Expansion Tank Undersized?

[John_187]

This project I inherited has a clearly undersized expansion tank on the water heater. It is 100 gallon water heater, and the expansion tank is 2.0 gallon with 0.9 gallon acceptance.

Now, I know the water heater has a T and P valve, but I want your take on how bad an undersized expansion tank really is? I see an expansion tank as a "Second level" of protection against water heater bursting, etc. Also, protects piping against wear, ect.

I have already mentioned that I caught this to manager. What are all the realistic risks of an undersized expansion tank? Thanks

 

[PEDARRIN2]

The expansion tank should be considered the first level since it would be utilized first (before the T/P valve). If your expansion tank is undersized, it will "accept" so much water expansion then not be able to accept any additional. This additional expansion would be accommodated by the T/P valve spitting which is a nuisance and potentially an issue if you have hard water and the deposits start to accumulate on the internals and potentially keep it from operating properly. That would not be good.

Recommend replacing the expansion tank.

 

[hydrae]

Or adding a second 2 gallon expansion tank

The standard 2 gallon expansion tank is sized for a 40 to 50 gallon water heater in that
the density of water at 5°C, (41°F) is 1.00 and at 70°C, (167°F) it is 1.02 so running the numbers a 2 gallon unit will do the job. Since most water heaters are 40 to 50 gallons this works out. With a 100 gallon tank that will need about 4 gallons, but I imagine those expansion tanks are hard to find since not many locations have 100 gallon tank that do not experience regular use.
The most extreme case is when all the hot water used quickly and then no water is used as the tank heats up again. But you may be fine if the heater set point is set to a very low value such as the energy conservation set point of 120°F, but commercial food service will not like that.

Hydrae

 

[bimr]

Signs that You Need a Water Heater Expansion Tank

1.If you have a closed system.
2.If you have a pressure regulating valve or a check valve.
3.If you have noticed fluctuating high and low water pressure.
4.If your local city codes require you to have an expansion tank.
5.If you want your water heater to last as long as possible.

Bottom line: If your home has any kind of backflow prevention device installed on your home’s main water line, your home has a closed plumbing system.

These “backflow prevention” devices stop the water from flowing out of your home’s plumbing and back into the municipal water lines.


Closed System