How a GARN Works

This is a summary of the different parts of the GARN® WHS and their purpose. Think of the GARN® WHS as a big tank of water that you heat by burning wood. The fire isn’t modulated. The entire wood load is burned quickly while the heat is dumped into the large water tank. The result is a very clean, highly efficient burn. That means less wood, less work, and less smoke. When a thermostat calls for heat, a pump turns on and draws the hot water out of the tank and into your heated space for distribution.


1.    Fresh air is ducted from the outside and enters the unit through an inlet in the back.

2.    The fresh air travels through a tube immersed in the water and into the air collar.

3.    The fresh air is circulated around the air collar before entering the combustion chamber. This keeps the collar relatively cool. The air collar distributes the air into the combustion chamber.

4.   In the combustion chamber the wood burns in its first stage.

5.    The smoke and exhaust from the fire enters the insulated reaction chamber. Additional fresh air is provided by the air collar. A high temperature secondary combustion occurs. The process is called direct gasification. Direct gasification burns the smoke and creates additional heat.

6.    After exiting the reaction chamber the clean exhaust travels through 30 ft of tubing. Heat transfers from the very hot gases to the water in the tank.

7.    The exhaust exits the GARN® WHS vertically or horizontally through a 6” insulated flue.

8.    A digital controller automatically stops the induced draft blower when combustion is complete. The water is now hot and ready to be used for heat in a building.



  • A GARN WHS needs to be installed indoors. A designated area in a building, a separate building, or an attachment to an existing building is required.
  • This is the front of the GARN WHS unit. The front is where you load wood, mount the blower, access the digital controller, drain the unit (when needed), and access the cleanout ports for periodic pipe cleaning. An overflow pipe is also provided so the first time you heat your unit the water that expands to the top of the tank comes out here. The GARN® WHS provides optional off-peak electric elements fittings up to 49.5 kW.



  • This is the back of the GARN® WHS. The back is where you connect to the 2” hot water supply (3” on the WHS 3200) and 1.5” hot water return (2.5” on the WHS 3200), duct the fresh air intake, and vent with the horizontal exhaust (if applicable).




  • This is the top of the GARN® WHS. The top is where you access the full-size manway (for water inspection and water chemical addition), anode rods (for corrosion protection), fill the tank with water, and vent with the vertical exhaust (if applicable).

  • A fire is started by loading the combustion chamber and adding some newspaper and small kindling. A simple match does the trick. The fire heats the tank of water.
  • Reloading after a fire is easy. Open the combustion door. The induced draft blower ensure little to no smoke backdrafts into your space. You can then pile on more wood to get the water up to your desired temperature.
  • When your building calls for heat, a pump in your system turns on and draws the hot water from the tank and distributes it for heat. A fire need not be maintained constantly. That’s the idea behind storing the heat.

  • The GARN WHS is not pressurized and so cannot be directly connected to a pressurized system. The GARN WHS can interface with a pressurized system by simply installing a heat exchanger.
  • When the GARN WHS unit drops below the operating temperature of your system, start a fire and heat the water again.