Designing for Solar Heating

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By Don Fal By Don Fallick lick

H

ow you plan for solar heatingow you plan for solar heating depends to a very great extent depends to a very great extent on whether you’re building a on whether you’re building a new structure or adding on to an new structure or adding on to an exist-ing one, and on the method of heat ing one, and on the method of heat transference. It might be said that transference. It might be said that these considerations loom so large as these considerations loom so large as to virtually determine the design of to virtually determine the design of the system.

the system.

Active systems

So-called “active” solar heating So-called “active” solar heating sys-tems produce the highest tems produce the highest tempera-tures. Their liquid heat-transfer fluids tures. Their liquid heat-transfer fluids lend themselves best to integration lend themselves best to integration with a conventional heating system. with a conventional heating system. They are relatively easy to retro-fit to They are relatively easy to retro-fit to an existing structure, are not an existing structure, are not especial-ly bulky, allow for traditional ly bulky, allow for traditional archi-tecture, and can be modified, tecture, and can be modified, enlarged, or even removed, at any enlarged, or even removed, at any time. They also require expert time. They also require expert installa-tion, and are

tion, and are inordinatelinordinately expensive toy expensive to purchase, run, and repair.

purchase, run, and repair.

“Active” disadvantages

Virtually all active systems require Virtually all active systems require electricity to run pumps and electricity to run pumps and con-trollers. Protection must be provided trollers. Protection must be provided to guard against freezing of the to guard against freezing of the circu-lating water, or to ensure that lating water, or to ensure that ponous anti-freeze solutions remain nous anti-freeze solutions remain iso-lated from the environment. Protection lated from the environment. Protection must be “fail-safe” against power must be “fail-safe” against power out-ages during severe winter weather. ages during severe winter weather. Lots of systems failed in Seattle, Lots of systems failed in Seattle, Washington, in 1991 when a coastal Washington, in 1991 when a coastal city that rarely sees any snow caught city that rarely sees any snow caught over two feet in 24 hours. Much of the over two feet in 24 hours. Much of the city was without power for several city was without power for several days during the coldest Christmas on days during the coldest Christmas on record. Provision must also be made record. Provision must also be made for emergencies caused by the failure for emergencies caused by the failure of these “fail-safe” systems.

of these “fail-safe” systems.

Active systems require direct Active systems require direct sun-light to function, so there must be light to function, so there must be some way to store heat during cloudy some way to store heat during cloudy periods as well as at night, and to periods as well as at night, and to recover the stored energy. All these recover the stored energy. All these requirements increase the cost of the requirements increase the cost of the system while decreasing its efficiency. system while decreasing its efficiency.

The trade-off between reliability, cost, The trade-off between reliability, cost, and efficiency has led many solar and efficiency has led many solar designers away from “active” systems, designers away from “active” systems, especially when designing for new especially when designing for new construction.

construction.

Passive systems

Besides allowing a wider choice of Besides allowing a wider choice of building sites and orientations, new building sites and orientations, new construction facilitates design of construction facilitates design of pas-sive solar heat systems, which use sive solar heat systems, which use sunlight to directly heat the building sunlight to directly heat the building itself, or the air within it. Passive itself, or the air within it. Passive sys-tems depend upon

tems depend upon convectionconvectionoror radi- radi-ation

ation to circulate the collected heat.to circulate the collected heat. While some “passive solar” homes do While some “passive solar” homes do use fans to assist circulation, most can use fans to assist circulation, most can function, at least minimally, without function, at least minimally, without electricity.

electricity.

Because they do not need to heat a Because they do not need to heat a liquid up to near boiling, passive liquid up to near boiling, passive sys-tems are good at extracting useful tems are good at extracting useful energy from even diffuse sunlight. If energy from even diffuse sunlight. If your area is frequently overcast in your area is frequently overcast in winter, a passive, warm-air system winter, a passive, warm-air system may be your only solar option. Many may be your only solar option. Many solar “experts” who make a living solar “experts” who make a living selling active systems refuse to selling active systems refuse to acknowledge how much energy such a acknowledge how much energy such a system can extract from a theoretically system can extract from a theoretically “impossible” situation. (See “A house “impossible” situation. (See “A house heating solar greenhouse” in

