boiler feed backflow preventer- code

[fixitron]

I recently replaced a boiler feed backflow preventer. There was a ball valve upstream of it but nothing to isolate it from the boiler system except the boiler feed pressure regulator. I thought that if the boiler is above the set pressure for the regulator, then the regulator valve should be closed. So I slowly backed off the union nut and very little water came out. I was able to replace the soldered valve with no problem.
Why does the code even require a backflow preventer? It seems to me that not only would the potable water side have to have zero pressure, but the feed regulator would have to leak as well, which would actually be constantly feeding water to the boiler if it did leak. So how could boiler water get back into the potable water side?

 

[Diehard]

It was leaking back due to a weak spring that is included in the overall design of the PRV.
And yes, many backflow applications are based on loss of pressure on the feed side.

Although a PRV may inherently prevent backflow it is not design for that purpose and under certain conditions could allow water to backflow. Supposedly, If the inlet pressure is lower than the discharge pressure, the pressure force coming back from the boiler can allow flow backwards through the regulator.

As you likely know, there are many different types of backflow preventers, with their use based on level of hazard and type of application.

The backflow preventer used on a non-high hazard residential boiler consists of Dual Check Valves with Intermediate Atmospheric Vent. If it was a high hazard, a RPZ (reduced pressure zone) type would be required. In other words an RPZ would be less apt to allow backflow.

In most cases not every backflow preventer type is 100% fool proof. It's just an acceptable level of minimum protection.


I know you didn't ask for this info but thought you might find it interesting.