Attic Cooling and Solar Performance

[russ]

Try a temporary fan first - You can see it makes no difference when you cool the attic - You have to cool the top layer of the roof - not the bottom

[SunEagle]

I always thought that the designed roof racking system was supposed to be high enough to allow a good air flow between the solar panels and roof.

As russ states for best results you want to cool the area directly below the panels as opposed to cooling your attic area.

[DavidT]

If I solely want to test the effect of cooling the attic, best approach would be for me to open up some windows on a hot summer day and run the whole house fan. If I don’t do the air intake and duct plumbing, I’m sure curiosity will get the best of me and I’ll have to experiment with the above. Bit of a pain to put our screens in, but I’ll need to know if it has any effect. Lol

Even if I’m not directly cooling the outer surface of the roof (the shingles), I would think cooling the underside would have to have at least some effect. Would you all agree? Even with air flow under the panels, seems there would have to be plenty of radiant heat hitting them. One question will be… just how hot will my attic get now, now that the Southern roof is 90% covered with panels. And if the attic is now going to be cooler due to the shading, will that lighten the load of the AC unit?..

[russ]

I would think cooling the underside would have to have at least some effect. Would you all agree? Don't think you would see any measurable difference.

Even with air flow under the panels, seems there would have to be plenty of radiant heat hitting them. There will be

One question will be… just how hot will my attic get now, now that the Southern roof is 90% covered with panels. And if the attic is now going to be cooler due to the shading, will that lighten the load of the AC unit?..

The shade will help the attic temp

[DavidT]

Try a temporary fan first - You can see it makes no difference when you cool the attic - You have to cool the top layer of the roof - not the bottom

Russ, have you tried this, or know of anyone who has?

[J.P.M.]

Russ, have you tried this, or know of anyone who has?

I have. Many years ago. Depending on how much air is moved through the attic on a sunny day, if it's cooler outside than inside, it can't not work. About the only question is how well it works and how much it reduces the temp.

[russ]

Russ, have you tried this, or know of anyone who has?

David - I have worked with many industrial heat recovery systems over the years - what you propose goes against all rules.

Too little differential temperature and using air as a heat transfer media - air is really poor for low delta T applications.

[Wy_White_Wolf]

IMHO - if your solar production increases with increased attic ventilation than you PV panels are mounted to close to the roof.

WWW

[DavidT]

I talked to one of the guys who works at the company that installed my HVAC system. We didn’t get into attic/roof temperatures, but he agreed that the deal breaker with using the whole house fan for cooling the house would be the concern of mold due to higher humidity levels. He talked as if whole house fans are ancient relics from a time long gone. I’ll have to ask my cousins (new home builders) if they install them anymore. We’re so spoiled.. We’ve survived thousands of years without AC and now we can’t live without it.

[Volusiano]

I talked to one of the guys who works at the company that installed my HVAC system. We didn’t get into attic/roof temperatures, but he agreed that the deal breaker with using the whole house fan for cooling the house would be the concern of mold due to higher humidity levels. He talked as if whole house fans are ancient relics from a time long gone. I’ll have to ask my cousins (new home builders) if they install them anymore. We’re so spoiled.. We’ve survived thousands of years without AC and now we can’t live without it.

Can you clarify to me where a whole-house fan would vent out to? The attic, or directly outside?

I can see that in very mild climates (or mild seasons like spring or fall) where AC is really not needed, running a whole-house fan with all windows opened to draw in fresh and cooler air is beneficial. Are you thinking about installing a whole-house fan and vent it up into the attic to help keep the attic cooler as well (and hopefully the roof and hence the solar panels, too) and kill 2 birds with 1 stone? If that's the case, I would surmise that if it's cool enough to not need to run the AC and being able to just run a whole house fan with windows open, then it's not hot enough up on the roof to cause the panels to heat up so much that keeping the panels cooler would be a concern.

If you want to install a whole-house fan just as another cheaper cooling approach for your house, that's fine. But doing so expressedly for the purpose of trying to cool the panels indirectly through the attic, then the roof, then the air gap between the roof and the panels, would be overkill and possibly with very little ROI.

[J.P.M.]

Can you clarify to me where a whole-house fan would vent out to? The attic, or directly outside?

I can see that in very mild climates (or mild seasons like spring or fall) where AC is really not needed, running a whole-house fan with all windows opened to draw in fresh and cooler air is beneficial. Are you thinking about installing a whole-house fan and vent it up into the attic to help keep the attic cooler as well (and hopefully the roof and hence the solar panels, too) and kill 2 birds with 1 stone? If that's the case, I would surmise that if it's cool enough to not need to run the AC and being able to just run a whole house fan with windows open, then it's not hot enough up on the roof to cause the panels to heat up so much that keeping the panels cooler would be a concern.

If you want to install a whole-house fan just as another cheaper cooling for your house, that's fine. But doing so expressly for the purpose of trying to cool the panels indirectly through the attic, then the roof, then the air gap between the roof and the panels, would be overkill and possibly with very little ROI.

