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Hot Water Expansion Tank


You may notice the next time you have your water heater replaced that it now has a sort of miniature 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.

The small tank 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 back-flow prevention devices have become part of homes built after 2005, a thermal expansion tank might save you a fortune. I’ll explain why. 

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 back-flow prevention device, 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.

Test the pressure once per year

Test the pressure once per year

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

Gratuitous scorpion mama-with-babies photo.

Gratuitous scorpion mama-with-babies photo.

Once a year, you can test your water expansion tank, 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 is easily done with a bike pump or air compressor. The PSI is right on the reservoir. Just make sure you have the shower or a faucet (hot water) on when you do this.
If you have any questions about this, or if you’re going through 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. 

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