What is a Thermal Expansion Tank?


You may notice the next time you have your water heater replaced that it now has a sort of tiny replica of itself perched beside it or on top of it, kind of like one of those baby scorpions riding on the mama scorpion’s back.

Building codes

That unit is a thermal expansion tank, and some cities are now requiring them as a part of their code, and for an excellent reason: hot water expands. A temperature and pressure relief valve is not a thermal expansion device. Now that backflow preventers have become part of homes built after 2005; a thermal expansion tank might save you a fortune. I’ll explain why. 

How it works

From 90 degrees to 140 degrees,  40 gallons of hot water will expand by almost half a gallon. (think of a half gallon of milk to get an idea of the quantity.) It used to be that the water would back up into the water main. With a backflow preventer, this is no longer the case. 
In a closed system, this extra water won’t have a place to go, because water can’t be compressed. The pressure on the system increases. 
If the system has a thermal expansion tank, no problem; the tank takes up the extra water, and the system is safe. But, without it, that extra half gallon has to go somewhere. Sometimes it will manifest as a leaky faucet. Sometimes, though, it will cause something more sinister, such as damage to your water heater.

The thermal expansion tank has a bladder inside it, which provides a place for water the flow. On the other side is pressurized air. 



Once a year, you can test your water expansion thank, too, with nothing more elaborate than a tire pressure testing gauge. If the pressure is zero, or if water is leaking out of the valve during the test, there is a problem.

If the pressure is zero, you’ll have to add air to the tank, which you can do with a bike pump or air compressor. The PSI is written right on the tank. Just make sure you have the shower or a faucet (hot water) on when you do this.

If you have any questions about your thermal expansion tank, or if you’re having to replace a water heater tank every few years or so and don’t know why it might be you don’t have a thermal expansion tank and need one. Give us a call, and we will come out and take care of that for you.

911 Heating, Cooling and Plumbing can service all commercial and residential HVAC and plumbing needs. Call 602-795-2900 today!

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