While i am not a "pro" I have found what i think are some area's on this boilers control system that i wanted to improve greatly at least for my own use. I am sorry this is not formatted very well to be a good technical document. But I am very sure that MANY people even pro's out there will like this. Enjoy!. At some point i might sit down and make a visio document to really simplify the explination. Right now though i am working a lot of hours and trying to get my CCNA (cisco cert yes i am a networking guy ) Additional controlls for your Harman pb 105 2 aquastats or similar controller that closes a switch at a given set point. 2 relays 1 SPST 1 DPDT or 2 DPDT ( I suggest the functional devices relay in box relays. They make mounting them a breeze and wiring is easy. They also normally have a led to indicate the coil is charged) Depending on how you wire this will depend on which model you need some use 10-30 volts for coil some have 120 for coil so find out what your voltages are before ordering one. Ok my pb105 has a overheat zone built into it they all should have this. It is the dark blue wire in the junction box on the boiler. This thing for me is 100% useless it has 120 all the time weather the light is on or not. So i could not put this built in control to work for me. I did not like running my circulator pump all the time even when there was no call for heat. My boiler is set up to connect to my oil burner so it delivers hot water to the oil burner so it doesnt have to fire up to deliver heat. So for me i went into the control box for my oil powered boiler and found the hot wire that connected over to the burner itself. I unstalled a switch and a transformer on that so when i turn the switch on it will allow the burner to fire up. When the switch is off it powers only the transformer which steps the voltage down from 120 to 12. This i then have wired out to the pellet boiler. I take the 12v from the oil burner and have that connected to a STSP relay which i use to power the circulator. Yes this does create a few seconds of lagg time because the hot water has to flow from 1 boiler to another but not having the circulator run 100% of the time saves a lot of wasted heat (even with the pipes all insulated) The user guide suggests keeping the water in the boiler over 125 i think it was. So to do this i employ an aquastate with imersion bulb fitting and simply wire it in series with the pump so if the water temp is below the set point on the aquastat the pump cant get power and prevents it from cooling off even more. This next part is where it is a little more complicated. Mostly because i really wanted to protect the boiler. This is where i prevent the boiler from getting out of control with its heat. i ran 2 wires form the living room zone valve and ran them out to the pellet boiler. 1 side goes to 1 contact on the aquastat the other wire connects to 1 wire of the coil on the DPDT relay. The other side of the coil connects back over to the aquastat. This way when the temp reaches set point the coil is charged up. Now i connected the NC connection of the relay to the power for the boiler. So in normal conditions this is powered up. When water temp is above set point this breaks the main power to the boiler. Then on the other pair of NC and NO connections i connected the NO connection to the pump and 120. With this configuration when the boiler reaches (mine is set for 200 but its adjustable) 200 the power to the boiler is shut off ( it takes a while for the fire to die completly but without the fan running it looses a lot of ghusto) and opens the living room valve and circulates the very hot water. The really nice part about this is how automated it is. Without this control when the pb105 gets too hot it simply shuts off and has to just very slowly cool off. It will not actually turn itself back on either. With these new controls in place when the pb105 hits the hi point the power to the boiler shuts off the circulator turns on and the living room gets a bit warmer (also alerting me and my wife something might be going wrong) and the boiler stays off until it is 20 (adjustable also) degrees below the set point. Then the boiler gets power again and resumes normal operation. Probably one of my biggest annoyances has been that depending on the pellets depends on where my feed rate has to be. Some pellets i need to leave it cranked at 5 or 6 some pellets i need to have it at 1 just so it doesnt overheat. I feel like i have to out smart the engineers that originally built this thing. Ok turns out i had an issue where my aquastat was not reading the temp properly for the boiler. Now it is behaving better.