Mystery Overflow Drain

[EricJK]

Yes that's called a "Dual Check Valve with Intermediate Atmospheric Vent". When pressure is higher on the boiler side than on the water feed side, it dumps some water, rather than going back into the potable water feed.
It may only be the result of lower pressure on the feed side due to high use but you could check to see if the pressure climbs above the typical 15 to 20 psi(assuming that's your typical-?). If it climbs high, it could be the result of a water logged expansion tank. But if that was the case it would like be releasing more water.
They do sell repair kits if it's just a matter of a weakening unit. I'd probably live with it, once I checked out the possible causes.

Thank you! We actually have pretty bad water pressure so this all makes sense. I'm pretty sure the valve has been working as designed. Mystery solved.

 

[wood4d]

Or you could put a new check valve. for about 40 bucks if you do it yourself this is a backflow preventer to feed your boiler. It separates your house water from your heating system. If the atmospheric vent is leaking why do you think the rest works?

 

[Diehard]

Or you could put a new check valve. for about 40 bucks if you do it yourself this is a backflow preventer to feed your boiler. It separates your house water from your heating system. If the atmospheric vent is leaking why do you think the rest works?

It's not a consistent leak. It sounds like it may be doing exactly what it was designed to do.
We're not talking about a high hazard application here. In fact many homes don't even have these backflow preventers installed, unless it is a newer or updated system. The PRV's act as a backflow preventer to a certain extend.

 

[EricJK]

Thanks again everyone. As stated - I think the valve may be working as designed, but our water pressure is so bad I think I need to get that fixed first. We may get a pressure tank or I may have an issue with our home water filter... still running that down. Once we get that sorted out, I'll look at the valve again. Thanks.