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Posted by Harold hydronicnetwork.net on January 09, 2004 at 12:01:20:
In Reply to: System Pressure posted by JH on January 09, 2004 at 10:17:54:
Pressure reducing valves are mistakenly called autofill valves. They would be more accurately called "auto flood valves when pipes break," a poorly trained plumbers auto leak filler," "an automatic boiler and pump corrosion device," etc.
In the days when commercial systems had drip seals on the circulators, some means of filling had to be done if the maintenance man was too lazy to check for a full system on a regular basis.
The instructions that come with the pressure reducing valve say to CLOSE THE HAND VALVE TO THE PRV. That being done, it is possible for the water to drop back to 6 psig when the boiler cools. The expansion tank is prefilled to 12 psig, so as soon as the water startes to expand due to heating by the boiler, the gauge will jump right up to 12 psig and go up from there as the water expands more. As long as the pressure doesn't rise above the reseating pressure of the safety relief valve - 25 psig for most residential relief valves - the pressure is OK.
However, air is always a component of boiler water. Fresh water is 5% air by volume. About 3% of that can be vented, but 2% of the fill volume remains air gases. Eventually 15% of that - the oxygen becomes part of the internal corrosion rust and is no longer free, so most of that 2% is nitrogen, carbon dioxide, hydrogen and some trace gases.
So you started out with a system filled with water, eventually 3% of that left, so when the system cools down, the pressure drops below 12 psig while the hand valve is closed.
You may now open the hand valve to the pressure reducing valve to allow it to monitor the water pressure during the topping off as you
MANUALLY fill the system while monitoring it. Then you will close the hand valve to the PRV to prevent AUTOFLOOD and AUTOCORRODE in the future until such time as you need to perform a circulator or other service.
AMAZING isn't it that HVAC guys can seal air conditioners that have pressures of 250 psig without leaking a GAS over 10 or more years; but some folks can't seal systems to hold water that get to 25 psig, so they leave a feed valve open to KILL the boiler in a few years? Are the pipes under your house spraying water and automatically refilling with the infinite volume of the upstate reservoir?
Written for all to see at:
http://64.226.150.132/hydronic/service/purgeair.htm