I couldn't find any discussions of solar chimneys so let me ask:
Has anyone here designed or built one?

I'm considering adding a combination solar chimney/trombe wall to the south side of my two-story house, with an outside vent at the top plus two inside vents at top and bottom.  By varying which vents are open I could control whether I'm heating or ventilating.  During winter it would pull cool air from the downstairs, heat it, then push it back into the house/upstairs (ala a trombe wall).  During summer it would pull air from inside the house (ideally from yet another vent upstairs which is ducted to the bottom of the chimney) and vent it outside -- I would allow fresh air to be drawn in from the shaded north-side of the house.

Any suggestions or feedback are appreciated!

What's your zip code? In many places a solar chimney and/or earth-tube approaches to sensible cooling only adds to the latent load (humidity) to the house in summer, which isn't exactly the right thing to do.

Using the high mass of a Trombe wall in summer to moderate the interior temps can work well if you shade the passive-solar glazing on the exterior and used night time ventilation strategy to passively cool the mass. I had some friends in Maine who did pretty well without AC using that approach in a passive solar home with a large Trombe wall. Don't have any hard data on it though. In winter they used a small woodstove in the sun room with the flue running inside the mass wall to supplement the solar gain.

I'm in San Diego so there's rarely any humidity to worry about. Mostly I'm just looking for 'free' (sans electricity) ventilation.

re trombe walls, I'm not thinking of using the thermal-mass aspect (to provide delayed heat transfer at night). Instead, I just want the daytime 'sun room' effect to cycle air through the 'warmer'. Then I'll rely on my insulation at night. For instance, during the winter, if I can heat the house to 75 F during the day it shouldn't cool down below 65 F even on the coldest nights, e.g. 32 F. That's the idea, at least. I realize my primary investment (in my forthcoming remodel) has to be upgrading the insulation.

Superinsulating to PassiveHouse levels is pretty cheap in San Diego. Using a sun-room to drive ventilation will add to the solar gain, and more appropriate to much colder climates where that gain can be put to good use. (Your Trombe-wall reference threw my assumptions off on your location & climate.)

Not that you'd be able to get there cheaply as a retrofit, but the "whole wall" R values (thermal bridging of framing etc included) that are usually cost effective in San Diego can be found on the "zone 3" row of table 0.2, p10 in this document:

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

Note, an R20 wall isn't a 2x6 wall with R20 batts. With thermal bridging factored in it takes about 1-1.5" of exterior rigid foam (depending on the type of foam) to get there. To get to PassiveHouse levels you may need to bump those numbers a bit, but most important would be adjusting the window area size & type appropriately.

If you're going to spend ten grand on the sun room to drive ventilation you might as well put a kilwatt of PV on the roof and run a heat-recovery ventilation system or exhaust venting. But But air-sealing, insulation, even window upgrades ( and maybe CA Title-24 cool-roof shingles ) might be a better investment still.

In summer the hours that you'd get the best cooling effect from ventilation are well after sunset, when the solar resource isn't available. In the winter you can probably get a big boost out of a thermal-air panel mounted on the south side of the building (either a commercial version such as Cansolair or SolarSheat or a DIY, maybe even integrated into the building.)

@Dana1, excellent points, all!

I think you're right about the relative value of this solar chimney idea :-( My problem is that my house is so poorly insulated now that even in the late afternoon it gets warmer inside than out, so ventilation seems like solution. Ideally I could upgrade my insulation and THEN decide what further improvements to make, but I really want to do everything all at the same time. (I have a wife and kids who will not suffer any more construction than absolutely necessary.)

Also, that Building Science doc is really amazing! It's the most up-to-date reference I've seen. Also, more useful than the latest California Title 24 regs which seem more focused on appliance's electrical efficiency (less so on construction techniques).

re San Diego weather and the need for heating vs cooling, I think there are more net-heating days than cooling but they're not very heat-intensive days! For probably 5 months of the year the day-time highs are in the 60s. In my current poorly-insulated house those constitute 'heating' days...

A simple description of a solar chimney is that of a vertical shaft utilizing solar energy to enhance the natural stack ventilation through a building.

San Diego is definitely heating-dominated, (about 1250 HDD to 1000 CDD base 65F) but neither heating or cooling intensive.

