Wednesday, October 2, 2019

Homematic - Automated bathroom ventilation - Proof of concept

My house is using a centralized ventilation system (a WHR950) with three operating modes. You're supposed (!) to switch to mode 1, 2 or 3, using a switch in the leaving room, whenever needed... which no-one ever does.

I was wondering... could I automate this?


The WHR950 (a balance ventilation system that delivers fresh air to the living areas and sucks out old air from the toilets, bathroom and kitchen) has three positions:

  1. Low - when there are no people in the house
  2. Mid - when there are people (default)
  3. High - whenever someone is cooking, taking a shower, or in the toilet

You're supposed (!) to switch to mode 1, 2 or 3, whenever needed... which no one ever does. I talked to my neighbors about this, and yeah, same thing there.


Keep in mind that the WHR950 is fairly noisy (thanks to stupidity of the building company who used the wrong size of piping, probably to save a bit of money, but that's a whole different story). So, what you definitely don't want is the system running in 'high' mode when someone takes a 'dump' at night, or a late shower.

So, rule number one: don't switch to high speed at night.


It's all about the bathroom but perhaps I could combine this with some other ideas. I could use a light detection in the toilets... which wouldn't work when someone doesn't switch on the light... Or an IR detector. Yet both would mean more sensors. Not worth it (though some of my fellow humans do smell 😎).

I think I'll add one central push button which, when pressed, switches the ventilation system to 'high' for 15 minutes. Just have to teach my housemates to push the button after they've answered nature's call.

Rule 2: switch 15 minutes to high speed after a button push.


Recently I noticed some stubborn mold in the bathroom (the little black spots). Typically these are a sign of insufficient ventilation.

It would be best to switch the system to mode 3 when it's getting too moist in there, which can be handled by a humidity sensor.

Rule 3: speed up during day hours if humidity too high.

Note it took the mold 10 years to show up. It's not super critical it seems.

Presence detection

It is possible to save some energy by switching the system to mode 1 whenever nobody is at home. That isn't that simple, unfortunately. Checking for mobile phone mac addresses isn't fail-safe,  but perhaps I'm able to read the alarm system status. Something for later, although the savings would probably be very limited, let's say 5 hours per day, 5 days a week:

    5 hrs x 365/7x5 days (only weekdays) x 0.06 kW x 0.20 e/kwhr = 15 euro per year

Not enough for the effort, unless I more or less get it 'for free' when combined with some other function... perhaps in combination with a 'presence simulation'?

Food for thought.

Rule 4: switch to low speed if there's nobody home. (No clue yet how to handle that.)

Positioning the sensor

It would be best if I could put the sensor somewhere in the air duct, or somewhere in the air flow between the shower stall and the air duct inlet. Unfortunately the air duct starts directly above the shower stall. Good for ventilation, bad for detection. Actually great for detection, but I bet an indoor sensor would fail rapidly.

I'll just have to settle for room humidity and place the sensor somewhere else in the room on a wall.

In hindsight perhaps I should have considered an outdoor sensor... Though I have a feeling those wouldn't survive much longer. Warm vapor clouds from showers are not the same as outdoor rain or fog.

Proof of concept

Will bathroom humidity detection work? Perhaps... for that I needed to collect some data and put that in a diagram.

I had some issues with missing data due to an incorrect default setting (more about that here) but ended up with a diagram. It's by the way amazing how long it takes for the in-room humidity to drop (without extra ventilation) after someone took a shower... Regarding shower duration, I think the sensor location is causing quite some delay, but it's probably going to work anyway (click to enlarge):

Notes & Conclusion

  1. Some people take longer showers than others 😑
  2. Sensor is NOT directly in the airflow, but elsewhere in the room
  3. With ventilation switch in position 2 (mid speed) it can take 6 hours to reach pre-shower humidity levels
  4. With ventilation switch in position 3 (high speed) humidity drops much faster (I estimate about twice as fast) but does level out after a while
  5. Empirical testing has shown that running on high speed for the first 15 to 30 minutes has most impact on humidity

So, the concept seems to work. Now I have to think how to wire up the WHR950 to my Homematic system...

Update. Fixed that too, see here.


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