Passive solar backup heat?

smile3January 20, 2006

I'm in the process of designing a passive solar house, what kind of heat would you use for supplemental heating? If I can get to 100% passive solar will I still need supplemental heating to meet codes? I am looking at possibly putting in a masonary heater but will building codes allow me to use heating that isn't automatic (such as a furnace or geothermal)?

The passive solar design I am looking at will have thermal mass and a large amount of south facing glazing that I will have computer controlled exterior rolling insulated shutters. I am looking for input on this idea, any cons to this I might not have thought of?

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I think it is very doubtful that you can achieve 100% passive solar anywhere in Canada. Given that the northern latitude gives us less sun in the winter and many parts of the country(mine especially) spent a lot of time under cloud cover due to storms.

You would have to check out your local codes about heating appliances. Would you really want to have a house that doesn't have a heat source that will run when you are not home? I wouldn't want to be tied to my house for the whole winter, or have to drain my water lines every time I go away for a few days.

    Bookmark   January 20, 2006 at 7:43PM
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I don't think I would be tied to my house at all in the winter. I may not be able to reach 100% passive solar, but I think I should be able to stay well above freezing.

I think exterior insulated shutters run by a computer system should prevent alot of unwanted heating/cooling that is caused by the large amount of glazing required for passive solar.

What kind of heating system does a person put in with passive solar though? It seems like a waste to put in ducting or underfloor heating for a few days a year that I can light a fire to make the house more comfortable.

    Bookmark   January 20, 2006 at 10:35PM
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Use solar heat collected to run radiant in-floor heat . . along with other solar passive techniques . . and put a boiler / whatever in the system as your "back-up" for poor solar times and to satisfy having "automatic" heat. Radiant in-floor works well with large thermal masses; which ties in well with passive solar. Get the best of both. Set the boiler / whatever to run off a t'stat that doesn't kick in 'til the temps get pretty low. If you truly get as much solar heat as you think; you will likely rarely need the boiler . . but it will be there if / when you do. Not only allows for the ability to leave the place for a few days with absolute peace of mind, but also allows for the possibility of failure in some part of your passive system(s) without losing all of your heat . . .


    Bookmark   January 21, 2006 at 11:06AM
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Passive solar heating does not work well in northern latitudes. I live in Michigan, lat 44, and have been toying with it for years. There just is not enough sunlight in the winter to power passive solar heating.

If you stuck some pipe in the ground 3' under, you could circulate 60F degree air out of it year round. But that's if you like 60F degree air. I prefer about 73F.

    Bookmark   January 23, 2006 at 11:22AM
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A 100% passive solar home is a very complicated matter. I believe it can be done, but not easily, and perhaps not affordably either. The building would need to be very efficient at capturing solar energy, but it would also have to be very good at storing this energy. Instantly we have a design conflict, all those places that allow heat in during the day will start to leak heat backwards when the sun is gone and the outside air temperature falls. Things can be done to correct this, but the complexity starts to increase dramatically. Also, to store heat you need mass. Concrete or brick are popular, but they're bulky and expensive... Well, expensive is relative, I'll conceed it may just be me who is looking at cubic foot of concrete and wishing it were cheaper! Such a house which collects lots of solar energy, stores it well and has enough thermal mass to remain warm overnight can be designed and built at some expense, but that's not the full selection of problems facing the 100% passive solar house, it also has to perform well throughout the whole season. The traits that make it work so well in winter may cause it to work too well in summer and overheat terribly.

Personally, I have discarded the idea of the 100% passive solar house due to it's complexities, expense and design limitations. But I have not given up on the idea of the 100% solar heated house.

Passive solar is still a very adaptable technology and can provide a substantial amount of free heating, but it has limitations and should only be used up to the point where it is doing what it should - providing free heat for a sensible cost. Going extreme and trying to squeeze that last 20% or 30% of your heating needs from it by adding vast quantities of expensive glazing, bulky thermal mass and inconveniant manual systems such as heat retaining curtains and solar shades that must be ajusted frequently is irrational. Active solar technology, such as solar air heating pannels can provide the remainder for a much lower cost and avoid most all the design issues. Active solar also has the advantage of being able to turn it off when not needed, a conveniance of fossil fuels that has allowed a lot of badly designed buildings to remain comfortable all year round. It could certainly help compensate for any small design flaws in a passive solar home. I suspect the only way to make something perfect is to design around any faults it has.

    Bookmark   January 23, 2006 at 5:48PM
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Hi Solargary, I have some questions for you.
"The link in the post above to the Saunders Cliff house is 100% solar heated in a northen, 6000 deg day climate."

- What is a 6000 degree day?

- How do you solar heat a house on a cloudy day? Or foggy day? Or at night?

- How do you retain heat if your south walls are mostly glass? Windows let a lot of heat out, even when they are closed and are double-paned.

I would like to know a little more. Thanks.

    Bookmark   January 26, 2006 at 11:47AM
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I posted a link below explaining degree days.

A good passive solar design has a way to store the heat you gain on sunny days. If you read about the Saunders house you will find that they have choosen to use thermal mass to store heat (in this case water and stones). You can also use solar heat collectors and store the heat in a boiler type system.

