I placed a sensor in the Output furnace duct. A second sensor was placed in the Cold Air return duct. This was fairly easy to accomplish, as there were holes already cut into the duct work from a previous duct cleaning.
The outside sensor was a little more tricky. I wanted to protect the sensor from the elements, and from direct sunlight, but still be able to obtain a reasonably accurate measurement. A bit of googling revealed the best way to accomplish this was to use a 'solar radiation shield' or Stevenson Screen. For what it consists of, it is fantastically expensive to buy.
I built mine using a similar method to how this guy (complete with pictures) built his, using the bottom of flower pots from Walmart (6 x 59¢), a 3' length of threaded rod, a 2' length of plastic hose, a couple feet of light chain, and a bunch of washers & nuts.
Line the plastic flower pot bottoms up, and mark them. You will need to be able to line them all back up again.
Drill 3 holes equally spaced through the pot bottoms. The hole you drill should be just large enough to put the threaded rod through.
Cut a large hole in the centre of 4 of the lids. This hole should be large a couple of inches across (if using a 6" pot bottom).
Drill an additional hole through the same 4 lids that have the large hole. This hole should be large enough to run your cable for the sensor through.
Cut the threaded rod into 3 equal lengths.
Put a nut on the bottom of each of the rods.
Place a washer on top of each of the nuts, and place the 1st lid (with large hole in middle) on the rod, lip side down.
Cut the hose about 1" in length. Put the 1" hose on each of the rods.
Place the next lid (with large hole in middle) on the stack.
Repeat the above 2 steps for the until you run out of lids with large holes.
Run the cable with the sensor up the cable hole. Position it so that it is in the middle of the last cut hole, and fasten it. I used wire ties for this.
Place the remaining lids on the top of the stack, continuing with the lid / hose pattern.
On top of the last lid, place a washer, and nut on, and tighten it up.
I drilled a small hole in an extra washer, and used this to connect the chain.
I ended up adding extra washers & nuts to the bottom, to help keep the centre of gravity low, and keep it from swinging in the wind too much. You might need to experiment with this on your own.
A dab of a product like LocTite will keep everything together (Word of warning - this product should be used in a fairly well ventilated area).
Use wire-ties where necessary.
I hung mine from the eve on the north side of my house. I hangs down about a foot or so, and is mostly shaded from the sun most times during the day.
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