Water Shutoff Valve - Affect on water heater

I am considering installing a Z-Wave shutoff valve on my main water valve.

My question is how shutting off the main water valve affects a gas water heater. That is, if the main water valve is shut off, whether there is a leak in the hot water system or not, is the water heater at risk? I assume that eventually (over weeks) the water heater will run out of water. Will the water heater be damaged if it runs out of water?

I know that I could also install a Z-wave switch on the water heater power line. But the location of my water heater is in a really bad Z-wave coverage area, and that is why I am asking.

Both hot and cold water in your house work off of the pressure from your water main. If your water is shut off, you can open your hot or your cold water at any sink and nothing will happen.

You water heater won't go dry if your water is shut off, it's a sealed container so nothing to worry about :smiley:

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There is some small risk, in theory. If the water system does have a leak, and that leak is below the water heater (or the heater itself is leaking) it could drain the unit.

If there was no leak, the system would be closed tight, and any temperature fluctuations would cause the pressure in the system to rise or fall. Too much pressure and it would likely relieve some small amount of water out of the relief valve on the water heater.

I'm not sure how likely either of these is to cause an issue under actual conditions, but I'm fairly sure that most manufacturers will recommend shutting the water heater down if the water supply is shut off.


One would hope, like a boiler, there's a low water shutoff... But in the immortal words of Ronald Reagan, "trust but verifiy."

I have never seen a residential grade gas fired water heater with a low water shutoff.

Yes but you run the risk of developing Legionella bacteria in the storage tank filled with tepid water over a period of time.

I agree with @dylan.c that since it is a closed system, the likely hood of losing water in the tank is not likely, but that would depend on how long and under what conditions the water supply will be shut off. Normally if I am only going to be away from the house for a week or two I would just shut off the water supply to the house and leave the water heater on with a temperature setpoint of at least 120 deg F.

The OP mentions a period of weeks, I suppose if I was going to be away for weeks or months I would consider shutting off the water supply and gas and draining the tank.


Everyone has their own level of comfort. Personally I believe the odds of having a leak that when the main water is shutoff the hot water tank drains is low enough to ignore (or at least not be concerned to the point of adding even more automation to avert).


Lots of good opinions in this discussion. Sometimes the "solutions" cause more trouble than the "problems." I remember one time in my early days as a homeowner we took a week-ish vacation. I set the water heater to the "vacation" setting right at the unit's controls. I suspect it never ran the whole time we were gone. When we came back and I put it back into normal, it wouldn't fire up. Spent a couple hours troubleshooting the thing to and fix the source of the issue. It turns out that the tubing between the draft inducer fan and the pressure switch was plugged. Now I just leave it alone.

Similarly, my in-laws once took a week vacation in January. We live in southern MN, and their water line runs underneath their street. With no snow cover and a couple days of frigid temps the frost got pushed pretty deep. Combined with no water flowing through their service line, it froze up. Took something like 3-4 days for them to locate and thaw the blockage. Excavation was involved, if I remember correctly.

Anyway, just a few examples of how changes from "normal" use can end up causing unintended consequences. I'm pretty sure I wouldn't mess with my water heater unless I were leaving for a month or more. And if I ever take a vacation in January, I might even leave the water running a bit...just in case. As @JohnRob said, everyone has their own level of comfort.


It certainly does make a big difference where you live. Having lived my whole life in southern Louisiana and central Texas, I would not even begin to make suggestions for someone living in Minnesota.


Thanks for the input.
Thought I would add a few more details to my original post. We live in San Diego, so weather is not an issue. All of automatic sprinklers are within the main water valve so we can't just turn off the main valve without turning off the sprinklers. We are retired and do leave for a couple of weeks to 8 weeks, at a time. Though, sadly, we have no HA in our RV. We manually turn the water heater down to the "vacation" setting before we leave. And we have previously experienced a couple of water leaks. We have water sensors in the house and so would be alerted to a leak. We could then remotely turn off the main water valve. But as per my original post, I have been concerned about the water heater running dry. Perhaps instead of turning the water heater to "vacation", I should just shut it off. ???

