Understanding Thermal expansion Tanks

takeadoe · Sep 29, 2015

If the pressure in the thermal expansion tank is less than the house, why doesn't it simply fill up with cold water? I'm totally confused. Is it magic? Why have I seen some say you are not to adjust the precharge on the expansion tanks, and yet others say it should match the static pressure in your house, provided the pressure in the tank doesnt exceed 80 psi. Please help clarify.

Thanks in advance!

Mike

phishfood · Sep 29, 2015

The air in the expansion tank is compressible. Which means that if you apply more pressure to the tank on the water side of the bladder, that the bubble of air will get smaller. But as it gets smaller, the pressure on the air side of the bladder will rise. So if the pressure in a water heater goes up, the higher pressure on the water side will compress the air on the air side. The pressure on the air side will go up, until it equals the pressure on the water side. Basically, it acts like a spring.

But if you don't have the tank there, then you don't have that "spring" to absorb the pressure created by the heating of the water. So the pressure in the water heater and the piping connected to it will go up dramatically, potentially causing damage.

speedbump · Sep 30, 2015

To further clarify; water can not be compressed.

Phish did a great job of explaining that. I would have had so many if's, and's, or's and butts in there that it would have been totally confusing.

takeadoe · Sep 30, 2015

Thanks guys. So, do you set the pressure in the expansion tank to match the house static pressure or not?

Thank you again for your help!

speedbump · Oct 1, 2015

I don't know how most Plumbers set them up, but some tanks can't take much more than a 30 lb difference between min and max pressures. If yours spikes much over 30 psi, the extra pressure could take the bladder out.