Plumber69
In the Trades
When 12 psi expansion tank isn't enough. Can someone explain the theory on inlet pressure matching expansion tank pressure.
Boiler pressure 3 story house
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Jadnashua
Retired Defense Industry Engineer xxx
The reason you need pressure in a boiler is to prevent elevation changes to lower the boiling point of the water. Elevation changes cause about 0.43#/foot of change. Just like in a car's radiator system, it needs pressure so the fluid doesn't boil. Depending on the elevation change from the basement to the highest point in the system on the third floor, you might need more than 12psi to prevent getting too low. Say you need to push water up 20' from the basement to the third floor...your static pressure would have to be at least 20*0.43=8.6psi to keep it above ambient. You usually want some leeway so, you might want it somewhat higher than 12psi in the basement.
The amount of precharge you need in the ET will depend on the normal, working pressure in your heating system. As described above, that may be more or less than 12psi (most all boilers will have a low pressure cutout switch to prevent it from boiling the water - steam on a hot water boiler is dangerous!). The goal with the ET is to have it at it's 'rest' position when things are static...i.e., not heating. This puts the least wear on the bladder since it's not being stretched a lot at that point. When the boiler fires, the water will expand, and push back on the bladder. Because air compresses and water doesn't, the bladder will collapse, raising the air pressure in the tank until it is back in equilibrium - IOW, the air pressure will rise to the system pressure of the expanding water. Since it's much easier to compress air, if the volume is large enough, the pressure won't rise much. When things cool down again, the ET will push water out of the tank, back to its equilibrium point.
The amount of precharge you need in the ET will depend on the normal, working pressure in your heating system. As described above, that may be more or less than 12psi (most all boilers will have a low pressure cutout switch to prevent it from boiling the water - steam on a hot water boiler is dangerous!). The goal with the ET is to have it at it's 'rest' position when things are static...i.e., not heating. This puts the least wear on the bladder since it's not being stretched a lot at that point. When the boiler fires, the water will expand, and push back on the bladder. Because air compresses and water doesn't, the bladder will collapse, raising the air pressure in the tank until it is back in equilibrium - IOW, the air pressure will rise to the system pressure of the expanding water. Since it's much easier to compress air, if the volume is large enough, the pressure won't rise much. When things cool down again, the ET will push water out of the tank, back to its equilibrium point.
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