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Thread: Passive Solar Heaters: boxes, walls, attics

  1. #1

    Default Passive Solar Heaters: boxes, walls, attics

    I just posted this exact posting on another site’s forum (to save time). Thought people here might have something to say about it too.

    I’m particularly curious about thoughts on my idea of sealing an attic space and using that to capture solar heat to reduce furnace demand. One thing, people without experience with metal roofing may not realize is that they shed the snow load, so that means that the metal could frequently receive direct sunlight throughout the winter. (And yes, I understand he conventional and traditional thinking and learned experience regarding attic design and ventilation, moisture, mould, dry rot, etc in attics.)


    My other posting - verbatim:

    I’ve long thought about building a stand-alone solar furnace / box to heat air and then blow it into our cabin. It would only likely take some of the chill off and provide a higher start temperature when I go out to the cabin but anything would help.

    Plus I have a fair bit of space to work with so I can think big (beyond get little 3x5 DIY examples that are common.) I could easily fit in something more like 12’wx8’h or larger, maybe even something looking more like a fence (say 30’w x6’ h) but I’d guess that the costs would quickly become excessive considering the infrequent usage of the cabin.

    So has anyone played around with these?


    Here’s a good article (below) and YouTube has lots of DIY heaters that all look pretty creative and all seem primed for some simple further improvements.


    Build a Simple Solar Air Heater - Renewable Energy - MOTHER EARTH NEWS


    I’m a retired aircraft engineer, but you don’t need a similar background to tackle this project. In fact, a solar air heater built into new construction or added to an existing building can be an easy and inexpensive heating solution. Following the simple principles and plan outlined here, you can heat your workshop, barn or even your home with free heat from the sun. If it works here in Bozeman, Mont., it’s bound to work wherever you are.


    https://www.motherearthnews.com/rene...r-zmaz06djzraw

    The best thing about wall mounted heaters is one, it could be designed to also increase or decrease wall temp and therefore alter both winter and summer temperatures in the building.

    Also:

    MY ATTIC RE-THINK - SOLAR ATTIC HEATER:

    For another thread, or maybe this one, would be a discussion of my thought to turning a conventional ventilated attic into a sealed attic cavity as in a typical gable ended cabin to passively heat a cabin, garage, etc. Add lots of insulation and run piping in it like one goes with fluid heated flooring and circulate it down into the living space. . Adding metal roofing to a our cabin and garage got me to rethinking that attic, but unfortunately I added whirley birds to our cabin when the metal went on.
    Last edited by KC; 14-10-2017 at 07:56 AM.

  2. #2
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    End of the day, it's probably cheaper and easier to just do PV panels on the roof instead of this concept. Or just build the house/cabin with better insulation so it's nearly net-zero to begin with.

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    I think great insulation, great windows works in the winter and keeps the heat out in the summer too. Having the house sealed and a heat exchanger to the outside helps too. If you have a very well insulated house you don't need much heat to begin with.

  4. #4

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    Quote Originally Posted by Marcel Petrin View Post
    End of the day, it's probably cheaper and easier to just do PV panels on the roof instead of this concept. Or just build the house/cabin with better insulation so it's nearly net-zero to begin with.
    That's what I did and I have the added benefit of water collection from the roof. So that its a solar room, a potential gardening area, and 3 seasons environment. Without any heating I can sit in there in comfort around +250 days of the year. Any sunshine at all warms it up, even on chilly days. I specifically built a slant polycarbonate roof directly facing the sun and thus maximizing incident sun exposure. Used a 2.5foot run over length of the 12ft panels consistent with recommendation. Works great for rain and snow. Snow tends to slide down any day with any sunshine. I occasionally remove snow manually.
    Last edited by Replacement; 26-10-2017 at 09:21 AM.
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  5. #5

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    Quote Originally Posted by sundance View Post
    I think great insulation, great windows works in the winter and keeps the heat out in the summer too. Having the house sealed and a heat exchanger to the outside helps too. If you have a very well insulated house you don't need much heat to begin with.
    Yeah I’d love to build new and superinsulate etc. The old 40watt bulb heating the house year round... and build on an acreage where there aren’t the urban constraints of lot size orientation etc.

