How to use the XH-M452 temperature and humidity module

This very common general purpose module has two sections - temperature and humidity.
Each section has its own volt-free relay contact. The heating section can control either heating or cooling (but not both) and the humidity section can control humidifying or dehumidifying.

If switching mains power or high loads I recommend using an off board relay with a 12V coil switched by the module's relay contact.
For safety reasons I always recommend having an extra safety thermostat in line with heating elements in case of relay or circuit failure. The safety thermostat can be a simple bimetallic one with a fixed limit temperature.

Each section has its own two buttons underneath its three digit display.

To set the heating/cooling mode and temperatures, start by holding the left button down until the display starts to flash. You can then adjust the relay-on temperature, with the left button incrementing and the button next to it decrementing the temperature. Holding a button down makes it count quickly after a slight pause. If you pause between presses too long the setting is stored and the display reverts back to normal mode.
Then set the heat/cool relay-off temperature by holding the second button from the left and adjusting as before.

If the on temperature is lower than the off temperature it will act as a heating control.
If the on temperature is higher than the off temperature it will act as a cooling control.

Then set the humidity settings in the same way using the two right hand buttons under the humidity display.

All settings are stored in non volatile memory.

Pressing the buttons briefly in normal mode will display the current temperature/humidity setting on that button.

To reset all the settings to factory defaults, press all four buttons down together until the display shows 888888.

Take care to mount the sensor module where it can't get wet or contaminated with oils or solvents. The humidity membrane may have its accuracy skewed if it is contaminated.

The search keywords to find these modules are "humidity M452". Typical cost for both the control module and sensor as a pair should be less than £10 shipped.

An interesting use for this module could be to wire the two contacts in series to directly control a small positive pressure ventilation fan. That way it could be programmed to turn the fan off if the air was too cold or humid.


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