In the days of gravity heat, circulators weren’t available, so installers used large pipes and let the water "turn" slowly on its own. But nowadays, heating pipes are much smaller and every hot water system has a circulator.

The only time hot water should leave a modern boiler is when a thermostat calls for the circulator to come on. If hot water is unchecked and allowed to "gravity circulate" out of the boiler when the circulator is off, the zone will overheat, and you’ll have a call-back.

So when you zone with circulators, you’ll use Flo-Control valves to keep the hot water in the boiler. Let’s take a look inside one.

This is B&G’s SA valve. "SA" stands for "straight or angle," which means, for piping convenience, you can use either the bottom or side tapping of the Flo-Control valve as an inlet. Naturally, there’s only one outlet.

As you can see, there’s a weight inside the Flo-Control valve. It’s made of bronze, and it rides up on the valve stem whenever the circulator operates. When the circulator shuts off, the bronze weight drops back down onto the seat. The weight prevents gravity circulation when the zone is off.

To work, the Flo-Control valve must be installed with the stem pointed toward the ceiling. You should always install the Flo-Control valve in the supply piping because the system water is hottest at this point. There are times, however, when you may need a second Flo-Control valve on the return side of the boiler because, believe it or not, gravity circulation can occur in a single pipe! It doesn’t need a complete loop.

The hot and cold water just flow past each other in the same pipe. You’ll usually notice this "back end" gravity circulation if there’s a radiator directly above the boiler on the return side. Adding a second Flo-Control valve to the return side of the zone piping will solve the problem every time.

If you turn the stem handle at the top of the Flo-Control valve counter-clockwise, you’ll manually lift the bronze weight from its seat. This will effectively take the Flo-Control valve "out of the loop" and allow the boiler to gravity circulate.

The only time you’d want to do this, however, is if the circulator failed. Turning the stem handle and lifting the weight will give the folks some heat during the time the circulator is down. But this is essentially a home owner feature because, let’s face it, if you’re there on the service call, you’re usually going to fix the circulator, not bypass the Flo-Control valve.

Turning the stem handle has no effect on the system other than to allow gravity circulation to take place. In other words, that stem handle won’t help you balance the system’s flow rate or direct the flow in any other way. Its only function is to raise and lower the bronze weight.

We mention this because we’ve seen guys try to make the water flow a certain way by pointing the stem in this direction or that direction. That’s not what it does.