    Unfortunately in terms of heating I’m in the position of having to work with what I have - in this case two old cabins where even some simple modern renovations are somewhat undesirable. Eg there’s no way I’d replace any of the original fir windows with vinyl framed windows.

    Building a sunroom across the south face might be one option to capture heat.
    Last edited by KC; 26-10-2017 at 10:08 AM.

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    Yah to maintain the look means trade offs. So perhaps either PV cells or thermal solar heating are options. If you just want heat you might look at evacuated tubes...
    http://www.apricus.com/html/evacuated_tubes.htm

  7. #7

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    Could a furnace cold air intake be embedded in attic insulation at some appropriate depth to pre-heat intake air using lost heat and without causing some dewpoint issues?

    Would a secondary advantage be that the air collected via attic level intake(s) would somehow cleaner than the current ground level intakes (maybe less dust and other pollutants. Radon?)

    Alternatively, on a south facing cold air furnace intake; could useful preheating be achieved by adding a metre or two of pipe, painting it black and using it to extend the ground level intake horizontally alongside the house foundation to capture the heat of the sun?
    Last edited by KC; 11-03-2019 at 10:38 AM.

  8. #8

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    The net gain would not be worth it. In fact, the losses at night as the duct work loses heat and becomes a cold sink.

    A heat recovery ventilator system (HRV) or an energy recovery ventilator system (ERV) is a better way to go.

    https://www.greenbuildingadvisor.com/article/hrv-or-erv
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  9. #9

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    In the attic it would have a flap at the opening and the duct could be plastic or otherwise somewhat non-heat conductive. However I guess it would have to head down into the basement. Still, would this be any different than the chimney (when the furnace isn’t running)?

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    Again, the answer to your concern is an ERV/HRV.

  11. #11

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    Maybe hook a duct from the top of your chimney to capture the waste heat and connect the other end to your furnace intake. A closed cycle is very efficient and keeps the carbon monoxide and carbon dioxide from getting out.


    no, don't do it.
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    If you want to capture passive solar radiation you're far better off to design a house that takes advantage of thermal gains through the proper placement of windows and heat absorbing surfaces. Doing something hokey like trying to heat your make-up air by running it through your attic insulation will neither work nor is it likely allowable by code.

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    There's no way your attic, at any depth is going to be warmer than the current cold air location. Not to mention it's going to be drawing cold air from the attic space to keep the furnace supplied. Drawing your cold air intake from outside, even if painted black is also a lose lose proposition. On a sunny day (when you would benefit), the furnace isn't on that much. The cold dark, windy nights is when it's working full time and you'd lose so much.

    HRV is the answer. There are hundreds of people far smarter than us who have done all the testing in controlled experiments.

  14. #14

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    If your attic is warmer, that is a bigger problem. You need more insulation R50-R60
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  15. #15

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    What you could use if you have a large south facing wall without unsighly windows is a solar wall. Never done one for a house but. ...

    avoid the attic and use a HRV as other have suggested.

    https://www.solarwall.com/technology...-single-stage/
    Last edited by BalancedOP; 12-03-2019 at 03:08 PM.

  16. #16

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    Quote Originally Posted by Edmonton PRT View Post
    Maybe hook a duct from the top of your chimney to capture the waste heat and connect the other end to your furnace intake. A closed cycle is very efficient and keeps the carbon monoxide and carbon dioxide from getting out.


    no, don't do it.
    Problem solved: Just wear face masks with tubes to bring in breathable air from outside.

  17. #17

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    Quote Originally Posted by nobleea View Post
    There's no way your attic, at any depth is going to be warmer than the current cold air location. Not to mention it's going to be drawing cold air from the attic space to keep the furnace supplied. Drawing your cold air intake from outside, even if painted black is also a lose lose proposition. On a sunny day (when you would benefit), the furnace isn't on that much. The cold dark, windy nights is when it's working full time and you'd lose so much.

    HRV is the answer. There are hundreds of people far smarter than us who have done all the testing in controlled experiments.
    I’m aftaid you are right. And that’s why we are still building “the same old, same old”. I’d guess that experiments have probably just proved that decent windows and insulation with the old mid efficiency furnaces make the most financial sense.
    (Payback on high efficiency furnace, chimney mods. etc are likely far, far out - another government mandated expense.)

