This is a common question SWMBO asks me, so I figured it's time to have it automated.
The Oven is a Rangemaster with Electric Ovens and Gas Hob.
As the proud new owner of an IotaWatt I can easily monitor the electical use of the oven circuit and work out if that has been left on.
The Gas hob is a bit more tricky. I don't know if there is such a thing as a gas flow switch, but I'm reluctant (and it's illegal in the UK) to mess with the gas supply feed. I thought about a temperature sensor somewhere over the hob, comparing to ambient in the kitchen, but that might be tricky to get located near enough to pick up the heat of a low gas ring and far enough away not to fry it.
Anyone done anything like this or have any "outside the box" suggestions? I know we can just get up and go and check the oven, but that seems defeatist! Would also be re-assuring if we are out of the house to be able to check.
I have looked at this problem many times and I don't think detecting burners being on is the biggest problem. We occasionally accidentally leave a burner on the lowest setting after cooking a meal. In the full brightness of the kitchen, the tiny flame is almost invizable so as we go off to eat, it goes unnoticed. For my situation, how would you decide when it's OK for a burner to be on and when it's not. Is a pot on the stove simmering for hours OK or not?
If this is about being on after you have left the house, how quickly and how forcefully can you be notified so that you can return to shut off the burner. You mentioned not being able to mess with the gas, but could you add a Dome shutoff to an existing gas shutoff valve?
I am really interested in what happens here and what others may have come up with.
1 How to detect the hob is on
2 How to write rules that correctly work out if it is a problem or not
I think for us initially at least, we just want to be able to check when we have sat down of an evening after our meal that it has all been turned off, so I guess after the motion sensors in the kitchen have been off for say 15 minutes and it's after 7:30 pm a rule could just check if it was all off and give us a TTS warning if not. We don't ever have the oven on when we are out so a pushover to my phone if anything was on when we were both out would work, or even better as the last one goes to leave.
Writing an app to cover most eventualities would not be a problem.
The problem is detecting the gas being on.
The only thing I can think of is tapping into the thermocouple that cuts off the gas if the flame gets blown out.
Not sure how easy/legal that would be
Possible alternative would be a temperature sensor in the cooker hood..
As you say, comparing against ambient temp in the room
You would need to test some scenarios to get some ‘base’ readings to work from
Purely brainstorming here. I wonder if there is any measurable difference in the temperature of the supply line when gas is flowing? If there is, a comparison with the ambient air temp in very close proximity to the supply line could give you an on or off.
Ok.. had a spare xiaomi temp sensor hanging around so have stuck that in the cooker hood.
Will let the temperature settle down for a few minutes then turn the smallest of the gas rings on and check the difference (if it registers anything)
For us I think we just have a collection of outliers. We may eat at 4:30 or 9:00 and everything in between. I would doubt we eat any 2 consecutive meals at close to the same times. We also work from home so we may each cook a separate lunch or breakfast. It will likely be a lot easier if your lives are more regular.
As for detecting, in addition to the temperature sensor in the hood, I had thought of running thermocouples under each burner. The thermocouples would be a very exact detection of a burner on condition.
If this works, then there are other things to think about..
If you are using all the gas rings (say cooking sunday lunch) then turn everything off.. how long before the hob cools enough to not register a different temp to ambient?
Trying to cover all eventualities will make writing the app complicated
I reckon once all of the saucepans etc are off the hob (ie anything with thermal mass), the temperature would drop fairly quickly. It's if it can detect the smallest gas ring being on that will be interesting.
That would be the most accurate solution I reckon, if it's feasible. As Andy says there are probably already thermocouples there but how you would tap into them I don't know.
In some ways, what you really want to catch are odd things. The easy things aren't likely to be issues. So you left the burner on low while you had dinner. You will catch it when you get back to the kitchen. When you leave the house, how quickly can the temperature drop be detected and the alarm sent.
Are there thermocouples anymore? Thermocouples were necessary with a standing pilot light, but at least in the US, with electronic ignition, I don't think there are thermocouples. There is in a gas oven, since the oven burners are turned off and on to regulate temperature.
Interesting potential breakthrough. I had a thought and I've just put one of my Hue motion sensors on the work surface next to the oven "looking" across the burners and it seems to be able to detect when they are on. It's damned hard to test with me having to move around to turn them on and off, but I reckon it is picking them up. With the smallest burner on the lowest setting it needs to be quite close to it but I reckon I might be onto something.
As far as detecting the temp change, I'm willing to bet that a very hot day would cause a greater rise in temp than a burner being left on the lowest possible setting. I'll be shocked if a standard temp sensor will be able to accurately and repeatedly differentiate the difference between that and ambient. There's just too much temperature variability in a room to detect such a small heat-source.
The only thing that would be reliable enough was some type of thermal camera like this one:
or this one:
You'd need an Arduino-type board (ESP8266 or ESP32) or a Raspberry Pi (even a Zero W would work) to run them, but this would give your absolute accuracy to detect the burner being left on at all. At $40 for the camera and say $15 for a Zero and required hardware, $55 isn't all that bad. This has the added benefit of allowing you to only detect when the heat source is very small...the burner left on, and ignore when the temp is high all over the place when you're cooking. Kind of like a camera motion sensor does that has light switch detection. A large enough area of high temp can be ignored. You would have to write your own program for it but like everything Adafruit, there is a python library for it.
In the UK yes,
To light the burner, we normally need to hold the knob down a couple of seconds before the burner will stay lit.
This is because the thermocouple need to warm up a little to keep the gas valve open on that burner.
This ensures that if an open window causes a draft and blows the gas out, then the gas valve shuts off to prevent gas leak
I have set up an Aqara motion sensor in the back left corner of the hob facing across the burners to the front right. It is reliably (so far) picking up even the smallest burner on it's lowest setting being on. It obviously also picks up motion in front of the hob, but that's not an issue as I'm only interested in when a burner is on and no one is in the room. So if the sensor is active and the other motion sensors in the kitchen are not, a burner is on.
We have an outdoor oven and firepit that operate that way but not cook tops. With the oven, I think the thermocouple operates manually with no electrical connection involved. I believe same somewhat happens with our hot water heater.
It’s a good idea, but you’re essentially using a Xiaomi motion sensor as a safety device. Unless you have that connection via a Xiaomi Gateway ( which is very solid), I personally would look for a sensor with a more trustworthy connection.
