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# staticpressure

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while at rest, (no pumps, and around 60 degrees F)

you take the highest point in the system and subtract the lowest point, and divide by 2.31 to get the actual static pressure produced by the weight of the water, at the lowest point in the system, and is the minimum pressure, you need to have in the system, to push air all the way out the top, if you happen to have a bleed valve there

so if you system was 18ft tall, you would need at least 7.79psi to push the air out the top,

to be on the safe side we add a few pounds so the default setting to the average PressureReducingValve is 12psi, and of course the air pressure in the expansion tanks when empty, would also need to be 12psi, so it's balanced

now when the water heats up the pressure will increase, and will increase more if you are pumping away from where the expansion tank is connected (known as the Point of Zero Pressure Change) or will decrease if you are pumping toward the point of zero pressure change

i like to keep up the pressure up to keep the air out, so mine are usually set for 17-20 and increase to 25 when the pump starts  as long as you keep the max pressure at least 5psi below the relief valve you are ok, we dont want the relief valve opening and causing fresh oxygenated water to come in which will corrode the system

dont worry about the pressure since the boiler can be run continuously at the rated pressure, and is usually long term tested at twice that, and pressure tested to 4 or more times that.

Now if you have a very tall building, and say, you need 40psi, then you can put in a heat exchanger that can take the pressure, or put the boiler on the roof, so that the pressure at it, is only a couple of pounds, whilst below, the pressure is much higher,

but then you ask, doesnt the law of hydraulics say, that pressure is evenly distributed in a cylinder, yes, but law of hydraulics does not take into account the weight of the fluid. And if the cylinder is very tall, the working hydraulic pressure may only a few pounds, but the static pressure at the bottom of the cylinder due to weight may be a lot more

ps if this is not sufficiently confusing there is a lot more bull where that came from ;-)