Expansion tank pressure?

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Greenmountain

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Instructions for my boiler say to fill system to 16-18psi.
If my boiler system pressure will be 17 psi filled, what should the pre-charge of my expansion tank be?
 

OldSalt

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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?
 

John Gayewski

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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?
Your reading about two different things. An expansion tank for a boiler is different than an expansion tank for your domestic hot water.

Your static pressure where your water enters your house is the reading you should take (the pressure will be highest here in a dynamic flow state). Then set your expansion tank to that reading or a little higher. The idea is not to have water in your expansion tank unless it has expanded into it. So if your house's inlet pressure is 50 then your tank should be at 50, 51, or 52. When the hot water expands it goes into the tank and raises the overall pressure.

The size of the expansion tank is based on the volume of water and the temperature rise. Don't check your expansion tank pressure with the system's static pressure on it, unless your checking to see if your above 80.If your above 80 then you need a bigger expansion tank.

An expansion tank should be installed with the schrader valve pointing down so it can be checked for water intrusion.
 
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HaveWrench

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For the domestic hot water expansion tank, you probably want to check the water pressure using a pressure gage on a faucet (preferably one at the same level as your hot water heater). For you boiler expansion tank, assuming it's on a hydronic boiler in the basement, the static cold pressure will be determined by the water column, i.e. the tallest/highest pipe on the circuit (so about 2.3ft/psi), plus a little extra (2-5psi) since you want all parts of the system to be under slight positive pressure. Your expansion tank AIR pressure (dry) needs to exceed this static value by a little bit (1-2 psi is ok) - this way the tank is "empty" when the system is cold. When it heats up, the pressure will increase and the incompressible water will push into the tank bladder, which will compress the air - if you get this to 80PSI - as the previous post mentioned - you need a bigger tank (or your tank is broken and you're pushing against the steel).
 
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