I decided to recover the heat from my dryer.

(Please read the attached: analysis.pdf for cost benefit analysis of this build.)

I addressed two problems with my solution:

I filter the dryer exhaust with a furnace filter
I reduce the moisture with a dehumidifier.

To use this system, every time you you turn the dryer on, you turn the timer on for 60 minutes.

Materials:

Furnace filter (I chose a 16 * 20 filter because it fit the space).
Dehumidifier. Dehumidifier must have the following features:
- It must remain "on" when powered down. Some must have the start button pressed when the power goes out. They won't work with the timer.
- It must allow for a drain hose.
Electronic timer with 60 minute option -- the kind used to time the exhaust fan in a bathroom.
Electrical box for above.
Extension cord, 3 prong if dehumidifier is. (I had one lying around.)
Dryer vent pipe -- a short piece, maybe 1 foot long. Situation dependant.Sheet wood - amount depends on size of your box/filter, type is really unimportant -- a great opportunity to diminish your scrap pile.
Pieces of 2 * 4, 2 * 2 (can be cut from 2 * 4), 1 * 2. Again, my scrap pile had all of this stuff.
Nails/screws. I used my pneumatic stapler.
Part of a garden hose, the male end. Enough to connect the dehumidifier to a drain.

Analysis.pdf
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Step 1: Build the Back of the Box First.

First I built the back of the box. The size of this panel should be as follows:

Height is dimension height of the furnace filter.
Width is dimension width of the furnace filter minus the thickness of the sheet material minus 1/2". That way the filter will stick out a bit so you can replace it more easily.

If a furnace filter says it is 16" * 20" * 1" that means that it'll fit that size of slot, the filter is actually smaller than those dimensions.

The box is surrounded by 2 * 2, just so that the top and sides have something to attach to.

Step 2: Build the Slot for the Furnace Filter

What I did (you don't have to do it this way, but it made for clean edges and easy calculating) was to build the side, attach the 2 * 2, then saw out the slot.

The height of the side should be the same as the height of the back. The length of the side should be the outside dimension of the width of the box.

The 2 * 2s must not run to the back of this panel, but must allow room for the back and its 2 * 2 frame. (see picture in next step.)

Step 3: Build the Back Side.

The right side (in my case) should be the same size as the left (in my case) was before the slot was cut out. The 2 * 2s need to have a slot in them so that the filter can fit in. Note that there is a gap at the back of the 2 * 2s to fit the back panel.

Step 4: Assemble the Box

A top and bottom panel need to be cut such that they overlap the sides and back.

Then 1 * 2 is attached inside the box to complete the top and bottom of the slot. On the right side I only attached a front 1 * 2 so that the filter had something to be pushed against. The fact that more air might go through the slot is inconsequential, and by doing it this way, the filter will push in more reliably.

Step 5: See How the Filter Fits In.

Ok, not much of a step.

Step 6: Add Dryer Vent Pipe

A hole is cut into the side of the box (really not fussy, I just used my jigsaw.) A piece of dryer vent pipe is placed into the hole. This is where the dryer vent will attach. This pipe can just go part way into the box, it is very much not fussy, but it can't but into the far end of the box.

Step 7: Create a Base and a Timer.

I made a simple rectangular box from 2 * 4s with a base on it. In my situation this fit nicely on my hot water tank. It is big enough to hold my vent box, and my dehumidifier. Note, my filter box doesn't sit inside this box, it sits on top of it. Otherwise its a pain to access the filter.

I mounted an electrical box into the side of it, ran an extension cord through it, and wired in the timer. (Note that the black wire in the extension cord is the one that should be switched by the timer.

Step 8: Assemble It in Place.

Mount your platform. Mine was screwed to the studs just above my hot water tank.
Mount your filter box in place. (It must sit on top of the 2 * 4s so you can access the filter.
Put the dehumidifier in place. It should sit inside the 2 * 4 frame so that it has nowhere to go. It should be close to the opening in the filter box, but not terribly tight. We are loose coupled remember.
Make sure that the dehumidifier is turned on. (It must stay on when there is no power. If not -- bad choice of dehumidifiers.)
Attach the drain to the dehumidifier and run to an available drain.
Plug the dehumidifier into the extension cord and the cord into the wall.
Spend about 3 weeks teaching and cajoling the users of the dryer to remember to turn on the dehumidifier as well. Maybe a sticky note attached to the dryer can reduce this to about 2 weeks.