A standalone dual-pass solar air heater using downspouts

[precaud]

karl, the output was insulated last week, the input yesterday.

I don't share your enthusiasm for the soda can collectors. Cheaper, yes. But a pain to work with. And higher flow resistance.

 

[Todd]

Very cool, thanks for sharing. I've thought of doing something similar, but I don't think we have enough sun to make it worth it.

 

[precaud]

Todd, NREL keeps pretty good records of available solar energy in many places across the country:
(broken link removed)
For example, a south-facing wall in Eau Claire gets 1,000 btus/ft^2/day in January. While that's less than the 1600 that we get here, you just have to scale your collector up in size to make it work. Two of the biggest solar devotees I know of live in places you wouldn't think solar was viable - Maryland and Pennsylvania.

 

[karl]

karl, the output was insulated last week, the input yesterday.

I don't share your enthusiasm for the soda can collectors. Cheaper, yes. But a pain to work with. And higher flow resistance.

Point taken, but $70.00 cheaper makes a huge difference, especially when you're wanting to make 5 or 6 of them.

 

[btuser]

btuser: That’s about $.40 worth of heat/day.

Measured in what? And what's your point?

I'm just running numbers through my head. Believe me, I'm jealous when I see stuff like this (you've already given me ideas for a greenhouse booster next Spring) but I've got a big giant pig of a house that is hungry hungry hungry. I need serious btus.

17.5k btu per day would mean about $.40/day for me. I heat with oil. 139k btu/17.5k btu is about .125 gallons of oil (I'm counting my oil system at 100% efficiency) which at todays market price of $3.5/gallon is 43.75 cents. If I count my wood stove its about 15 cents/day if I have to buy wood, natural gas about 20 cents, electric about 50 cents. A pair of 100 watt lightbulbs would put off the same amount of heat at a constant 24hr rate rain/shine night/day for a cost of $.86/day with an electric rate of $.18 per Kwh.
A heating space would have to have a heat load of 750 btu/hr, so your basement at 200sqft would need a heat load requirement of about 3-5 btu/hr per square foot. That's not enough for even 1 air change per hour.

The main problem I see is its only going to heat during the day, but if its heat gain it counts. What happens when its 20 degrees outside?

 

[precaud]

Sounds to me like you're not a candidate for this.

I need serious btus.

Sounds like you need serious insulation.

...so your basement at 200sqft...

Who said I had a 200 sq ft basement?

As to the rest of it, why must everything be measured in $$ ? On average over the season, I expect this little heater to replace one load of wood in the stove per day. That's a lot of wood I don't have to gather and process. Very worthwhile to me.

The main problem I see is its only going to heat during the day, but if its heat gain it counts. What happens when its 20 degrees outside?

You have a way of casting everything in a negative light. And what's the point of your 20 degrees question? Several things can happen. If it's sunny, I have enough passive solar to heat the house to 75F even on sub-zero days. If it's not sunny, I have woodstoves.

 

[karl]

As to the rest of it, why must everything be measured in $$ ? On average over the season, I expect this little heater to replace one load of wood in the stove per day. That's a lot of wood I don't have to gather and process. Very worthwhile to me.

The main problem I see is its only going to heat during the day, but if its heat gain it counts. What happens when its 20 degrees outside?

Ok. I'm definitely not a naysayer as my previous posts show. I'm truly interested and would like to build a larger system for my house. I hope you keep posting results. There are a bunch of us who would love to know so we would have the extra push to go build one ourselves.

Anyway, I got to ask. What sort of Barbie doll playhouse of stove do you have if it one load of wood is only 17,000 btus or so?

 

[precaud]

Anyway, I got to ask. What sort of Barbie doll playhouse of stove do you have if it one load of wood is only 17,000 btus or so?

You're making erroneous assumptions. It's not all about quantity, but location and timing too.

Location: This particular heater is vented directly into my lab in the basement where I work. All of its heat output has to pass by me in order to get to another room down there. My barbie-doll X33 is located more centrally in the 1250 sq ft basement so a good chunk of its output doesn't make it into the lab.

