Monday, September 01, 2008

Energy in your Attic

As those who read this column regularly know, I have a bit of an obsession with attics, and with the energy available in our attics. So it will be no surprise that, while here at the farm in central Texas, I’ve been doing some investigation above the ceiling.

Previously, of course, I build a solar heat collector in the conventional way-an insulated box with clear polycarbonate sheeting on top. The collector did a credible job as a solar heat collector, generating temperatures inside the box over 80 degrees F above ambient. Though I wasn’t able to generate the desiccant or absorbent cooling I was hoping for, and didn’t get around to quantifying the amount of energy gathered, it is clear that given enough area and storage volume and no worries about aesthetics, it was practical to supply essentially all the heat required by a typical house very cost effectively with this type device. And I’m still convinced it could generate most of the cooling required as well.

But, what if you could generate these benefits in your attic? Without the clear sheeting? Without the additional box/framing? Without the aesthetic issues? Obviously, without the clear sheeting you would collect less of the energy hitting the surface and collect heat at lower temperatures, but might you be able to improve economics and aesthetics by reducing cost and making the collector invisible from outside the attic?

The first step to finding out was to take some temperature measurements. Obviously, the best attics are built with good ventilation. This preserves the integrity of the insulation and minimizes heat transfer into the house, reducing your utility bills. But, it also reduces the temperature in the attic. So, to effectively collect heat from the attic, especially in winter, you would need to insulate below the roof decking and collect heat from the space in between.

To get a feel for whether the temperature would be high enough to make collection worthwhile, I stuck a small patch of fiberglass insulation under the roof decking and inserted a thermometer in the space between. On clear days, the temperature measured was about 150-160 degrees F, or about 50-60 degrees above ambient. Even on partly to mostly cloudy days, the temperatures were about 40-50 degrees above ambient.

This means that it is quite possible to obtain all the heat needed for hot water heating during the summer. And, south of the Mason-Dixon, where high temperatures in the winter average around 60 degrees, it is possible to collect the heat needed for space heat, and much of what is needed for water heating.

Of course, possible is far different from practical. It remains to be seen how much this type collector would cost, and the amount of heat which would be generated from each square foot of collection area. The answers to these questions will have to wait, as I’m moving on to new adventures, but with collector costs a fraction of those for conventional collectors, it seems a good bet that this could be practical as well.


The Visioneer said...

Very interesting! I'm glad you're still developing these theories. The thermodynamic axioms of your plan seem well within reach...I would just love to build a full-scale prototype within a reasonable budget to see what could really be gleaned from all this free heat. Perhaps plans for such a prototype could be integrated into a certain garage apartment’s attic framing before the first nail was driven?

max said...

Hey, I'm slow to notice your comment. Too much focus on hurricane and Wall Street excitement, I guess.

I'm convinced a prototype could be built for a reasonable sum and would pay for itself. Of course, by definition, a prototype could lead to expensive lessons and learnings as well.

If you are still interested, I'd be glad to help you implement it. Just shoot me a note and let me know what your plans are.

XRing said...

I was just thinking about heating my basement up a little during the winter (in New England) using the attic heat.
My idea was to install a PV panel on the roof and place a DC fan (and screen filter) inside the attic up near the peak.

The fan would be connected to a 15 foot long duct going down to the basement.

Right now, the basement is getting a good geothermal effect from the ground, keeping it around 60 deg F. But, if I use an indoor Heat pump(for day time space heating upstairs), the basement is going to get a bit colder.

I would feed the attic air right to the heat pump unit.
With will likely be a $399 Samsung 25,000 BTU AC.. :)

max said...

XRING, I like the way you are thinking. With a little innovation I believe we can mitigate the energy crisis and improve our standard of living at the same time.

I'm not sure what you mean by an "indoor heat pump." Are you talking about putting the cold side of the heat pump in your basement and the warm side in the main part of the house? I've never heard of that, but it might work if you have enough heat exchange area in the basement walls and floor to absorb the cooling without lowering the temperature too much. Of course, by lowering the temperature of the basement you would increase the heating load somewhat, but if the area between the basement and house is well insulated, it should not be too significant.

