# Overheating with Solar



## conscious (Jan 4, 2005)

So I've got a situation I'm trying to resolve. My solar heating system is supposed to work like this: The mass (water) is in the attic above my collector (glass). The heat from the sunspace rises up into the attic and heats the mass. When the mass/attic gets up to a warm enough temperature the air radiates or alternatively is blown down through a duct to heat the living space. 

So far, every thing seems to be working correctly except, that the living space starts overheating every time I get the attic over 90 degrees. I assume this is because the mass/attic needs to be insulated from the living space to slow down the radiation of heat to the living space.

Before I start removing all of that water and spending money to insulate, I want to make sure I'm thinking correctly. Any thoughts are greatly appreciated. Thanks.


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## J2E1 (Oct 15, 2009)

Just so I'm clear, you have a solar collector heating the water in the attic and your sun room contributes heat to the atic air space, with the understanding that any hot air will also heat up the water there as well. Sounds to me like you've got a lot of heat and no where for it to go but down into the living space. You want to keep that hot air up there to last you through the night but not have it radiate down during the day.

Maybe the first thing to try is some radiant barriers on the attic floor as a start then if that doesn't work do fiberglass and have the radiant barrier on top? Do you have rafters or trusses?

If you do insulate you're going to be using the duct for more of your heat transfers even during the day because the heat won't be radiating down on its own.


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## conscious (Jan 4, 2005)

J2E1 said:


> Just so I'm clear, you have a solar collector heating the water in the attic and your sun room contributes heat to the atic air space, with the understanding that any hot air will also heat up the water there as well. Sounds to me like you've got a lot of heat and no where for it to go but down into the living space. You want to keep that hot air up there to last you through the night but not have it radiate down during the day.
> 
> Maybe the first thing to try is some radiant barriers on the attic floor as a start then if that doesn't work do fiberglass and have the radiant barrier on top? Do you have rafters or trusses?
> 
> If you do insulate you're going to be using the duct for more of your heat transfers even during the day because the heat won't be radiating down on its own.


I have rafters. 

That's a good idea, thanks. Do the radiant barrier first.

It's obviously about trying to find a good balance between passive and mechanical transfer. I don't want to completely stop the radiation but I also don't want my house overheating. Right now there's definitely too much radiation.


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## J2E1 (Oct 15, 2009)

Yeap, definitely a balance, and what can be a pain is that balance is as variable as the weather. I don't know about Indiana, but here across the border about 30 miles in Ohio, all we get is clouds, clouds, and more clouds. The radiant barrier, (I think) will slow less heat than insulation will so it's the first 'smaller' step. You'll still get the conduction/convection of the air on the radiant barrier and the barrier on the rafters, into the drywall and into the air in the living space.


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## david_r (Jan 6, 2010)

Paul,
Curious what your water temp is when the attic gets to 90? I'd be really surprised if it was anywhere near 90. Air is .018btu/ft3 F. An equivalent amount of water would take 62 btu/F so that's about a 3400:1 ratio.

My point being is that maybe it isn't your thermal mass that is causing the heating but a lack of insulation issue.


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## conscious (Jan 4, 2005)

david_r said:


> My point being is that maybe it isn't your thermal mass that is causing the heating but a lack of insulation issue.


Hi David, Yes, that was my theory from the start of this thread. I don't remember anyone saying it had anything to do with the mass. 

I also believe it's a heat transfer issue from the air to the mass. It needs to be speeded up with higher temps. 

Does anyone know if more mass will increase heat transfer?

Thanks.


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## conscious (Jan 4, 2005)

david_r said:


> Paul,
> Curious what your water temp is when the attic gets to 90?


I'm not following you on that because it totally depends on the period of time and the water-air temperature differential.


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## J2E1 (Oct 15, 2009)

More mass won't necessarily increase heat transfer, what will is surface area. A larger surface area will allow more heat from the air to be taken into the water faster. The reverse is also true though, as the air temp decreases, the water will give up its heat to the air in proportion to the surface area.


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## conscious (Jan 4, 2005)

J2E1 said:


> More mass won't necessarily increase heat transfer, what will is surface area. A larger surface area will allow more heat from the air to be taken into the water faster. The reverse is also true though, as the air temp decreases, the water will give up its heat to the air in proportion to the surface area.


In this case I would be adding more containers not just increasing volume which obviously means more surface area. The system as sized for 1000 gallons and I only have 500 at this point so I'm assuming that doubling the amount of containers is going to significantly increase transfer.

And is too much surface area a bad thing then? Is there a balance that needs to be had between not enough surface area and too much? 3/4 of my water will be in 2 liter pop bottles. The rest will be 55 gallon drums. 

Thanks for your thoughts its much appreciated.


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## J2E1 (Oct 15, 2009)

I'm sure you've done the numbers and are safe, I'm just thinking about all the weight of 1000 gallons of water on top of the rafters in addition to dealing with your roof weight. 

I don't think you can have too much surface area, especially since you're dealing with sealed containers and not having to worry about evaporation to a large degree, maybe some expansion due to the temp rise in the water.

