I am saying that if you really want a belt-and-suspenders approach, you could use a thermal cutoff to (say) cut power to the relays calling for heat. Do I personally think that is required? No, I don’t really but it is your house & family etc. I know my current furnaces have thermal cutoffs (I’ve seen them) so I don’t worry.
If you’re in a serious mood, don’t read this! …
@timb – I think what you also need is an off-site backup controller, in case the part of your house that holds your Sparkcore controller gets nuked or something. Let’s set up a hot running spare at my place here in New Zealand. You’ll just need to add a separate Sparkcore somewhere to send my unit your backup temperature sensor readings. Put that one inside a vented concrete and lead-lined box, I suppose. Then we can work together to come up with a protocol whereby the off-site backup at my place can take control when it thinks something is wrong, whether you like it or not. Yeah baby. 
Deal?
Awww. OK. Sorry guys. :’( I just couldn’t resist.
The possibilities with Sparkcores and the IoT are mind boggling! 
We could devise a kind of mesh control network of thermostat controllers so reliable that we wouldn’t even need furnaces any more, for all the BTUs our little regulators would be pumping out.
OK. I’ll shut up and let you get back to work now. hehe
Man. You’re building exactly what I’m building. I’m a bit luckier though I think. I’ve got access to 8 dampers from my basement. Nice and close to the source so I don’t waste the energy heating the ducting all the way up to the registers.
My plan.
- Temperature/humidity sensors everywhere. Ideally one per room. I’m using the DHT22 with a spark core.
- I want to build these into light switch boxes so they stay pretty well hidden…and powered.
- All data is gathered in my cloud service at http://sccb.azurewebsites.net
- Have a core near all the dampers and have it control a stepper motor connected to each. I’m hoping I can control 8 motors with one core. TBD.
- Have the cloud service trigger function execution when temperature values reach certain thresholds. These functions are to open and close the dampers.
That’s the super basic logic version. From there I’ll analyse the effects and evolve the logic.
Today I’ve got data gathering from one sensor with three more on order and got a stepper motor controlled from my cloud service.
Now to figure out how to mount a flat faced stepper motor on a round duct.
OP here. Go back and read the thread. There was nothing wrong with my HVAC unit and it was relatively new to boot. The thermal safety cutoff in gas units trigger between 165-170 degrees. If there was anything wrong at all with our unit, it was that it was a bit over sized for the home (which is common) and as a result the fan was able to pump enough air volume through the system so that the unit wasn’t much hotter than the hottest locations in the home, my daughters room happening to be the hottest because of the greatest airflow.
In terms of inspecting, an inspector would find that the unit has a safety cutoff. There is no way to test them as doing so requires they be replaced because they are thermal fuses. Further, it doesn’t matter because it triggers to stop a fire, not to stop melting things and causing medical emergencies. As far as the thermostat, an inspector isn’t going to pull out a JTAG interface and do a boundary scan of every chip in the thermostat so there’s no way they’d catch that a critical component was nearing a failure point.
In terms of melting items in the house. Do some Google searches. It happens more than you might think and in fact my insurance agent was even aware of cases his firm had paid-out on when I’d mentioned it to him.
Here’s a case as an example:
Things go wrong enough that there’s even been consideration towards legislation:
http://www.achrnews.com/articles/print/123645-acca-others-oppose-thermostat-law
Come on dude, don’t be a buzzkill. We’re way past that now. We’re building things here man!
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LOL! That made my morning - thanks for that! In reading through the thread that’s grown from here I do have a couple observations. First, when used with a heat pump with an inductive backup I think the risk of harm is reduced although I do worry about an unintentional or intentional oscillation of the reverse flow switch in the pressure line. Accidentally leaving on the induction coil without the unit running could also be an issue. Gas furnaces are my primary concern because of their sheer ability to quickly pump out the BTUs. Second, I think your initial reply about my possible over reaction (btw, the temperature was confirmed with a handheld IR thermometer and it was above 120 in many spots) did in fact touch on a very important point. You made the assumption that for my daughters room to get so hot that the unit itself would be tens of degrees hotter without necessarily considering the size of the air handler on the HVAC unit or the plumbing of the ducts. These days if anything, HVAC installers error in putting larger air handlers onto units with less BTU handling. As a result, the temperature by the thermal fuse is in fact not necessarily significantly hotter than the actual home.
