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Expansion Tank for Water Heater

Rick18071 said:
Wouldn't this include a check valve?
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I'm not sure what you're asking. Water hammer arrestors do not have check valves.

It doesn't matter how many check valves or shut off valves you have downstream of your water heater (technically, everything downstream of your water heater is a closed system when all the faucets, spigots, showers, etc, are off).

The expansion tank MUST ALWAYS be connected by a permanently open pipe to the water heater tank: NO check valve or shut off valve should be placed in the piping between the expansion tank and the water heater tank. The expansion tank is placed on the cold supply side of the water heater so that the tank bladder is not subjected to high temperatures from the hot water, but pressure-wise, the hot side and cold side are at the same pressure.

You can have as many shut off valves and check valves as you want on the water piping (cold or hot) upstream or downstream of the water heater as long as NO valve exists between the expansion tank and the water heater tank. The purpose of the expansion tank is solely to absorb the additional water volume created by thermal expansion within the hot water heater as it heats cold water. Without the expansion tank, that increase in volume due to the water heating has nowhere to go, and since liquids are essentially incompressible, it builds pressure inside the tank. At best, the pressure will cause your T&P safety valve to pop-off to relieve the pressure (but realize that even when the T&P opens, the stress of repeated cycles of 150 psi pressure WILL weaken the tank and cause it to fail prematurely). At worst, you could have a catastrophic water heater tank explosion.
 
No, a check valve only prevents backflow. It is a one way valve and does not discharge any water.

It is the end appliance that requires a hammer arrestor.
 
Rick18071 said:
Doesn't a check valve close very rapidly and need a water hammer arrestor?
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Swing-style check valves have the potential to create water hammer because, under certain circumstances, the water can start flowing backwards before the swing gate closes. This is why swing check valves are not recommended for home water piping. Spring check valves do not have this problem.
 
Many moons ago I installed a swing-type check valve between my well and pressure tank after the coupling for the submersible pump came loose and all the water siphoned out of my system. I learned quickly that I should have installed a slow-closing check valve when I heard a loud CLANG every time my well pump shut off.
 
Ty J. said:
No, a check valve only prevents backflow. It is a one way valve and does not discharge any water.

It is the end appliance that requires a hammer arrestor.
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Thanks. The code doesn't say what kind of quick closing valve of where it is located to require a hammer arrestor. I also never saw any valve listed as a quick closing valve. To turn off my lab faucet I just need to push it in and it closes pretty quick. Would this be a quick closing valve?
 
Rick18071 said:
Thanks. The code doesn't say what kind of quick closing valve of where it is located to require a hammer arrestor. I also never saw any valve listed as a quick closing valve. To turn off my lab faucet I just need to push it in and it closes pretty quick. Would this be a quick closing valve?
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Depends....

Definition of "quick acting" is subjective. Code is not always clear, is it.
 
Typically if a valve has a solenoid that closes it quickly such as a clothes or dishwasher or icemaker, etc. It is quick close and considered quick close. If you do it by hand, it is not.
 
Michael.L said:
One other thing to consider regarding whether or not to install an expansion tank. If the homeowner (or whoever) shuts off the cold water supply valve that feeds the hot water heater, but forgets to also shut off the water heater thermostat, then there's a risk of the tank bursting if there is no expansion tank.

This is another reason why I advise simply installing an expansion tank for every tank water heater. ALWAYS install the expansion tank between the water heater's cold water supply shut off valve and the water heater.
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Wouldn't your temperature and pressure relief that is required take care of this. The bladders is in the expansion tanks can and do fail.
 
Long, long ago I worked for a pool plastering company in the SF Bay area. At a mountain top property we plastered a water tank that was built for fire fighting reasons to protect a mansion. I was told that the water pressure was up there. My hoses swelled to twice their normal diameter. I could stand them like a flagpole. Nothing blew.

I was in the laborers union and worked 70+ hour weeks for months on end. I did it for about three years.
 
Gregg Harris said:
Wouldn't your temperature and pressure relief that is required take care of this. The bladders is in the expansion tanks can and do fail.
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The expansion tank is your first line of defense: it prevents the build-up of high pressure. The T&P valve is your second line of defense: it relieves pressure after it has already reached high levels. You MUST have a T&P valve. You SHOULD have an expansion tank.

Let's say you don't have an expansion tank and only rely on the T&P valve. If the water piping is a closed system (due to a check valve, backflow preventer, closed supply valve, etc.) then every time the water heater fires, the water pressure has the potential to reach 150 psi before being discharged by the T&P valve. Normal water pressure is typically in the range of 45-60 psi, so you are more than doubling the water pressure. Because pressure acts on surface area (pounds per square inch), the water heater is subject to a significant amount of force due to the tank's large internal surface area. This repeated expansion and contraction (due to high/low pressure cycles) fatigues the tank, especially at the welds. Note that it can also crack the glass lining, subjecting the steel tank to internal corrosion. All of this can weaken the tank, resulting in early failure.

In a worst-case scenario (closed system, run-away thermostat, malfunctioning T&P valve), the water heater tank can violently explode.

EVERY mechanical device is subject to failure, including the bladder in an expansion tank. That's why regular system inspection and maintenance are important. It's also why it's better to have layers of defense, rather than relying on a single safety device.
 
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