8.7kW system coming Orange County, SoCal

[sensij]

The flat vent is an O'Hagin Vent, which is something of a premium product, based on the price that some roofers had quoted when discussing vent options for my re-roof. I would guess that the net free ventilation area of those is less than the dormer... they wanted to replace my two dormers with three O'Hagins. Since it is just a simple attic air vent, rather than asking them to move it (and re-finish the roof where it is right now), or move the panels (if their current location looks good, and avoids most shade), I would just ask them to add more venting elsewhere, and try to at least maintain the level of venting you had before the installation. With the solar panels over it, that vent won't be as effective as a vent in the open, but it should still let more air move than if the roof was closed up.

The flashing is Prosolar's Fastjack E-Series. According to the installation instructions, those bottom holes are only to be used in case of high wind (they must expect that the edge won't be tight to the roof), and no nails are needed at all for a standard installation. The instructions don't call for any sealant around the edges, and while the deflected edges look unprofessional, I'm not sure they are really much of a problem. If there are actually nails along that bottom edge that have pulled out, that would worry me... the sealant is doing almost nothing to protect a penetration there.

With respect to hot attic air venting onto the panel... I'm going out on a limb here, but I would think that the hottest surface is the roof itself, and that the vent will be cooler than the surrounding roof, even with attic air coming out of it. Assessing whether the particular mix of heat transfer paths occurring at that spot is better or worse for the panel without the vent is beyond my knowledge or experience.

[posplayr]

With respect to hot attic air venting onto the panel... I'm going out on a limb here, but I would think that the hottest surface is the roof itself, and that the vent will be cooler than the surrounding roof, even with attic air coming out of it. Assessing whether the particular mix of heat transfer paths occurring at that spot is better or worse for the panel without the vent is beyond my knowledge or experience.

I think you can be assured that any summer heat in the air of the attic is because of the solar load on the roof heating the roof and heating the air underneath. The air temp will always be less than the roof temperature.

I used to have a fairly large house with a tile roof but near the coast. In the summer the tiles would heat up and the house would warm up. We had no real need for A/C where we were but still in the high heat of the summer (85 degrees) at the end of the day the house was hot. So I installed thermostatically controlled fans in the attic probably set to 85 degF to turn on. Those things would run till 10:30 to 11:00 PM in the evening. I eventually raised the shutoff temp to probably over 90-95degF just to stop them from running so long.

The point is that it was the tile was keeping the air warm, and a simple exchange of attic air was not going to change that. With the thermal mass it took along time to cool down. The fans probably did next to nothing to keep the house cooler despite running for 8 hours a day.

If the roof is cooler because it is covered by a panel then the air in the attic will probably be cooler in proportion. So the panel temperature is going to be much more correlated to roof temperature and is not going to diverge much based on any venting scenario. My educated conjecture.

[J.P.M.]

Thanks J.P.M! I will ask an inspector to make sure.

You're welcome.

[s_man]

The flat vent is an O'Hagin Vent, which is something of a premium product, based on the price that some roofers had quoted when discussing vent options for my re-roof. I would guess that the net free ventilation area of those is less than the dormer... they wanted to replace my two dormers with three O'Hagins. Since it is just a simple attic air vent, rather than asking them to move it (and re-finish the roof where it is right now), or move the panels (if their current location looks good, and avoids most shade), I would just ask them to add more venting elsewhere, and try to at least maintain the level of venting you had before the installation. With the solar panels over it, that vent won't be as effective as a vent in the open, but it should still let more air move than if the roof was closed up.

The flashing is Prosolar's Fastjack E-Series. According to the installation instructions, those bottom holes are only to be used in case of high wind (they must expect that the edge won't be tight to the roof), and no nails are needed at all for a standard installation. The instructions don't call for any sealant around the edges, and while the deflected edges look unprofessional, I'm not sure they are really much of a problem. If there are actually nails along that bottom edge that have pulled out, that would worry me... the sealant is doing almost nothing to protect a penetration there.

With respect to hot attic air venting onto the panel... I'm going out on a limb here, but I would think that the hottest surface is the roof itself, and that the vent will be cooler than the surrounding roof, even with attic air coming out of it. Assessing whether the particular mix of heat transfer paths occurring at that spot is better or worse for the panel without the vent is beyond my knowledge or experience.

