Anyone ever try harvesting the heat in a south facing attic for space heating?

The Steam Whisperer · May 27

I've been spending a lot of time in our now partial attic/ partially finished second floor space. We have a large south facing slope and its gets quite warm ( mid 80F's) in that attic when it is still quite cool outside. I thought of just running a bare round duct across the space with a fan, but the heat transfer calcs through the ductwork only show a small amount of heat transfer. So next, I was thinking of just using a small fan pulling from a duct with the duct open near the top of the attic blowing down into 1st floor living space and having a intake vent at the bottom of the attic (on the opposite end from the supply) to draw air from the living space. I strongly suspect I would want a very good filter before the fan to avoid bringing that lovely old 130 year old attic smell into the living space. With the low heat load for our home, I bet that would supply enough heat to keep the home warm on sunny days for much of the spring and fall here in Chicago. The roof area is about 650 sq ft and near optimum solar collection angle facing directly south.

This would be a heck of a lot cheaper and reliable way of utilizing that solar energy over solar panels.

JUGHNE · May 27

Where is your attic insulation located? On the floor or is the roof insulated?

If on the floor then how is the attic vented? In my case the objective is to keep the attic at the outside cold temp in the winter by ventilation to cut down on humidity.

Jamie Hall · May 27

That works very well indeed. Depending on where you arrange insulation and how your fans are set up, you may find that you can get enough heat to be comfortable even on remarkably cold, but sunny, days. Much simpler and cheaper — as you note — than any fancy solar collector with water and pumps and who knows what. And more effective…

And yes, I have tried it, quite effectively. A number of times, in southern and central New England.

The Steam Whisperer · May 27

Insulation is in the floor of the attic ( 1st floor ceiling) and all around the second floor ( which is under the roof). We have ventilation, just probably not as much as code requries, but we have had no signs of condensation.

Jamie, did you use a filter on your systems?

hot_rod · May 27

there was a company selling air to water solar systems years ago, I forgot the name, Solar Attic rings a bell.

It was a hydronic unit heater basically, hot attic air to hot water.

This gives you dhw or hydronic options, and easier to pipe around to other locations

You need to look at quantity and quality of of that hot air

Larry Weingarten · May 28

Hi, This might be the sort of thing @hot_rod was talking about.

Yours, Larry

EdTheHeaterMan · May 28

I have found that the attic of my ranch home in South Carolina can get to be over 120°F on some summer days. I have tried to use a large net to harvest some of that heat so I can release it into my home in December and January, however the large thermos bottle that I store that hot air from July, does not seem to hold that heat that long. I think I need a bigger thermos bottle

Jaws - You're gonna need a bigger boat - Jaws gif

kcopp · May 28

These folks have some interesting ideas..

Plenty of stuff to experiment w/…

BuildItSolar: Solar energy projects for Do It Yourselfers to save money and reduce pollution

Kaos · May 28

If your attic is hot in the winter, there is a good chance the heat is from air leaks from the house. Instead of trying to come up with a Rubegoldbergian way to putting the heat back in, fixing the air leaks is a much simpler option.

Half story houses tend to have notorious air leakage, most of it can be sealed up with pieces of rigid foam, canned foam and a lot of elbow grease.

Also I would not want to pump any of that attic air into the house, not the cleanest air and in case asphalt shingles, it will smell like tar when the sun hits the roof.

Jamie Hall · May 28

https://forum.heatinghelp.com/discussion/comment/1856653#Comment_1856653

In the actual solar houses I have worked with (for instance with William Shurcliff, years ago now!) or the ones I have built, no filters were needed. Air circulation was pretty continuous.. Some of these used heat from the area under the roof, as you ae enquiring about; some just south facing glass in an otherwise "normal" house (including one very successful flat roof mid-century modern one!).

A MUCH bigger problem in all cases using the "attic" or loft was overheating — a somewhat counter-intuitive result (which, I admit, we messed up on on one house and had to do some retrofit modifcations… can't win them all…).

