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Posts: 185
Reply with quote  #1 
I found out last spring that with reflectors added to my hot air collector that it can experience meltdown if the power goes out.  The box smoked up my whole basement.

I'd like to add a small 12v photovoltaic powered fan to my system, but I'm having a hard time figuring out how/where to add it so it doesn't block the current airflow or reverse the airflow.

I'm not looking to increase overall airflow. I'm just wanting to add a fan that will run any time the sun is shining on the collector and the temp gets over a certain temperature.  I can use a snap switch for the temp control.

Here's the current configuration.

Airflow diagram.jpg  I'm wondering if I put a small fan in the intake duct if it would block the airflow too much during the running.  Maybe the best thing to do would be to run a separate small intake duct with the solar fan.


Denver, CO

Double screen hot air collector


Posts: 563
Reply with quote  #2 
I put a solar powered fan on my air exchanger preheater.  It has a snap disc.  When air is being drawn through the collector it pulls the exhaust flap shut.  When there is no air flow and it heats up, the solar fan comes on and vents the collector.

Your collector gets hotter and would need a bigger solar panel and fan for stagnation venting.  The other thing would be figuring out the right temp for the stagnation snap disc so it doesn't kick in under the high end of normal operating temps.

I should have designed a vent into my new collectors.  They run on 12V DC and a few weeks ago the power supply failed to turn on.  Luckily I had the collector partially covered with a blanket since it was getting up to 82F that day.

Kevin H

Posts: 185
Reply with quote  #3 
Thanks Kevin.  Your plan helped me think this through.

I think what I'm going to do is run another small intake duct with an inline fan coming from the basement window on the inlet side and into the collector.  I'll put a damper on the existing 6" inlet so that if the main fan stops, the main inlet will close.  I'll use a 12 volt fan with a snap switch powered by a car battery with a battery tender.  I decided the PV panel was too expensive and would cover too much of the collector. I already have a spare car battery and battery tender. I can just get a snap switch that opens at 140 degrees to control it. With the main fan running, I've never seen a temp above 130 even on a full sun with the reflectors.

So it would look something like this.  

Airflow diagram with backup.jpg   

The only thing I wonder about is how long the fan will run on battery power.  Our power rarely goes out for more than 2 hours and if it did, I'd usually be able to come home and put the cover back on the collector. If the main fan failed and the power stayed on, the battery tender would continue to power the backup fan.

Denver, CO

Double screen hot air collector

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Posts: 2,240
Reply with quote  #4 
Hi, MC,

Say your DC fan is 0.5 amp, if your battery is 65 Ah, and in good health, then the fan would run for 130 hours...

However, since you would not normally run a battery below half-charge, half this to 65 hrs.

That ought to be enough for the power to come back on !


(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilĂ " unless talking musical instruments...

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Posts: 2,898
Reply with quote  #5 
Bear in mind you only have an overheat risk when the sun is high enough to heat the collector. So your 65 hours of battery should buy you at least a week...
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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