  18. #18

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    Quote Originally Posted by Edmonton PRT View Post
    If your attic is warmer, that is a bigger problem. You need more insulation R50-R60

    My attic is stuffed with insulation. Nonetheless the heat rises through the insulation and then, if it’s still above the ambient temperature, into the attic airspace. Therefore I was wondering if some of that heat loss could be cycled back into the house rather than escaping into the attic.
    Last edited by KC; 12-03-2019 at 04:21 PM.

  19. #19

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    Are HRVs Cost-Effective? - GreenBuildingAdvisor
    “Compared to a simple exhaust fan, a heat-recovery ventilator saves energy — but it probably won’t save enough to justify the high cost of the equipment”
    https://www.greenbuildingadvisor.com...cost-effective


    Broken Ventilation Equipment Goes Unnoticed for Years - GreenBuildingAdvisor

    https://www.greenbuildingadvisor.com...iced-for-years

  20. #20
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    ^^If your attic temperature is above the indoor temperature of your home, your attic is either insufficiently ventilated, the attic insulation is not as good as you think it is, or both.

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    Quote Originally Posted by KC View Post
    Are HRVs Cost-Effective? - GreenBuildingAdvisor
    “Compared to a simple exhaust fan, a heat-recovery ventilator saves energy — but it probably won’t save enough to justify the high cost of the equipment”
    https://www.greenbuildingadvisor.com...cost-effective


    Broken Ventilation Equipment Goes Unnoticed for Years - GreenBuildingAdvisor

    https://www.greenbuildingadvisor.com...iced-for-years
    This is probably true. For the fairly insignificant amount of outdoor air drawn into a new high efficiency furnace an HRV or air to air heat exchanger is probably not going to save all that much in energy costs.

    Forced air heating is by design a relatively inefficient way to heat a house. Hydronic heating is more efficient and allows for much more design flexibility. When we build our new house in BC I'll be designing a hydronic heating system with an outdoor air make-up to go into it.

  22. #22

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    Quote Originally Posted by East McCauley View Post
    ^^If your attic temperature is above the indoor temperature of your home, your attic is either insufficiently ventilated, the attic insulation is not as good as you think it is, or both.
    No, in the winter the house is warmer than the attic airspace. (I would guess though that the temperature on the top surface of the insulation in the attic is above that of the general attic airspace but there’s probably a ton of factors at work there too.)

    At the time additional insulation was blown in, the depth of the old and new was about 18” (maybe more - I can’t recall as it’s been two decades). Soffits are perforated, small gable end vents and two Whirlybirds per level.
    Last edited by KC; 12-03-2019 at 04:47 PM.

  23. #23
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    Okay. Wasn't sure what you meant by ambient temperature.

    We added extra ventilation to our attic when we had our shingles replaced last summer. It reduced the ice damming problem we were having before at the bottom of the ridge line of the roof facing south and west.

    I also had a contractor come out last fall to look at adding additional insulation to the attic floor on top of about a six inch layer of loose cellulose insulation that is over 30 years old. He recommended against putting new insulation on top saying loose cellulose that old has packed down and lost much of its insulating value. He was also concerned about the additional weight on our ceilings of not removing the old insulation first. But of course removing the old attic insulation before installing new increased the cost considerably so I delayed making a decision until this year.

  24. #24
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    Quote Originally Posted by KC View Post
    I’m aftaid you are right. And that’s why we are still building “the same old, same old”. I’d guess that experiments have probably just proved that decent windows and insulation with the old mid efficiency furnaces make the most financial sense.
    (Payback on high efficiency furnace, chimney mods. etc are likely far, far out - another government mandated expense.)
    Uh, we are not building "the same old, same old" in terms of new product. HRV/ERV's are pretty much a requirement in all new homes, along with all high efficiency gas fired appliances, higher insulation requirements, better windows etc. Every few years the government has been turning the dial up on that kind of thing in terms of better/tighter codes. As far as existing homes go, yes, it's often the case that upping furnace efficiency by only a few percentage points isn't going to yield an appreciable payback and you're better off reducing heat loss instead.

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