Usually, but not exclusively, for cooling mode, whole house fans in moderate climates are used at night and/or when outside temps are cooler than inside if cooling of the conditioned space is desired. The fans run until the desired cooling effect is achieved, sometimes all night and beyond. Then, during the daylight hrs., the fan is operated until the outside is warmer than inside. The house is then buttoned up and the A/C turned on as needed. Sometimes fans are vented to the attic, but more often directly outside. Operating a whole house fan during the day in regular mode would probably be counterproductive as it would mean drawing warmer outside air into the conditioned space. Attic venting is usually a separate issue. I can see where diverting the fan inlet/outlet for the purpose of cooling solar panels would be a real Rube Goldberg setup and not lead to a good, workable outcome.

[SunEagle]

Usually, but not exclusively, for cooling mode, whole house fans in moderate climates are used at night and/or when outside temps are cooler than inside if cooling of the conditioned space is desired. The fans run until the desired cooling effect is achieved, sometimes all night and beyond. Then, during the daylight hrs., the fan is operated until the outside is warmer than inside. The house is then buttoned up and the A/C turned on as needed. Sometimes fans are vented to the attic, but more often directly outside. Operating a whole house fan during the day in regular mode would probably be counterproductive as it would mean drawing warmer outside air into the conditioned space. Attic venting is usually a separate issue. I can see where diverting the fan inlet/outlet for the purpose of cooling solar panels would be a real Rube Goldberg setup and not lead to a good, workable outcome.

When I was growing up many years ago my parents had a whole house fan and we used it just like you stated on Summer nights to helps us keep cool. We also used it during the day but only opened the windows on the shady side of the house. The air was not really cool but the movement from the fan was nice.

Later my parents got rid of the fan and went to a 3 ton AC unit. Now that was real nice during the summer.

[DavidT]

I have a whole house fan installed already. The “idea” would be to connect an exterior air intake to the duct work and use the fan for house cooling. The fan is in the ceiling of an open foyer. Flip on the switch and louvers open up and the fan sucks air out of the living spaces and pushes it into the attic area. From there it escapes out the attic vents (soffit and ridge). It’s mainly used when the smoke alarm goes off due to something being in the oven too long. One time my wife turned on the fan but didn’t open any windows. I came home and heard a “whup whup whup…” sound. It was the blower in the dryer going backwards due to the vacuum being created in the house.

If the fan is used during the night, I wonder how much harder the AC unit would have to work in the morning to condition all the new air. Running a humidifier wouldn’t be a viable solution for that since the overall objective is to lesson electric consumption. Something else I need to keep in mind, my solar panel system will over produce as it is with my current consumption. Any production beyond my consumption has diminishing returns for me since with net metering I get reimbursed at my electric supply rate, not supply plus delivery. So if I do search for and find ways to lower my electric consumption it’s not as beneficial for me as it would have been before my solar panels. So that makes it even harder for the fan idea to work for me.

BTW – Today was the best day yet for my young solar system – the 33 panels put out 59 kWhs today. I can’t wait to see what they’ll do during the long summer days.

[J.P.M.]

When I was growing up many years ago my parents had a whole house fan and we used it just like you stated on Summer nights to helps us keep cool. We also used it during the day but only opened the windows on the shady side of the house. The air was not really cool but the movement from the fan was nice.

Later my parents got rid of the fan and went to a 3 ton AC unit. Now that was real nice during the summer.

Understood. You want nice, you pay nice and it sure does feel good. Depending on season/location you can reduce a lot of A/C use. I'm north of San Diego and inland some from the coast. Summers are quite toasty but not brutal. My A/C operates about 120-150 hrs./yr. depending on weather. Night time ventilation does the rest along w/ 78 deg. F. daytime thermostat setting. Cooling degree days on a 60 deg. F. base is about 1800 or so. 3,000 ft.^2 conditioned space.

[J.P.M.]

I have a whole house fan installed already. The “idea” would be to connect an exterior air intake to the duct work and use the fan for house cooling. The fan is in the ceiling of an open foyer. Flip on the switch and louvers open up and the fan sucks air out of the living spaces and pushes it into the attic area. From there it escapes out the attic vents (soffit and ridge). It’s mainly used when the smoke alarm goes off due to something being in the oven too long. One time my wife turned on the fan but didn’t open any windows. I came home and heard a “whup whup whup…” sound. It was the blower in the dryer going backwards due to the vacuum being created in the house.

If the fan is used during the night, I wonder how much harder the AC unit would have to work in the morning to condition all the new air. Running a humidifier wouldn’t be a viable solution for that since the overall objective is to lesson electric consumption. Something else I need to keep in mind, my solar panel system will over produce as it is with my current consumption. Any production beyond my consumption has diminishing returns for me since with net metering I get reimbursed at my electric supply rate, not supply plus delivery. So if I do search for and find ways to lower my electric consumption it’s not as beneficial for me as it would have been before my solar panels. So that makes it even harder for the fan idea to work for me.

BTW – Today was the best day yet for my young solar system – the 33 panels put out 59 kWhs today. I can’t wait to see what they’ll do during the long summer days.

The usual, main idea of a whole hose fan is for cooling. The logic is to operate it when it is cooler outside, usually at night, cool the house interior by sucking the cooler night through the house so that hopefully the A/C runs less and the house is cooler in the morning. I'm not sure I'm following your logic on this.