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Posted by Harold hydronicnetwork.net on January 19, 2004 at 07:44:57:

In Reply to: VIBRATION SOURCE posted by ROB LEWIS on January 19, 2004 at 06:19:57:

Firstly, my rant, stop calling a pressure reducing valve an auto fill valve (the term is OBSOLETE) - if you wish, call it an auto FLOOD valve, because it is not there to be constantly open to make up for a leak in your system that is supposed to be SEALED. A low water cutoff protects the boiler from burnout. Read:
Rant over.

The Lochinvar is a small-tube boiler. This means that if there is insufficient flow in any tube of the many of them, the water will sizzle, causing a vibration. The water turns to steam bubbles at the hot spot and collapses again, hammering the tube until it breaks.

Sometimes the trick of increasing the pressure to 20 psig will stop a sizzle; but it is not a permanent fix, as the hand valve to the pressure reducing valve should be closed, so you can tell when your system starts to leak by the pressure drop.

Most small-tube manufacturers include a bypass scheme to circulate a minimum quantity of water through the boiler to prevent this. Observe if such a method of protecting the boiler has been installed. If not, increase the water flow through that tube by some means; perhaps even following the manufacturer's directions.

For many installers, a boiler is a mysterious thing that is covered by sheetmetal, so the insides are evaluated the same as cast iron boilers. However, making a boiler light and efficient changes the piping needs at times. This can also happen in large cast-iron or steel boilers, so it is a common fix for a more rarely encountered problem.

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