Saturday, June 11, 2011

Solar Heat from the Attic

I think everyone has gone up to the attic and been surprised by how hot it is.  Its just a small step from observing the heat to wondering if you could make use of it somehow. 

I added a new section that gathers together a few schemes for using attic heat for space or water heating, and other things -- here are a few samples -- see the link below for all the details.

Tim just did a project on EcoRenovators that makes use of attic heat in a nice simple way --he  just blows hot attic air down into the house when the attic is hot and house needs heat -- nice and simple, and it works.

Tims heat from the attic systemThis is Tim's setup -- it uses a chimney that is not in use anymore to channel the air down to the kitchen.  A homemade differential controller decides when to turn the blowers on.












The picture to the left is the "Black Roof" solar heating scheme.  This scheme basically harvests attic heat for space heating with just a few simple components and a blower.  It is said to be effective in cold climates that get quite a bit of sun -- eg Colorado.




There have been a number of schemes to use the attic heat to preheat domestic water.  This is a recently announced commercial offering that uses PEX tubes installed in the attic vent system to harvest heat.




Another idea that I like is picking up the inlet air for the clothes dryer from the attic.  Electric clothes dryers are big time energy users -- they not only use about 4 KWH of electricity per load, but they also end up sucking cold outside air into the house as they blow the air they have just heated out into the cold (wasting all the heat they just added to it).  If you could work out a way to pull hot air from the attic for the dryer it would eliminate all this cold air infiltration and the drywer could use less electricity since its inlet air would be hot already.  The nice thing about this is that you can time the drying to coincide with times when the attic is hot.

The ability to pull useful heat out of the attic will depend a lot on the climate you live in.   In our very cold climate, the space heating applications would be limited to the shoulder seasons, but this still could be an important source of heat.  The link below has my logged hourly attic temperatures for about a year.

If the use of attic heat idea could be incorporated in the initial design of the house there are all sorts of interesting and cost effective things that could be done to make the attic an effective heater.  Things like designing the attic to be a more efficient collector through the design of the roof surface, the attic vent system, and selective use of insulation on non-collecting surfaces could make the attic a fairly efficient and very large solar collector for not a lot of extra expenditure.  Introducing glazing could make the attic collector competitive with commercial air heating collectors.   

Got any ideas on any of this?  Have you used attic heat?  How?


All the details on the schemes above as well as several more interesting ideas are detailed here...

Gary

6 comments:

  1. I don't think the attic-pex domestic water preheater will be taking off anytime soon, common sense will kill it.

    By the time you've installed everything required for this system, all you have to add is $500 for a solar collector, and voila, you have a real solar system that works all year round.

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  2. Hi Kevin,
    I suppose it might go over better with HOAs.

    Have you seen any attic heat recovery methods that do work?

    This fall I'm going to try closing off the back of a couple rafter bays with polyiso board with inlet at bottom and outlet at top to see if it makes a simple thermosyphon collector. If it works, idea would be to collect hot air in a plenum along the ridge and then use duct and fan to blow the heat into living area.
    I'm sure this won't do much in the mid winter, but we have shoulder seasons that seem to go all the way to July :)

    Gary

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  3. Hi Gary,

    I used to live in a house that almost got no sun; an old style villa with its northern (I am in dunedin, new zealand) side built 1.2m from the neighbouring house, with no windows on that side; it had poor insulation as well. Due to a government scheme, the landlords got decent insulation put in the ceiling space, and a moisture barrier and reflecting layer under the house. The house was freezing; basically an ice box in the lounge, where the attic space man hole access was. The roof was dark grey decramastic tiling over the top of an existing iron roof. The house had a chimney in the lounge (double sided brick) which i removed. I designed a system in C#, using Dallas one wire sensors and my HTPC; serial in took temperature measurements from the ceiling, the lounge and one from outside for fun; I used a program called log-temp to spit out xml files which my c# program read every 10 minutes to control a parallel port based relay system. This, in turn, controlled a bilge fan connected to a computer PSU in the roof space - i replaced the existing manhole cover in the lounge with one made of Medium density fibre board with a vent for the fan. The difference in the house was massive - basically made the place livable. The roofspace on a reasonably sunny day often got to the 45 degrees celcius mark, and even with the outside temp reaching only 12 degrees or so, the house would stay at 20+ degrees celcius until late evenings when we went to bed. We gained roughly 7 to 10 degrees on a day with sun over not having the unit, and of course the house was drier and easier to heat due to the removed moisture. A simple differential controller and a bilge fan can make a MASSIVE difference in the right house, thats for sure!

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  4. Thanks Brendon -- nice to know there is some real potential there for a simple system to work at least in some climates and homes.

    Gary

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  5. Attics have two strikes against them.

    #1. Attics are typically poor solar collectors because they must be vented by code. That means that when they heat up, the warm air spills out by design.
    #2. New codes allow unvented attics, but also require impermeable insulation at the roof sheathing plane. That kills all solar collection potential.

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  6. Hi Kevin,
    Agree with both points, but I wonder if one could monitor attic relative humidity and find a reduced level of attic ventillation that allowed for more heat collection without moisture problems.

    For the 2nd point on new construction it seems like ther must be a good way to integrate a collection channel below the roof deck but above the impermeable insulation that should allow for fairly efficient collection -- at least as efficient as an unglazed collector can be.

    I plan to do some experimenting in my attic this fall/winter with using some of the rafter bays as collector channels. Dave in VA (who has a very nice logging system plans to do the same.

    With all that area, it just seems like there has to be a good way to make use of it.

    Gary

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