This winters
project was adding interior re-circulation to an earth
berm passive
solar home with basement and added active thermal. By pulling more air to the basement the basement walls becomes a better battery for storing heat. This is an effort to increase the house's total storage capacity in advance of adding more active thermal air collectors. The existing collector is bringing in roughly 10,000 BTUs at is peak. Write up on that much
Here. The plan is to expand it by another roughly another 4 feet wide glass and 15,000 BTU's. At the other end of the house add a 2nd collect for another 4 feet wide with hopefully another 15,000 BTU's of heat. If the house is too warm I can always choose to not open either collector. But what I really want is to pull that heat to the basement with
enough air flow to maintain roughly even temperatures rather than it stratifying and thus heat the basement. If those walls and a bit in the floor are warm heat rising
should add to the stored heat from the passive to carry thru longer runs of gray days. Eventual goal probably 5 more projects down the road is to eliminate the need for the heater completely while using minimal power to do it and staying completely comfortable. Collector fans draw 30 watts and only need to run when the sun shines. Worst case the re-circulation fans draw 48 watts and might need to sometimes run a full 24 hours a day(if I get better fans I can drop that to 20 watts and still move the same air). Fans are all 12V preparing to eventually have them run on PV and batteries
First picture here is a drawing looking at the west end wall cross section of the house. The coldest air in the house is right next to the basement floor so the bottom of the duct and the sweep pull down to within 1 inch of the floor. By adding the sweep extending under the guest bed that adds 5 feet of sweep length. The bottom of the duct is 19 inches wide inside so that gives me nearly 7 feet by 1 inch of area for basically Cross sectional area is 79 square inches. The air is the pulled up to past the breaker panel where the cross sectional area is 66 square inches and is the bottle neck.(breaker panel and bed location were limiting factors) It then goes by an adjustable upper air intake. Thinking is I may want to mix warmer upper air with the coldest floor air. (Since the floor joist duct is the next bottleneck at 14.5x9.5=137 square inches may want additional draw high) From the duct the air goes thru an air filter. Designed its box so I can get 6 different filter sizes in by just changing boards in the hole.(all are cut and ready). The air then runs through the joist cavity, being pulled by a fan and pushed to the far wall where it encounters an anti-back draft door. Simple air pressure and flow lifts the light weight foam door. Turn the air off and the door closes. This has the air upstairs now. Eventually I think I want to get some more ducting added creating a thin line parallel to the sloping ceiling so I blow the air out mixing it with the warmest air in the room over a wide area. But for now I am stopping here to see if that is needful? Because of all the furniture upstairs that much would be a pain to do so I would like to avoid it if I can.
The air duct photo shows the long box. The bottom part is card board because the eventual goal is to turn that area into a built in end table with the battery bank for PV under it. So for now it is built as a very temporary function. The green vertical is a
yard stick and the top of the end table should be about 33 or 34 inches so below that should be an empty cavity with doors eventually to hold the battery bank. The sweep duct under the bed is
wood and built to tie into this eventually. For now the
cardboard is simply stapled to it. Till it is warm enough to open the house up have avoided painting to avoid the fumes plus I figured this showed better for pictures. The gray box on the right in the picture is an old computer case. Its power supply is running both the
solar collector fans and the interior re-circulation fans. It will eventually be home to an Aurdino automating everything here. By pulling the coldest air off the floor right there it will come thru right up past the batteries cooling them, giving dual function to the re-circulation fans eventually.
The air duct partial picture shows the adjustable rotary door partially open. Added it in case I wanted to mix warmer high air with floor air plus it should allow a bit more draw. This picture also shows the lid off the computer case.
The air duct filter photo, shows the upper cover off so the filter shows. It is held to the edge of the breaker panel with magnets and currently a screw at the top because I ran out of magnets. Eventually it will simply have 3 magnets holding it so it just snaps off as assembly The board above is filter access and is screwed up for now to but that should be replaced with magnets so it simply pulls off to change the filter. Because access to the fans in the floor joist cavity is necessary it needed to be removable. The computer case below is mounted on a pair of french cleats so it is removable too leaving a mostly clear path up into the joists to work on the fans. This was also built this way in case I found I wanted to purely pull the highest air in the room for some reason. Simply grab the cover and pull it off with the magnets
Last picture is the lower french cleat mounting the computer case. If the upper cover is in place the computer case can NOT be lifted enough to come off the cleats. Still need to add a pin to keep it from sliding sideways off the cleats.
Finally here is a link to a
video showing the anti back draft door working upstairs.
Anti back draft door.
The air upstairs has to travel most of the length of the house to get to the stairs to go back down. Giving time to mix. So far I find I can change the temperature down by the floor with this by about 4 degrees on a gray day with the upstairs warmer. Hoping by pulling this close to the floor I can warm the floor some too. But between stratification and boundary layer as best I can tell is it will have minimal affect on floor temperature. But will it help hold if the floor starts warmer in the fall?? Time will tell.
The fans were chosen to bring the total (with re-circulation and collector) air flow to just over 10 air changes per hour with the basement. The Ceres green house info suggest that is the minimal air to successfully store heat reaching a constant temperature. Standing in the basement bedroom doorway that is a fair little breeze. Now since the 4 quiet fans needed to reach the number were nearly $100 I went with 2 $6 fans with a much higher noise rating for testing for now. But accidentally plugged them into the 5V side of a connector instead of the 12V side. Running on lower voltage they are far quieter and move 1/5 the air as best I can measure. So the control circuit just got a bit more complicated as I now want high and low fan power choices.
Need to complete list here. Paint, Building the control system. Lining the joist area ahead of the fans with drywall for reduced fire risk. Adding a fire door that falls shut just beyond the fans to reduce fire risk. Building the end table/battery cabinet thus increasing sweep distance. The sweep is built to easily lower so the air can come from even closer to the floor once the added floor length from a cabinet is there.
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