What are you currently heating/cooling with? In a decently air-tight and even moderately insulated house in that climate a small continuously-variable speed ductless mini-split or multi-split air-source heat pumps can handle both the heating & cooling loads at high efficiency, with a comparable or lower operating cost than natural gas in heating mode (depends on the local utility rates for either.) The 97.5th percentile outdoor design heating temp for San Diego is +44F, a temp at which the better mini-splits are achieving coefficients of performance (COPs) of about 4. (Sure it gets colder that that but rarely long enough or cold enough to matter.) At a COP of ~3 a heat pump is more source-fuel efficient than condensing gas burners, and at 4 it's blowing them away. With R20+ walls and reasonable glazing fractions you'd typically only need one interior unit per story, (maybe a multi-split, or 2 mini-splits) since the room-to-room balance issues are much lower than in air-leaky old-school less insulated R10-ish timber-frames. With tight R10 there may be some balance issues, but they won't be terrible. A smaller mini-split runs ~$4K+ to install but the comfort level is much higher than with bang/bang on/off furnaces (but not as high as with radiant floors or low-temp panel radiators.) They're also pretty quiet in cooling mode compared to window shakers (and 2x the efficiency.)

Getting the smallest one that meets the load will provide both highest comfort & efficiency, so before upgrading the mechanicals it's best to do at least the first serious round of envelope upgrades, and use the existing system's energy use to measure the actual heating load. This more accurate than typical heat-loss calculation software, since those are based on many factors that can easily be mis-measured and mis-entered, whereas energy use & efficiency of the existing system plotted against heating degree day weather data reflects the actual performance of the building, independent of where the heat losses are. Heat loss calcs done perfectly will overshoot by 15-25%, but it doesn't take too many errors on assumptions or measurements to end up 2x oversized. A tight 2400' 2 story R10 building with an R25+ attic and not excessive glazing fractions will have a heat load of less than 15000KBTU/hr @ 44F, but they don't even MAKE forced air furnaces that small, so most are many times oversized for tight houses in your neighborhood. But 10-15KBTU mini-splits abound, with min-BTU ratings in the 3-4KBTU/hr range. At R20 wall/R50 attic your 44F heat load may be under 10K in a tight, not overglazed house.

First things first- get a blower door & infra red imaging test and fix all of the leakage you can, and figure out all of the thin spots & gaps in the wall insulation using infra-red imaging. The testing tends to run ~$500 in my neighborhood, but it's subsidized by utilities in some places (maybe in San Diego). The advantage is that you can then fix the most eggregious parts first, in a cost-effective manner. If you can get the air leakage down to 2 air exchanges per hour @ the calibrated standard 50 pascals pressure you're golden, but even 3 is pretty good as a retrofit. Leaky older houses are often well over 10ACH/50, and most can be brought under 6 ACH/50 doing just the dead-obvious. Start with big holes first (leaky or absent flue dampers, & backdraft preventers on dryers or bath & ktchen fans, box over and seal any recessed lighting cans penetrating the attic, foam seal all plumbing & electrical penetrations in exterior walls and attics, etc.), which can easily add up to 5x all of the leaky windows in the house (but fix those too, :-) ) When you're not sure what's left, that's the time to schedule the test, and let the pros have at it. Some insulation companies will specialize in air-sealing as a service, which is usually combined with before & after blower door tests. By the time you've spent even $1500 in air-sealing you may have cut your heat loss/gain by 1/4 or more.

Then insulate: Stud cavity insulation is usually upgradeable with blown-fiber drilling from the exterior, with or without existing batts in place. Higher density goods like cellulose is a better air-retardent than low density blown fiberglass, independent of rated R value. Some super-fine new-school fiberglass such as Optima or Spider do very well when "dense packed" to 1.8lbs/foot or more, but it's at a price premium. Air-sealing the attic and getting as much high-density fiber up there that fits(without overloading the truss chord or joist load ratings) is also pretty cheap & effective. If you can't get at least a foot (R40) of insulation all the way over the outer edge of the studwall it might be necessary to insulate at the roof deck and go with an unvented attic to get to high-R, which is much more expensive, but there are different ways of going about it, with different price-points.

