If you have a watt-210 valve the pressure relif valve goes on the COLD side on the exterior hose bib. Pressure is pressure equal on both hot and cold.

When water is heated it expands. On a ‘closed’ system the expansion tank, on the cold side, absorbs that extra expansion . If you did not have an expansion tank the pressure relief valve would open


The bladder will last longer on the cold side.


  1. Install the THERM-X-TROL® on the city supply line to the water heater at a point
    between the water heater and backflow preventer, check valve or pressure
    reducing valve (see Figure 1).


Yeah ,that was all I could find on it…

When water is heated it expands. On a ‘closed’ system the expansion tank, on the cold side, absorbs that extra expansion . If you did not have an expansion tank the pressure relief valve would open.

The expansion goes to the cold side.



plumbing-hot water heater-expansion tank.jpg

Chuck, by looking at the picture above you will see that water is represented by the red color.
As far as I know the tank itself does not have a check valve though you see one on the cold water side of the piping in this illustration.

In Chicago expansion tanks are required if there is a check valve by the way.

Please explain using this illustration as a guide or reference how only the cold side would expand.

Interesting that it is the hot water doing the expanding.
Another interesting fact is that cold water when cold enough turns to ice on the opposite end of the spectrum and also expands.
Yet an expansion tank would do no good in that case.:slight_smile:


The expansion tank is just that an expansion tank it has a bladder in it, water pressure on one side and air pressure on the other, has the air valve on it to set pressure. When the water heats it expands and we all know you cannot compress a liquid (water). On the hot side (closed system) of the system there is nowhere for the expansion to go. If there is a check valve on the cold side, again (closed system) again there is no where for the expansion to go. This would cause the Pressure relief valve to open with no expansion tank installed. With an expansion tank installed the expansion goes back through the cold side and puts pressure on the bladder, thus expanding into the tank. If the expansion tank was installed on the hot side the same results would occur as the expansion could not go through the cold side due to the check valve.

As for the freezing and the expansion. Freezing on a system with a check valve would most likely occur on the supply side, not water heater/expansion tank side of the check valve thus causing damage to the water lines and components.


what i was trying to say is that the pressure relief valve should be installed on the tank TPR valve means temperature pressure relief valve the hotter the water gets the more pressure the tank builds up inside and if there is no relief valve the tank could explode thats what i meant

OK ,better explanation as you are correct in that it expands toward the cold side when there is no check valve.

(Reason I assume is ,that a typical city water pressure is weaker than the piping or tank construction)(of course a TPR is set in between the two).

Of course with a check valve the pressure would all be equal and have no where to go but out the TPR or if that malfunctioned would choose the weakest point.

If you put a pressure gauge on the hot and cold side they will read the same assuming their is no valve or checks between them(only the tank).

Another question would be why a TPR valve is even needed if there is no check valve.?

Would not the extra pressure created simply push back against the flow of water coming in from the city.?

After all the HWH is only a closed vessel if the shutoff are closed.

The only check valves I ever see are on boiler feed water lines so I 'm not sure I understand but I agree with you.

Of course that assumes the city and their BIG pumps aren’t pushing back :wink:

Well I kept out of the number game because it is different elsewhere ,but here the water pressure is around 30Lbs.

I see no way the pumps could adjust and push back or the check valves would not be needed.

So basically HWH’s only explode if…
Both ends are closed .

H.W. relief valves are temperature sensitive.