I recently purchased a home with a CNG hot water boiler system. The house has had, on two separate occasions, two additions built which added a second living room and a master bedroom with bath. There are two thermostats, one in the center of the original house and one in the new master bedroom.
When I moved in the heat in the master bedroom didn't work because of a faulty zone valve actuator. While replacing the actuator I noticed something that didn't make sense to me, but hopefully someone can explain it. Coming off of the boiler (heated side) there is a single tube which branches into three tubes. Of those three tubes, two of them have zone valves. All three continue on out of the room and I'm unable to see where they lead to. My question is, what is the third tube (the one without the zone valve) for?