The comfort difference after air-sealing and upgrading the cavity & attic insulation is usually something you'd feel right away.

If you're re-siding, that's the time to add exterior rigid foam and upgrade windows at the same time. If you won't be re-siding for a decade or more, and you have a bunch of single-pane windows that don't leak huge air, retrofitting tight, high-quality exterior storm windows is much cheaper than swapping them out for even the cheapest double-panes, and cuts the heat-loss from windows by a bit more than half. If you're sure you'll never be residing and adding insulation, buying decent grade (U0.34 max) replacments would be in order. Note: Casements & awning windows leak less an seal tighter than double-hungs or sliders, and give more egress & ventilation area per square foot of glazing to boot. Push-out versions are usually cheaper and tighter than crank-outs. Fixed windows (where appropriate) are tighter & cheaper still.

@Dana1, wow, that's above-and-beyond responding to my original query re solar chimneys!

My house was built in the mid '60s and has no wall insulation, only about R10 in the attic, and several bath/kitchen fans without dampers. I would like to upgrade the electrical and add network cabling, but I don't know yet if that means I'll be tearing open the walls to such a degree that I might as well re-do them -- in which case I could do the wall insulation properly.

I also have a typical gas furnace, a relatively new one that the previous owner installed in the past decade. I would like to replace it with a heat pump, so your suggestions on that are appreciated. In my dreams I would install a ground-source model but the installation costs are probably prohibitive and not justified.

Speaking of costs, yes, the local utility offers a small rebate for energy testing. But you only qualify if you spend over a certain amount. Still, every little bit helps :-)

Posted By bsmith1051 on 10 Jun 2011 01:17 PM
@Dana1, wow, that's above-and-beyond responding to my original query re solar chimneys!

My house was built in the mid '60s and has no wall insulation, only about R10 in the attic, and several bath/kitchen fans without dampers. I would like to upgrade the electrical and add network cabling, but I don't know yet if that means I'll be tearing open the walls to such a degree that I might as well re-do them -- in which case I could do the wall insulation properly.

I also have a typical gas furnace, a relatively new one that the previous owner installed in the past decade. I would like to replace it with a heat pump, so your suggestions on that are appreciated. In my dreams I would install a ground-source model but the installation costs are probably prohibitive and not justified.

Speaking of costs, yes, the local utility offers a small rebate for energy testing. But you only qualify if you spend over a certain amount. Still, every little bit helps :-)

Installing retrofit cellulose into wall cavites does not require ripping open the walls- either the exterior or interior wall is drilled to accomodate a hose or nozzle, and it's blown in with hoses under pressure with air. Since the air exiting the stud-bays follows the paths of least resistance, all of those air leaks get clogged with cellulose, making it far tighter than you can get with batt-installations.

If there is literally NO insulation in the walls,  find a cellulose installer and get it quoted as both standard density (2-hole method) or dense-pack (1-hole, where they insert a hose in the wall and really pack it in there to 3lbs/cubic foot or more).  A standard density blow is good, but a dense-pack will reduce air-infiltration in the walls by something like 99%.

Have them do an R40-R50 open blow in the attic while they're there (but only if you've alreadygiven it your best shot on air-sealing the attic floor/top-floor ceilng first.)

Only THEN bring in somebody for a blower-door & IR test.  Otherwise it'll be wasted, since those are all measures that you would be taking anyway, and it should reduce the air infiltration pretty dramatically.  That way the testing will find the parts that got missed, rather than the dead-obvious, making it more valuable.

If nobody is doing cellulose, see about using Certainteed Optima or JM Spider new-school fiberglass, and have them dense-pack it to 1.8lb density to minimize infiltration.  It'll give about R1 better whole-wall R than dense-packed cellulose, but it's usually more expensive.

drilling holes in the walls would still involve refinishing afterwards. I just need to decide if I would rather refinish the inside or the outside. Any suggestions about rewiring, e.g. CAT6 networking? Can that be done without opening the walls? If so, maybe I'll go for the outside-drilling to fill the walls. And then get a quote for wrapping the house in styrofoam+stucco -- as shown in that Building Science doc!

