Flow Control Valves: How They Stop Gravity Circulation in Zoned Systems

Hot water rises. In a zoned heating system, that's a problem the moment a circulator shuts off, because the heavy, cool water in the return and the lighter, hot water in the supply can start a slow circulation on their own, with no pump involved. The result is zones that creep warm when nobody called for heat, and a boiler that bleeds heat into the house all day. A flow control valve is the fitting that stops it.

What a Flow Control Valve Does

It's a weighted check valve. The internal plug is held shut by its own weight, so it stays closed against the small pressure that gravity circulation creates. When the circulator runs, pump pressure is enough to lift the plug and let water through. When the pump stops, the plug drops back down and the loop is closed off. It prevents backflow, and it also blocks that slow convective drift that a plain spring check valve may not hold against.

The Manual Open Lever

Most flow control valves have a small lever or stem on top. Turn it and the valve is held open, which lets water flow either direction. You use it when you're flushing or purging a zone, filling the system, or diagnosing a problem and you need to bypass the valve temporarily. It's easy to leave in the open position after service, and that's the first thing to check when a zone heats up on its own.

Straight or Angle Pattern

The two common body styles solve a piping problem rather than a performance one. A straight valve goes in-line on a straight run. An angle valve turns the flow 90 degrees, which saves an elbow when the piping already has to change direction at that point. Bell & Gossett's lineup includes both, for example the 107019 straight/angle valve in 1-1/4" and the 107024 straight valve in bronze with 3/4" sweat connections. Pick the pattern that fits the layout, then match the connection type and size to your pipe.

Where It Goes

• On the discharge side of each zone circulator, ahead of the zone, so each circuit can only flow when its own pump runs.
• On the supply or return of a zone that's served by a shared pump and a zone valve system, wherever gravity flow would let a loop circulate through the boiler.
• In piping orientation that the manufacturer specifies. Most weighted valves need to be installed with the plug moving vertically so gravity does its job, so check the orientation arrows and notes before you commit to a position.

When You Don't Need a Separate One

Some circulators ship with the flow check built in. Taco's 007-ZF5-3IFC priority zoning pump and the 0010-SF3-IFC stainless circulator both carry integrated flow control, which removes a fitting and a leak path. If your replacement pump includes one, adding a second flow control valve in series is unnecessary. If you're swapping a pump that didn't have one for a pump that does, remove the old external valve rather than doubling up.

Don't Confuse It With a Zone Valve or a Flow Switch

A flow control valve is passive. It has no motor and no switch, and it doesn't decide anything. A zone valve opens and closes on a thermostat's call, and a flow switch reports whether water is moving. They often appear in the same piping, but replacing one with another won't solve the problem the original was installed for.

National Boiler Supply carries flow control valves from Bell & Gossett and others, in straight and angle patterns and several sizes. Send us your pipe size and connection type and we'll match one.