heating solar greenhouse” in BH BH M M

# 12.) # 12.)

Designing for solar heating

Passive system Passive system Act

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“Passive” disadvantages

The disadvantages of passive solar The disadvantages of passive solar stem from the same qualities as its stem from the same qualities as its advantages. Because passive systems advantages. Because passive systems operate on small temperature operate on small temperature differen-tials, they require excellent insulation tials, they require excellent insulation to retain those “extra” degrees of heat. to retain those “extra” degrees of heat. Often, such houses are earth-bermed, Often, such houses are earth-bermed, or even built underground, to take or even built underground, to take advantage of insulation which is advantage of insulation which is liter-ally “dirt cheap.” Because the house ally “dirt cheap.” Because the house or the air is heated directly, the system or the air is heated directly, the system must be built into the building, or at must be built into the building, or at least be a major

least be a major architectural addition.architectural addition. The large areas of glass needed to The large areas of glass needed to capture diffuse sunlight make it capture diffuse sunlight make it impossible to disguise the nature of a impossible to disguise the nature of a passive solar heating system. passive solar heating system. Regardless of one’s preferences in Regardless of one’s preferences in architecture, or the actual design, architecture, or the actual design, pas-sive solar homes come in only one sive solar homes come in only one basic style. Worse, those large glazed basic style. Worse, those large glazed areas can turn the building into an areas can turn the building into an inferno in the summer unless inferno in the summer unless provi-sion is made for shading and sion is made for shading and ventila-tion.

tion.

To prevent nighttime heat loss, To prevent nighttime heat loss, glazed areas must be covered with glazed areas must be covered with insulating shutters or blinds. Opening insulating shutters or blinds. Opening and closing shutters requires tedious and closing shutters requires tedious daily attention. Owners of passive daily attention. Owners of passive solar homes dare not leave them solar homes dare not leave them unat-tended more than a day or two. Many tended more than a day or two. Many ingenious systems have been invented ingenious systems have been invented to solve this problem, and some work to solve this problem, and some work fairly well, but all complicate the fairly well, but all complicate the sys-tem, add to its cost and upkeep, and tem, add to its cost and upkeep, and detract from its efficiency.

detract from its efficiency.

Glazing

Glazing is indeed the weak point in Glazing is indeed the weak point in a passive solar design. Many a passive solar design. Many compa- compa-nies have introduced so-called “solar” nies have introduced so-called “solar” plastics, designed not to cloud up, plastics, designed not to cloud up, even after decades in direct sunlight. I even after decades in direct sunlight. I know of none that actually delivers as know of none that actually delivers as promised. Those that come close have promised. Those that come close have shown tendencies to weaken and shown tendencies to weaken and weather. The only thing that really weather. The only thing that really works right is tempered glass, which works right is tempered glass, which is heavy, fragile, expensive, and hard is heavy, fragile, expensive, and hard to seal permanently.

to seal permanently. Do not useDo not useordi-

ordi-nary window glass, which becomes nary window glass, which becomes deadly when shattered. Tempered deadly when shattered. Tempered glass isn’t much stronger than window glass isn’t much stronger than window glass, but it breaks

glass, but it breaks into rounded piecesinto rounded pieces or powder, instead of deadly, pointed or powder, instead of deadly, pointed missiles.

missiles.

One of the most important One of the most important advan-tages of passive designs is that they tages of passive designs is that they ten d to u s e

ten d to u s e “lo w tech n o lo g y . ” I f “lo w tech n o lo g y . ” I f you’re capable of designing and you’re capable of designing and build-ing a “normal” structure, you can ing a “normal” structure, you can probably do just as well designing and probably do just as well designing and building your own passive solar home. building your own passive solar home.

No free lunch

Al

Al ll solar heated designs mustsolar heated designs must include provision for a backup

include provision for a backup heatingheating system, for those periods when

system, for those periods wheninsola- insola-tion

tion (incoming sun energy) just isn’t(incoming sun energy) just isn’t enough to overcome weather of enough to overcome weather of extreme severity or duration. There extreme severity or duration. There are no 100% solar heated homes. How are no 100% solar heated homes. How

close you come to 100% will depend close you come to 100% will depend on how much time, money, and on how much time, money, and exper-tise you have available to put into the tise you have available to put into the project, as well as the weather.

project, as well as the weather.

There’s no free lunch. A solar There’s no free lunch. A solar assist-ed home is one which receives less ed home is one which receives less than 50% of its heat from the sun. The than 50% of its heat from the sun. The less you depend on your solar system, less you depend on your solar system, the cheaper and easier it will be to the cheaper and easier it will be to build.

build.