Thanks sensij! Probably opening another vent in another place and keep the other one as you suggested is a good idea, I will talk to them about it. Also now looking at the FastJack installation guide I understand it more. I also has the same concern about unprotected penetration when I found those screws coming loose on the bottom edge. I guess if it could not be secured down then they would have to seal it at the hole under the jack to protect the penetration.

[s_man]

I think you can be assured that any summer heat in the air of the attic is because of the solar load on the roof heating the roof and heating the air underneath. The air temp will always be less than the roof temperature.

I used to have a fairly large house with a tile roof but near the coast. In the summer the tiles would heat up and the house would warm up. We had no real need for A/C where we were but still in the high heat of the summer (85 degrees) at the end of the day the house was hot. So I installed thermostatically controlled fans in the attic probably set to 85 degF to turn on. Those things would run till 10:30 to 11:00 PM in the evening. I eventually raised the shutoff temp to probably over 90-95degF just to stop them from running so long.

The point is that it was the tile was keeping the air warm, and a simple exchange of attic air was not going to change that. With the thermal mass it took along time to cool down. The fans probably did next to nothing to keep the house cooler despite running for 8 hours a day.

If the roof is cooler because it is covered by a panel then the air in the attic will probably be cooler in proportion. So the panel temperature is going to be much more correlated to roof temperature and is not going to diverge much based on any venting scenario. My educated conjecture.

It makes sense. Thanks posplayr! I was afraid moisture or hot air coming out of the vent just accumulate more underneath the panels but hopefully the space is not too cramp to cause ventilation problem.

[J.P.M.]

On roof temps, vents, heat, etc.:

1.) Spaces under roofs, particularly those with insulation between the space - call it an attic for now - and the living space or conditioned area under it can and usually will experience large temp. swings in response to outside conditions. One such input is solar radiation. Solar radiation will heat structural members such as roofs. That heat will be transmitted to the interior (underside) of the roof and that heat will be transmitted via convection, radiation and some conduction to the rest of the adjacent attic space and the air in it. Some of that heat will pass to the rest of the dwelling below, more for those attics with no insulation between the attic floor and ceiling of the space below.

2.) In warmer times of year, it is usually desirable to remove that excess heat. This is often and usually done my venting the heated air, via natural or forced convection, or both, and in the process replacing it with outside air. The outside air thus introduced will then pick up heat from the space and in a semi continuous process, remove that heat, hopefully at a rate that at least matches the energy input.

3.) When properly sized, numbered and located, roof vents that do this via natural convection assisted by wind can often do an adequate job of keeping the space temp. down.

4.) Natural convection roof vents work best when there is wind blowing over them and probably a bit better away from the vent, and in the "downhill" direction on a sloped roof, sort of enhancing the venturi effect. Such vents still work (that is, remove air from the space under the vent) in still air, on flat roofs and when covered by objects such as solar panels. They just do not perform as well with impediments.

5.) Forced convection (fans) can work well. One common method is a fan located in a gable ends, or better yet 2 fans, one at each end located high in the gable, one at each end, one blowing in, one out and both blowing in the same direction as the generally prevailing winds that are coincident with the most heat events if possible.

6.) A common shortcoming in thinking results in using a single vent or vents and not allowing or considering that the exhausted air will not leave without a way for air to get in to replace it. If something leaves, there must be a way to replace it. In the case of an attic, vents, forced or mechanical without matching inlets of some sort will not be as effective. In such cases, the air they do manage to exhaust will probably then come from the conditioned space below, or adjacent vents reducing or eliminating those other vents usefulness. That air, if it come from the conditioned space, which is probably cooler due to mechanical refrigeration, will be replaced by warmer or hotter outside air sucked into the conditioned space through cracks, vents, leaky windows, etc., increasing the cooling load. This is usually counterproductive to at least one of the purposes of cooling the attic in the first place.

7.) In the case of multiple nat. vents, without lower inlets in the eaves or gable ends, or some other arrangements, one or 2 of them will feed the other(s). This will usually tend to lower temps. some but not as much as when used in conjunction with lower arrangements. Nat. vents and mechanical ventilation used simultaneously often act in a similar fashion. Since forced convection commonly is more effective than nat. convection, a fan at a lower level than the roof can suck air through the roof vents in a somewhat reverse fashion. This will sort of short circuit the air flow and the attic temp., while lower than the no vent situation, will be higher than optimal.

