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Retired Defense Industry Engineer xxx
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New England
The boiling point drops as pressure drops. Some cooking instructions have special times for higher altitude locations (longer) because the water boils at a lower temperature. Conversely, use a pressure cooker, and water doesn't boil until the temperature gets higher, cooking things faster.

Say the boiler is in the basement, and you have a two-story home. The water may need to go up 16'. Water pressure drops about 0.43#/foot of elevation change. So, the pressure at the top floor could be 16*0.43=6.88# less. Start out at 2psi, and there would be a vacuum above, and the water could instantly boil. The pressure needs to be high enough to ensure you always have positive pressure at the highest point it gets pumped. Throw in a pump, and the suction side may have a lower pressure, making things worse.

I had a GB142, and finally trashed it as it kept having parts fail more often than I thought they should.
Hey, wait a minute.

This is awkward, but...

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