I briefly touched on using a humidity controlled fan in my bathroom install article. Here's some more details and an example of how well it can discriminate shower usage.
Here's my bathroom in the Cortex plan. Since I've moved to a new PC running XP and a larger desktop, I've resized the plan and changed to slightly larger icons. You can see the shower fan and shower light represented on the right hand side, with the light level, humidity and temperature below them. On the left is the motion sensor, the IR receiver and the over sink light. The main light is represented by the LD11 icon in the centre. The final icon shows the bathroom door:
The first thing is to set up the thresholds in the humidity dialog. It's quite helpful to have some data to work with, here's a graph showing two showers in relatively quick succession:
Since the graph shows pretty good transitions, I decided to control both the fan and the shower light from it. The first thing to do is set up the thresholds. Here you choose the transition points and the actions to take on each transition:
The graph shows very steep rising edges, so the actual threshold is not going to be that important, I selected 75% to minimise the chance of false triggering. It is possible to set a list of actions to take place on each transition, so I add both light and fan on the upward transition:
I want the fan to run on longer than the light, based on the data from the graph, so I set separate downward thresholds. The thresholds can have hysteresis in case the behaviour is a bit noisy, but the humidity curve is pretty clean, so it has not been needed here. The next thing is to set the bathroom fan for automatic control, this is done in the behaviour dialog:
The other thing that can be set here is an operating period. I've used this here to ensure the fan won't run in the middle of the night, whatever the provocation.
The last thing to be set up is the bathroom switches, to allow override or forcing behaviour:
Here you can see three different control mechanisms being used. I'll discuss the lights control in another entry, but you can see that the shower fan can be toggled from the button - either to force running or to turn it off early. In practice it hasn't been necessary.
So how well does it work ? To show this I have zoomed in on the humidity graph from the top of the page and also shown a graph of the fan activity over the same period. You can see that the control is pretty effective:
I have considered changing the shower light to also be controlled on presence, and then master enabled by the humidity thresholds. However, since the triggering just from humidity seems to work very well, this seems an unnecessary complication.
The one thing that I would quite like to see on the graphs is to have the thresholds optionally displayed. When checking operation this would be quite useful. However, since an arbitrary number of thresholds can be set up, some selection would be required to avoid the graph becoming over-cluttered.