8.) Roof materials with a lot of thermal mass, like concrete tile, and particularly those of a dark color as is common, heat up (in the morning) and cool down (in the evening) slower than lighter weight materials. Basically the roof mass, like any thermal mass, and not unlike a capacitor in an elec. circuit, changes the thermal time constant of the system. The thermal mass by itself has little if any effect on the overall heat transfer rates, concrete having pretty good thermal conductivity. The roof color has a larger effect on heat gain.

9.) Properly sized and placed vents and fans fans will help deal with any unwanted heat gain. They will probably not get rid of all of it. If on thermostats, they will operate with a time a lag for systems with a large(r) thermal mass. Kind of a geeky thing to do, but analogous to an elec. circuit the thermal time constant can be used to est. the magnitude of that time lag. Another geeky thing to do is to put a control surface around the roof and do an energy balance on it. Can be lots of fun to some.

10.) From my own geeky investigations, under full sun at various times of the year, but at min. solar incidence angle on the array (and thus the roof), the concrete tile under the array is, as a 1st approx. for the purpose of brevity, about halfway between the average panel temps. and the amb. air temp. on the roof. That ambient air temp. on the roof is about 3 - 5 C. warmer than the air ground temps. Under full sun, the panels run about 25 C. +/- a few degrees C. warmer than the surrounding air, and the solar exposed roof deck runs up to about 35 C. above local roof amb. air temp., depending on wind vector, time of year and solar flux. The interior ceiling temp. of that portion of the array directly over that cathedral ceiling with no attic/crawl space above that ceiling is about 3 C. warmer than that portion of the same ceiling which is under solar exposed roof.

11.) As a practical matter, an attic or for that matter, any control volume, will be cooler if there is less net energy input. A solar array, or any radiation barrier or shading will reduce that input. Roof temp. and panel temp. are driven by mostly the same inputs, but there is not usually a strong causal relation between them, beyond some radiation heat transfer between the two, often parallel surfaces that sends them closer to one another with respect to temp.

12.) Not absolute, but I kind of doubt that attic air exhausting near or under a panel will do any damage to a panel or have a measurable effect on performance in warm weather. However, in cold weather, I'd guess there might be some small potential for damage from condensing moisture freezing on the underside or the electronics of a panel in a way and with a frequency not accounted for in the original design. That could raise warranty concerns.

In any case, it just seems like a dumb thing to put a panel over a vent, or not move a vent before placing a panel.

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

[s_man]

Great reading material. Thanks J.P.M! Thinking back, I regret just let them come and do it on their schedule and Icould not be there. A lesson learned; don't trust any vendor to take care of your interest better than yourself!

[posplayr]

Great reading material. Thanks J.P.M! Thinking back, I regret just let them come and do it on their schedule and Icould not be there. A lesson learned; don't trust any vendor to take care of your interest better than yourself!

In the same vein, I adopted a rule no concrete gets poured unless I'm on site. Too much bad stuff can happen, like washing out concrete and having it run down the street to mess all your neighbors curbs.


BTW with my forced convection attic coolers, I added 3 large vents in each of three gable ends and had two fans in two of the three gables pushing out creating a low pressure area behind. In theory this would pull fresh air from the third shaded gable and avoid a high pressure center that might push attic dust into the house. The roof was concrete tile with a lot of thermal mass and it's own schedule for heating and cooling little affected by the attic fans.

[sensij]

BTW with my forced convection attic coolers, I added 3 large vents in each of three gable ends and had two fans in two of the three gables pushing out creating a low pressure area behind. In theory this would pull fresh air from the third shaded gable and avoid a high pressure center that might push attic dust into the house. The roof was concrete tile with a lot of thermal mass and it's own schedule for heating and cooling little affected by the attic fans.

The downside to intentionally creating a low pressure zone is that if the interior ceiling/attic floor is not perfectly sealed (or if there are other paths through the wall spaces) there will be a tendency to pull the conditioned air into the attic. Balancing the push and pull is probably a better design practice, when workable.

FWIW, when I redid my roof ahead of my PV installation, I took out a powered fan and dormer vents and replaced it with a full ridge vent. I haven't increased the soffit vents yet to match, kind of waiting to see how things go when the Santa Ana's kick up in next couple months. So far, it has been comfortable, but the weather hasn't been bad, and the reflective shingles + the shade from the array are probably helping too.