Good electricians can fish CAT6 through lots of stuff, but it's much easier in empty wall cavites & partition walls than in dense-packed cellulose. Dense-packing after it's wired isn't much of a problem though.

Drilling & filling from the exterior is the most common method of retrofitting. Where cosemetic appearance is an issue selectively removing small sections of siding/clapboard/shingles whatever makes the patch up look better, but if you're going to strip the siding and rigid foam over it or you don't care if the plugs are visible, installers will drill trough the siding an pound in a wooden plug that sticks out an inch or so, to be trimmed flush & painted over later. A cellulose fill can happen pretty fast, and isn't much of a hindrance/mess in an occupied house when done from the exterior.

You might scan for other ideas & approaches here:

http://thousandhomechallenge.com/case-studies

It's often good to stage some of it if you're going to be living in there or working strictly out of pocket. Air sealing and retrofitting cellulose would be phase-I, and would be very cost effective relative to the foam-overcoat.

If the ducts of the existing heating system run in the attic, (or even if they don't, and are accessible), sealing every seam & joint with duct mastic, and taping all the seams & doors on the air-handler with FSK tape (2" aluminum duct tape) will make the thing quieter and more efficient. In a survey of CA homes by PG & E the average duct leakage was something like 25% of the air volume, and 40% or more wasn't uncommon in indivitual homes. Those sorts of leaks can drive air infiltration rates significantly by depressurizing some rooms, pressurizing others. A couple of weekend days and $50 in tape and mastic would pay off in under one heating season in most cases (and more likely in in yours than in tighter houses.)

Geothermal is very expensive to install, and in your climate would not be more efficient than an R410A refrigerant heat pump with continuously variable motor drives. A ductless mini/multi split and 2 indoor units will run about $6-7K to install. In a high-R version of your house you may be able to use a simpler mini-split with only a single indoor unit. In your climate the you would get an average coefficient of performance of better than 3.5 out of them, maybe 4.0. (Most will hit 4 or better at 45F outdoor temps.) With geo once the pumping power is added in the true COPs of geo might make it there too, but it's less guaranteed, and usually a lot more money up front, with a lot riding on the skills of the designer.

The passive ventilation scheme used for night ventilation cooling on phase -II of this project using a wind powered turbine ventilator might work in your instance. Flip through the whole thing, but the vent stuff starts on p.30.  Key to making that sort of system work is the tightness of the ducting & operable vents, or it'll depressurize the house and drive infiltration 24/365. 

Note that they added a thermal-air heating on their roof that pre-heats ventialtion air when space heating is needed.

Since your furnace is relatively recent and low-cost to operate, it's worth doing much of the rest first, which will lower the heating load, making the size & cost of the replacement system smaller.  The more building envelope upgrades that get done  before the heating system swap, the less likely that you'll end up with something 3x oversized and running at lower efficiency.  But if you're switching from a gas-burner to a heat pump in that phase, be sure to decommission and seal the flue, since that would otherwise be a passive infiltration-driver.

"A standard density blow is good, but a dense-pack will reduce air-infiltration in the walls by something like 99%."

Best case scenario. A blower test is a must for insulation you cannot see.

@Dana1, that Beeler house case-study was very interesting. Their use of Solar Wall units sounds like my idea for the solar chimney, actually.

My conclusion from all this is that -- after insulation! -- my proposed 'solar chimney' will be overkill during daylight hours, and I already knew it would be unused at night. "Ideally" I would fix the air infiltration plus insulation, then monitor energy-use for a year before doing the next stage. Not sure if I really want to wait, though.

San Diego, the easiest climate in the world to build a zero energy house. Right deep earth temperature, no extreme weather, mean temperature in coldest month 58 degrees.

Control air seal everything, shade or have radiant roof, 2 inches of polyeurothane in roof cap, insulate attic floor with cellulose, radiant shield or shade south and west wall, thermal mass would help, earth tubes all the time no solar in windows during summer. I think that would do it.

Note with California solar rebates, and federal tax credits Solar has a short payback with a net meter and they can shade the southern roof top.

Brian

@zehboss, thanks for the encouragement :-)

I understand what you said about the shade and insulation. But earth tubes? Don't those suffer from mold? I thought they'd been largely discarded.