To reduce heat loss through windows a number of different techniques can be used. Some of the techniques that are used include: insulated curtains, removable insulation, low e windows, or a sunspace that while connected to the house is closed off during cold times.

That being said you can use passive solar to different degrees depending on your needs. The most common is moving windows to the south side of the house with some sort of shade for summer months. This takes very little planning and just helps heat the house on sunny winter days, much as a fireplace or a woodstove only heats your house when they are burning. From this basic passive solar you can than add extra south facing glazing, along with thermal mass, in systems from the simple to the complicted depending on your wants. For more information on any of this Gary's site is very informative and is well worth your while to visit.

Here is a link that might be useful: Degree Days

    Bookmark   January 26, 2006 at 1:01PM
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I guess I wont know until I build how close to complete passive solar I can achive so I am planning on an supplementry heat source. I like the idea of in floor hydronic heat, but it seems like it will be really expensive to put in compared to other systems. What do you think of electric radiant? I realize that it costs more to run than hydronic, but I think that will be offset by the lower initial cost. I also cant see it costing too much as it will hopefully only be used a few times during the coldest/cloudiest times of the year. I also know I will need somekind of HRV/ERV, do they need to be ducted to every room? If so would it just be cheaper to install forced air heating? Any other thoughts you have for my house would be appreciated.

I am trying to keep my building costs down so any ideas to help me do this would be greatly apperciated.

Some of the ideas I have for my house so far:
1) Hot water recovery
2) Making my own concrete floor tiles
3) Making my own concrete counters (with hydronic heating/cooling for baking purposes)
4) Masonary heater with built in pizza/bread oven (I'm not sure how feasable this is yet. I'm looking at possibly building my own or putting it in at a later date)
5) South window wall and clearstory with computerized exterior rolling shutters and a sunspace in one corner (sunspace may be added at a later date) I know this will be one of the most expensive things in my house but I think the benifits will outway the cost.
6) Some sort of large thermal mass (not sure exactly what I will use yet, I want it sylish or hidden with someway for heat transfer)
7) The house will be aprox 1200sf 1 1/5 story with an unfinished basement (well for now anyhow)

    Bookmark   January 30, 2006 at 6:48PM
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Radiant heat costing too much . . . you can spend as much as you want, someone will be willing to take it from you. In-floor radiant heat in a slab will cost you basically the tubing . . a pump . . some plumbing . . which can be all DIY. Heat the water with electric, propane, whatever . . and it can easily be supplemented via solar collection. Visit Radiantec web site for some good typical examples of how it's done . . . they now have a "sister" site more tailored towards solar collecting. Lots of good info there.

ERV / HRV: Can easily be done without central ducting; if you think it out first. I have radiant heat and thus, no ducting. I chose a unit which hangs in the attic, centrally located. The fresh air coming it gets blown into the great room at one end of the house where there is cathedral ceiling. The outgoing air is sucked from a back bedroom at the other end of the house . . nice flushing action. Depends a lot upon your floor plan etc. But, if you build tight as you wish to do, you WILL need one. I run mine on a timer, as running constantly would be more air exchange than is needed. Made a HUGE difference in this place, and doesn't cost much to run. I'm on a grid-tied PV system, so I watch ALL my usage carefully . . .

Hot water recovery . . I've heard bits and pieces about it, and one thing I have heard is clogging of the heat exchanger by way of all the stuff that goes down the drain . . not sure how real that is or if there are ways around it or not. The basic idea is sound, though I don't know how financially justifiable it is.

Good luck . .


    Bookmark   January 31, 2006 at 6:16AM
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I've looked into the drain water heat recovery a little and the more complicated systems seem like they would be expensive and problematic. Attatched is a link for a simple system that preheats your water only while you have hot water flowing out of the system.

Here is a link that might be useful: Drain Water Heat Recovery

    Bookmark   January 31, 2006 at 10:19AM
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I agree with Bob, hydronic heat in a slab is fairly cheep and easy to install. Hydronic heat also gives you more possibilities with what you use as a heat source(solar, wood, gas, oil, electric) if you use electric resistance you are stuck with your electric company. One of the many reasons we decided to build off grid is the amount of power outages per year. If you go with a hydronic system you will be able to add a generator if needed in times of power outages and still have heat.

Do you have reliable electricity in the winter? I am wondering about your electric shutters. Can they be run manually?

    Bookmark   February 1, 2006 at 9:48AM
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Our power is very rarly interupted. Yes all the shutters will have a manual override and I am hoping to put in a small pv system to run the necessities during outages.

    Bookmark   February 1, 2006 at 10:06AM
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I don't know a tonne on the subject but I was surfing my old magazines and found an old harrowsmith that mentioned the heat retentive properties of ZEOLITE. Apparenly used in many industry applications. Zeolite is supposed to have a heat retaining ability 400 some odd times better than water or stone. Worth investigation maybe. What about a heatpump system? Parents have one at their cottage home over 16 years no trouble and effective heat and cooling, particularily when your away.

    Bookmark   February 20, 2006 at 8:58PM
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