greentheo

I recently built a space heater for my office which will not only heat up the office, but likely a good portion of the house as well.  It cost me ~$100 and could have been built for less.

Additionally, one can get 2X-4X the heating power for a mere $30 dollars per multiple of heating power with some concentrating mirrors.

Follow the instructions below and within 4 hours (less if you are a bit more skilled than I), you can be enjoying free heat supplied by the sun!

Almost all materials can be found free or really cheap if you look around.  However, if preferred they're really not too expensive at your local hardware store either.

Materials needed:

• 3 - 1"x4"x8' boards ~ $1.50/each
• a box of 1.5 inch screws ~ $2
• a few feet of flexible dryer hose ~$5
• 1 - corrugated tin roofing panel (8'x26") ~ $10-$15
• 1 - polycarbonate clear plastic roofing panel (8'x26") ~$20
• 2 - corrugated roofing panel firring strip ~ $2/each
• 1 - 1" extruded polyethelene insulating panel (8'x24") ~ $5
• 1 - sheet of plywood or particle board or hardy board ~ $10
• 1 - can of high temperature black paint ~$5
• 1 - bathroom fan (40 CFM ought to do fine) ~ $10
• 1 - attic fan temperature control unit ~ $20
• 1 - tube of caulk ~ $2
• 4 - 1'x4' long dressing mirrors ~$5/each (optional) - 4 mirrors = 2X, 8 = 3X etc...

Total cost without mirrors: $98, with 2Xconcentrating mirrors: $118.
Step 1.

Clear a little room in the garage and cut down your plywood panel (4x8') into one 2.5'x8'2" square.  This maybe the most difficult portion of the project.

Step 2.

Take one of the 1"x4"x8' boards and cut it into the following lengths: 2X26", 2X22".  These boards form the short ends of the box, and are necessary for keeping the box stable.  Join the 22" boards and the 26" boards so that on one side there is a 1" space for the long side1"x4" s to join.  It's not imparative, but does strengthend the overall box.

Step 3.

Screw together all 4 sides of the box.

Step 4.

Screw the box and the plywood sheet together.  We should now have a rectangular box with one side left open.

Step 5.

Lay the insulating  Extruded Poly Ethylene Sheet inside the box.  It should fit pretty nicely.

Step 6.

Mark where the inlet and outlet holes will go on the back of the plywood.  These should be 4" wide for the standard dryer hoses to fit into.  Using a key saw or jig saw cut out the holes.  Cut out holes in the EPS which correspond to the holes in the plywood backing.

Step 7.

Caulk the inside of the box so that it's as close to airtight as possible.

Step 8.

Cut one of the firring strips for the tin roofing to the proper length and fit it so that it fits inside the wooden frame and its bottom is about 1" down from the opening ede.  Screw it in from the sides.  This will be the resting spot for the corrugated tin roofing.

Step 9.

Lay the roofing inside the box.  It should fit just right.  If needed, use tin snips to trip the sheet on one side.  Spray paint the sheet metal with the high temperature black paint.  Caulk up the edges of the sheet metal to make it more airtight.  If needed put a couple of screws into the sheet metal to keep it secure, but keep in mind that metal contracts when hot.. it will distort the whole box in the heat of the day.

Step 10

Attach firring strips to outer edges of box and lay polycarbonate sheet on top and fasten.  This sheet helps keep heat from convective losses.  Caulk or secure it around the edges.

Step 11

Place the attic fan temperature sensor inline with the bathroom fan (electrically).  The actual temperature can sit inside the box, or simply inside of the return duct.  When it gets hot enough it will turn on the fan (to circulate the warm air back into the house).  When it get's too cold it will automatically shut it off.

Step 12

Attach the hoses to the box, and the bathroom fan to the inlet side (which should be on the bottom of the box).  Plug in the bathroom fan/temp control apparatus.

Step 13 - Optional solar concentration.

To concentrate extra solar power make sure that the face of the heater is pointing south (the shadow of a tree at midday points to the north is an easy test).  Take  your mirrors and lay them on the ground in front of the box.  If needed tilt one edge up to make sure that the reflection is right on the center of the collection face.

Add as many mirrors as you like, but beware.... more than 3X concentration could quite easily ignite drywood (don't point it at your house!).

Step 14

Enjoy your free heat!  Monitor your system for the optimal cut in and cut out temperatures.  Take note of your heating bill and watch how low it goes!

But does it work?

I haven't had a full year to test it, but so far my experience has been done under the following conditions:

• Time of testing: Mid February - sunrise 7 AM, sunset 5:30PM ~10.5 day light hours
• Avg. February high temp - 48 F
• Avg February low temp - 20 F
• Avg resting temperature for my office ~ 55 F

And now for the results:

• Cut on time ~ 7:45 AM
• Cut off time ~ 3:15 PM (my neighbors house and tree causes too much shade :-/ )
• Cut on/off temperature (as measured by the thermostat) ~ 70 F
• Final temperature of office at cut off ~ 65-70 F
• Temperature of office 5 hours after cutoff ~ 63-65 F

So we're off to a good start!   It may not be 'piping hot' inside my office, but it is comfortable... and it will eliminate $30-$50 of electric baseboard suplied heat per month.  Additionally, as it warms the rest of the house, another $10-$20 per month will likely be saved.

Update:

The heater has been running for about 1 month now and I have a few updates to make on it's performance:

•  Maximum observed temperature in room ... ~74 degrees
• Out of the last 30 days in my office I've only had the heater on 2 times.
• It's been a bit warmer and sunnier than usual (2 F greater average temp in Feb in 2009 than 2008)
• Estimated energy savings on electric heating bill in February, March (and by inference Oct and November) ~50-75% of total bill (~$40-$70/ month).
• Number of hours of heat delivered per day ~8
• Time when maximal temperature is reached ~3 pm
• Time when temperature in room drops below 65 F after a full day of heating- ~10 pm-1am.
• Maximum temperature inside heater ~110-130 F (even when it's 30 F outside)
• Possible efficiency improvements:
  • glass on front of box instead of polycarbonate, with better sealing to minimize convective heat loss and maximize solar gain.
  • air must be collected from bottom of room for optimal heat exchangement
  • neighbor's tree and house must be removed for more direct sunlight.

But overall I'm quite happy with the project.  It has practically already paid for itself and best of all... my wife thinks that it's an acceptabley asthetic project completed on a nice timeline to warrant further tinkering in the garage