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WK400 owners manual (pdf)
WK400 brochure (pdf)
WK400
Add On/Stand Alone
Wood Forced Hot Air Furnace.
The WK400 is an add on wood furnace that can be added to your existing oil furnace, or installed as a stand alone wood furnace.
Click here for WK400 installation.
The Benjamin line of ADD-ON units, when used properly, will increase the convenience and
pleasure you receive from heating your home with wood, and yet still give you the convenience
and reliability of heating with fuel oil.
Because of the unique Benjamin wood burning design, which will utilize more of the BTUs
available from each stick of wood, you will soon discover that you can reduce the amount of wood
that you use to heat your home.
How does the Benjamin WK400 ADD-ON Wood Furnace do this?
In several ways..
- The unique combustion chamber.
- Hand welded steel construction.
- The heat exchanger passage design.
- WK400's operational procedure.
1) The WK400's unique wood combustion chamber.
- It's oval shaped.
- It's lacks firebricks or wood grates.
- It's thoughtful design and construction.
Hand welded, and constructed of 3/16", and ¼" thick steel, the combustion chamber is the
heart of the unit. It measures 18" High, 21" wide, and 24" deep (This means that it can handle
more than one 24" log!)
As you may already know, smoke contains much of the wood's total energy, and it wants to spiral
by it's very nature. Benjamin has built a uniquely different combustion chamber and heat exchanger
that facilitates that effect. A conventional square combustion chamber, or box frame heat exchanger
restricts this natural flow. The smoke will rise up to the corners and swirl around there, cooling
as it does so. This cooling process allows creosote and soot to form on these surfaces. They act as
an insulator, and restrict the ability of the combustion chamber and heat exchanger. Benjamin's oval
combustion chamber design allows the smoke to spiral and release it's heat cleanly and efficiently.
The oval design also eliminates the need for firebricks or wood grates. These were also found to reduce
efficiency. Wood grates will allow too much combustion air to enter under your burning wood. Your wood
will be consumed ineffectively and the heat will go up your chimney, instead of into your home. Firebrick,
in essence, will do the same thing. BTUs and time will be lost as the unit is forced to heat the unneeded thermal mass.
Benjamin has a better idea.
Benjamin wants you to always keep a layer of coals and ash in the base of the wood burning combustion
chamber. These are to be used in place of the conventional wood grates or firebrick. This will slow your
wood burn down to a manageable level, allowing you to effectively remove heat from all three of the stages
of combustion, and it will allow the smoke to do what it wants to do naturally.
Also note that the door to access the wood burning chamber is made out of cast iron. Benjamin is one of the
few remaining wood burning manufacturers that still utilize a cast iron door. They do this because the cast
iron will not flex the way a steel door will. This ensures a tight gasket seal and a reduction in lost BTUs
due to a constant air leak draft.
2) The WK400's Hand welded steel construction.
The one piece, hand welded design of the Benjamin WK400 ADD-ON furnace is constructed of 3/16" and ¼" thick steel.
Benjamin's safeguards the WK400's consistently high degree of product quality, because each weld is individually
inspected by the welder after it is created. The 3/16" steel thickness extends all of the way out to, and includes,
the exhaust collar. Let's put it this way, the collar on the unit will probably be more rugged than the smoke
stack it connects to. Because Benjamin uses thicker steel than many other manufacturers, they have been able to
extend their product line's life span beyond that of which is expected.
Thanks to it's revolutionary design, the Benjamin WK400 is classified as an "ADD-ON" appliance. This enables it
to be connected to an existing oil or gas furnace, and will allow it to exhaust into a shared solid fuel chimney
flue. Maine code allows for this use, and insurance companies are agreeable to it as well.
To learn more about why Benjamin uses steel see the article "Steel Vs. Cast Iron"
3) The WK400's Innovative heat exchanger passage design.
Another innovation from Benjamin. 
As we mentioned before in our discussion regarding the combustion chamber, smoke likes to spiral by its very
nature. Benjamin has extended the combustion chamber's unique oval shape into the heat exchanger as well. This
design will reduce the amount of creosote and soot build up within the heat exchanger, increasing it's ability
to work effectively, thus boosting efficiency.
The heat exchanger also features a gradual incline from the front to the rear of the unit. This allows for
the natural rise of smoke.
There are two cleanouts on the front of the unit,and one on the front of the combustion tunnel, accessible
after removing the front jacket cover (4 screws). To guard against the rusting and seizing of the heat
exchanger cover removal nuts, Benjamin has opted to use dissimilar metals made of brass and steel.
A unique unit requires unique tools, and Benjamin has you covered. Each unit comes equipped with it's own
custom designed and built steel clean-out rake!
4) How the WK400 operates.
The WK400 operates using two thermostats (or a single two stage programable thermostat) in your living
space. One thermostat will operate the wood side of the system and one will run the oil. This means that
you will be running two separate set-points.
Let's assume that you will be running primarily with wood, with oil backup.
The oil side of the system will be set at your "set-back"" temperature. This is the temperature that you
want the house maintained at when you aren't around, or when you go to bed at night. Let's call this
temperature 58°F. (If you want to run the oil side as your primary heat source and only want to
utilize the wood portion in emergencies, you simply use the oil thermostat much like you would any other
while completely ignoring the wood thermostat)
Now, let's say that you want to fire up the wood heat portion of your furnace when you get home from work.
Your house is being maintained at the 58°F oil set-point. You set the wood thermostat at your target
room temperature. For now, we will call this 72°F. You go down to the basement and start your wood fire.
(If you want to preheat the chimney to achieve a better draft for the wood fire, turn the oil thermostat up while
you are loading the wood firebox.) Once you have a wood fire running that you are happy with, go back upstairs
and go about your normal activities. The wood thermostat, set at 72°F, will maintain the house temperature
by opening and closing the air draft control on the wood combustion chamber. (The silver box and chain connected
to the top left corner on the front of the unit.)
When the wood fire goes out, the temperature of the house will start to drop. The wood thermostat will be calling
for heat, but there is nothing on the other end to provide it. The temperature inside will continue to drop.
When the house reaches 58°F (our oil set-back temperature) the unit will automatically switch over to the
oil burner and maintain this set-point, until you reload and light another wood fire.
These set points are fully adjustable and can be set anywhere you want them to be. With the automatic switch over
from wood to oil, you no longer need to worry about "rushing home to stoke the fire". The Benjamin will take care
of everything for you. Go out with your friends, stay late at work, relax, and enjoy a slower pace to life. When
you come home, the heat will still be on.
With a Benjamin in the basement, these are no longer concerns. You can remain toasty warm inside without the fear
of frozen water lines, as both the FS140 combination fuel furnace, and the
CC500 combination fuel boiler, when properly installed, can still operate effectively without power. They were designed to utilize gravity flow
convection, if the need arises.
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