My warning at the time was to head off problems like I ran into as there was a lot of buzz happening in the tech press and whatnot without any discussion of safety. I think this thread has accomplished bringing the topic of safety forward and there are some good ideas surrounding secondary independent switches which trip at a much lower temperature. My advice is pretty much this. If you are single, don’t have pets, and like to tinker, go for it. If you have a spouse, children, and or pets think a bit about what you are doing and make sure you understand enough about electronic fail safes and the dangers of HVAC units before proceeding. The singular cheap control which trips at too high of a temperature in your HVAC unit will not save you so don’t consider it a control. They are a last resort intended only to keep the house from quickly burning down. If you do proceed and screw up, consider the ramifications, even if no one is harmed it’s going to be a lot of explaining to the spouse.
patcheudor, we get it. In fact we got it a long time ago. Instead of lamenting, let’s make this about ideas to improve safety. You talk about the DANGERS but the purpose here is how to MITIGATE those dangers. Seatbelts were put into cars because too many people died and insurance companies complained. Nothing changes until the pain of change is outweighed by the pain of staying not changing. Please, use this topic to propose solutions and be the genesis for change!
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I get that & my response was to timb who made some assumptions and discounted the risk based on a lack of understanding of HVAC system diversity & fail-safe controls.
I’m glad the OP came in here and slapped around @timb a bit… it was necessary 
I mean, you are all hijacking the OP’s thread which originally served as a warning for those building thermostats. If anything, the continued posts are diluting that warning, especially when you are effectively brushing off his continued discussion. Last time I checked this was his thread
I don’t see any reason you can’t disagree with him, but it’s not really polite.
I would suggest branching off from this thread and start a new one. It also might be a good idea to link back to this one for a rollercoaster discussion on safety.
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BDub et al, I agree with starting a new topic where we can discuss how to IMPLEMENT “safe” designs. Safety is not just about life and limb but also property… This could lead to concrete methods for code and hardware. 
With wifi, a cloud dependency, and a maturing platform, the 
does present a new set of safe design and safety issues. Not that it's unsafe, but mitigating issues such as a cloud disconnect, a buggy TI firmware, and other what-if scenarios would make for some enlightening challenges and learning exercises. "What if my relay melts down and the Core keeps resetting due to TCP packet loss while connecting or maintaining connection to the cloud?"
Last time I checked, it wasn’t impolite to disagree with someone or have our own set of opinions. Not that we are even disagreeing with his overall point! His post served as a warning about the dangers of a failed thermostat and our posts are about trying to account for those dangers and build something safe.
@patcheudor I didn’t misunderstand anything about HVAC failsafes. I did make a few assumptions on your specific situation, but we’ve already covered that and moved on. I apologize.
The reason I revived this thread instead of starting a new one is because I thought there was validity to your warning.
So, we can do one of two things:
-
We can start a new thread, where nobody will be able to read your warning in the first post.
-
We can continue here.
Since it’s your thread, I’ll let you decide if you want us out.
Not all of them, thus the need for a new thead in my opinion.
I'm not going to argue about it any further... do whatever you feel is right.
The first two quotes are my opinions/assumptions and we’d already covered that. The last quote is me pointing that out.
We had a good discussion going and now look. This is what I was trying to avoid. Ah well, I’ll just go ahead and do up a new thread, no worries. 
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On the topic of smart thermostats, I came across this thing on Kickstarter, might be interesting for some of you fiddling with their systems to get some inspiration or whatnot from.
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Searching the page for the word "safe" yields 0 results.
I asked them: "What has been done in software and hardware to make the Kumostat fail-safe?"
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There was a kickstarter project, plug.ee, did used the Spark Core but did not get funding (asked for too much $$ IMO). They created a safety feature using a dual controller approach called a "proxy":
The proxy controller manages housekeeping tasks like automatic thermal shutdown or current compliance (a circuit breaker) and enforces very strict safety policies. The firmware of the proxy controller is locked and cannot be changed after manufacturing to satisfy safety requirements.
I believe this is one piece of creating safe designs. I bet Weemo doesn't do that and I suspect neither does Kumostat.
Again, if you have a smart thermostat, you can add a home automation controller for extra security: if the outside temperature is below, say, 60deg.F, raise an alarm/try shutting down the heater if one or more interior temperatures goes above, say, 80deg.F. With this, I don’t think you even need to check to see if the heater is running.
Our daughter did die from a situation similar to this. Can you please contact me? 8173089193 or kerivolmert@gmail.com
Well our’s was serviced, only two years old, and our 17 month old daughter died from a malfunction!! His warning is right on and completely justified!!!