What is the correct pressure for the pre-charge on a domestic hot water expansion tank? I have read in several places that it should be pressurized for the incoming water pressure. This would involve getting a pressure measurement of the building water service, and pumping the tank up to that pressure accordingly. Alternatively, I have also read in a few places about 12 PSI and it being the fill pressure for a 2 story house. Why would the fill pressure not be the incoming water service pressure?
Check the system pressure
At this point you’ll want to close the faucet you opened, and open the shutoff valves (make sure you cap the extension pipe, or close the valve on the extension, or you’ll have water everywhere). Turn on all the hot water fixtures in the house until water flows normally (no sputtering), to insure the system is filled and at full pressure. Check for, and repair leaks.
Attach a pressure gauge to any part of your plumbing system. Some gauges contain threads for a garden faucet, or you can attach one to the extension where the expansion tank will be installed. Once the gauge is attached, open the valve to get a reading on the gauge. Note the pressure. If the pressure is not within the normal range of 40 – 80 psi, you’ll have to take steps to correct it (which is not covered in this article).
Pre-pressurize the tank
Most expansion tanks come pressurized to 12 – 40 psi, but before you install the tank you have to match the system pressure. Start by removing the protective cover from the air valve on the tank. Use a tire gauge to check the pressure. Use a bicycle pump, or compressor, to fill the tank to match the system pressure you noted earlier.
From Bell & Gossett FHD501A (http://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2012/07/fhd-501a.pdf):
Most residential diaphragm tanks are pre-charged to 12 psi to match the fill-pressure needs of a typical two-story house. When the feed-water pressure reaches 12 psi, the system will be filled to the top floor and will be under several pounds of pressure. At that point, no more water will enter the system because the pressures on both sides of the diaphragm will be equal. It pays to check the air pressure in a diaphragm tank before you install it, because some of the air may have escaped during shipment and storage.
It would seem to me that if you simply went with the 12 psi and your service pressure was, say, 60 psi, your tank would be completely waterlogged and provide no real function.