attic exhaust fan control

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Old 03-06-17, 09:47 AM
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attic exhaust fan control

Hi, I have x2 attic exhaust fans 6 ft. apart from each other on a roof that has a ~45 deg. incline. Not how i would have designed ventilation, i replace the motors every 1.5 years or so.

Anyhow, there are x2 cheap analog control boxes, one for each fan, and when one switches on the other is off, and the one fan will suck the outside air out of the other vent (and basically do nothing but burn electricity) until the attic heats up enough to activate both. I can't get the sensitivity so they both turn on at the same time, it is really coarse controls.

What i would like is a digital control that can handle the amperage on both at the same time, and even better, I would like a control that kicks on the fans at low RPM at lower temps like 80C in the attic, and ramps up the RPM to full with temperature as the attic temp goes up, does such a thing exist? Presumably the right digital switching would prevent motor burn-out at less than full RPM?

I don't like that i have zero ventilation until the attic gets hot, but having them blast full on at lower attic temps is not what i need either. I can't really install other vents (like ridge vents) because the power vents would just suck air from them instead of pulling from the soffits in the side attics of the house like i need them too.

Any advice would be appreciated!
 
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Old 03-06-17, 10:26 AM
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Welcome to the forums.

Two fans ?
Must be a massive attic..... but only 6' apart ?

You have a lot of exhaust power there.
How does the fresh get in to the attic to replace what is being discharged out ?
 
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Old 03-06-17, 10:56 AM
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Where are you located?

And I hope you meant to say 80f instead of 80c. It's very unlikely your attic will ever reach 80c unless the house is on fire. You might see 70c with a black roof, poor ventilation on a hot day.

You are talking pretty good expense to add the controls including variable speed that you want. You'd be better off just continuing with what you have. And depending on your location, size of the attic, and inlet and outlet area you may be OK without powered fans. Or, you can disconnect the heat & humidity sensor/controls that you have and wire both motors to a single controller. There are numerous analog and digital controllers that can control a decent sized load for a reasonable price. Trying to get fancy and add variable or multiple speeds will add considerably to the cost while offering minimal energy savings.

Keep in mind that if you have properly sized air inlets that air will be circulating and exhausting out your fans even when the motors are not running. Also, your house is protected by the insulation on attic floor so your goal is not to keep the attic cool. You want it cool enough so it does not harm the shingles.
 
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