As stated, the Coal-trol will settle in to a FR based on demand and setpoint. MIN and MAX are the limits at which the Coal-trol will feed fuel when at FR 0 and FR 99, respectively.
So, taking your example:
MIN 6 and MAX 40
Setpoint 72 degrees
Outside 20 degrees
Lets say this produces a settled FR of around 60.
If you were to increase MAX to 60, yes the settled feedrate would be lower, maybe FR 45ish. However, in trying to achieve that setpoint the Coal-trol may will dial up a period of high feedrate which would potentially result in burning coal falling off the grate or inefficient burning of the fuel in general due to the pushing rate being too high. MAX should be set no higher than your stove and grate system can handle. It is reasonable, in some cases, to reduce MAX from the ideal. Please see below.
As to overshooting and undershooting. In general if you're finding that it will not settle on a setpoint:
- Make sure the thermostat is mounted on an interior wall. The unit must be normally oriented and have no obstructions on the top or bottom edges. There is a hole on the left side on the top and bottom which allows air to enter the thermostat and pass over the sensor. If these are blocked or the unit is not flat against a wall in a normal orientation then the air flow will be restricted and it can impact performance. Also make sure it is not sitting on a couch or shelf or something like that as these can act as thermal sinks which slow the room temperature from getting to the sensor and result in over and undershoot behavior. Likewise make sure the thermostat is not in direct sunlight, not in a draft, or a "dead spot" in the room where convection air flows will not reach easily.
- Sometimes a stove can be "too much" for a house or heating demand situation (particularly happens in Fall and Spring seasons). Try lowering your MAX setting from the normal setting. Some people have had to reduce it by as much as half (MAX 40 to MAX 20) in order to get proper response. This can also help eliminate overshooting when there are circulation problems. If you do this, you may find the need to increase MAX when the weather gets more consistently colder.
- Better circulation of heated air. If your stove is in a relatively closed off area of the house with poor circulation to the rest then transfer of heat will be slow and the room it is in will over overheat significantly, particularly in the thermostat is not in that room with the stove. When the rest of the house does get up to temp, there is a pool of overheated air in the stove room which will slowly result in an overshoot. If the thermostat is in the room, then the stove will heat the room up to temp (perhaps with overshoot, due to the "overpowering stove" scenario above) and then over time the heated air will mix with the rest of the house and slowly bring it up to temp. While this happens there may be a series of over and undershoots which occur, as each heating event mixes with the house slowly. It is entirely possible for this to continue on indefinitely if the mixing is too slow (so slow the outdoor demand changes enough to result in never settling). Slow, steady, but deliberate flow is best for heating applications.
- HLF is an advanced setting which generally should not be used. In some cases, when all else fails, you can try using it. IT will only affect behavior when a setpoint change is made (either manually or based on programmed settings). It does not affect steady state behavior at all. Higher settings will result in stronger "pre-loading" based on the amount of setpoint difference between the new and old setting. If, for example, a change from setpoint of 60 to 65 would normally result in an immediate FR increase of 20 points (it doesn't, just tossing numbers to illustrate), the jump in FR with HLF set to 1 might be 25 points, HLF 2 might be 32 points, and HLF 3 might result in a 42 point jump in feedrate. And similarly for FR decreases due to reduced setpoints. The idea being that underpowered stoves or larger dwellings would benefit from a bigger "boost" than normal on setpoint changes. Another case it can be useful is if you're wanting quicker response to setpoint changes, and are willing to live with more overshoot than normal to get there.
Tester, please give us a call to discuss your situation. We want to get you working as best as possible. Our number is 315-299-3589.