# Adjustable resistor value for central heating thermostat

1. ## Adjustable resistor value for central heating thermostat

Hello,

I'm looking to replace my current central heating boiler remote control (it's not really a thermostat) (Viessmann WS in combination with Viessmann Trimatik MC) with an Arduino. This remote control is in my living room and tells the boiler which temperature I want for day and night settings. It does that by changing the resistor value between three cables: one ground and one for day temperature and one for night temperature. There's 24V (according to the manual both AC and DC, I assume this is because there are more expensive remote controls that allow you change more settings) on those wires. The resistors can be set as follows:
- night 287 to 317 Ohm = 7 to 23°C
- day: 290 to 318 Ohm = 14 to 26°C
The wires need to be 1.5mm2 according to the manual

My question is: how can I incorporate this in an Arduino? I can't use digital potentiometers directly on the wires due to the high voltage and currents. If I place them in parallel with a resistor, I assume I need to measure the current in order to keep the total resistance at the desired level? Or is there another, more straight forward way?

Pictures:

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2. ## Re: Adjustable resistor value for central heating thermostat

So you want to lower the heat with a resistor? That's a waste of energy and the resistor should be a very high power one.

You should use PWM to control the heating (or just turning it on and off to get an aproximate temperature). The temperature could be measured by a cheap linear sensor amplified and connected to your MCU ADC. The program is simple; when the temperatures is lower than the one you want, you turn on the heater, and you turn it off when you reach the required temperature. I don't understand why you need different resistor values.

About the timing control, you can use the MCU timer and some variables to count the time (use crystal oscillator if accuracy is required). Then the temperature range can be modified with one condition: Day or night. You must define those conditions, I mean you have to put a determined time (hours and minutes).

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3. ## Re: Adjustable resistor value for central heating thermostat

in the old days I used an HP 16 chan. reed relay with custom resistors to make a DAC for a floating remote control to Lambda power supplies far away. They were programmed in something like 200 Ohms per Volt.

Check the manual but it looks to be around 0.5 deg C per Ohm for both
Set the minimum value such as 18C and max 26C and then a 4 bit latch can be used to set the resistance. with 16 steps and 4 reed relays to Gnd. I believe. Check.

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