[posplayr]

The downside to intentionally creating a low pressure zone is that if the interior ceiling/attic floor is not perfectly sealed (or if there are other paths through the wall spaces) there will be a tendency to pull the conditioned air into the attic. Balancing the push and pull is probably a better design practice, when workable.

FWIW, when I redid my roof ahead of my PV installation, I took out a powered fan and dormer vents and replaced it with a full ridge vent. I haven't increased the soffit vents yet to match, kind of waiting to see how things go when the Santa Ana's kick up in next couple months. So far, it has been comfortable, but the weather hasn't been bad, and the reflective shingles + the shade from the array are probably helping too.

In Santa Barbara, within 1 mile of the coast it is pretty moderate weather/temperatures (75-80 deg) ; nobody has AC. I'd me more worried about dust.

[sensij]

In Santa Barbara, within 1 mile of the coast it is pretty moderate weather/temperatures (75-80 deg) ; nobody has AC. I'd me more worried about dust.

Yeah, that makes sense. I don't have A/C either... when I had my powered attic fan running, I would open the hatch to the attic and intentionally draw some of the warmer air out of the house and create some circulation, like a poorly implemented whole house fan. It was better than nothing, although more ceiling insulation is the right next step at this point, I think.


Edit: Sorry for the tangent.

[J.P.M.]

Yeah, that makes sense. I don't have A/C either... when I had my powered attic fan running, I would open the hatch to the attic and intentionally draw some of the warmer air out of the house and create some circulation, like a poorly implemented whole house fan. It was better than nothing, although more ceiling insulation is the right next step at this point, I think.


Edit: Sorry for the tangent.

When you did that, if the outside ambient was warmer than the inside, you wound up adding heat to the dwelling. If you have a tight dwelling and didn't crack a window or open a door, the tactic was less than optimally effective. If not warmer outside, the tactic cooled the interior and the attic, especially with a cracked/opened window/door.

On the ceiling insulation, don't forget the vapor barrier and remember that when sealing, details and attention to them are king. Not so much around here, but in colder climates for other readers, condensation from air leaks can be a long term nightmare.

[J.P.M.]

Great reading material. Thanks J.P.M! Thinking back, I regret just let them come and do it on their schedule and Icould not be there. A lesson learned; don't trust any vendor to take care of your interest better than yourself!

You're welcome. Take it for what its worth or less.

[sensij]

When you did that, if the outside ambient was warmer than the inside, you wound up adding heat to the dwelling. If you have a tight dwelling and didn't crack a window or open a door, the tactic was less than optimally effective. If not warmer outside, the tactic cooled the interior and the attic, especially with a cracked/opened window/door.

Yes... I used an inexpensive differential temp relay to make sure the fan wouldn't come on if outside was warmer than inside. Playing around with different combinations of window and doors open and closed to direct the circulation was sort of fun, but probably not all that productive. The house isn't truly tight, but probably the best lesson I learned out of it is that just because there is no A/C, doesn't mean the windows/doors should be open all day. A tight house can be a comfortable house. Now, if I could just convince my wife...

[posplayr]

Yes... I used an inexpensive differential temp relay to make sure the fan wouldn't come on if outside was warmer than inside. Playing around with different combinations of window and doors open and closed to direct the circulation was sort of fun, but probably not all that productive. The house isn't truly tight, but probably the best lesson I learned out of it is that just because there is no A/C, doesn't mean the windows/doors should be open all day. A tight house can be a comfortable house. Now, if I could just convince my wife...

Yes it seems that keeping the house closed before noon it stays cooler. Then as the roof gets hot and the heat builds up in the house, you have to open some doors to get the radiated heat form the roof to blow out the house. But in my case trying to blow out the attic probably helped some but at the cost of running the fans for several hours?? It certainly was not real effective is cooling the roof.

Most of my air inlet was from the north end of the house that by afternoon was shaded by tall trees. While it may not have been much different that the temp in the house, the north side air was certainly going to be cooler than the air in the attic. Turning on thermostatically based on a fixed temperature, I don't think it is possible for the ambient temp to be more than the attic temp for temps above 85 degF.

In your case as well, ambient might have been more than inside the house for a while, but it is always less than the attic air temperature which is what is heating the house in the first place.