Looking to determine whether gas water heater is on

[Corwyn]

I am working on a arduino based solar hot water system. One little side project is to determine whether the hot water I am getting is heated solely by solar, or whether the Bosch Aquastar tankless hot water heater has turned on to boost the temperature.

My first thought was to use a CdS light sensor, appropriately placed to see the light of the gas flame. The readings I am getting are too widely fluctuating to work based just on a threshold value.

Anyone have any ideas for a better way to determine whether the heater has kicked in?

Thank You Kindly,

Corwyn

[Zener]

PIR sensor. The kind they use in motion sensors. You will still get a fluctuating output but it should be usable. I never tried it though. You could also use a thermocouple I suppose. Or even a bimetallic temp switch "thermodisc".

[franklin97355]

Seems this is a great use of a thermocouple.

[Corwyn]

Thanks, I may try that. I do worry that getting close enough to the flames to be able to differentiate hot water from flames will be closer than the thermocouple enjoys being.

[Zener]

If it has a flue exhaust fan then you can sense that.

[adafruit_support_bill]

Or a even a flue exhaust damper should be detectable.

[Corwyn]

It has neither. No electrical connection at all. Thank You Kindly.

[Zener]

Did you try any of the other ideas?

There is also this from Home Depot

"Honeywell-Replacement-Thermopile-Generator-750-Millivolt-CQ200A"

I could not post the link because I got this message which I am not making up:

"Your message contains banned spam words. Please rephrase your post without the following words : xxxxx6986" where the xxxxx was 20318

[tastewar]

Sheesh, another guy trying to peddle that honeywell thermostat again! Will it never stop?! Guess we'll have to add "honeywell" or "thermostat" to the banned word list...