Help starting solar attic fan

CruznTXCruznTX Registered Users, Users Awaiting Email Confirmation Posts: 4
Hi All,

 My roofer just installed a new solar attic fan.  I monitored it from morning to evening to see if it works.  Even in the middle of the day the fan doesn't kick in.  So I've tried to test the solar panel and the fan separately.  I found out that the solar panel was giving 19V (this was at noon and there were no clouds).  The fan had these details:
Broad Ocean Motor 
V909334 ZWK472B82
24VDC 1.265A 1000rpm
Class A T.P. 85 deg C

Am I correct that the panel is not strong enough to power the motor?  I have tried using a transformer and I've verified the fan works when I attach it to a 24V dc source.  The solar panel is the one that came with the attic fan.  Unfortunately, I don't know what the brand of the solar attic fan nor can I get it from the roofer.  I hope someone can help me rectify this problem.  Will adding a capacitor help kickoff the motor?
Thanks in advance.


  • BB.BB. Super Moderators, Administrators Posts: 32,749 admin
    Some fans may have a switch (to prevent them running in cold/winter--I assume yours does not?).

    It does sound like the solar panel is too low of voltage (this would normally be a "12 volt" solar panel with Vmp~15-17 volts).

    Can you see the panel ratings (Vmp, Imp, Watts, etc.)?

    The panel, if it does not have high enough current output (>1.3 amps or so??) would have difficulties starting/running the motor.

    Is the 19 volts from the panel with the motor attached or open circuit (Voc or voltage open circuit)?

    At this point, can you just call the roofer and tell him to repair/replace the fan?

    If you have hand 120 VAC power near the attic fan, you may want to change to a thermostatically controlled AC powered fan. Generally they will move a lot more air and the motor can last longer (depending on the quality of the DC motor).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CruznTXCruznTX Registered Users, Users Awaiting Email Confirmation Posts: 4
    Thank you for the quick reply Bill and apologies for mine.

    My fan doesn't have an on/off switch.  The solar panel is just connected directly to the motor.

    I will have to go up the roof and check the solar panel to see if it has any ratings.  I cannot see anything from the attic side.

    How can I test the amps?  Do I just disconnect the solar panel connector and attach the terminals to the connector from my multi tester?

    When I took the 19v measurement from the solar panel, it was not connected to the motor.  So I guess it's 19Voc.  I disconnected it from the motor, set my multimeter to 200V and connected the terminals.

    I will look into the thermostatically controlled fan.  Do they install from the attic or do I need to go up on the roof?

    Many thanks.
  • NANOcontrolNANOcontrol Registered Users Posts: 222 ✭✭✭
    Placing the meter on the 2A setting and reading the panels in sunlight is the real test.  If it can't supply at least 1/3A it won't run the motor. I wouldn't expect the motor to run at its rated speed and they are under powering it by design
  • BB.BB. Super Moderators, Administrators Posts: 32,749 admin
    As NANOcontrol said--Just set your DMM to 10 Amp full scale and measure the solar panel output (you are shorting the panel out with the current meter--But that is OK for solar panels--Don't do this to anything else like batteries or AC mains--You will not be happy with the results--Can be dangerous). Set to 2 amp or lower scale if panel is "small".

    I do not know about options for 120 VAC powered attic fans. Much of the work is getting the roofing and flashing right (you do not want the installation to leak). For the most part, I think you need to access both top (to install flashing and fan assembly) and underside (electrical work) of roof.

    Solar DC roof vents, from what little I have read/seen, do not really move much air. We had one person that installed a few roof fan (and improved soffit inlet venting?)--And found after a few years, the DC fans failed, but the attic remained cooler (due to improved ventilation).

    In general, Solar DC roof vents seem to be expensive relative to their benefits.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CruznTXCruznTX Registered Users, Users Awaiting Email Confirmation Posts: 4
    Thank you NANOcontrol and Bill for y'alls insights.  I've read some articles about the power attic ventilators ( and it talked about what Bill has just mentioned, best to put money on sealing the ceiling and have good ventilation.

    I'll check the amperage rating and see what the panel gives out, at least I know the specs of my components. 

    Worst comes to worst, I'll just remove the motor so the fan blades are not blocking so that I've got passive ventilation working.  =)  Once again, thanks.
  • Raj174Raj174 Solar Expert Posts: 795 ✭✭✭✭
    Didn't you say this was newly installed by your roofer? Is it not covered by a warranty? Can you call the roofer and get them to take care of it? With a 12 volt panel and a 24 volt fan, it sounds like either the fan or the panel were ordered by mistake and should be replaced.

    4480W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH nominal LiFePO4, Kohler Pro 5.2E genset.
  • NANOcontrolNANOcontrol Registered Users Posts: 222 ✭✭✭
    Does the solar panel look like one solid element with stripes?  These are known to have unreliable connections to the glass element. Multiple individual cells are more durable.  Imagine how many of these are on roofs and not working.
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited December 2017 #9
    The only cases I've seen where powered attic ventilation makes economic sense is in cases of excessive moisture in the attic/ceiling (ie mold) or problems with ice dams - and others fixes are expensive.

    I am available for custom hardware/firmware development

  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭
    Even moisture and ice dams are better addressed with improved natural airflow (eg. soffit and ridge venting vs gable only) IMHO. The only place mechanical ventillation makes sense to me would be in something like an equipment penthouse.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    I'm not aware of any way to use natural attic air flow to increase attic pressure to the point where it exceeds the inside the house stack effect induced pressure.    It's not just about moving air, it's about pressure differential and preventing ex-filtration (Winter) and infiltration (Summer).

    I am available for custom hardware/firmware development

  • CruznTXCruznTX Registered Users, Users Awaiting Email Confirmation Posts: 4
    @NANOcontrol yes it's one solid element with stripes.  This might be one of them that does not work fully.
    @Estragon and @jonr from what I've been reading, what you guys have written makes sense.  I'm now putting this project in the back burner since the effort to make this work does not warrant the time and benefits I'm putting into it.
    @Raj174 I did contact my roofer but because they are inundated with Harvey projects mine is not high on their list.  I also understand that they have other work that is more important than mine so I didn't push hard.
    Thanks all for your insights.

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