Converting a Clothes Dryer to Use Solar Heated Attic Air

Randy describes his new clothes dryer setup that uses an attic solar collector to supply hot air for his dryer.  This system saves the energy associated with heating air for clothes drying and also the energy required to heat or cool the air that a normally connected dryer pulls into the house as it vents air outside.

The dryer system works in conjunction with Randy's solar attic based heat collector that is used for space heating.

dryer gets solar heated air hooking dryer up to an attic hot air supply

Dryers are a major energy user in homes.  A Canada study shows that dryers typically use 930 KWH a year to do 416 dryer loads (2.23 KWH per load).  Nearly all of this heat energy simply expelled outdoors (wasted).   In addition, as the dryer vents air outside, it pulls fresh air into the house which has to be heated or cooled (depending on season) to room temperature -- this can easily add another 300 KWH plus per year.  Its amazing to me that this very large energy sink does not get more attention.

Randy's setup is simple and effective, and it appears that it can save a serious amount of energy. 


If you can't do Randy's solar dryer, here are some other options for saving clothes drying energy...

The details from Randy:

Modifying the Dryer

Here are some pictures on how I converted a used 110 volt Kenmore gas dryer I purchased at a garage sale for $35. It can now be used with a solar collector or attic heater. The the pictures below show how the normal dryer air intake was blocked and heated air from the attic solar collector was used instead.

After converting the dryer, I blocked the intake and was surprised that it was not even close to air tight. The design of the dryer and the worn drum gaskets caused the exhaust to output more air than came in the intake. To overcome the problem I decided to add a fan to the intake duct coming from the attic heater. That combined with the powerful dryer fan should do the job.


converting dryer to operate on attic air
Gas dryer before conversion to use
attic air supply.

Back of the dryer before removing the cover.

The black metal duct on the left is the
dryer exhaust duct, and the silver
colored duct on the right is the
dryer intake duct.

Circle marks the point where the duct
supplying attic air will be hooked up.

Cutting hole for attic air supply duct.

Making a flange to connect the duct from
attic to the dryer.

Aluminum tape to secure the flange
tabs in place until adhesive is applied.

Showing how tape secures the new flange
in place.

RTV high temperature sealer/adhesive to
secure and seal the flange in place.

The new flange sealed in place.

The regular intake that pulls air from
the cabinet is closed with aluminum tape.

Closeup of normal air intake sealed up
with aluminum tape.

One thing to bear in mind is that this conversion will not let you use gas heat as a back up, when solar is not available. I will have to tackle this later on, when the permanent dryer install is done.   

Hooking the Dryer Up to Attic Air Supply

I used a 6 inch. insulated duct 23 ft. long to bring the heated air down from attic heater to the dryer.  A 6 inch. to 4 inch. adapter was used at the vent attached to the dryer intake.

The fan installed in the attic plenum works great. I picked it up at the local salvation army store for $20. It looks like new. It was installed in some kind of portable filter cart. The dryer and fan cord are plugged into a timer, which in turn is plugged into a switched plug strip next to the dryer.  The timer insures that we don't leave the dryer on too long.

dryer connections for attic supplied dryer
This shows the new intake connection for the dryer.  The upper duct connects
to the attic air supply and provides all of the heated air needed for drying.
The lower duct is the normal dryer exhaust and is vented through the wall to the outside.

dryer with attic vent in place
The completed dryer with the attic air
intake duct installed.
ducting for attic dryer supply air
The attic air intake duct.

Connecting the dryer air supply duct
to the attic collector plenum and blower.

The dryer exhaust vent through the wall.
blower for attic intake dryer
The blower used to provide sufficient hot air
flow for drying clothes.

Timer to control the fan.

The dryer was installed in the garage, next to kitchen and laundry room door for convenience.


The dryer is in! It is functioning better that I hoped.


solar clothes dryer being loaded
The dryer was installed in the garage, next to
kitchen and laundry room door for convenience.
solar clothes dryer finished load
Dry clothes come out hot like a normal dryer.
solar dryer temperature
Left side gage is dryer inlet temperature,
right side gage is attic collector temperature.

Loads of towels, sheets, pants are running at 90 minutes for a full load.   

The booster fan is 110 volt and 1/15th hp.   The cfm rating unknown.

Power and energy consumption:

Dryer alone power use                264 watts

Attic blower alone power use     141 watts

Total Power Use (fan+dryer)      396 watts

Energy used for one 90 minute dryer load              0.6 KWH

Cost for one dryer load at 10 cents per KWH       6 cents

This compares to 2.2 KWH for an average electric dryer load without counting the additional energy consumed to heat or cool outside air that a normal dryer pulls into the house.  If one allows 1 KWH for heating or cooling infiltration air, then the total energy use per load for a conventional dryer load would be 3.2 KWH, or more than 5 times as much energy as the dryer supplied with attic air!

The measurements on the attic dryer were taken with a Kill-A-Watt meter:

electricity consumption solar dryer

My home is shaded at 4pm, but there is enough stored heat to last until about 6:30pm. Shade will effect the drying schedule.

Something I had not thought about earlier, is that a dryer draws 100-150 cfm. When the dryer is operating in the home this causes the home to draw in outside air. If outside air is 17deg. or 100 deg., I not only pay to run the dryer, I also must now heat or cool the air drawn into the home. This is a hidden cost. Of course if the dryer is in a room with a door, you can crack a window, if you have one. But what do we usually do? We turn on the fan in the utility room to vent the heat and moisture. That will now draw even more air. If dryers were made to vent in air from outside the home, couldn't this prevent the draw from the home and the hidden costs associated with it?

What I Would Have Done Differently

If I had it to do over, I'd make these changes:

  1. Locate the dryer intake vent 4 inches higher to match the drum intake holes.
  2. Block off the intake with foam after cutting the hole.  This would have saved an hour of work removing the duct and taping up many holes.  It is tricky to reinstall.
  3. Install a new gasket on the drum to close the gap where air bypasses so that the additional fan is not needed.
  4. Use sheet steel vent to the intake instead of sheet aluminum, its much stronger.

Other Possibilities

Here is a possible attic water heater design. This won't work in winter and will not drain back at night. The only way to drain it is if I kept each run separate and used air pressure to empty the lines, in winter. Still thinking. I like that it is low cost and uses few connectors and has lots of pipe in contact with the heat source.


I'm also thinking about a 2nd attic heat collector, as I have more roof area available.

The details on Randy's first attic solar collector are here...

Randy    July 14, 2011


Please feel free to add any comments, suggestions, corrections... below.

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