Is there any device that can detect usage of the hot water line? I want to be notified when somebody turns on the shower.
To detect if shower is turned on I would try it with a humidity sensor, e.g. from Aqara.
That will not be a realtime notification, but should be within seconds.
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Yup, I’ve a humidity right above the shower head. Works great.
Example to detect a (quick) increase of the humidity value:
Also a (Aqara) water leak detector instantly reports the shower running. It has one disadvantage, it takes hours (overhere) for the floor to dry enough for the detection alarm to turn back to false.
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As I use Plugwise for my heating, I can use the ‘state’ flowcard that indicates heating and warmwater usage whilst monitoring the central heater.
I doubt you have Plugwise, but perhaps you have some other way of monitoring your boiler / heater.
I use a sensor on my watermeter and a temperature sensor on the outflow of my boiler. This way you kinda know how much hot water usage there is (takes a few seconds for the temperature sensor to go over the threshold i set so it’s not 100% accurate)
I use the data straight from the boiler (combi-CV) using opentherm:
After 5 minutes shower the lights start to flicker, and after 8 minutes showering the temperature is regulated to an uncomfortable temperature. Works like a charm with my teenage kids!
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That’s a great way for the kids indeed 
but what if if you have some private time with the lady under the shower?
if the partner gets cold it is the right time to, “warm her/him” up
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Not a likely scenario with teenage kids living in the house.
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I have a smart plug at my CV, and when the hot water runs, the power is always higher than with standard heating. In this way I detect the hot water. Homewizard via a virtual device then measures the amount of hot water. And, of course, if someone flushes the toilet then that’s counted as hot water too…
I installed a flow-sensor in the hot-water pipe of the shower.
After 5 minutes 1 of the hue lights in the bathroom starts to blink.
2 minutes later a Popp water-stop turns off the hot water in the bathroom.
If the watertap is turned off during showering, the timer pauses.
Ofcourse you need the space to install the sensor and the flow-stop.
An advanced flow handles the complete process including a manual override button if I want to shower a little longer.
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Don’t worry….it’s a thermostatic valve, so no hot water is no water at all…
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Which flow sensor do you use?