I think you'll still need to insulate though as the path of least resistance - attic air through drywall to living space is way less than the attic air to attic water as david_r's numbers would show.

I'd love to hear more opinions that have a bit more real world experience than I have. I definitely agree with david_r though, more water is a good thing but insulation is needed.


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## david_r (Jan 6, 2010)

Hi Paul. Sorry for the late return. Solargary has a chapter on his website book talking about attic solar rooms. I believe it is chapter 7
http://www.builditsolar.com/Projects/SolarHomes/PasSolEnergyBk/PSEbook.htm

Unless you can have a complete coverage of water between the hot area and the living area, I think you are going to have to insulate and use some form of active transfer. With that type of coverage, you would be using the water as the insulating layer. 

Something else you might consider is also controlling your attic temperature as part of your total system.


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## david_r (Jan 6, 2010)

Paul,
found this a little further down this forum.


Quote:
Originally Posted by SolarGary View Post
Hi,
You might take a look at this house by Norman Saunders -- it has a similar construction that uses a sunspace to heat thermal mass in the attic -- this house works very well, and is essentially 100% solar heated without overheating problems:
http://www.builditsolar.com/Projects...saunders-1.pdf



Gary
Oops! I meant to post the above note to the thread on solar overheating from the attic storage.


Gary


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## conscious (Jan 4, 2005)

david_r said:


> Paul,
> found this a little further down this forum.
> 
> Quote:
> ...


Thanks Gary (and David). Yes, my solar heating system was designed after the Saunders house. One difference that may have to do with the overheating is that his mass is isolated in a room above the sunspace. In my case, my mass is in the attic above my living space.

Have you been in Norm's house?


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## SolarGary (Sep 8, 2005)

Hi,
I've not been to Norm's house -- would love to see it.

It may well be that you need some way to insulate the living area from the attic, and then provide for a way to actively (as in a fan and duct) transfer the heat down when needed. This would allow the storage temp to vary over a wider range, since you are not living "in the storage".

One (perhaps a bit goofy) way that Nick Pine tells people to slow down the heat transfer from an overhead heat store is to attach a layer of aluminum foil to the ceiling of the living area. This slows down the heat transfer rate by radiation from the ceiling into the living area. Insulation would do the same thing.

Gary


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## moocow (Jan 6, 2010)

1000 gallons in your attic?? I think a gallon of water weighs 8lbs. Due the math thats 8000 lbs in you attic!! Hope you have extra support up there. My house pops and cracks when there are three guys up there let alone 8000 lbs.


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## conscious (Jan 4, 2005)

moocow said:


> 1000 gallons in your attic?? I think a gallon of water weighs 8lbs. Due the math thats 8000 lbs in you attic!! Hope you have extra support up there. My house pops and cracks when there are three guys up there let alone 8000 lbs.


That 8,000 lbs is spread out over 80 sq ft. which comes out to 100 lbs/sq ft. Kind of like having refrigerators side by side up in the attic. 6x6 ceiling beams/joists carrying the load down through interior walls to quadrupled and tripled floor joists take care of the extra support. I know it sounds like a lot but believe it or not, the attic floor decking is 2x6 tongue & groove which itself supports up to 100 lbs/sq ft. according to the mfg.


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## conscious (Jan 4, 2005)

SolarGary said:


> It may well be that you need some way to insulate the living area from the attic, and then provide for a way to actively (as in a fan and duct) transfer the heat down when needed. This would allow the storage temp to vary over a wider range, since you are not living "in the storage".


Thanks Gary.

To me it seems counterproductive to be "living in the storage" as you put it (ie heating the attic with supplemental heat) which is what happens if I don't insulate the attic from the living space. Why heat that space? Am I thinking right?


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## SolarGary (Sep 8, 2005)

conscious said:


> Thanks Gary.
> 
> To me it seems counterproductive to be "living in the storage" as you put it (ie heating the attic with supplemental heat) which is what happens if I don't insulate the attic from the living space. Why heat that space? Am I thinking right?


Hi -- not sure exactly what you mean, but what I was trying to say was that if the attic storage and the living space are tightly thermally coupled (ie not insulated from each other), then you can't really let the storage in the attic get to hot, or it will overheat the living space. 
With the attic storage and living space insulated from each other, you can run the storage up as hot as the collector will make -- this means you need much less storage weight to do the job.

Hope you will tell us how this comes out, and what you conclude is the best way to go.

Gary


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## survivalpro (Nov 14, 2009)

Just a thought:

Since you are using a fan to blow the heat into the living space you could use a simple thermostat in the living area to turn off the fan. This could be a three way thermostat that will also open an roof vent to cool the attic and water tank down.

Might work!


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## speedfunk (Dec 7, 2005)

How are things working latley? Have you insulated attic? 

another thought that might be simplier is to move your water (mass) out into the sun room. Then by opening and closing doors you could regulate heat. It would also have added benefit of moderating the collector (sun room) temps. Very simple and passive..less to go wrong 

I hope it figure it out... You have some cool idea's 
Jeff


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