Sub-zero Roasting

If you live in northern climes, many an ardent home roaster puts away
their roasting gear at the first hint of a snowflake - forced to buy their beans
until warmer weather returns once again.

It does not have to be so.

I am happily roasting with my HWP roaster at -20C and can easily obtain
active second crack or beyond despite the frigid winter temperatures
outside.

How can this be? Many home roasters complain about slow or stalling
roasts when temperatures fall below 10C or 50F. The secret is
cogeneration - the use of waste heat to pre-heat the intake air going
to the roaster.

Does does this entail the use of some sort of complex heat exchanger
device or other heating devices?

Nope. All you need is a box with four flaps.

Lest you give your local fire chief a heart attack, you will want to
build a steel, fire proof, insulated, double walled box. Do not be
tempted to use an ordinary cardboard box...

Your ahem..steel box should be at least as tall as your roaster. A bit
of wire can be used to hold the side flaps tight enough together so
that the end flaps can be used as air deflectors. Next, you
have to make yourself some dampers to control the amount of cold air
coming into the box. Cut one or more three-sided holes so essentially,
you've got a flap that you can open or close like a damper.
Stick a spare dial thermometer through the side of box - close to the
bottom where you can measure the air intake temperature.

Yes, -20C is very cold. That's a little colder than zero on the old
Fahrenheit scale.

Cold is cold so we are going to have to cheat a little.

You will need one unheated garage and a car that has been driven from
work and is now transferring its heat to the enclosed garage.
That car better be turned off mister because if you're dead of carbon
monoxide poisoning, how are you going to hit the cool button?

If your garage is attached to your house, you've automatically bought
yourself several more degrees as heat leakage will always keep
your garage a bit warmer than outside.

Now place the roaster in the box and move the two end flaps at about a
45 degree angle. Fire up the roaster. Needless to say, do NOT leave
your roaster running unattended.

The idea is to recirculate some of the roaster's hot exhaust air and
keep it in the box. In just a couple of minutes the air inside the box should be around room temperature (about 20C) or higher. You can get fancy by
moving each flap up or down to control the amount of recirculated air -
ie; move the flaps vertically if you want the temperatures cooler or
close the flaps a little if you want temperatures a bit warmer. Think of all
the profiles you can create. Want to ramp up to first crack rapidly?
Extend the pause between first and second? Slow a rolling second? You can do it all! Ah the joys of winter roasting!

It is a good idea to open the garage door around first crack or so
because coffee smoke may not be good for the lungs.

Roast as normal until you hit the cool button. Open up the flaps and
open the damper(s) and allow that cold air in to rush in and cool those
beans fast!

There are some disadvantages though.

Recirculating coffee smoke through your roaster fan motor is not likely
to extend the life of the unit and inhaling large amounts of coffee
smoke may shorten your lifetime too!

Is it worth it? Do I need to ask?