@Kaos does have a point, but this doesn't mean that the attic space can't get hotter than the upper floors from sun load. It can and it will (just a quickie from today, 27 April — it was thin overcast (not full sun), house interior temperature 65, outside air temperature 70, attic temperature 105). The approach is to yes, seal up uncontrolled air leaks and insulate the attic floor or knee walls as the case may be — and then provide controlled air circulation to the rest of the house, to harvest the heat.

winnie · May 28

I've often wondered if a heat pump could be specifically designed with an evaporator coil that is intended to go in to the attic space. Obviously this couldn't be a reversible heat pump ( you wouldn't want to try to pump heat into the hot attic, only draw heat from the hot attic).

EBEBRATT-Ed · May 28

1.08 x cfm x TD of attic air versus room air will give you the BTUs

You will find you have to move a ton of air unless the TD is very large to get much out of it

The Steam Whisperer · May 28

https://forum.heatinghelp.com/discussion/comment/1856664#Comment_1856664

I agree completely. The house is now very tight after spending the past 15 months getting the second floor ready for habitation. As an example…. I just finished putting in the EMT and boxes for all the electrical in the bath. All conduit connectors are sealed with caulk and all the boxes and trim rings are sealed with caulk at all holes, knockouts and box edges. The exterior walls ( and knee walls) are all sheathed with 1 inch foamboard on the inside, electrical conduit is sealed into the foam and electrical boxes sealed to the foam. Window egdes are all sealed to the foam board. Firestops/blocking at the ends of the floor joists going into exterior walls are all sealed to the ceiling below and the foam board above. Where the 1st floor partitions meet the ceiling this has been sealed as have all the 1 st floor electrical boxes, plumbing and conduit penetrations. I know the value of air sealing to improve thermal performance and eliminate condensation in assemblies. My last home( built in 1905) with 3200 sq ft and 800 sq ft of original windows ( mostly) cost less that $1,000.00 per year in total gas bills in rural Northern Illinois ( 6500 DD).

However, 650 SQ ft of dark brown collection surface facing at a nearly perfect angle south is going to collect alot of btu's.

I just ran some quick numbers….that roof space is probably collecting about 200,000 btu/hr at peak collecting time on a clear day.

Even using vertical south facing glass, heat gain for 650 sq ft is about 1,000,000 btu/day, based on my old mechanical engineering texts.

My design day heat load for about 2100 sq ft home is about 35,000 btu/hr, so this extra solar heat could really cut fuel usage.

Kaos · May 29

I think you are overestimating the amount of heat you can gather. 650sqft of perfectly angled roof sees about 200kbtu for each sun hour. Around winter time is about 3.5 sun hours per day, so not a whole lot of heat. That is the energy the hits the roof, most of that is lost to outside air and radiated away, so only a small fraction of makes it into the attic.

Even if that attic is hot right now, as soon as you start pulling heat out, the attic will get colder especially if you are replacing it with outside air.

There is also the quite significant risk of pressurizing the house by brining this air in which will push warm moist house air into your walls. Generally, when you do this, bad things happen to walls.

One way you can make it work is like the poster here, but again don't expect a lot of BTUs out.

https://www.greenbuildingadvisor.com/question/erv-as-an-attic-heat-collector-an-experiment

This falls into similar category as Trombe walls. Not worth the effort and complexity.

Jamie Hall · May 29

I respectfully disagree. First, direct solar radiation is around 340 BTUh per square foot, the area being measured in the plane perpendicular to the sun incidence. There are losses, of course, in re-radiation if the collecting surface can do so. If it cannot — if, for example, the collecting surface is behind a IR opaque glass, that is not a factor. Previous work and studies of mine have found that a good approximation for a collector behind glass, whether it is room contents, a Trombe wall, a floor… whatever — can be expected to absorb around 250 BTUh per square foot. This figure will be higher if the solid temperature of the collector is lower — so cooling the collector by moving the heat elsewhere, for instance with air circulation, is always advantageous.

Now given that figure, that 650 square feet of effective collector will absorb somewhere in the vicinity of 162,000 BTU per hour, or on a day with 3.5 hours of sunshine (taken as the average daily insolation in New England) around 570,000 BTU. That is sufficient to supply the entire heat load of a well-insulated, 1100 square foot house in most areas south of about 45 degrees north latitude.