A water leak is normally considered to be an urgent situation. If you have the system set up to turn off the main water valve, you should receive alerts via voice, text, and email indicating that that has occurred. The lead should be addressed as soon as possible, not several days later.

If the leak is at the gas hot water heater itself rather than somewhere else in the system and a hot water faucet is opened, the leak can continue until the hot water tank is completely drained. With either an electric or gas hot water heater, this would be a potential safety hazard should the tank overheat. However, this is not something that is going to happen quickly. If you respond to the leak in a timely manner, there should be no issue.

I do recognize that if this is in an unoccupied site (a vacation cabin for example), you might not be able to respond immediately. If you are going to be away for an extended period of time, it might be best to turn off the water and the gas and drain lines subject to freezing if that is a potential risk.

My wife has a friend who owns a condo 2000 miles away from her main home. They have had water leaks on two occasions at the property. The damage done ran into many thousands of dollars.

Have you considered getting a Dome shutoff valve for the water heater natural gas line in addition to the Dome on the water line coming into the house? You could synchronize them so when the water valve closes, the water heater valve would close also. Need to make sure the gas supply to the furnace is not shut off also.

Just checked on Amazon and the only one available is used. EBay has several, both used, open box and new.

I don't know how you house is situated but for me I would have the leak sensor immediately shut off the water, then notify you. Logic: if there is a major leak and you can/don't respond in minutes a lot of damage can result. Likely much more than the cost of a water heater.

In the meantime you might consider a simple test. Turn the main water off and simulate a leak (using a faucet, hose connection etc.). If the heater doesn't drain then you will have a more comfortable feeling shutting off the water.


Can’t speak to on-demand, but the residential tank water heaters I’ve dealt with have always been water pressure driven; i.e. if the water is shutoff there is no force to drive water out of the tank. Suppose if the tank was located in an upper story you might be able to get some pull out by opening a lower valve but not enough to effect the tank. Only scenario that I can think of that empties the tank is one in which the tank has already been compromised.

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As long as the hot water heater itself does not develop a leak, you do not have to worry about it draining as the water inlets and outlets are on the top of the tank. However, if a leak developed in the tank liner or at the drain valve at the bottom of the tank leaked, then the tank might empty over an extended period of time. If the drain valve broke off completely (unlikely), then the tank might drain in about an hour. Personally, I would be far more concerned about the damage the water leak itself could cause than the risk of damage to the water heater.


One caution. I have leak sensors behind the toilets in case of a leak. When mopping the floor of the bathroom, be sure to take up the sensors. My wife has forgotten them on a couple of occasions. The mop water is sufficient to trigger the sensors which turns off the water supply valve. She has learned to pick up the sensors before she mops, but I often come back hours after the floor has dried to find the sensor up on a shelf where it does not do much good.

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I have a "housekeeper" mode that disables leak-sensor based automations. You could consider a "housecleaning" mode or variable that does the same ...... maybe control this mode/variable using a virtual switch that is linked to Alexa/GH, or a button controller.

The "housekeeper mode" would be a possibility, but I never know when my wife is going to clean each of the bathrooms. I would need a button controller in each bathroom to turn off the leak sensor for a specific period of time and then have an automation turn it back on after enough time has passed for the floor to dry. However, you still have the issue of the wet mop knocking around the sensor if you do not pick it up.

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Ah ... my sensors are in fixed spots.

Speaking of sensors... we had the PRV fail on our water heater a few years ago. The drain on the PRV goes up and out of our basement to the "blind" side of our house (the side we rarely visit), and we didn't notice. Of course, it failed just before we left for vacation, and was leaking the entire time we were away (two weeks). It leaked so badly that the ground outside the house was saturated (and 110F) about six feet in every direction, and leaked back through the foundation wall into the finished basement, which is how we discovered it.

The new heater and PRV now have a Tasmota-based temp sensor on the pipe, and that sets off notifications if it's much above ambient. I use the same Tasmota rig to monitor temperature at the end of my hot water recirculation loop as well, and use that to reduce pump runtime and wear.

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So it seems the moral of these stories is ......... don't go on vacation.