Chofu Wood-Fired Water Heater
This Chofu model is a precision built wood-burning water heater designed specifically for hot tubs. It circulates water using the principal of thermosiphon (the pumping action created by rising hot water), eliminating the need for a circulating pump or electricity. This unique feature opens up a whole new range of possibilities for alternative hot tubs. Now you can have a basic soaking tub without pumps, chemicals, or high maintenance. With the Chofu heater you can retrofit an existing tub or put together a low-cost soaking tub, using a wooden tub or stock tank.
The stove body of the Chofu is made with high quality stainless steel, grade (316), so it can be used outside, without worry. And since the firebox is also made of stainless steel, it will withstand years of service without danger of burning-out. The smoke outlet on the Chofu features a unique vented collar that provides secondary combustion of unburned gases, for reducing smoke emissions and chimney sparks.
The Chofu utilizes a sophisticated heat exchange design for efficient water heating. The stainless steel stove body surrounding the firebox is a double-walled water-jacket with a 1-inch space between, making the entire firebox (except the front) a heat-transferring surface. Additional heat transfer takes place in a water-filled baffle plate that runs horizontally through the firebox. The baffle deflects the path of the fire, so it gives up more heat into the water jacket before going up the chimney.
The Chofu will connect to any tub using accessories provided (see connection kit): flexible neoprene tubes, stainless steel pipes, and thru-wall tub ports. (Thickness of the tub wall must be specified to determine the length of thru-wall ports.)
Installation requires these four basic steps:
(1) Preparing a brick or cement foundation next to the hot tub to receive the Chofu heater.
(2) Cutting 1 7/8-inch holes in the side of the tub wall for thru-wall ports.
(3) Installing pipes to connect heater and tub.
(4) Installing the stove pipe, (8 feet recommended)
The Chofu operates like a conventional wood stove, achieving its fastest heating rate from dry hardwoods. It uses 17-inch wood with an 18-inch x 14-inch x 14-inch firebox. The most efficient heating comes from using 17-inch x 1 1/2-inch x 1 1/2-inch wood, loaded at 45 min. intervals.
The Chofu has a high and low speed draft control to regulate the heating rate. It operates with the draft wide open for fast heating, and closed down to maintain temperature once the water is hot.
High grade stainless steel stove body
1/8-inch steel stove front
Heavy cast iron door and grates
Hot Tub Connection kit (see photo)
Generous size firebox, 18"L x 14"W x 10"H
Vented smoke-outlet for secondary combustion.
3/4-inch drain for freeze protection.
An ash drawer for easy removal of ashes.
A long handled ash rake
Dimensions: 16" wide x 23" long x 18" high
Stove body: Grade 316 stainless steel, inner wall 20-gauge, outer wall 22-gauge
Stove Front: 1/8-inch steel
Firebox door and grates: cast iron
Firebox dimensions: 18"L x 14"W x 10"H
Heat exchange surface area: 9 sq. ft.
Smoke outlet: 4 5/8-inch (reduced to 4-inch)
Circulating pipes: 1 3/4-inch O.D.
Drain: 3/4-inch I.D.
Heating Rate Information
Although the heating rate is variable depending on dryness of wood, frequency of loading, etc., the average heat output of the Chofu CHS is 32,000 BTUs. A 200-gallon tub can be heated at approx. 20ºF per hour.
Note: The heat output of the Chofu increases after an initial 45-min. warm-up period that heats the stove body and establishes a bed of coals.
55ºF to 105ºF
||1 1/2 hours
||2 1/2 hours
||3 1/2 hours
||4 1/2 hours
The Chofu Wood-Fired Heater must always be protected from freezing to prevent damage. We provide special through-wall ports with threaded plugs that allow you to keep water in the tub, while draining just the heater. To drain the heating unit, plugs must first be inserted into the ports; then remove the threaded drain plug at the rear/bottom of the heater to drain water out of the Chofu. It must be noted that firing up the Chofu when it is empty of water will quickly damage the heater. The Chofu water jacket must always be filled with water and the tub ports open to allow water to circulate through the water jacket and therefore, cool the firebox metal.
Alternative Freeze Protection Methods
• Drain the tub This will remove about 2/3 of the water from the Chofu water jacket. The remaining water can be drained by removing the drain plug at the rear/bottom of the heater.
• Covering the Chofu This method can be used instead of placing plugs in the through-wall ports and draining the Chofu, however, it requires that you make an insulated cover that can be placed over the Chofu.* This may be adequate protection for nighttime temperature drops just a little below freezing, but for long term protection, the enclosure must have an external heat source, such as a light bulb or heat tape. The tape can be wrapped around the bottom stainless circulating tube, causing heat to rise up through the water jacket and into the tub.
A protective cover may be either a soft insulated (tea cozy) type or a hard shell, wood structure. To provide complete freeze protection, the enclosure must snug up against the tub to protect the circulating pipes, as well as the heater.
*When the Chofu heater is not in use (and also to allow it to be covered), the stove pipe may be removed. This can be accomplished by lifting off the pipe at the connection between the black enameled collar and the stainless 4-inch pipe adapter. This connection has a very snug fit, thus does not need to be secured with sheet metal screws, as do all the other connections.
Chimney Cap and Stove Pipe Support Bracket
Secondary combustion in the Chofu Wood-Fired Heater eliminates most sparks; however, a chimney cap may be necessary when the heater is being used in a fire hazard area or when the stove pipe exhaust near trees.
Please Note: A chimney cap will cause a restriction in the air flow (draft) through the firebox, which will result in a slower heating rate. To compensate, an additional 2-ft length of stove pipe may be used to maintain an optimum draft.
8-ft of stove pipe is recommended for optimum draft. When all connections are secured with sheet metal screws (supplied with every Chofu), the pipe does not need support, however, if additional stove pipe or a chimney cap is used, the stove pipe will become top-heavy and must be stabilized by using a Stove Pipe Support Bracket. (See price list)
top of page