As for keeping the basement warm by ducting heat from the attic, you would have to look at your average daily temperatures during the winter months in your areas and see if they are high enough relative to the basement to generate the heat needed to justify the expense. Keep in mind, the 50-60 degrees boost I quoted was from the space between the roof decking and insulation and was south of the Mason-Dixon. The temperature in theyour attic would depend on how well ventilated it is, but would probably give a differential of only 20-30 degrees, and only when the sun is shining.

max said...

XRING: You don't give me a lot of details on your house or location, but, just for grins, I checked the climate data for Boston to get an idea how it might look.

The annual average temperature is about 50 degrees, which means the ground temperature is probably in the 55 degree range. That would mean that your basement is being raised about 5 degrees above the ground temperature by heat leakage from the house.

Also, the average high temperatures in January and February are in the mid 30s. Based on that with a 20-30 degree temperature rise in your attic, you would be hard pressed to raise the basement temperature much by blowing directly from the attic peak.

I'd look at a traditional glazed collector, which could give you the heat you need directly and could work for both hot water and space heat. I'm not sure direct attic heat would work consistly enough to be justified for New England.

XRing said...

I'm just north of Boston. And we had some sun today but it was only 77 up in the attic. (half way between the floor and peak)..

Maybe a DIY closed ground loop buried in the back yard?? (next summer)!

Anyways, the indoor heatpump is my pipe dream this month. :)

Here's a URL at DIY.

If I could pump 25,000 BTU out of the basement slab for just 4 hours a day (and let it recover for 20 hr), I think payback would occur pretty quick.

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max said...

XRING, I looked at your string on DIY. I believe your indoor heat pump would work. You can probably expect about 1 BTU/hr/sq ft/degree diffential to be absorbed by your basement floor and walls (I'm assuming no insulation, including carpet or other flooring.).

So, if your ground temperature is 55 degrees as I previously calculated and you cooled the basement down to 45 degress, you would disperse about 10 BTU/hr/sq ft of surface area.

Assuming your basement is 40'x40' you would have about 1600 sq ft of floor area and say 1000 sq ft of wall area. So 2600 sq ft should disperse about 26000 btu/hr under these conditions.

You might have a problem with icing, as someone mentioned. Even though the basement would be 45 degrees, the evaporator would be quite a bit cooler. But if you did have a problem with icing, you could probably eliminate it by adding a fan to increase air flow and raise the temperature of the evaporator. Running it only part time, as you suggested, could help with the icing but would reduce your benefits. I think you could run it pretty much full time.

Keep in mind also that the 30 degree temperature differential between the basement and the house would lower your efficiency somewhat, as well as increase your temperature losses through the floor. Even so, I'm pretty sure it would be a cheaper heat source than resistance heat or heating oil. I'd be inclined to give it a go, since it seems you have pretty much everything you need to try it.

I will point out though, that in the long run a conventional ground loop might be a better investment. Even though the initial cost would be higher, the higher exchange rates which could be acheived with water would improve the efficiency quite a bit. And, you would reduce that heat loss through your floors to the basement.

Good luck.

XRing said...

There's a little problem. I do have some insulation. But, it's not really so effective that it worries me much.
But it will slow down the recovery time.. =less run time.

55 is close to what I got. The average mean air temp in Woburn for one year is 56.70 And I hear that's equal to the underground temp.
Another problem. Smaller house. I think it's closer to 20,000 sq ft of floor+wall surface.

I've used my dehumidifier a few times during cold weather and it never had any compressor problems etc.
But, I never had the basement down to 45 either.. While cleaning up flooding, I always run the heat zone down there.
(That basement is a HARD place to heat).
That 30 degrees is a big differential, but starting up in the morning after a night of oil heat, the heatpump would only
need to increase the upstairs 5 or 6 degrees. And then cycle off and on as needed.
Upstairs is pretty tight. Most of the flooring has good insulation.

