Curious - hot H20 heat system overpressure problem(s)

Old 11-05-06, 11:51 AM
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Smile Curious - hot H20 heat system overpressure problem(s)

I'm just curious - I've read a lot about maintaining specific water pressure in hot water heat systems and would like to know what problem(s) high pressure causes. Cavitation, leaks, water loss thru blow off, etc.

And what are the evils associated with too low of pressure?

Also, why not install a blow off valve set closer to the max desired pressure? According to what I read, I should have about 15#'s and the blow off is rated at 30#.

Thanks! Never hurts to learn something every day!

Last edited by mtujohn; 11-05-06 at 11:52 AM. Reason: typo correction
Old 11-05-06, 12:08 PM
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water pressure

The water pressure needed is determined by the number of stories in the buiding. Usually 12# is used for single story, and up to 18# for a three story building.

You don't want the pressure relief set on the edge for many reasons. First, even with an expansion tank, you will have some pressure increase on a heat call as ALL of the water in the boiler, piping and radiation is heated and expands. Secondly, when water is blown out of the relief; fresh make up water will be added to the system during a cool down as the pressure drops. The fresh water has oxygen, which isn't a good thing (rust, corrosion) for a closed loop system with ferrous metal parts. Relief valves can fail when operated, they may drip or leak bad enough to flood a cellar. Running the pressure high enough to cause them to release pressure is not a good idea. 30 pounds is code, and provides ample safety margin.

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