An example

The house heating solar greenhouse The house heating solar greenhouse mentioned earlier provided about 75% mentioned earlier provided about 75% of our heat, at a cost of about $1000, of our heat, at a cost of about $1000, including the use of lots of recycled including the use of lots of recycled materials and

materials and nnoo hired labor.hired labor. Convective distribution of heated air Convective distribution of heated air was assisted by floor grates, costing was assisted by floor grates, costing nothing. (See “Heat your upstairs by nothing. (See “Heat your upstairs by cutting holes in the floor,”

cutting holes in the floor,” BHM BHM #14.)#14.) These additions, plus a

These additions, plus a lotlot of insula-of insula-tion, helped convert a cold, drafty, tion, helped convert a cold, drafty, century-old house into a fuel miser century-old house into a fuel miser that used only one cord of hardwood that used only one cord of hardwood to get through a bitter Wisconsin to get through a bitter Wisconsin win-ter! We could have increased our ter! We could have increased our “solar percentage,” but the cost would “solar percentage,” but the cost would have far exceeded the return in saved have far exceeded the return in saved energy!

energy!

There are many contractors who will There are many contractors who will install a prefab “solarium” for you, install a prefab “solarium” for you, usually for a stiff fee. These range usually for a stiff fee. These range from really good ones to strictly from really good ones to strictly deco-rative units that leak more heat than rative units that leak more heat than they produce. One of the best national they produce. One of the best national brands is sold by Four Seasons. They brands is sold by Four Seasons. They

Two ways to add a south-facing Two ways to add a south-facing greenhouse to a

greenhouse to a non-south-facinon-south-facingng structure

structure

Taking advantage of a low spot on the horizon Taking advantage of a low spot on the horizon

by orienting away from true by orienting away from true southsouth

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work, they don’t leak water

work, they don’t leak water oror heat,heat, and they’re guaranteed. They’re not and they’re guaranteed. They’re not cheap, though.

cheap, though.

Doing it yourself

The weak point of virtually all The weak point of virtually all do-it-yourself greenhouses is sealing the yourself greenhouses is sealing the roof glazing panels. “Sunlite” solar roof glazing panels. “Sunlite” solar plastic by Kalwall is still the best plastic by Kalwall is still the best solu-tion, in my book. It will eventually tion, in my book. It will eventually degrade and need replacement—in 10 degrade and need replacement—in 10 to 15 years. It costs nearly as much as to 15 years. It costs nearly as much as glass. But you can get it in rolls, and glass. But you can get it in rolls, and stretch a single sheet over a 2 x 2 stretch a single sheet over a 2 x 2 sup-port grid, sealing it down with silicone port grid, sealing it down with silicone seal and gasketed roofing nails. It’s seal and gasketed roofing nails. It’s picky work getting it on right, but not picky work getting it on right, but not difficult, and it leaves a glazed roof difficult, and it leaves a glazed roof that can’t leak, except at the edges. that can’t leak, except at the edges. These can be sealed just like any These can be sealed just like any glaz-ing. The plastic is nearly 1/8" thick, ing. The plastic is nearly 1/8" thick, lasts much longer than the corrugated lasts much longer than the corrugated stuff most of us are familiar with, and stuff most of us are familiar with, and looks lots better.

looks lots better.

Existing structures

Solar additions to existing structures Solar additions to existing structures can be practical, even if there is no can be practical, even if there is no south-facing wall to attach them to. south-facing wall to attach them to. The greenhouse need not be built The greenhouse need not be built par-allel to the building wall. A triangular allel to the building wall. A triangular or irregular plan can work just as well or irregular plan can work just as well as a rectangular one, as long as the as a rectangular one, as long as the ratio of the greenhouse volume to ratio of the greenhouse volume to glazed area is kept around 9:1, plus or glazed area is kept around 9:1, plus or

minus a bit. In my experience, ratios minus a bit. In my experience, ratios higher than 10:1 won’t provide higher than 10:1 won’t provide enough heat to justify the expense and enough heat to justify the expense and trouble of construction, while ratios trouble of construction, while ratios lower than 8:1 will be difficult to lower than 8:1 will be difficult to insu-late at night.

late at night.

Other orientation

It may not be necessary to orient the It may not be necessary to orient the glass to face due south anyway. glass to face due south anyway. Generally, solar heating systems work Generally, solar heating systems work best with a maximum southern best with a maximum southern expo-sure to the sun, but there are sure to the sun, but there are excep-tions.

tions.

