Słomkowski's technical musings

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Buderus 2107 as an electric water heater controller

Originally published on . 2 min read, 470 words.

I adapted a Buderus oil furnace controller to operate an electric immersion heater as a temporary solution for domestic hot water (DHW) in my house.

In autumn 2024, I remodeled the boiler room in my house. I replaced an old Buderus oil boiler with a new Bosch boiler fueled by natural gas. During this time, I needed to have domestic hot water available. As a temporary solution, I used the Buderus 2107 controller, which had previously been attached to the oil boiler, to control the electric immersion heater in the hot water tank I had installed.

Buderus 2107

The Buderus Logamatic 2107 is a house heating controller that manages the furnace/boiler, hot water production, and up to two heating circuits. My device is also equipped with two modules: the FM244 for solar collector support and the FM241 for the second heating circuit with mixing.

As usual, links to the documentation:

By the way, there exists a third-party Wi-Fi module for this controller.

The fix

During the remodeling, I removed all the equipment in the room except for the DHW tank and the solar collector piping. However, the solar collector didn’t provide enough heat on cloudy days, so I had to add an electric immersion heater. Since it was a temporary solution, I opted for a budget-friendly option: I bought a cheap 2” three-phase electric heater from Aliexpress and connected it to the mains through a contactor LC-Tec MC-25/40. I connected the contactor’s coil to the PS output on the controller, which drives the DHW tank pump.

Chinese 2" electric immersion water heater.
LC-Tec MC-25/40 contactor.
Buderus 2107 controller with resistors for boiler and external temperature.

To trick the Buderus controller into driving the electric heater, I needed to emulate the presence of the oil boiler. The controller has several temperature sensors to measure the water temperature in various parts of the heating system. However, with the oil boiler gone, I had to simulate these sensors.

The controller operates as follows: when the DHW tank temperature is lower than the set-point and the boiler temperature is sufficiently high, the controller turns on the tank pump (PS on the schematic). I set my hot water thermostat to 45°C, so the simulated oil boiler temperature needed to be higher. Interestingly, to have water circulation working, the outdoor temperature sensor was needed as well. The table below summarizes the necessary resistors:

Symbol: Input: Resistor: Description:
FK Boiler temperature 2.2 kΩ Simulates temperature about 60°C.
FV2 Supply temperature 2.2 kΩ 60°C, required if mixer module FM241 is present.
FA Outdoor temperature 22 kΩ Simulates about 7°C.

The temperature sensors that can’t be simulated are: