bartman99
New Member
Posts:57
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| 28 Apr 2010 10:00 AM |
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Hi,
We live in Virginia and have a 4 panel solar thermal setup that contributes to our winter heating needs and provides ample hot water in the summer. We are able to use the panels for heat as we have hydronic radiant floor heat. Anyway, Virginia now has a solar thermal rebate and because of that we can add 3 more panels for essentally free since all the plumbing, etc has been done. Out current system uses 4 AET flat panel 4'x10' modules that weighing ~155lbs each. The are mounted on a 2 rail/track system with room to add more. I am worried about the point and dead loads of adding 3 more panels.
Our house is brand new. The rafters are hand cut (ie. not trusses) 2x12's. I assume the roof meets Virginia code. The panel rails are installed perpendicular to the rafter(each rafter carries 2 points, 1 from the lower rail and one from the upper rail). This past winter we had an (unusuallly) large amount of snow in Virginia.
I am not asking for engineering advice, but if a setup of 7 4x10 panels mounted on the roof exceeds the experience of this group.
BM
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Dana1
Senior Member
Posts:6991
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| 28 Apr 2010 02:32 PM |
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Most roof structural framing can easily handle the load. With any mounting pitch over ~6:12 snow loads will slough fairly quickly, and won't accumulate to significant depths on flat panels, and you needn't add snow load to the total static load. If you follow the latitude +15 rule of thumb for setting space heating panel angle they'll be considerably steeper than that. At 155lb/40ft^2 you're at about 4lbs/ft^2 loading which is roughly equivalent to 2' of fresh (unsettled) maritime-state snow. (Methinks you had more than that up there this past winter, eh?) |
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bartman99
New Member
Posts:57
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| 28 Apr 2010 03:23 PM |
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roughly equivalent to 2' of fresh (unsettled) maritime-state snow. (Methinks you had more than that up there this past winter, eh?) Yea, we had a total of 56" for the winter. The roof is 5/12 and the panel are propped up at a steeper angle than that. That being said, with one of the larger snows, the panels we covered for about a week. Must of been really sticky snow! |
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jbaron
Basic Member
Posts:122
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| 28 Apr 2010 04:02 PM |
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IMO, the panels don't provide enough weight - above the amount of snow and the like that you've already accomodated - to be significant.
What might be a problem is the extra wind load, and how the roof was engineered to accept an uplift force rather than a downward force. Unfortunately, I can't really advise you on this.
Jeff |
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Dana1
Senior Member
Posts:6991
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| 28 Apr 2010 05:12 PM |
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Posted By bartman99 on 28 Apr 2010 03:23 PM
roughly equivalent to 2' of fresh (unsettled) maritime-state snow. (Methinks you had more than that up there this past winter, eh?)
Yea, we had a total of 56" for the winter. The roof is 5/12 and the panel are propped up at a steeper angle than that. That being said, with one of the larger snows, the panels we covered for about a week. Must of been really sticky snow!
Sticky snow can get it to hang onto the panel for a bit, but sloughs whenever it hits a weight threshold. The layer that stuck for a week probably wasn't anything like a foot deep (or was it?). Composition roofs of equally high pitch can sustain much higher snow loads than glazed panels- panel glazing doesn't provide nearly as much grip. At a pitch of 5:12 the snow will stay on the grippier roof deck pretty much forever- it melts before is slides. At 10:12 or higher snow will slide off composition roofs- sometimes in a
single avalanche, other times in a slug-slow crawl. 10:12 (~40 degrees) is near the center of scare-zone steepness for us backcountry skiers, since it can often build to significant depths before releasing, and old snow layers of depth hoar deep in the snowpack even from months prior can create a weak-bonded sliding surface just waiting for a climber or skier to trigger an avalanche. At pitches 10 degrees higher it reliably sloughs before building much depth, 10 degrees lower it's far less likely to slide- the bonding forces of snow on ground or snow on ice being far more favorable to holding the slab o' white & frozen in place. The bond to glass or plastic glazing is far far weaker- it sloughs sooner than on granite, ice, or dirt. What jabaron said about engineering for wind-loading. A solar panel mounted well off the deck can be viewed as a big sail with more than a little bit of lever-arm. |
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bartman99
New Member
Posts:57
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| 29 Apr 2010 10:14 AM |
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Thanks everyone for the information. There's not typically too much wind where we are, but you never know. I doubt the roof was engineered for more than the code specs in Virginia (are there uplift code specs?). Is there any way to "beef up" the roof for uplift? Strapping the rafters to the walls perhaps? |
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Eric Anderson
Basic Member
Posts:441
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| 29 Apr 2010 11:52 AM |
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Virginia appears to base the state code on IRC 2006 edition code.
