Grainstoves from J. A. Mitchell Marketing

Chapter Eleven: Starting the Fire and Basic Operation



As you can imagine, getting a grain-fire started is a difficult and complicated process. Fortunately, Harman has eliminated the confusion and frustration associated with this part of the process by creating an automated fire-starting system, designed specifically for grain. The process starts, of course, when the stove is turned on. The stove can be set to room or stove temperature, but for purposes of starting the fire, it does not matter which you select. At the beginning of the of the ignition process, the blower turns on for roughly five seconds. It turns on just long enough to create a vacuum and close the pressure switch in the fire box. If all the airways and ducting are clear, the vacuum will allow the pressure switch to close and the stove can begin ignition. If there is a blockage or problem in the airway or ducting, the pressure switch will not close and the auger and igniter will not turn on. This safety feature will simply not allow you to start a fire when there is a problem with the airflow.

If there is no problem, the auger will start its initial charge and will feed grain continually for roughly two minutes. At the time the auger starts its initial charge, the igniter turns on to begin the combustion process. Harman uses a heavy duty 8 inch pencil igniter to start the fire in the PC-45. To give you an idea of how much more heat is required to burn grain relative to pellets, Harman only has to use a 1 ½ inch pencil igniter to start fires in its pellet stove.

The combustion air pump also turns on at the same time as the igniter. We are using the igniter and air pump to create sufficient heat to drive the moister out of the grain and to burn the volatiles. We have to create a fire that is hot enough to drive the moisture out of the grain and heat the burn pot and surrounding elements to the temperature necessary to sustain the grain-burning process.

PC-45 Combustion Blower

After the initial two-minute charge, the stove switches from a timing based system to a temperature based system. In other words, in the initial phase, the auger, igniter and combustion fan run full-out for approximately two minutes. After two minutes, the auger stops and the stove begins to monitor the exhaust temperature (through the ESP) and respond accordingly. When the auger stops after the initial charge, no more grain is being fed into the burn pot, but the air pump and igniter will continue to run full-out to create as much heat as possible. The auger will not feed any more gain until it senses that the exhaust temperature has risen above 200º.

Initial Charge

However, the auger will feed grain for roughly 1/10 of a second every 15 seconds just to stir the grain (because the stir stick is connected to the auger). When the exhaust temperature reaches 200º, the auger will begin a regular feed cycle while the igniter is still running.

It should be noted that this process is unlike any other grain-burning stove on the market. The reason we start the regular feed cycle before we turn off the igniter is that we want the burning process completely established before the igniter goes out. Most other grain stoves do not have an igniter, and those that do run only on a timing system. The igniter turns on for a certain amount of time and when that time is up, the igniter turns off and only then will the auger begin its regular cycle. In these timing systems, the initial charge of grain may burn out by the time the igniter shuts off. Then, when the stove starts a regular feed cycle after the igniter turns off, the new grain may smother the fire. As explained above, in the PC-45, the igniter continues to burn after the auger starts its regular feed cycle, so this infusion of grain will not cause the fire to go out.

PC45 Igniter - a 400 watt element
housed in a stainless steel lance

In fact, the igniter will not shut-off until the exhaust temperature reaches 249º. In other words, after the auger has begun its regular cycle when the exhaust temperature reaches 200º, we still wait for the exhaust temperature to reach roughly 249º before the igniter will shut-down. Sometimes it takes a while before the stove reaches 249º. During start-up it can be very difficult to get enough heat into the burn pot to create a self sustaining grain-fire. When the fire is started from scratch, much of the heat generated by the igniter is being sucked off to heat the refractory and metal surrounding the burn pot, and until these elements reach the requisite temperature, the fire in the burn pot cannot get hot enough to sustain a grain-fire. For that reason, we have developed this process where our heavy-duty igniter remains on until after a regular feed cycle has been established and the exhaust temperature has reached a sustainable 249º.

PC45 Igniter Assembly

At this point we should point out again that very few grain-burning stoves even have an igniter. Most stoves require the operator to manually light the fire using pellets. Those few competitors that do use an igniter do not use a heavy-duty pencil igniter and do not utilize an ignition process nearly as sophisticated as ours. As we stated before, these other grain-burning stoves that have an igniter simply use it to burn the initial charge of grain. They do not establish a regular feed cycle before the igniter turns off and as a result the stoves constantly flare up and die down in the ignition process. One of the reasons we are able to utilize such a sophisticated automated start-up process is because we, unlike all our competitors, use an ESP to monitor exhaust temperature.

PC45 ESP Probe

Even if these competitors began using better igniters they still could not compete with the ease and success of our ignition process because none of our competitors use an ESP to monitor exhaust temperature. Without monitoring the exhaust temperature, it is impossible to know with any precision when the fire has reached “critical mass” and is able to support itself. Our competitors’ ignition systems work strictly on timing systems, which are imprecise and cannot possibly adjust to the wide variety of grain grown in Western Canada.

As much as we enjoy discussing the reasons why the Harman PC-45 is the best grain-stove on the market, we should get back to explaining the ignition process. We left off with the exhaust temperature reaching 249º and the igniter shutting off. As we stated above, during this process, the entire stove is being heated, so depending on the characteristics of the grain you are burning, getting the exhaust temperature to 249º may take some time. When the igniter turns off, the stove has reached the point at which it can sustain itself. When the exhaust temperature reaches 251º, the distribution blower will turn on and begin to distribute the heat into the area surrounding the stove.

At this point, if you have not set the feed adjuster rate to the maximum rate the stove can handle for this particular batch of grain, you should do that now. Once the feed adjuster rate has been set, all you need to do now is set the room or stove temperature to the desired setting. The fuzzy logic process will adjust the feed rate (up to the maximum you set using the feed adjuster) to reach the desired temperature. However, the adjustment will take a little time. The auger feeds on 15 second cycles, and begins by feeding for 2.1 seconds out of that 15 second cycle. It will then adjust up or down by a 10th of a second every 15 second cycle. Therefore, if the processor determines the stove should be feeding 3.3 seconds out of every 15 second cycle, it will take 12 cycles, or 3 minutes, to reach the desired setting. The chart below summarizes the ignition process. Please note these are the key ESP activation temperatures when dip switches 3,4,7 and 8 set to on.

(print this page)

Warning, J.A. Mitchell Marketing Ltd. is not the manufacturer of the Harman PC-45 or any other product. J.A. Mitchell Marketing Ltd. is not responsible for any loss or damage caused by the Harman PC-45 or any other product. J.A. Mitchell Marketing Ltd. provides no representations or warranties. If you are confused or unsure at any point, contact your local Harman dealer.