When the sun is low in the sky, the When the sun is low in the sky, the earth’s atmosphere filters out much of earth’s atmosphere filters out much of its heat energy. That’s one reason why its heat energy. That’s one reason why it’s hotter at noon than in the morning it’s hotter at noon than in the morning or the evening. Near the US/Canadian or the evening. Near the US/Canadian border, the winter sun is close to the border, the winter sun is close to the horizon

horizon allall the time, so there is lessthe time, so there is less reason to prefer noonday sunlight. It reason to prefer noonday sunlight. It may be possible to increase the total may be possible to increase the total duration of direct “insolation” by duration of direct “insolation” by ori-enting the building slightly away from enting the building slightly away from due south, taking advantage of low due south, taking advantage of low spots in the horizon.

spots in the horizon.

My house in Washington State did My house in Washington State did just

just this. this. (See (See BHM, BHM, The The Best Best of of thethe First Two Years: “Semi-underground, First Two Years: “Semi-underground, solar house has some ideas worth solar house has some ideas worth crowing about.”) High canyon walls crowing about.”) High canyon walls would have blocked the low winter would have blocked the low winter sun, but the house was oriented to take sun, but the house was oriented to take advantage of a rift in the canyon rim, advantage of a rift in the canyon rim,

giving a boost to the total insolation, giving a boost to the total insolation, just wh

just when it en it was needewas needed most.d most.

The only way to tell if such The only way to tell if such “mis-orientation” will help is to compare orientation” will help is to compare suntrack charts for all sites under suntrack charts for all sites under con-sideration.

sideration.

Sky heat

Depending on your intended use, Depending on your intended use, you may wish to orient a passive solar you may wish to orient a passive solar collector to take advantage of either collector to take advantage of either morning or afternoon sun. “Sky heat,” morning or afternoon sun. “Sky heat,” which is the primary source of the which is the primary source of the energy collected by warm air systems, energy collected by warm air systems, is greatest in the afternoon, when the is greatest in the afternoon, when the earth is re-radiating the solar energy it earth is re-radiating the solar energy it h as s o ak ed u p in th e d ay tim e. h as s o ak ed u p in th e d ay tim e. Greenhouses intended primarily for Greenhouses intended primarily for house heating should be oriented to house heating should be oriented to take advantage of sky heat, as well as take advantage of sky heat, as well as the more direct sunlight around noon. the more direct sunlight around noon. Sky heat may not be necessary for Sky heat may not be necessary for greenhouses intended primarily for greenhouses intended primarily for raising plants. Seedlings need long raising plants. Seedlings need long hours of light. Heating them is hours of light. Heating them is sec-ondary. In fact, a greenhouse ondary. In fact, a greenhouse opti-mized for heat may get too hot for mized for heat may get too hot for raising plants, and require venting to raising plants, and require venting to keep from killing them!

keep from killing them!

Vertical glazed walls

It’s easiest to build vents into It’s easiest to build vents into verti-cal walls. Everybody “knows” that cal walls. Everybody “knows” that solar glazing is supposed to be angled solar glazing is supposed to be angled to catch the sun as directly as

to catch the sun as directly as possible.possible. The angle usually recommended is The angle usually recommended is equal to 90° minus the latitude of the equal to 90° minus the latitude of the site. For the continental US, this site. For the continental US, this results in a glazed wall between 65° results in a glazed wall between 65° and 45°. Such walls are difficult to and 45°. Such walls are difficult to build, glaze, seal, insulate, and build, glaze, seal, insulate, and venti-late.

late.

Fortunately, studies have shown that Fortunately, studies have shown that an identically glazed vertical wall an identically glazed vertical wall loses only about 5% to 10% of its loses only about 5% to 10% of its heat-gathering ability. Improved heat-gathering ability. Improved abili-ty to shutter or otherwise insulate a ty to shutter or otherwise insulate a vertical wall against nighttime heat vertical wall against nighttime heat loss may more than make up for the loss may more than make up for the slight loss in heating efficiency.

slight loss in heating efficiency.

But the real plus for vertical walls is But the real plus for vertical walls is their greater ease of ventilation. their greater ease of ventilation.

