How to set up a basic humidistat on KNX: a spare panel thermostat instead of a separate device

A humidifier needs a humidistat: something that switches it on when the air is dry and off once the humidity is back. A dedicated KNX humidistat is one more device on the wall or in the cabinet, and one more line on the quote.

Often you can do without it. KNX panels and sensors usually have built-in thermostats, and not all of them are in use. A thermostat does not care what it controls: it compares a number with a setpoint and switches an output on or off. Feed it humidity instead of temperature and you have a humidistat.

Below is how this was done in a 72 m² apartment with five climate zones. Each zone has a Zennio Z35 v2 panel with a built-in humidity sensor and a humidifier switched through a relay on a Zennio MAXinBOX 24 v2 actuator. We show one zone; the other four are set up the same way.

What it has to do

• Humidity below the setpoint — the humidifier runs. Setpoint reached — it stops.
• The panel shows the current humidity. The setpoint can be changed, and humidification can be switched off altogether.
• The panel shows whether the humidifier is running right now.
• The relay does not click every couple of minutes.

Why a thermostat can do the job

A thermostat in “Heating” mode is simple: value below the setpoint — output on; above — output off. A humidifier is to humidity what heating is to temperature: switch it on and the value goes up. So “Heating” mode is exactly the humidistat mode.

KNX sends temperature and humidity in the same format, a two-byte floating-point number (DPT 9). To the thermostat, 40 % humidity and 40 °C are the same number, 40. It does not need to know that it is a percentage.

Seven group addresses

The humidistat needs seven addresses: command and status for on/off, command and status for the setpoint, the humidity, the actuator channel command and its status.

Step 1. The humidity sensor

The Z35 v2 has the humidity sensor built in. Its parameters are on the “Humidity → Configuration” page. The screenshots come from a Russian-language ETS, so the tables give the parameter names in English.

Step 2. A thermostat as the humidistat

The Z35 v2 has two thermostats. The first one runs the room temperature, the second is free, so that is the one we turn into a humidistat. ETS labels the setpoint in °C, but that is only a label: the thermostat works with a number.

Thermostat 2 → Configuration

[System 1] Heating control

The result is a 37–41 % band. The lower hysteresis is wider than the upper one, so the humidifier starts less often and runs longer each time, and the relay does not click on every small swing in humidity.

Thermostat and sensor objects

Address 5/0/18 has type DPT 9.007 (humidity), while the “Temperature source 1” object is DPT 9.001 (temperature). ETS lets you link them because both objects are two bytes in the same format. Two objects listen to this address, the thermostat and cell K6 on the screen, which is why it shows three links.

Step 3. The relay and the humidifier

The humidifier is wired to the relay through its ON/OFF control input, a dry contact. Its power supply stays on; the relay only enables or disables it. That way the relay never switches a power load, and the humidifier starts and stops itself the way it is meant to.

Relay objects

Output J19 takes its command from 5/0/19 and sends its status to 5/0/20. The other four zones work the same way on outputs J20, K21, K22 and L23, each zone with its own address group.

Step 4. The panel interface

The Z35 v2 itself deserves a word here. Its pages are built from cells, and each cell can be given its own function — a button, an indicator, a number field, an enumeration — and its own label. That lets you build an interface on the panel for almost anything, not just lighting and climate.

Four cells were enough for the humidifier.

What happens in operation

The table doubles as a test plan with the ETS group monitor.

Things to keep in mind

• Calibration. Check the panel sensor against a hygrometer before you pick a setpoint. The sensor measures the air at the wall where the panel hangs and can be noticeably off from the real humidity.
• The humidifier’s own humidistat. If it has one, set it higher than the KNX setpoint. Otherwise the humidifier stops before the thermostat decides it should.
• Where the humidifier stands. Keep it away from the panel: a stream of moist air on the sensor switches the humidifier off before the whole room has reached the setpoint.
• Bus failure. With “No change” the relay output stays as it was: if the humidifier was running, it keeps running. If that is not acceptable, set the output to switch off.
• Labels in ETS. The setpoint and hysteresis are labelled in °C. That is not a mistake; the thermostat simply works with a number.

Commissioning checklist

• Humidity sensor checked against a hygrometer and the calibration entered
• Humidity sending period set (60 s here)
• Thermostat 2: Heating, Basic, Temperature source 1, “Thermostat always on?” — No
• 2-point control: 3 % below, 1 % above
• Humidity linked to the thermostat’s temperature source and to the indicator on the screen
• Control variable linked to the relay output, relay status linked to the indicator
• In the group monitor: the humidifier starts below 37 % and stops at 41 %
• Humidification switched off while the humidifier was running: 0 went out on 5/0/19 and the humidifier stopped
• Bus power switched off and on: setpoint and on/off state kept

When this is not enough

A spare thermostat makes a simple humidistat: on when it is dry, off when it is enough. If humidification has to take other things into account — airing, the air conditioner, a night mode — that logic is easier to keep in a controller. In this same apartment that is how the air conditioners work: a Wiren Board runs them, while humidification gets by without it.