Mine is installed in a horizontal line. Not sure how much difference it makes other than the override lever wanting default position. Most residential boilers run closer to 14-15# cold, but 20 could be okay. You need to check the setup manual for your boiler. As long as the pressure is within the min/max of the acceptable range, it should be okay. Keep in mind when checking the static pressure in the expansion tank, you have to remove the pressure from the system first, otherwise, you'll just be reading the system's water pressure, not the static air pressure. The fill valve is (I think) adjustable within a certain range. Also, your pressure gauge might not be all that accurate.