I was hoping to take advantage of the cool earth temp by burying a concrete or brick wall on the north side, and with the top several feet exposed as a sort of 'radiator'. Then position an inlet window directly in front of it so that cool(er) air is drawn into the house by natural convection (and maybe a wind-powered ventilator or solar chimney!). Everything shaded, too, of course.

The 'edge case' for my summer cooling is the twice or thrice yearly Santa Ana winds -- 90-100F weather that blows into the city from the desert and can last for a week or two. Hard to stay cool for more than a couple of days in that environment! Overnight 'lows' are still in the upper 70s and low-to-mid 80s.

People who say they are problematic simply do not understand how to design, construct and use them.

A 40 foot cargo container mounted on end provides a great solar chimney. If the goal is all natural convection you want 40 to 50 feet of rise from the lowest input to the highest exhaust area. Otherwise you do not get enough natural stratification to drive the air.

One foot thick earthen walls would give you enough mass to keep the house cool for those 2 week stents. But you are not building new.

Having a large berried tank and a radiant floor installed could keep the house cool. The only issue is designing the control system to keep the floor above the dew point.

A live roof and or live wall strategy works as well.

Your current problem is you have a poorly designed home in an ideal climate. Note that 95% of homes built in Southern California are poorly designed homes. They got away with it because of the climate. Building a self-heated and cooled home in such a climate is a no brainer. Unfortunately fruits and nuts abound.

Many simple solutions to your issues exist, you have to look at what is the most cost effective and acceptable for you and your neighborhood.

Brian

Posted By bsmith1051 on 21 Jun 2011 06:09 PM
@zehboss, thanks for the encouragement :-)

I understand what you said about the shade and insulation. But earth tubes? Don't those suffer from mold? I thought they'd been largely discarded.

I was hoping to take advantage of the cool earth temp by burying a concrete or brick wall on the north side, and with the top several feet exposed as a sort of 'radiator'. Then position an inlet window directly in front of it so that cool(er) air is drawn into the house by natural convection (and maybe a wind-powered ventilator or solar chimney!). Everything shaded, too, of course.

The 'edge case' for my summer cooling is the twice or thrice yearly Santa Ana winds -- 90-100F weather that blows into the city from the desert and can last for a week or two. Hard to stay cool for more than a couple of days in that environment! Overnight 'lows' are still in the upper 70s and low-to-mid 80s.

The PassiveHouse folks are big on earth-tempering the ventilation air, and that may be a reasonable approach in a San Diego climate.  It doesn't have much going for it in term of bang/buck- you can't "air condition" with it without a very large underground array- you need both the thermal mass of the soil and the surface area for heat exchange.  Earth tubes will often be ADDING heat to the incoming air stream at night, when you'd be able to get more sensible cooling out of-un-tempered outdoor air.  For the dollar invested you'll still get more comfort & energy savings out of tightening & insulating the house.

The propensity for mold growth in earth tubes and how much effort in design & implementation  it takes to manage the problem it is dependent upon outdoor air dew points relative to the subsoil temps.  In parts of the southeastern US with mean summertime dew points in the high 60s ore even low 70s with deep subsoil temps in the low-mid 60s there is quite a bit of condensation and very high RH in the tubes to deal with.  In San Diego  the mean summertime dew points are in the low-60s, and deep subsoil temps are in the mid-high 60s, so condensing hours are much fewer, and the mean RH in the tubes is much lower.  Filtering the incoming air to reduce the amount of organic matter in the tubes is necessary for any earth-tube system.

Ventilation needs to occur in any house. Free dehumidification of the intake air is a big advantage. Systems need to be designed for the specific climate and building it is to be used for. It is not simple, you need experience and a real understanding of humidity, air flow, temperature differentials, thermal conductivity, dynamic modeling of the system. It needs to be an holistic approach to the whole system so that the end product works. Tube length, sizes, number, convective drive, counterflow allignment, drainage, heating and cooling load, recharge capability, mass heat flow all have to be understood and implimented to design earth tubes that work. Designed and installed properly they work well with substantial gains in efficientcy of the total system. There are very few individuals who have a clue about sizing, laying out and installing earth tubes.

Brian