Expansion Tanks

Concerning expansion tanks on water heaters. In anyone’s experience do they actually prevent damage to water heaters or is it more that in theory a water heater could be damaged if the tank was not present. Yes, they are required but in a house where water is being used constantly do they really solve a problem or is this a case of code gone overboard? Secondary question does anyone know when the national plumbing code started to require them?

Many water heaters don’t warranty the unit if an expansion tank is NOT installed. I look at the expansion tank as a 2nd line of insurance, the pressure relief valve would be the first line.

Not sure when the code went into place.

Expansion tanks really are used to prevent backflow into the public water system. In my opinion over kill. Installing an expansion tee will do the same thing.

The codes are not well enforced in my area so I do not know.

  1. Use the term “an approved device for thermal expansion control.” That is how the code references it. “Expansion tank” is inappropriate as some valves have thermal expansion control built in.

  2. Expansion control is not to prevent backflow. Instead, thermal expansion control is required whenever the house is a closed system (and the water cannot back up into the municipal water system).

I talked with the chief plumbing inspector for charlotte nc for a long time about this.
They do not require one if the water pressure is ok at the time of their inspection.
This of course could change but that was the answer last year.

If the house has a hot water circulation system it will have some check valves so a thermal expansion tank is very necessary. Many houses can have no hot water usage during the period where a colder tank is reheating and get very high pressures.

If something is going to blow apart it will most likely be a pipe, fixture, toilet line, washing machine hose or fridge water line. An old water heater could blow but newer ones are not the weak link in most houses.

Battle of the CMI’s. :smiley:

So debate as to whether they are installed or even needed. Fun debate.

For those that have these installed, do you inspect them for failure? Just asking.

:mrgreen: :mrgreen: :mrgreen:

14000 North 94th Street Unit 118 008.JPG

14000 North 94th Street Unit 118 008.JPG

14000 North 94th Street Unit 118 007.JPG

Good question, the life of the rubber bladder is hard to estimate.
Checking the air pressure would be hard on most since they are typically wrapped in insulation not to mention the fact that you have to turn off the water valve and drain the pressure off.

Regular home maintenance item even though very few homeowners do anymore than paint, clean gutters and replace filters.

There is an easier way. We just had a Master Plumber give a talk on water heaters at a local meeting.

Most of us take a reading on the water pressure. His method is to install your pressure meter on the drain for the water heater. Turn the dishwasher on, run the faucets and bath until about half the tank is used. Not hard to do. Once the hot water is run to that point, turn off all fixtures and take your pressure reading. This should be the normal static pressure of the system. After 20 minutes or so, the hot water should be replenished, check your reading again. If the pressure has increased more than 8-10 pounds or more, the unit is bad and should be replaced.

Sure it is beyond the SOP, but if I am taking a pressure reading anyway, why not just leave the gauge on a little longer and see if there is a problem? Comments?

Not a bad method and I may start doing that as these units around here get more age on them.

It does leave the possibility of showing “good thermal control” when it may be bad. Possible things that would give a false test:

Toilet plumbed with hot water, toilet flush valve leak causing water usage that keeps the thermal expansion from showing up.

Someone turning on a hot fixture when you did not know it.

Yep. You need to make sure that nobody uses a fixture during the test that would relieve any pressure.

The plumber also said you could just open a faucet for a short time and then turn up the thermostat on the water heater. Same results. Just don’t forget to turn the thermostat back down…:shock:


Good article Nick. All correct. But what I’m asking is the expansion tank really a needed item or is it code overkill? I’ll continue with code etc. for inspections but personally I see overkill and wonder what others have seen in the field. I’m especially interested if ther are any plumbers out there with field experience with expansion tanks or lack of them.

When they put a Back Flow device and or Regulator on the water supply to the house, and you have Copper or CPVC Piping then yes they are needed.

PEX Piping, then no they are not.

When they put a Back Flow device and or Regulator on the water supply to the house, and you have Copper or CPVC Piping then yes they are needed.

The type of piping is irrelevant. The code doesn’t specify a piping exception.

How do you define “code overkill”? There is no way to know what occurs without dissecting the water heater to see if damage is present. I always assume people smarter than I am wrote the codes and don’t question it, as I’m not privy to the information they used in deciding to promulgate it.

Wasn’t the expansion tank for DHW systems brought in when some codes/municipalities required a backflow/check valve on the municipal water supply line? We don’t have the backflow/check valve requirement here, hence, no expansion tanks on the DHW system.

Do you have pressure regulators? Those valves create a closed system at the house and would meet the criteria for requiring an expansion control device.