Before we get into discussing the various Viessmann controls, there are many common features of the controls, but the most important is the 41 plug. Therefore we will start with a discussion of its importance, especially when it comes to troubleshooting no-heat calls when there isn’t a burner fault or an active fault of the boiler control.
One of the first best troubleshooting lessons all good techs should learn about the Viessmann controls and their influence on the power burner is the 41 Plug. This plug is also known as the “Call For Heat” circuit since the boiler control sends a 120-volt signal to the burner when the boiler temperature is below the setpoint temperature.
Using the basic schematic below the 41 burner circuit operates through the following steps:
Power is established at the 40 plug of the boiler control
With the Power (On/Off) switch set to ON, the power continues through the fuse (6.3 amp)
The fixed high limit (manual reset safety aquastat) allows power to continue down to the burner control (in this case it is an LFL, but it could be the LAL or Viessmann burner control) via the L1 terminal
The power is returned to the boiler control from the burner control through the T1 connection on the 41 plug
The power will continue through the Adjustable High Limit (AHL) operating aquastat if the boiler temperature is below the AHL setting.
The power is then sent to the K1 relay, which is the burner relay
If the AHL is closed and the control calculates a higher setpoint than the actual boiler temperature, reported back by the #3 boiler sensor, the K1 relay closes
The K1 allows power to travel through the T2 terminal and to the burner allowing the burner to operate.
When the burner is firing, power is then sent back to the boiler control via the B4 terminal. This is the signal to the hour counter and also records each burner start. (this is helpful in diagnosing short cycling: Burner starts divided by run hours equals the number of burner starts per hour or put another way Burner run hours divided by the number of burner starts equals the average burner run time each time it starts)
To be clear, the image above shows a burner terminal connection, which will be different for the various types of burners in the field.
Video Explanation: 41 Plug Call for Heat
Presentation Showing the 41 Plug Call for Heat Path (Provided by Greg McNeil. Thank you, Greg!)
NOTE: For the full effect of the presentation, please view the Slideshow!