Timing: When I'm not down there working, I don't make fires. But the solar heater will produce whether I'm there or not, raising the baseline temperature (except in periods of prolonged cloudiness, which aren't often), making the space easier to heat to comfortable temps. I can see this already. Last year I started making fires down there by the second week of October and this year I haven't had to yet. Shorter wood heating season = less wood used.

 

[maple1]

IMO, these downspout heaters are only a good choice for heaters with horizontal form factors. It's better if you can go vertical, take advantage of natural thermosiphoning, and use black screen for the collector/heat exchanger. Cheaper and easier to build, too.

I'm curious about the bolded - would you or someone have a link to more info about that? Does that mean replacing downspouts or pop cans with screening?

 

[precaud]

Many ways to do a screen collector - here's a simple one:
http://www.builditsolar.com/Experimental/AirColTesting/ScreenCollector/Building.htm

and test results:
http://www.builditsolar.com/Experim...llector/110111TestScreenCol/ScreenColTest.htm

 

[btuser]

I've got a pretty tight house with r50 in the attic and r19 in the walls with modern double glazed windows and pretty good Southern exposure. My negative outlook comes from a cold hard winter.

I"m in NH, which is a lot different than New Mexico. My degreed days are 7000 and my design temp is -10. 17,000 btus is a little more than 3 pounds of wood in an average stove, which is like a whole piece. How many cord do you burn?

Yeah, its about money.

 

[precaud]

Perhaps you're thinking of southern New Mexico. I am at 7,080 feet, degree days are 6487, of which heating are 6092. Forecast low tonight is 29. We had about 4 hours of sun today so I won't need to light a fire.

Each year for the last three, I have been steadily lowering my wood consumption by adding solar air heaters. Last year I cut upstairs wood use by 60%, documented in this thread:
https://www.hearth.com/talk/threads/57663/

I still don't understand why you're on this thread. You're clearly not interested in the subject matter and you seem intent on minimizing it.

This is the only forum I participate in regularly where people who aren't interested in a subject feel the need to weigh in on it anyway. I don't get it.

 

[madrone]

But even in its present form, it is giving me 17,200 btus per day for the cost of running a 20 watt fan for 7 hours. A pretty good deal.

How big of a PV would it take to run 20 watts for 7 hours?

 

[karl]

In your other link. You mentioned using window screen as a collector. Why did you switch to gutter material on this one?

 

[precaud]

In your other link. You mentioned using window screen as a collector. Why did you switch to gutter material on this one?

Different form factor. I think I explained this in the first post; this one had to be horizontally-displaced (low and long) to fit the space, with input and output on the same end. The other two were vertical, built into the house structure, and could take advantage of natural convection. This one is small; less than 1/4 the size of the others.

Madrone, I've never worked with PV panels so I don't know. Worth looking into some day, though.

 

[btuser]

I still don't understand why you're on this thread. You're clearly not interested in the subject matter and you seem intent on minimizing it.

This is the only forum I participate in regularly where people who aren't interested in a subject feel the need to weigh in on it anyway. I don't get it.

I'm very interested in the subject, and if you fancy yourself a scientist you shouldn't have a problem defending your project. I was impressed with the results of your other project so of course I'd be interested in this. But I'm not a cheerleader, I'm looking for BTUs. 60k/day for one of your other collectors is what you claim which seems to be a pretty good return. 17k on a good day is about the same as a window with an insulated shade. My original invasion of this thread was simply putting the thermal gain in perspective. That's a lot of work for 40 cents on a good day, but you heat with gas so you're solar air heater is more valuable to me than it would be to you!

Sounds like its cold where you live. What's your heat load when its -20? Mine is about 100k/hr (typical construction for our area). I heat with oil/wood. In the last 4 years I've reduce my usage from 1600 gallons to 823 last year. You say you've dropped a substantial amount of wood use since the solar air projects. Do you have hard numbers because that pile of wood I estimate between 1-2 cord. I don't know many people in a heating enviroment that can heat their house on only a cord of wood/year.

 

[precaud]

I'm very interested in the subject, and if you fancy yourself a scientist you shouldn't have a problem defending your project.

Thanks, but I have better things to do with my time. This project is what it is. If you're not interested, move along, please.

