Water boiler pressure/bleeding questions

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Mosh Pit

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1. There should be an expansion tank in the system to account for variations in water volume based on temp - the pressure should be stable. 20psi is on the high side of normal (often around 1 atmosphere - 14-15# - but usually at least 12...there's a pressure relief valve usually set at 30psi that will dump water if the pressure gets that high).
2. Every system is slightly different. You may need to run more water through it if you don't have heat to all zones to purge out the air. If there's heat everywhere, any small amount of air left should get purged if the air scoop or extraction device is working properly.
3. Dropping the pressure likely caused the boiler water to flash into steam - mini steam gas explosions. Long term, this will cause problems similar to what happens if the timing in your car is off or you don't have high enough octane and it pings or what would happen if you opened the radiator cap when the coolant was at 230-degrees. If everything works, you're probably okay.
 
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Jadnashua

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Variations in pressure may indicate two things: your expansion tank has failed or is failing or is saturated with water and there's little room for expansion (a bladder tank is fairly small and has a fitting like you a tire on the bottom - it should normally be full of mostly air; an old style expansion tank is much larger, and those need to be drained periodically and their air valve may be corroded and needing replacement); OR, you may still have some air in the system. Both the air and the water expand when they get heated...air will expand more than the water, and it can raise the pressure more than your expansion tank can handle.
 

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Leave the autovent opened...it is designed to purge small amounts of air that get trapped as it passes, and once enough has accumulated, it automatically opens momentarily to vent it out.
 

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If you have a leak, the autofill will mask it by keeping the system full of water. It's better to know you have a leak. If the pressure drops, on many boilers, there is a sensor that will prevent it from running. This is similar to the effect you get on your car's radiator with the pressure cap. The pressure keeps the water from boiling. Boiling (steam) can damage boilers designed for hot water, so many have a sensor to monitor the pressure (but not all). Ideally, once the heating system is filled with water, you'd never have to add any ever so leaving the fill valve closed isn't a big deal.
 
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