Double Spring Heavy Duty Automatic Greenhouse Window Opener. Opens Roof Vents and also Top Hung Side Vents by Thermofor at Halo Grow Box. MPN: HD3400. Hurry! Limited time offer. Offer valid only while supplies last. This quality greenhouse vent opener automatically opens and closes your greenhouse window as the temperature changes. This model is fitted with a heavy duty double spring mechanism designed for use in exposed locations or on lightweight polycarbonate glazed vents which do not have sufficient weight to prevent the wind opening the vent unintentionally. With the same unique action and smooth twist free operation as the HD3200 this unit incorporating double upper arms has a stronger bracket
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