Logamatic 2107 burner no start when boiler cold

gschabel

Greetings folks.  Here's a good one!



I am running a Buderus Logamatic 2107 with a Riello R40 burner that I maintain, and all runs well, normally. Boiler is a Buderus 250.



Periodically, when the temp of the boiler drops to around 80 degrees, (fall 50 degree nights after 70 degree days) the Logamatic appears to experience a "cold" lockout and will not fire up the burner. Not much thermal load is needed at these times. The circulators continue to run continuously, the flame appears on the LCD screen, the relay clicks, but no power to the burner. The boiler temperature continues to fall to ambient temperature around 60 degrees. Eventually the display will read Burner error., little help! Burner does not fire up in manual mode either. Pressing the High Limit reset does nothing and is NOT tripped. Powering down and back up makes no difference. Adjusting the aquastat knob on the 2107 does not help either. This situation also occurs when I purge/air bleed the system, causing the boiler temp to go below 80 degrees. In summary: Logamatic turns on, burner relay clicks, flame comes on LCD, and no power to the burner! Yes, the connections are tight!!



Here's the kicker.........  I then short out the two end wires on the high limit shutdown box to the left of the aquastat knob. Immediately the burner fires up! I keep the wires shorted until the boiler reaches a temp of about 100 degrees. I then remove the patch wire and the burner continues to run until it reaches the proper temp and runs fine for months on end, perfectly, or until I bleed/flush the system causing the boiler temp to drop under 80, or another marginally cool night comes along. Them I have to force the burner on again to get the boiler temp to about 100. Note that this only occurs a few times a year as well!!



Can someone tell me what's up with this? It really seems like a "cold lockout". I can't find anything about this in the manuals. I know about the safety high limit reset, but this is happening when cold!!  



I have found some similar posts but they seem to have had other causes like external aquastats, and DHW sensor issues.



This has me stumped!



Thanks in advance!!!!!!!!!!