Hi-Tech Wood & Oil Furnace

The Details:
This is our prototype wood and waste oil furnace that is currently heating our shop. I wanted the option to heat with a wood fired hot air furnace controlled by thermostat in addition to heating with waste oil. I recently reviewed the current wood and oil fired furnaces on the market today, and what I found is they have not yet caught up with todays modern controls and technology that is available. While some of the dual fueled heat exchangers have some pretty good designs they are lacking in modern temperature controllers to effectively control the combustion of the wood. An air tight firebox and controls are the heart of making this type of furnace work, and while it is convenient to have the oilburner stay in the firebox, it creats a place for air to get in. This design has a fully firebrick lined firebox, which protects the firebox metal from the intense heat of an oilburner, while it helps a woodfire burn more efficiently by allowing higher combustion temperatures, and increases the usefull life of the heat exchanger. The firebrick also helps in controlling radiant heat which is the nature of wood burning furnaces, even when it is in sleep mode. When the furnace is used to burn waste oil, the firedoor is removed and a special door is bolted in it's place which has the flanges for the oil burner. A fuel oil burner will mount to the same flange, and it only takes a couple minutes to change the doors. The images above show the major parts of the system.

Electronic Furnace Controller:
Shown in the first image above, this control interface is the heart of the furnace. It utilizes 3 digital programmable temperature controllers that monitor the stack temperature which is a direct indication of how the wood fire is burning. The controllers will react accordingly to changes in temperature. The first control, called the low fire control, maintains the fire while the thermostat is not calling for heat. It basically goes into a sleep mode. The second controller, called the high fire controller wakes the fire up when the thermostat sends a call for heat. As long as there is a call for heat this controller keeps the fire active and also sets the high limit of the fire. The third controller is called the fan control. This controller turns on the furnace blower that distributes heat through the ductwork when the temperature comes up during a call for heat. These controllers maintain the fire completely. All I have to do is put the wood in, and it does the rest.

Stack Temperature Sensors:
I have found through testing that the stack temperature is the best point to monitor the status of the wood fire. It provides reliable temperature readings to the controllers through the use of 3 thermalcouples. These can be adjusted in and out and once set, they don't have to be moved.

The Draft Fan & Air Control:
It is essential to control the airflow into the combustion chamber in order to control the burn rate and temperature of the wood fire. This blower is manipulated by the high and low fire controllers in order to keep the fire under control. The blower is connected to a 2" flange and the gate valve is used to set the allowed airflow at the lower firing rate. It provides a good seal if it has to be closed, and it would be closed during oil burner operation. The only other place air can get into the firebox is through the door seals, so it is important to keep the rope seals in good condition.

How It Works:
When the thermostat calls for heat, it activates the high fire controller which activates the draft fan. As the flue temerature comes up to a set point, the fan controller activates the furnace blower distributing the heat through the ductwork. When the thermostat is satisfied, it shuts down the high temp controller which shuts down the draft fan. The furnace blower will run cooling down the heat exchanger and flue temperature comes down. Once down to a set point the fan controller will turn the furnace blower off, and the fire goes back to a sleep mode. The low fire controller works to keep the fire from going out, activating the draft fan as needed.