I don't have a copy of that code book, but I would be fairly sure that there is an uplift proscription in the code that the builder had to meet. The previous code cycle , IRC 2003, which I have the CT code book for, requires uplift restraint. ( what used to be called hurricane ties)
IF the attic is open and you can look, you should find metal connectors at the rafter ends tying the rafter to the top plate of the wall. I think it is unlikely that the extra windage, or uplift from the collectors would tear off the roof, I think it is more likely that the mount would tear off the roof first. Depending on the exact scheme for attaching the panel mount, it is likely the weak link, not the rest of the roof. To me the more important question is what type of heat dump do you have planned for the summer time? You are going to have way more then you can use. As I type this I wonder whether you would be better off increasing the tilt of the panels(more vertical) to bump up winter gain. With 10 panels, you are still going to make plenty for hot water in the summer, even over tilted and you will need less heat dump.
Eric |
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| Think Energy CT, LLC Comprehensive Home Performance Energy Auditing |
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bartman99
New Member
Posts:57
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| 29 Apr 2010 03:18 PM |
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Posted By eric anderson on 29 Apr 2010 11:52 AM
To me the more important question is what type of heat dump do you have planned for the summer time? You are going to have way more then you can use. As I type this I wonder whether you would be better off increasing the tilt of the panels(more vertical) to bump up winter gain. With 10 panels, you are still going to make plenty for hot water in the summer, even over tilted and you will need less heat dump.
Eric
Eric, Thanks for the research. I will go in the attic and look for the hurricane ties. My memory is that they are there. With regards to heat dump, we'll have 7 panels total if we do the upgrade (not that it matters much). We have a drainback system that shuts off at 150F. So far (1 summer) it's worked fine, turning off the circulating pump when the temp reaches goal. B |
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Dana1
Senior Member
Posts:6991
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| 30 Apr 2010 01:41 PM |
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Posted By bartman99 on 29 Apr 2010 03:18 PM
Posted By eric anderson on 29 Apr 2010 11:52 AM
To me the more important question is what type of heat dump do you have planned for the summer time? You are going to have way more then you can use. As I type this I wonder whether you would be better off increasing the tilt of the panels(more vertical) to bump up winter gain. With 10 panels, you are still going to make plenty for hot water in the summer, even over tilted and you will need less heat dump.
Eric
Eric,
Thanks for the research. I will go in the attic and look for the hurricane ties. My memory is that they are there.
With regards to heat dump, we'll have 7 panels total if we do the upgrade (not that it matters much). We have a drainback system that shuts off at 150F. So far (1 summer) it's worked fine, turning off the circulating pump when the temp reaches goal.
B
Is there something in the controls to limit stagnation temps in the collectors? Sometimes it makes sense to super-heat the hot water storage during the day to keep the panel temps from going nuts, then pump at night to radiate heat away with the panesl to reduce the temp of the storage. But the ratio of panel area to storage has to be right to make this work. The other favorite heat-dumping scheme in some quarters is to heat a (sometimes uninsulated) backyard hot tub with the excess thermal uptake. |
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bartman99
New Member
Posts:57
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| 01 May 2010 11:53 AM |
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Posted By Dana1 on 30 Apr 2010 01:41 PM
Is there something in the controls to limit stagnation temps in the collectors? Sometimes it makes sense to super-heat the hot water storage during the day to keep the panel temps from going nuts, then pump at night to radiate heat away with the panesl to reduce the temp of the storage. But the ratio of panel area to storage has to be right to make this work.
The other favorite heat-dumping scheme in some quarters is to heat a (sometimes uninsulated) backyard hot tub with the excess thermal uptake.
No nothing to limit stagnation, though I thought that that was only relevant if there was water in the collectors? Our tank is a 500gal poly tank with a temp limit of ~160f, so we can't really superheat that water. What are the potential downsides of heating empty collectors? And unfortunately, no hot tub or pool  |
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Dana1
Senior Member
Posts:6991
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| 05 May 2010 02:14 PM |
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Posted By bartman99 on 01 May 2010 11:53 AM
Posted By Dana1 on 30 Apr 2010 01:41 PM
Is there something in the controls to limit stagnation temps in the collectors? Sometimes it makes sense to super-heat the hot water storage during the day to keep the panel temps from going nuts, then pump at night to radiate heat away with the panesl to reduce the temp of the storage. But the ratio of panel area to storage has to be right to make this work.
The other favorite heat-dumping scheme in some quarters is to heat a (sometimes uninsulated) backyard hot tub with the excess thermal uptake.
No nothing to limit stagnation, though I thought that that was only relevant if there was water in the collectors? Our tank is a 500gal poly tank with a temp limit of ~160f, so we can't really superheat that water. What are the potential downsides of heating empty collectors?
And unfortunately, no hot tub or pool
Letting collectors run at very high stagnation temps can mechanically stress & fatigue the materials, cutting into lifespan. They're designed to not self-destruct quickly, melt/catch fire etc merely from stagnation, but that's not to say it's a great idea to cycle them through high temps 100x/year if you don't need to. Shading them during the months when the heating load is near-zero is probably a nicer way to treat them. The data sheets for panels usually have a maximum stagnation temperature specified- IIRC that's sometimes a "do not exceed" number, other times a measured number under a specified test condition. |
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