My original invasion of this thread was simply putting the thermal gain in perspective. That's a lot of work for 40 cents on a good day, but you heat with gas so you're solar air heater is more valuable to me than it would be to you!

I don't heat with gas. Another bad assumption on your part. And we clearly measure the value of things differently. Hence my disinterest in engaging with you.

 

[begreen]

As to the rest of it, why must everything be measured in $$ ? On average over the season, I expect this little heater to replace one load of wood in the stove per day. That's a lot of wood I don't have to gather and process. Very worthwhile to me.

The main problem I see is its only going to heat during the day, but if its heat gain it counts. What happens when its 20 degrees outside?

Ok. I'm definitely not a naysayer as my previous posts show. I'm truly interested and would like to build a larger system for my house. I hope you keep posting results. There are a bunch of us who would love to know so we would have the extra push to go build one ourselves.

Anyway, I got to ask. What sort of Barbie doll playhouse of stove do you have if it one load of wood is only 17,000 btus or so?

Please take a few minutes to read a few of precaud's past postings. If you did you would realize how incredibly off the mark and insensitive this question is. We are lucky to have people like precaud posting. He takes combustion, burning and efficiency very seriously and has contributed a lot here with previous posts on stove efficiency and improving performance. I've lost track of how many stoves he has owned.

 

[begreen]

I still don't understand why you're on this thread. You're clearly not interested in the subject matter and you seem intent on minimizing it.

This is the only forum I participate in regularly where people who aren't interested in a subject feel the need to weigh in on it anyway. I don't get it.

I'm very interested in the subject, and if you fancy yourself a scientist you shouldn't have a problem defending your project. I was impressed with the results of your other project so of course I'd be interested in this. But I'm not a cheerleader, I'm looking for BTUs. 60k/day for one of your other collectors is what you claim which seems to be a pretty good return. 17k on a good day is about the same as a window with an insulated shade. My original invasion of this thread was simply putting the thermal gain in perspective. That's a lot of work for 40 cents on a good day, but you heat with gas so you're solar air heater is more valuable to me than it would be to you!

Sounds like its cold where you live. What's your heat load when its -20? Mine is about 100k/hr (typical construction for our area). I heat with oil/wood. In the last 4 years I've reduce my usage from 1600 gallons to 823 last year. You say you've dropped a substantial amount of wood use since the solar air projects. Do you have hard numbers because that pile of wood I estimate between 1-2 cord. I don't know many people in a heating enviroment that can heat their house on only a cord of wood/year.

Perhaps you should try setting up your own solar heater project? Armchair critiques aren't too helpful. Not everyone lives in your situation. This thread is exceptionally helpful to many that live in milder climates for example. At least precaud it actually trying out systems. Maybe they won't all succeed or have the best results. But I see a steady approach that improves on designs with each successive season. That is in the spirit of this forum. His sharing info with others is very helpful.

FWIW, in your region, my brother-in-law built his own, 2300 sq ft house in 1980. It used about 2 cords of wood/yr to heat. And this was so for 3 decades. With the addition of an attached solar greenhouse a few years ago, they are now down to about 1.5 cords/yr. The house was designed for this eventual addition of the greenhouse and the plan has worked out beautifully.

 

[btuser]

Point taken. Sorry I came off like a grouch, I was only trying to quantifiy it for myself and was thinking out loud. My own solar project involves cutting and burning the trees shading my house. I'll post pics when I'm done.

No doubt I would not have built my home as it stands. Unfortunately even with the harsh Winters here we don't build em right, or even like they used to with a center hearth.

 

[begreen]

It's hard to plan for solar when you are in a valley of shade. But there are lots of folks with good southern exposure that have the potential for a lot of free warmth. Wish our house wasn't on the north slope of the hill, but we do our best.

 

[precaud]

So true. I have had to do some serious limbing and felling to a row of large, leggy Siberian Elms on the south side of my property to clear the way for unobstructed solar exposure. I had zero remorse doing it - I hate the species. They have taken over the entire perimeter of my property, and every year I spend a day or two clearing them back. But they are winning the war, no question.