A lower efficiency is what I expect. That's one of the reasons I want to use a 25,000 BTU pump.
If I can get around 20,000 out of it for the first few hours of the morning, I would be a happy man..
We are south facing, so there is some solar gain a few hours after sun up.

The idea of a DIY ground loop is where I started.. :) But, getting the right heat exchanger setup to work with a stock window AC does not look easy. Might even be easier just to buy a compressor and condenser unit and build my own glycol/water-to-freon heat exchanger.. Etc etc.

I'm looking to skip all that work and expense and scavenging the basement's heat to the extent it's possible.
If there is not enough warmth there to make it worthwhile, I might be able to find out, by using that small
5,250 BTU unit. Maybe I'll find that only a 12,000 BTU heat pump will work..?.

Thanks for your help.

mikehoskins said...

How about a KISS approach?

Why not simply attach a thermostatically controlled fan and damper system that joins attic air to your cold air return?

It could work like this:

1.) heat is set to "on" for house thermostat and furnace or heat pump kicks on

2.) attic air is above a set temperature, say 80-90 degrees Fahrenheit (26 to 32 Celsius) and would be controlled by a thermostat in the attic

3.) damper and fan in attic turns on only when #1 and #2 are both true, simultaneously

4.) air is forced into house cold air return from attic only when #1, #2, and #3 are true, simultaneously

Wouldn't this be simpler and only kick in when the heat is on and the attic is warmer than the house?

I don't know about attic air pollutants, such as dust, mold, building materials, outdoor air pollutants, and insulation particles. On the other hand, would there be more fresh air from outside, replacing indoor air pollutants, and would the air filter catch enough of this to not matter?

I also don't know how attic air humidity in the winter would affect house air humidity. So, a whole house humidifier with a humidistat would be in order.

mikehoskins said...

Of course, this is even simpler but only heats certain rooms:

max said...


Thanks for your comments. I do think the KISS approach would give you some heat, but I think something a bit more complex might be more cost effective, for several reasons:

1. I do worry about the pollution and humidity effects from pumping air directly from the attic into the house.
2. A well designed attic would be well ventilated and therefore would offer heat only 20-40 degrees above ambient. While that would give you some heat, the times when it would be avaible to heat the house would be pretty limited.
3.You have only the limited thermal mass of the house to store heat from these limited time periods.

For these reasons, I'd lean toward insulating under the south facing roof and circulating water through pipes between the roof and insulation to capture the heat. This would more closely approximate the 80 degrees above ambient potential represented by my measurements. Then you could store the hot water to provide heat during the evening when the sun is not shining. This also allows for use of the heat for hot water in the summer, and eliminates the pollutant and humidity issues. You might even be able to satisfy a good part of your a/c load.

Granted, all this would be a bit more costly and complicated, but I think the increased heat capture and utilization rates would more than pay for itself.


mikehoskins said...

Agreed, there are trade-offs for each design -
1.) cost
2.) labor
3.) maintenance
4.) complexity
5.) permits/legality/NIMBY's
6.) utility/effectiveness
7.) ...(a gazillion more)...

In business, people often calculate ROI and TCO.

Unfortunately, labor, complexity, and maintenance are hard to calculate.

For example, what if you spring a leak, on a water-based system or have a boil-over that ruptures a pipe? Or, what if you have an air leak in your attic and it never gets warm enough to blow into your house on an air-based system?

If I were to do any of these, I'd look strongly toward the system that follows the KISS principle (or #4, complexity) the closest before committing to something that's harder.

I'd rather have something that works OK, instead of having a new boat anchor that has to be shut down.

Having said that, I do think there is a great potential use for wasted attic heat.

This guy does it but the project scares me off, for a couple of obvious reasons (see step 4 and the idea of putting a ton of sand in the attic):