Software for ESP8266 or ESP32 based devices designed to prevent Mitsubishi Electric heatpumps from doing too short heating runs and de-frosting the outdoor unit every 30 minutes or so. This .yaml file is designed for https://ESPhome.io, which is needed to generate, compile and update sources. Please follow the information on their site how to set-up environments.

Many Finns suffer from the poor design of Mitsubishi heat pumps. In normal winter at -15 c degrees especially MSZ-LN series de-frosts every 30/40 minutes even when there is practically no frost in the outdoor unit, ie. no need to defrost. Frequent de-frosting leads to poor heat production and most likely very poor effiency (“COP”) compared to a unit where defrost happens only once every 2-3 hours under the same conditions.

This software started from a topic in the lampopumput.info forum: https://lampopumput.info/foorumi/threads/msz-ln-sulatushuijaus.31223/

The design of these Mitsubishi MZN-LN25/35 etc. heatpumps is poor both in software and mechanically. The software in the unit does not suit the Finnish climate - and most probably not any climate where the pump needs to be used for heating and the temperature goes below -5 degrees. The pump heats mainly with short cycles (under 60 minutes) and tries to melt ice from outdoor unit's heat exhanger even when there is no ice. This is a waste of energy and makes heating inefficient.

This is due to the poor “fuzzy logic” algorithm which:

• Instead of using temperature sensors it tries to predict de-frost need from previous cycles.
• The mechanical side of the probems are related to the defrost temperature sensor. It is used to detect when the ice has melted from the outdoor heat exhanger. The defrost temperature sensor is installed poorly so it touches non-waterproof insulation material which gets wet from melting water/condensation and then freezes. The frozen defrost sensor does not measure correct value needed for the “fuzzy logic”. Thus it starts to affect the device's “fuzzy logic” which starts to think that heating has been too long as outdoor heat exhanger is too icy and thus starts to decrease duration of heating… and this leads to a situation where the pump does not heat much but tries to melt the outdoor unit's heat exhanger often. Basically wasting energy by melting really often means it heats the outside air by taking heat from inside.
• Also looks like Mitsubishi has limited the temperature sensor's lower range to only -15°C.

MitsuRunner cheats the heat pump by switching a 33k resistor in place of de-frost thermistor (measuring outdoor unit's heat exchanger's temperature) of the heat pump. That makes the heat pump think that the temperature is above zero celsius and there is no need to de-frost. MitsuRunner itself uses two thermistors to measure the temperature difference (delta) between the outdoor heat exchanger and the outdoor air. When the difference (delta) is big enough, it switches the heat pump's real de-frost thermistor back and lets the heat pump de-frost. After de-frosting the 33k resistor is switched back until MitsuRunner triggers the next de-frost.

• 1x ESP32 or ESP8266 based controller, for exampple WeMos D1 Mini
• 1x USB power source for the controller
• 1x relay compatible with the controller, for example Wemos D1 Mini Relay Shield
• 2x waterproof DS18B20 temperature sensors, one meter wire is plenty enough.
• (Optional: 2x non-waterproof DS18B20 for backup in case waterproof sensors reacts too slowly)
• 1x 33k (33000 ohm) resistor to be switched in the place of de-frost thermistor (“hack” resistor)
• 1x 4k7 (4700 ohm) resistor as 1-wire pull-up resistor
• enclosure for contoller, suitable for outdoor use

And some basic hardware store items like

• wire (multicore wire like CAT-cable or phone cable)
• etc.