PS - we had very little sun today, so I'm going to have to make a take-the-chill-off fire upstairs. First of the season.

 

[begreen]

Ha, I have an English elm directly to the south of the house that has equal disdain from me. They spread like wildfire underground.

 

[billjustbill]

Precaud,

Thank you for taking the time to post your construction pictures. Seeing it go together is great.

Years back, for $10 each, I picked up four of those vertical, chrome framed, double pane freezer doors you see in your local grocery store's frozen food isle and thought they would be great for such a project like yours.

Since then, I've read much about air heaters because I live in Texas, and my home's roof has a great Southern exposure. I went through the articles and the commerical versions of the soda can heaters and do like the thin shell of the cans allowing heat transfer so well. This type of thin material used in the thin wall tubing allows the makers of A/C units to raise their effiency values, too

Because of all the seams to be seal with all those cans, it appears that the next best thing would be using your working concept with three changes I'd like your comments on: 1. use a duct booster fan (found at Home Depot & on line) farther up into the intake to help with intake noise and some models can be used with a speed control that addresses the noise and sunlight conditions that might benefit a slower air speed. 2. use metal downspout material to improve heat transfer. 3. use a spiral piece of metal down the center of each downspout to swirl more air against the hot downspout surface area.

All the gas water heaters I've seen have some type of swirl or zig-zag baffle in the tube that allows the combustion fumes to go from the bottom burner to the double wall vent pipe. The rising heat slows and transfers the heat of the combustion fumes to heat more of the water before it goes out the roof vent. Maybe if you could insert some type of baffle through the use of a hole saw to cut a hole in line with each downspout, and pluged with expandable foam and fully sealed with an over lapping board, it would raise your efficency rate even higher.

Having built a cyclone dust collection system for my workshop, using 4", 6", and 8" ducting, the learning curve about air movement taught me that using standard 90* elbows or T's lessen effeciency. Using wye's or long sweep elbows allow air to move much better.

Just some ideas for your thoughts. Keep up your good work,
Bill

 

[precaud]

Hi Bill, thanks for your comments and questions, replies inline.

Since then, I've read much about air heaters because I live in Texas, and my home's roof has a great Southern exposure. I went through the articles and the commerical versions of the soda can heaters and do like the thin shell of the cans allowing heat transfer so well. This type of thin material used in the thin wall tubing allows the makers of A/C units to raise their effiency values, too.

Yes, I generally like the approach of using low-mass heat exchangers.

...three changes I'd like your comments on: 1. use a duct booster fan (found at Home Depot & on line) farther up into the intake to help with intake noise and some models can be used with a speed control that addresses the noise and sunlight conditions that might benefit a slower air speed.

I like the booster fan concept, but reports from users show they don't perform as described. Their cfm rating is under low-load conditions, which of course isn't how they will be used. And I like the idea of varying the cfm to light conditions. There are many ways to go about that. Using a PV panel for at least one of the fans in the system is one way I've thought about it.

2. use metal downspout material to improve heat transfer.

That's why I used aluminum downspouts and not the steel; I wouldn't recommend the vinyl ones for this.

3. use a spiral piece of metal down the center of each downspout to swirl more air against the hot downspout surface area.

Introducing turbulence (resistance) in the air path is great, in theory, but must be weighed against the effect it has on the system as a whole, especially the static pressure, which will require more power from your fan system to overcome. Other variables are in play, but that is basically what it comes down to with these systems. All of the resistances in the system are cumulative, and as you noted, every element in the system (diameter, length, elbows, other transitions) contribute to it. So you can add as much resistance as you want, as long as you're willing to deal with the pressure consequences.

Actual cfm needed to pull through these heaters is higher than one might think going in. When I reviewed horizontal collectors others have built and viewed measurements of their performance, the cfm requirements were always higher than they expected, and the fan performance under load was always worse. One guy built a 30ft-long downspout collector and even with 190+ cfm of throughput, it is in the low 40's for efficiency. So a major priority with my unit was to minimize system pressure while increasing air-to-exchanger contact area, allowing lower system air velocity. Instead of increasing resistance, I increased the amount of time the air spends going through the exchanger.