I was preparing to do some outside work which will require me to pull out my 100' compressor air hose, and decided it was time to re-check the pressure on my indoor expansion tank (for a Rinnai RUR98i tankless unit, with recirc option), since I'll have an easy way to add the air if needed. I had originally set it to 25 PSI, don't recall where I read that, but it might have been "those instructions" referenced by @Breplum
. However, under the assumption that I probably did it wrong six years ago, I started reading the inter-web sites for instructions on calculating the proper pressure (including Rinnai, who predictably had nothing constructive to add). I discovered that ... well, there's no definitive answer, just lots of assumptions, with the occasional convoluted calculation by what I assume is an advanced Mathematics Ph.D (Pa = H(Dc / 144) + 5
). (That calculation comes out to 6 PSI if I did it correctly for my application. )
My house water main pressure is about 50 psi, which is what I assume is the pressure for the system. Most expansion tank units have warnings not to exceed 80 psi. If @Breplum
is correct, I should have set the expansion tank pressure at 50 PSI, meaning that my tank is probably/has been likely waterlogged. However, I've read that it also depends on the storage tank size (mine is -ZERO-, since it's a tankless unit, but then, the entire recirculation hot water line represents "storage", so, not exactly zero, either). The tank pressure also varies by water temperature, and ours differs significantly throughout the 4-season climate year up here in Idaho. Oh, yeah, and the calculation also depends on the distance of the tank from the ... I dunno, water heater? check valve? Whatever ... I read also that these residential tanks are preset to 12 PSI for the average two story house, but then again, looking at the tanks themselves, I see presets of 20 PSI, 40 PSI ... again, no standard, nor rhyme nor reason.
So, not I'm getting kind of anxious about my water heater expansion tank pressure, don't know what it should be, and ... I completely regret being overly curious about the issue in the first place. Does anyone have a good answer (or maybe, just a better one), than a flat 25 PSI?