One more expansion tank question, I swear! I just found out my expansion tank is too small. I installed a Watts PLT-12 expansion tank (4.5 gallon tank) a few months ago, and I decided on that one based on what I was told at the plumbing supply store and the Watts sizing chart for the PLT and DETA expansion tanks on their website. The chart says that 40 PSI air pre-charge is standard, and the value 40 PSI is automatically placed on the chart (even though it can be changed). The problem is, all of the PLT tanks have an air pre-charge of 20 psi. Using the 40 PSI value my tank is adequate, but the 20 PSI pre-charge makes my installed tank too small for my system. The only tanks they have with 40 psi pre-charge are the DETA (commercial) series even though the chart says 40 is standard!. Instead of my $40 expansion tank, it looks like I need a $150 tank just because of the pre-charge. Why does the pre-charge make such a difference when you just end up adding air to it to equal the incoming PSI from the water supply line? Can anyone help me make sense of this?
You can adjust/change the air pressure in your tank. 20psi is just what they leave the factory with.
I did adjust it according to my incoming pressure, but why does a tank with a pre-charge of 20 psi vs a tank with a pre charge of 40 psi affect the performance of the expansion tank? If I'm just going to increase the pressure to 60 psi anyway, why does it matter? It makes over a 2 gallon difference in how much the tank will take in according to their chart.
There is a flexible rubber bladder that separates the air and water. Since it's flexible the pressure on both sides will always be equal. It may leave the factory with a 20 psi pre charge but when you put 60 psi water into it the bladder will stretch and the air will be compressed into a much smaller volume until it too reaches 60 psi. The air is now squeezed into a much smaller space so it has less room to do it's job and can handle less expansion (tidal volume).
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Boy, this is hard to explain without being able to wave my hands around.
Thanks for the help, The bladder psi should match your incoming water psi, I added 40 psi from my air compressor to the pre-charged bladder of air of 20 psi to equal the 60 psi incoming water pressure, how is that any different from me adding 20 psi from my air compressor to a pre-charged bladder of 40 psi to equal the 60 psi incoming water pressure?
It depends on the btu I believe by the expansion. Where did you get that calculator?
Change the pre charge to your incoming psi in the calc... What does is say now?
The pre charge always has to mach your static psi.
Use this...........................
Amtrol- Thermal Expansion Tank Sizing
Potable Water Expansion Tank Sizing - Support - Water Safety #38; Flow Control -Watts
The higher the number in the air pre-charge box, the smaller the size expansion tank you need.
I know it says air pre-charge, but do you think it means for you to enter the same number as your supply pressure psi?
I know it says air pre-charge, but do you think it means for you to enter the same number as your supply pressure psi?
Yes any expansion tank for DHW needs the air charge change to what the house psi is. Always.......
Tags: size, expansion, tank, expansion tank, equal incoming, expansion tank, incoming water, bladder equal, bladder equal incoming, bladder equal incoming water, bladder equal incoming water pressure, chart says, compressor pre-charged