Designing f

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Screens, window cranks, and other Screens, window cranks, and other technology are readily available and technology are readily available and cheap. If you make most of your main cheap. If you make most of your main glazing openable, you may not need glazing openable, you may not need any other vents. If you can’t do this, any other vents. If you can’t do this, as a rough rule of thumb, you’ll need a as a rough rule of thumb, you’ll need a ratio of about one square foot of vent ratio of about one square foot of vent to nine or ten square feet

to nine or ten square feet of glazing.of glazing. Vents should be placed so as to Vents should be placed so as to cre-ate a cross draft, with low vents facing ate a cross draft, with low vents facing the prevailing wind, and high vents on the prevailing wind, and high vents on the downwind side. Greenhouse the downwind side. Greenhouse sup-pliers sell heat-operated, non-electric pliers sell heat-operated, non-electric vent operators. I’ve seen them in use, vent operators. I’ve seen them in use, and they work well and last forever. and they work well and last forever. Check with your supplier before Check with your supplier before designing your vents, as they have designing your vents, as they have weight limits.

weight limits.

Convection

Hot air rises. This is the basic Hot air rises. This is the basic princi-ple behind convective heat ple behind convective heat distribu-tion. If you can make the solarium the tion. If you can make the solarium the lowest part of the structure, with lowest part of the structure, with grad-ually rising living areas, natural ually rising living areas, natural con-vection will tend to distribute warm vection will tend to distribute warm air from your passive collector air from your passive collector with-out any other assistance.

out any other assistance.

However, you must also design in a However, you must also design in a way for the cooler air to return to the way for the cooler air to return to the collector for reheating. In our house in collector for reheating. In our house in Wisconsin, warm air from the Wisconsin, warm air from the

collec-tor flowed through the kitchen/dining tor flowed through the kitchen/dining room, up a step to the living room, room, up a step to the living room, then up the stairs to the bedrooms. then up the stairs to the bedrooms. The final bedroom had a floor register The final bedroom had a floor register which allowed the relatively cool air which allowed the relatively cool air to descend to the living room floor, to descend to the living room floor, where it was sucked across the kitchen where it was sucked across the kitchen floor and back into the collector. It did floor and back into the collector. It did create a relatively cool draft across the create a relatively cool draft across the floor, but the temperature differential floor, but the temperature differential was only about 10° F, so it wasn’t was only about 10° F, so it wasn’t unpleasant.

unpleasant.

Convective solar additions Convective solar additions frequent-ly have problems like this, but ly have problems like this, but design-ers of new structures can plan for ers of new structures can plan for con-vection to eliminate winter floor vection to eliminate winter floor drafts.

drafts.

Thermal mass

Solar heating systems are cyclic. Solar heating systems are cyclic. They gather heat in the daytime, but They gather heat in the daytime, but use it primarily at night. To bridge the use it primarily at night. To bridge the gap, there must be some way to store gap, there must be some way to store excess heat and distribute it later. excess heat and distribute it later. Many methods have been tried, but Many methods have been tried, but allall work on the same principle: moving a work on the same principle: moving a fluid such as air or water past a fluid such as air or water past a previ-ously heated thermal mass, or ously heated thermal mass, or heat heat sink

sink . The mass adds inertia to the sys-. The mass adds inertia to the sys-tem, soaking up energy during the day tem, soaking up energy during the day and radiating it slowly at night.

and radiating it slowly at night.

Thermal mass can take many forms, Thermal mass can take many forms, but there are only two ways to get the but there are only two ways to get the heat energy into it. Active systems heat energy into it. Active systems cir-culate high temperature liquids culate high temperature liquids through or around the storage through or around the storage medi-um, heating it up. Because the storage um, heating it up. Because the storage mass is relatively small, it must be mass is relatively small, it must be heated quite hot to retain enough heated quite hot to retain enough ener-gy to last the night. Heat transfers gy to last the night. Heat transfers only from a relatively hotter medium only from a relatively hotter medium to a relatively cooler one, so

to a relatively cooler one, so the liquidthe liquid must be very hot indeed for the must be very hot indeed for the process to work. This is the source of process to work. This is the source of all the problems with active systems. all the problems with active systems.

Passive systems use a slightly Passive systems use a slightly differ-ent system. Instead of heating up a ent system. Instead of heating up a fluid and using it to transfer heat to fluid and using it to transfer heat to the storage mass, they use sunlight to the storage mass, they use sunlight to heat up the mass directly. If the mass heat up the mass directly. If the mass is just large enough, it will hold just is just large enough, it will hold just enough energy to radiate all night enough energy to radiate all night long, returning to its

long, returning to its original tempera-original tempera-ture by morning. If the mass is too ture by morning. If the mass is too small, it won’t hold enough energy to small, it won’t hold enough energy to last all night. If it’s too big, the mass last all night. If it’s too big, the mass will require more hours than there are will require more hours than there are in the day to reach the proper in the day to reach the proper tempera-ture. In other words, it will never get ture. In other words, it will never get warm enough to function properly. warm enough to function properly. Fortunately, a lot of mass is required Fortunately, a lot of mass is required to heat a whole house by radiation, so to heat a whole house by radiation, so it’s hard to build in too

it’s hard to build in too much.much.

