Need a reliable "summertime" angle based on my location

[Jocamo]

Greetings,

I'm installing a solar attic exhaust fan, and I need some mounting advice.

My house is in Houston (about 30 deg latitude), and I stop running the AC around late October. This in mind, I don't see the need to have a variable mount put on the roof.

My roof is unshaded and points approximately 255 deg SW (true). According to one site, I need about 6.5 to 7 deg of pitch toward the south for Summertime exposure.

I think I can get away with shimming the bolts on the roof for such a small tilt.

Q: does anyone have a reliable site to calculate the best angles?

That previously mentioned site gave 6.5-7 for a (summertime) fixed angle, and it gave a steeper pitch for summer were I to adopt a twice-a-year adjustment.

thanks in advance

[silversaver]

Your roof is SW 255 with what deg of tilt? Why would you pitch 6 to 7 deg just for little gain on the output? I think that will add the install cost + might look odd.... Just add 1 or 2 panels extra, then you get more output. My roof is SW245 with 23 deg tilt, I don't think there is anything wrong.... btw, I add 2 more panels for little more output.

[Mike90250]

You will want to have enough pitch to have the panels be self-rinsing. It's a pain to get up on the roof to wash panels off.

It's also expensive to construct racks to alter the tilt your roof naturally provides.

[control4userguy]

You will want to have enough pitch to have the panels be self-rinsing. It's a pain to get up on the roof to wash panels off.

It's also expensive to construct racks to alter the tilt your roof naturally provides.

Why would this matter? He is installing a ventilator...

[Mike90250]

> Why would this matter? He is installing a ventilator...

uh. well. gee.

[control4userguy]

> Why would this matter? He is installing a ventilator...

uh. well. gee.

I get that it has a solar panel but, from what I've seen, the DC motor will spin enough even if it's tilted off desirable angle.

[Jocamo]

It's about 31 deg of pitch from the gutter to the ridge.

It's a solar vent not for house power.

I only get one panel and would like it as close as possible to max exposure.

[sensij]

It's about 31 deg of pitch from the gutter to the ridge.

It's a solar vent not for house power.

I only get one panel and would like it as close as possible to max exposure.

You could use PVWatts to play with tilt until you get the maximum values in the summer months. A quick look suggests something between 0 and 5 deg from horizontal tilted south will get you the highest values in June, July and August, but the difference in that range or even up to 10 deg really isn't much.

Solar attic fans aren't really known for moving much air. I had installed one a couple years ago that was grid powered, so it moved plenty of air, but recently took it back out again and have focused on some more reliable methods to make the house more comfortable. Ridge vent for ventilation, increased soffit vents to supply the ridge, and more insulation. I don't have AC, so for me, it is about whatever keeps the house naturally cooler. If you run A/C, it becomes more about what you spend up front and the amount of electricity reduction it might achieve over time. In general, spending money on insulation is probably more cost effective than an attic fan, solar or otherwise.

[Jocamo]

You could use PVWatts to play with tilt until you get the maximum values in the summer months. A quick look suggests something between 0 and 5 deg from horizontal tilted south will get you the highest values in June, July and August, but the difference in that range or even up to 10 deg really isn't much.

Solar attic fans aren't really known for moving much air. I had installed one a couple years ago that was grid powered, so it moved plenty of air, but recently took it back out again and have focused on some more reliable methods to make the house more comfortable. Ridge vent for ventilation, increased soffit vents to supply the ridge, and more insulation. I don't have AC, so for me, it is about whatever keeps the house naturally cooler. If you run A/C, it becomes more about what you spend up front and the amount of electricity reduction it might achieve over time. In general, spending money on insulation is probably more cost effective than an attic fan, solar or otherwise.

Thanks

I have soffits and a ridge vent.

The company from which I purchased is attic breeze. It purports 1300cfm at peak.

That's 15 exchanges and hour which is far more than I think I will need.

[sensij]

Thanks

I have soffits and a ridge vent.

The company from which I purchased is attic breeze. It purports 1300cfm at peak.

That's 15 exchanges and hour which is far more than I think I will need.

I'm not questioning whether a 20 W fan is capable of moving 1300 cfm (others might). Even when it does what is is supposed to, the savings is probably not there. One good study to read is here.

