Attic ventilation - Direct power DC fans?

[Mr4btTahoe]

Hey alls...

So I'm having a bit of trouble with my new roof. Its now 2 years old (metal roof) and last year, the house was MUCH hotter during the summer than years past. After doing some digging, I think I found the reason.

Our previous attic vent system used a single exhaust fan in the middle of the roof with 2 inlets (one on each end of the roof). This allowed the fan to pull air from both ends and it worked decently. When the new roof was installed, the contractor went with a full ridge vent instead. Problem is (after doing some digging) is that the house now has no way for air to enter the attic space.

The main portion of the home is a manufactured home. There are no or minimal overhangs and no soffit vents (no place to add them). So.. now the attic is full of stagnant air. Sure some heat can escape via the ridge but with no air coming in.. it doesn't do much. I assume they made this mistake due to our house having a large addition that does have proper soffit vents and originally had a ridge vent.

Anyways.. now I'm trying to sort out a way to correct the issue without cutting holes in the roof. Currently, there are no gable end vents. My plan is to add gable vents and run 2 direct powered dc fans blowing into the attic from the gable ends. I may also add a 3rd fan inside the addition's attic space (the roof structure is cantilevered over the manufactured home so there is direct roof access from inside the attic of the addition).

I have a selection of 9" 80w 12v fans and 6 100w 12v panels. I'll do some testing with the fans to see how many panels each fan would need to function and if they'll handle the full voltage swing of the panels without causing trouble. Being an automotive fan, I know they can handle 14.7v constant so a few more volts likely won't cause them harm.

The attic space isn't very large. It is insulated.. the interior of the manufactured home has vaulted ceilings but there is still an attic space. I'd say 2-3' in the peak between the insulation and the ridge. That being said, I don't think it will take much as far as the fans go to move enough air to get a decent exchange and cool the attic space down a good bit.

I'm open to other ideas as well but this sounded like the easiest/cheapest option and I like to tinker. I've got the fans and panels laying around so very minimal investment.

Hopefully this is the right place to post this.

So... thoughts? Here is a pic of the house and roof system in question (right after they finished the roof). We are currently running on a 10kw grid tied system and getting ready to make some major HVAC changes (ditching the 2 systems we currently have - 1 electric furnace/central ac and 1 large package unit heat pump for the addition) and going with a 6 head mini split system. We are doing everything we can to reduce power consumption and with the HVAC crew coming out in a few weeks, I'd like to get the attic vent system sorted out soon.

Thanks for any input!

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[Mike90250]

Try just adding the gable vents and see if the air flow picks up

Or you could get a pallet of solar PV panels and install them to shade the roof !!

[littleharbor]

I have a pair of 80 watt 12 volt fans in my garage. I found if I connect two fans at a time in parallel it kept the fans from running so damn fast that I thought they would self destruct. To be a little more exact, I have the two fans hooked up to a 65 watt panel. They have been running daily for over 5 years and seem quite happy being hard wired this way.

[J.P.M.]

As Mike suggested, and as you write, add gable vents. I'd make the total inlet area ~= to the fan outlet area.

As you figured out/already know and what the installer may/may/not know, or simply tried to cheap out, the world's most powerful exhaust fan/blower/fluid mover will not work to pump air out of a ventilated space without a way for air to easily get into that ventilated space. What gets pumped out = goes in. If not much goes in, not much will come out.

Ridge vents are nice and pretty, but without sufficient draw and/or on a low slope roof, and if the area is wind sheltered by trees or other features, they won't suck much. Looks like your roof slope is low and you've got trees as a wind buiffer.

As for ideas, I'd think if the conditioned space is hotter than it was after the changes, that the attic insulation could be better. But before I checked that, I'd make sure the sealing between conditioned space and attic space is tight and all leak paths such as stove or bathroom vet fans are accounted for. And, back the what goes in = what goes out rule, I'd also do a general tightening up of the conditioned space.

For a rough est. of attic air temp., there are ways to est. required air vol. flowrates and so fan flowrates and also ways to estimate required vent areas that'll be necessary to keep a certain temp. diff. between ambient air temp and attic air temp. and thus help keep heat gain from attic to conditioned space at required levels.

[solardreamer]

I have a 40W fan gable mounted with bi-metal thermostat and powered by 50W 12V panel. It has worked well for several years now. Keeps my house temp down by 2-3 degrees on hot days. I had to put a buck/boost DC-DC converter to the fan because the solar panel voltage can go up to 22V and it killed the first fan motor since the fan motor can only handle up to a bit above 20V. The DC-DC converter is also good for controlling the speed of the fan for desired noise level and max power consumption.

100W panel should be enough to get 80W fan running at decent speed but likely below full speed. But I imagine 80W fan at full speed is probably pretty loud.

[Mr4btTahoe]

Update...

Did some tinkering around with one of the little fans I've got and a 100w panel. The panel has no trouble spinning up the fan. In direct sun at noon on a cool day, the highest voltage I saw at the panel with the fan running full tilt was 12.7v. Being an automotive fan, I know that it can take 14+ without issue.

I let the fan run all day and everything stayed nice and cool. Motor stayed cool as did the fan wiring. Didn't seem to mind the voltage changes.

The house is surrounded by trees and does have a fairly low pitch roof which was one of the reasons we went with metal.

I'm going to order some good sized gable vent louvered covers and get to cutting here soon as the weather cooperates. Supposed to rain all weekend again. These little fans are rated for 900-1000cfm each at full tilt. I'd say 2 or 3 should be plenty to get a decent exchange of air in the relatively small attic space.

As for insulation.. for sure it leaves much to be desired. That will be getting improved upon soon as well. The windows and HVAC took it's toll on my bank account so it'll be waiting a bit. Maybe this fall...

[bcroe]

If you want quiet, use big fans running slowly, not tiny ones reved out. Bruce Roe