The ideal place to put thermal mass The ideal place to put thermal mass is just inside the glazed area. Thick, is just inside the glazed area. Thick, concrete slab floors make excellent concrete slab floors make excellent thermal mass, but must be well thermal mass, but must be well insu-lated from the cold earth beneath. lated from the cold earth beneath. They should be painted dark colors They should be painted dark colors forfor heat absorption, or decorated with heat absorption, or decorated with dark rock slabs, set well into the dark rock slabs, set well into the con-crete for good conductance of heat. crete for good conductance of heat.

Trombe walls

One way to add

One way to add lotslotsof thermal massof thermal mass (without the disadvantages of concrete (without the disadvantages of concrete floors—see below) is to build in a floors—see below) is to build in a short,

short, veryvery thick partial wall betweenthick partial wall between the solarium and the rest of the house. the solarium and the rest of the house. This is called a

This is called a TrombeTrombe (rhymes with(rhymes with “bomb”)

“bomb”)wall.wall.It soaks up heat directlyIt soaks up heat directly from the sun shining on it during the from the sun shining on it during the day and re-radiates it into the house at day and re-radiates it into the house at night.

night.

The author's solar-heated house in Wisconsin The author's solar-heated house in Wisconsin

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Trombe walls are usually painted Trombe walls are usually painted dark on the collector side, but may be dark on the collector side, but may be any color desired on the radiation side. any color desired on the radiation side. They are frequently covered with They are frequently covered with brick, to add yet more thermal mass, brick, to add yet more thermal mass, beauty, and additional radiation beauty, and additional radiation sur-face. Often there is a brick-enclosed face. Often there is a brick-enclosed planter on the wide top of the trombe planter on the wide top of the trombe wall. The earth provides yet more wall. The earth provides yet more thermal mass, and the houseplants thermal mass, and the houseplants grown there disguise the solid nature grown there disguise the solid nature of the Trombe.

of the Trombe.

The water Trombe

Concrete has two great Concrete has two great disadvan-tages as thermal mass. It’s very tages as thermal mass. It’s very expen-sive in the quantities needed, and it’s sive in the quantities needed, and it’s virtually impossible to add or subtract virtually impossible to add or subtract mass after it sets. Ken Kern, author of mass after it sets. Ken Kern, author of The Owner Built Home, invented the The Owner Built Home, invented the “water Trombe” to solve these “water Trombe” to solve these prob-lems.

lems.

A water Trombe is a wall consisting A water Trombe is a wall consisting of stacked steel water barrels. Since of stacked steel water barrels. Since water is only half as dense as water is only half as dense as con-crete, a water Trombe needs to be crete, a water Trombe needs to be about twice as thick as a concrete about twice as thick as a concrete Trombe wall. This can be easily Trombe wall. This can be easily accomplished by stacking the barrels accomplished by stacking the barrels horizontally, on their sides, with one horizontally, on their sides, with one end facing the collector and the other end facing the collector and the other facing into the house. Only the facing into the house. Only the collec-tor ends need be painted black.

tor ends need be painted black. Caution!

Caution! A d r u m f u ll o f w aterA d r u m f u ll o f w ater weighs about 460 pounds. If you build weighs about 460 pounds. If you build a water trombe,

a water trombe, be surebe sure you build ayou build a rack that is strong enough to hold the rack that is strong enough to hold the weight. A 25-foot Trombe wall with weight. A 25-foot Trombe wall with barrels stacked in two layers weighs barrels stacked in two layers weighs close to

close to five tonsfive tons. Be sure your foun-. Be sure your foun-d atio n an foun-d f o o tin g s ar e u p to it. d atio n an d f o o tin g s ar e u p to it. Because of these weight Because of these weight considera-tions, a Trombe wall of any kind is not tions, a Trombe wall of any kind is not something you can easily retrofit after something you can easily retrofit after the building is built. It is possible in the building is built. It is possible in some cases, but I wouldn’t some cases, but I wouldn’t remend it without the services of a mend it without the services of a com-petent architect.