Since you have a ridge vent already, keep in mind that some amount of the CFM that is moved will probably be in almost a straight line from the ridge vent to the fan. Getting a airflow pattern that sufficiently cools the whole attic space is difficult, and if you try to compensate by moving more air, you can create negative pressure that could pull the cooled air out of your house (if you are using A/C) and further decrease the net savings.

How much insulation do you have? You can see what the department of energy recommends by checking out this page. Enough insulation and good sealing are probably two of the more cost-effective ways to reduce your A/C bill.

[SunEagle]

I'm not questioning whether a 20 W fan is capable of moving 1300 cfm (others might). Even when it does what is is supposed to, the savings is probably not there. One good study to read is here.

Since you have a ridge vent already, keep in mind that some amount of the CFM that is moved will probably be in almost a straight line from the ridge vent to the fan. Getting a airflow pattern that sufficiently cools the whole attic space is difficult, and if you try to compensate by moving more air, you can create negative pressure that could pull the cooled air out of your house (if you are using A/C) and further decrease the net savings.

How much insulation do you have? You can see what the department of energy recommends by checking out this page. Enough insulation and good sealing are probably two of the more cost-effective ways to reduce your A/C bill.

I had two solar attic fans that were mounted in the gables, each with a remote 10watt panel. They worked very well for me for 3 years. I just recently replaced my roof shingles and added ridge vents. They work as good so I didn't remount the panels and wire them back to the fans. I am not sure if adding an attic fan for you would be over kill and if it is a "through the roof" mount might become a point for water to leak in.

[J.P.M.]

I had two solar attic fans that were mounted in the gables, each with a remote 10watt panel. They worked very well for me for 3 years. I just recently replaced my roof shingles and added ridge vents. They work as good so I didn't remount the panels and wire them back to the fans. I am not sure if adding an attic fan for you would be over kill and if it is a "through the roof" mount might become a point for water to leak in.

A well designed ridge/soffit vent system can be every bit as effective as well placed mechanical ventilation. It will probably take more work/time/money to do it right, but it's lower maint., quiet, and still works in a power outage/rolling blackouts/etc. In most applications where the attic/crawl space is unoccupied, the goal of adequate ventilation isn't so much livable temps up there as much as moisture control (particularly in colder climates in the winter) via air circulation, and in warm, sunny climates, keeping the temp. of as much of the roof deck/structure as possible as low as possible. Doing that often has the secondary effect of reducing the temp. of the crawl space which may well somewhat reduce the cooling load of the occupied space. However, reducing the cooling load of the occupied space is a somewhat secondary goal. Lowering the HVAC load to the rest of the dwelling from an unoccupied crawl space, is a task often/usually done best by insulation on the floor of the unconditioned space.

Ridge and soffit vents are often used as a team. The air leaving at a ridge must come from somewhere - if that somewhere is at the soffits/under the eaves, more of the roof deck will probably stay cooler because more air will circulate over a larger surface. Also, if the barrier between the crawl space and the space below it is well sealed and insulated, the HVAC load will be reduced to the greatest degree.

An attic fan without some other attic ventilation to let the air INTO the crawl space that the fan is trying to get OUT of the crawl space won't work as effectively. In the unvented case, some, maybe a lot of that air that the fan (or ridge vents or other schemes) is attempting to remove will come from the conditioned space. That may very well, in effect, increase the cooling load of the conditioned space as cooled air is sucked into the crawl space from the conditioned space and replaced in the conditioned space by warm outside air sucked in through cracks/air leaks. If the house is super sealed, including the crawl space, the fan won't exhaust much at all - it'll be starved for air and won't meet the intended duty - sort of like a pump running with the inlet chocked off.

Take what you want/need of the above. Scrap the rest.

[SunEagle]

A well designed ridge/soffit vent system can be every bit as effective as well placed mechanical ventilation. It will probably take more work/time/money to do it right, but it's lower maint., quiet, and still works in a power outage/rolling blackouts/etc. In most applications where the attic/crawl space is unoccupied, the goal of adequate ventilation isn't so much livable temps up there as much as moisture control (particularly in colder climates in the winter) via air circulation, and in warm, sunny climates, keeping the temp. of as much of the roof deck/structure as possible as low as possible. Doing that often has the secondary effect of reducing the temp. of the crawl space which may well somewhat reduce the cooling load of the occupied space. However, reducing the cooling load of the occupied space is a somewhat secondary goal. Lowering the HVAC load to the rest of the dwelling from an unoccupied crawl space, is a task often/usually done best by insulation on the floor of the unconditioned space.