The problem isn't collecting enough energy. The problem is storing it and, related, moving it to where it can be stored if need be. It should be stored at as low a temperature as is feasible, to begin with, and it is ideal if the collector and the storage are the same, such as masonry floors or walls inside the heated envelope. We were working in New England, and were pessimists (engineers have to be), so for the houses and other buildings with which I was involved, all of which worked, by the way, we aimed at a storage of around 2000 BTU with a delta T of not more than 10 degrees per square foot of building footprint. That was sufficient for 4 sunless days in a row.

There are at least a dozen structures — houses, academic facilities, commercial facilities — which we designed and built in the late 1970 and 1980s which are still in use and still working as intended. But that was then, 50 years ago, and two of us are dead and the third is tired…

I might point out in passing, however, that all that is for new builds. Retrofitting rarely works.

EdTheHeaterMan · May 29

@FarmerBob, @farmboy and @Illinoisfarmer might know more about harvesting stuff. They are all men who are outstanding in their field.

hot_rod · May 29

An example of a top brand flat plate solar collector, how much energy strikes the collector and how much you actually harvest at the tank.

On a roof with singles, wood sheathing, no glazing or insulation around the "collector" I suspect a small percentage is getting "in the bank"

The Steam Whisperer · May 29

https://forum.heatinghelp.com/discussion/comment/1856747#Comment_1856747

The south facing glass numbers are from AHSRAE data for Feb 21 at 45degrees north latitude on a sunny day, so not much guesstimating there. The ERV idea is interesting. I just wonder if I can run direct air heating and use a very good filter to prevent contamination, especially from the air coming out of the attic, but also on the air circulating into the attic from the living space. For other methods, just sticking a big tank of water up there would probably take care of domestic water needs most of the year. It also wouldn't be very hard to make up a grid of piping to increase collection of heat… maybe add heat transfer plates like a radiant floor system or set tubing in plates right on the bottom side of the roof sheathing.

Jamie Hall · May 29

You can do pretty much what you are thinking just above, @The Steam Whisperer . I really don't think you need the filters if you are circulating air up these — but clean the attic well before you start!

I have used overhead ("attic") water heat storage on a couple of our projects and it worked well. But… it's heavy. Watch your load on the attic floor…

As I sort of noted above — if anyone actually read my comments! — the bigger problem is actually overheating, but if you have enough air circulating that shouldn't be a problem.

The Steam Whisperer · June 4

The attic is full of old rockwool insulation, about 30 year old fiberglass, some blow in fiberglass and some new fiberglass, so I probably don't want that stuff getting downstairs. Yep floor loading could be an issue with a tank up there, but I could keep it in the smallest span right next to the outside wall to keep the bending stress down on the joists…or maybe take some of the weight right into the old balloon frame studing ( I installed lots blocking for fireblocking and bracing during the remodeling this past year). maybe I could just thread together a piping grid and hang it on the bottom of the roof and use a pump or gravity ciruclation to a tank down stairs. My main concern would be during the cold months when it is drained down that it could turn into a great place to grow bacteria.

I think i'll start with just the fan set up to do space heating… It would probably cover all of our heat load in most of the spring and fall with minimal work to install.

Jamie Hall · June 4

Be careful on that loading. Bending stress can be a problem, and is usually the limiting factor for timber — but shear stress, which is concentrated at the ends, needs to be considered as well, particularly in older buildings.

The Steam Whisperer · June 5

Yep, that imaginary 45 degree angle line from the point of support to the top of the joist. And since it is balloon framing also the shear force on that 1x4 ribbon set into the studs. That's why I was thinking trying to at least support some of the load directly on the exterior wall… which would reduce shear in the joists but then create a bending moment in the wall. The pipe grid … set up for gravity circulation to a tank in the basement is probably the better solution. I'll try the air heating first… inexpensive and light weight. I suppose I could even circulate the air over the tank placed on the 1st floor.

Anyone ever try harvesting the heat in a south facing attic for space heating?

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