hot side shutoffs prohibited of DHW?

[Hearthman]

Ok Jerry, I have one that was raised on an HVAC forum: does installing a shutoff valve on the hot discharge of a DHW tank isolate the TPR and cause a potential problem? Can you cite a prohibition of this practice? A point was made the downstream of a hot side shutoff would require a separate TPR downstream of that shutoff. I did not see anything in the 2009 IRC /Ch. 28 but do not see where it would violate P2803.6
TIA,
Hearthman

[Jerry Peck - Codeman]

Hi Bob,

A good question was raised but it appears to me that their thought process of all water heater connections was not completed.

I have one that was raised on an HVAC forum: does installing a shutoff valve on the hot discharge of a DHW tank isolate the TPR and cause a potential problem?

Let's start with the typical domestic water heater plumbing installation:
- Cold supply line to the water heater has a shut off valve in it to allow for shutting off the cold water supply and removal/replacement of the water heater without having to shut off the water to the entire structure. Not only is that cold water supply shut off valve required, but it makes very good sense too.
- The cold water supply is connected to the cold water inlet of the water heater.
- The T&P relief valve and its discharge pipe is connected to the specified T&P relief location. This piping is completely independent of the cold water piping and is completely independent of the hot water piping.
- The hot water outlet from the water heater is connected to the hot water system of the structure - no shut off valve is required, nor do most codes prohibit a shut off valve from being installed (I believe the Chicago Plumbing Code may prohibit the installation of a valve in the hot water outlet piping, but then, Chicago walks to the beat of a different drummer and their reasoning is not always based on rational analysis, and frequently seems to be based the strength of their respective unions in allowing or prohibiting certain materials and methods used in construction).

A point was made the downstream of a hot side shutoff would require a separate TPR downstream of that shutoff.

Not sure how that thought process came about as the T&P relief valve is completely separate from the hot water out piping, in fact, the T&P relief valve is installed in its own separate opening in the tank and its discharge piping is not allowed to be connected to anything else.

So, let's look at it this way: let's say that a plumber disconnected the hot water piping of the structure and installed a plug into the hot outlet opening - that would pretty much be the "worst case scenario" would could encounter, right?

Okay, you now have a water heater connected to the cold supply, with a T&P relief valve installed in its opening, with the hot outlet plugged. You open the cold water supply valve and let the tank fill with water, which it will have a difficult time doing as the air in the tank will pressurize, so to aid in filling the tank we hold the T&P relief valve open to allow the air trapped in the tank to escape while the water fills the tank. When all the air is out of the tank and only water is coming out the T&P relief valve, we allow the T&P relief valve to close. The cold supply valve is still open, so the tank pressurizes to the supply water pressure.

Now we energize the water heater (whether gas, electric or solar) so the water in the tank heats up.

We know that the water will expand as it heats up, so if no provision is installed for that thermal expansion the water will: a) push back into the supply pipe; b) possibly manage to drip out the T&P relief valve (although we must remember that the T&P relief valve is set to open at 150 psi (which is considerably above allowable operating pressure for a water system designed for a maximum pressure of 80 psi) and it also opens at 201 degrees F (which is considerably above the maximum intended temperature for domestic hot water of 120 degrees and is even above the 140 degree F temperature some people set their water heaters at). Those settings mean the T&P relief valve *should not* open due to thermal expansion. Which leaves pushing the water back toward its supply - which would be a problem if the system is a closed system with pressure reducing valves installed and/or with check valves installed.

Thus, other than removing a limited amount of small water piping from serving as a thermal expansion area from the water heater which is now plugged directly at the hot outlet, there is nothing different than if the hot water piping was installed and connected to the tank, and certainly no different than if a shut off valve were installed at the hot water outlet.

The above simply shows the necessity for a thermal expansion tank, but nothing from installing a shut off valve in the hot water outlet from the tank which would affect the T&P relief valve.

The only way I can see there being a problem is if the AHJ allows, or does not prohibit, the T&P relief valve from being placed downstream from the tank in the hot water piping (the T&P relief valve *is required* to be in the top of the tank, so allowing the other location would be a code violation, and, in addition to allowing the tank to go BOOM! in the night, it would simply be a dumb (real stupid) idea to allow the T&P relief valve to be located anyplace other than in the top of the tank as is required.

So, yes, *IF* the T&P is allowed to be installed downstream of the water heater, and *IF* there is a shut off valve installed in the outlet of the water heater before the T&P relief valve, you would now have TWO totally dumb and really stupid acts which could lead to a water heater going BOOM! in the night.

But that is acknowledging that it would be installed all wrong to start with, and the valve in the hot water outlet of the water heater would not be the problem, the location of the T&P relief valve would be the problem.

If you can envision something I am missing, please pass it on to me.