Ridge and soffit vents are often used as a team. The air leaving at a ridge must come from somewhere - if that somewhere is at the soffits/under the eaves, more of the roof deck will probably stay cooler because more air will circulate over a larger surface. Also, if the barrier between the crawl space and the space below it is well sealed and insulated, the HVAC load will be reduced to the greatest degree.

An attic fan without some other attic ventilation to let the air INTO the crawl space that the fan is trying to get OUT of the crawl space won't work as effectively. In the unvented case, some, maybe a lot of that air that the fan (or ridge vents or other schemes) is attempting to remove will come from the conditioned space. That may very well, in effect, increase the cooling load of the conditioned space as cooled air is sucked into the crawl space from the conditioned space and replaced in the conditioned space by warm outside air sucked in through cracks/air leaks. If the house is super sealed, including the crawl space, the fan won't exhaust much at all - it'll be starved for air and won't meet the intended duty - sort of like a pump running with the inlet chocked off.

Take what you want/need of the above. Scrap the rest.

I had just added a thermal barrier in my attic and some additional insulation above the ceilings. I needed to get hot air that collected at the peaks out so I looked at the solar vent fans. I worked with a Mech Eng and we determined that I had enough open space in the larger of my 3 gable openings to provide most of the required CFM to equal what 2 of these fans needed mounted in the other gable openings. The rest of the air flow came up through my soffit vents. I did the work myself so for about $200 each (Costco is a good resource) and a couple of Saturdays it was finished. I was very surprised that between the thermal barrier and vent fans my attic was a lot cooling during the summer. Here is an information doc of a fan very similar to what I installed.

Solar Ventilator-Information.pdf

When I replaced the roof they installed 3 long roof vents which pretty much did the job of keeping my attic cool so I never re-installed the solar panels to run the fans. The fans are still in the gable openings if I need them in the future and it wouldn't take long to put the panels back up if it gets too hot again.

[J.P.M.]

I had just added a thermal barrier in my attic and some additional insulation above the ceilings. I needed to get hot air that collected at the peaks out so I looked at the solar vent fans. I worked with a Mech Eng and we determined that I had enough open space in the larger of my 3 gable openings to provide most of the required CFM to equal what 2 of these fans needed mounted in the other gable openings. The rest of the air flow came up through my soffit vents. I did the work myself so for about $200 each (Costco is a good resource) and a couple of Saturdays it was finished. I was very surprised that between the thermal barrier and vent fans my attic was a lot cooling during the summer. Here is an information doc of a fan very similar to what I installed.

[ATTACH]5688[/ATTACH]

When I replaced the roof they installed 3 long roof vents which pretty much did the job of keeping my attic cool so I never re-installed the solar panels to run the fans. The fans are still in the gable openings if I need them in the future and it wouldn't take long to put the panels back up if it gets too hot again.

Understood. All the added vent. gets rid of a lot of heat.

FWIW, consider if the fans do get turned on, they may have something of the opposite of, or reduce the intended cooling effect by, depending on relative locations, sucking air inward through the long roof vents and reducing flow through the soffit vents, perhaps increasing the overall roof temp. which is the source of the attic heat in the first place.

[sensij]

When I replaced the roof they installed 3 long roof vents which pretty much did the job of keeping my attic cool so I never re-installed the solar panels to run the fans. The fans are still in the gable openings if I need them in the future and it wouldn't take long to put the panels back up if it gets too hot again.

Your research on this sounds similar to mine. If fans are going to be used, a pair of gable fans as you've described sounds like the most effective way to do it. Just make sure to close off the ridge vent so you don't pull in air from the top.

One of the "benefits" to the way I had installed my fan is that it was oversized to the soffit vents, and pulled air out of the living space. Since I don't use A/C, this was OK, and I would actually leave open the attic hatch when the fan was running to encourage a preferred airflow path through the house. It was creating a poor man's version of a whole house fan, which I might need to look into installing properly now that I've pulled the attic fan out and am instead using the ridge vent for attic ventilation.

I haven't put up a radiant barrier on the rafters yet, but did install high reflectivity / high emissivity shingles when the roof was re-done. I want to wait until after the solar panels are installed and I get some rain to inspect for leaks before putting the RB up. Once the foil is there, inspecting for water damage seems much more difficult.

I'm still kicking myself for missing the opportunity to put some good insulation in the south facing exterior wall when I had it open a few years ago, before I appreciated how much difference it could make.