Gravity vs Motorized Shutters for Greenhouse Exhaust Fans

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Gravity shutters are louvers that the fan's own airflow pushes open and that close by their own weight when the fan stops. Motorized shutters are opened and closed by a small actuator, usually wired to the same thermostat or controller as the fan. Gravity is simpler and cheaper to run; motorized closes more positively and opens before the fan starts.

Either type has one job: seal the opening when the fan is off. The fan buying posts rarely cover this detail, so this guide sets out the trade-offs and the compatibility checks to make before you order.

What the shutter is for

An exhaust fan leaves a large hole in the wall. With the fan off, that hole leaks heat out in winter and lets wind, dust and pests in. The University of Massachusetts greenhouse ventilation fact sheet is blunt about it: if the shutters do not close tightly when the fan is off, they will significantly increase the winter heating bill. It also says dirty automatic shutters that do not fully open greatly restrict airflow.

So a shutter has to do two opposite things well. It has to open all the way when the fan runs, and shut firmly when it stops. Which type does that better for you depends on your wiring, your weather and how much maintenance you will do.

Gravity shutters: how they behave

Gravity shutters hang from hinges or pivots. Airflow from the running fan pushes the blades open. When the fan stops, the blades swing back and rest closed.

• Strengths: no wiring of their own, no motor to fail, and they cost nothing to run.
• Limits: they only open as far as the fan pushes them, so a weak or restricted fan may not open them fully. Wind pressure from outside can rattle or partly open them, and a sticky pivot can leave them cracked open.
• Mounting: they must be installed in the orientation the maker specifies so gravity closes them. Do not mount them on a slope or upside down unless the instructions allow it.

Motorized shutters: how they behave

A motorized shutter has an actuator that drives the blades. It is normally wired so the shutter opens when the thermostat calls for the fan, then closes when the call ends.

• Strengths: the blades do not depend on airflow to open, and they can be told to close firmly. UMass notes that fast-building negative pressure can hold a damper shut and suggests a time-delay relay so the fan starts only after the louver has opened.
• Limits: more parts and more wiring, so more places for problems. They also need a power source, and you should know what each does if power fails: stay where it is, or return to closed. The maker's manual says.
• Wiring: mains-voltage wiring and outdoor-rated components should follow the manual and local code, and a qualified electrician should do any fixed wiring.

Matching the shutter to your fan and wall

Compatibility is where buyers get caught. A shutter that is the right type but the wrong size or flange will leak or not fit.

1. Fan size and frame. Match the shutter to the fan's listed size, and ask whether the fan maker offers a shutter built for that model.
2. Wall opening. Measure the hole you cut or plan to cut, not the fan housing alone. Allow for the shutter's flange and mounting depth.
3. Controls. For motorized types, check that your thermostat or controller can power or signal the actuator. See our greenhouse thermostat guide for controller types.
4. Weather exposure. Confirm the product is intended for outdoor use, and think about the wind direction on that wall.
5. Intake side. A fan needs air coming in. UMass says intake louver area should be at least one and a quarter times the fan area, especially in poly-covered houses, and recommends the intake louvers be motorized and tied to the fan thermostat.

That intake guidance is separate from the fan's own shutter, so a full exhaust set-up may have two sets of louvers. Our small greenhouse exhaust fan guide and CFM ventilation math post cover sizing the fan itself, and how many vents a greenhouse needs covers the natural openings.

Which type suits which greenhouse

A small house with a plug-in thermostat

Gravity shutters are usually the practical choice. The fan and thermostat stay a simple plug-in pair, and nothing else needs wiring.

A cold-climate house you heat all winter

The seal matters most here. Motorized shutters can close more positively, but a well-fitted gravity shutter with clean blades also works. Check how tightly either closes, and consider a second seal such as an insulated cover in the coldest weeks if the maker allows one.

A windy site

Gravity blades can flutter in a gust against the fan wall. A motorized shutter or a fan placed on the leeward side avoids that. UMass notes that exhausting into the wind cuts fan output by 10 percent or more, which is another reason to think about where the fan sits.

A house with automation or a controller

If the controller already switches vents and fans, adding a motorized shutter fits naturally. Read the manual for the sequence it expects.

Upkeep and what to inspect before winter

• Blades are clean, because dirt keeps them from opening or closing fully.
• Pivots move freely, with no rust or binding.
• Closed blades meet evenly with no gap you can see light through.
• Actuator and wiring are intact and dry, for motorized types.
• The fan runs with the shutters fully open, with no rattle or partial opening.

The same fact sheet advises making sure vents close evenly and tightly before the heating season. If a gravity shutter still leaks when clean, that is a reason to move to a different type or seal the opening in cold weather.