I bought a nest gen 2 thermostat to play with a open source project that revives old nest thermostats (https://nolongerevil.com/). Since I don’t want to install it into the home, because it will be a toy. I was thinking of building a test rig using a arduino or esp32 to simulate a HVAC and indoor temperature. I’m IT guy, not a HVAC guy, I think this would be a good learning project. Any suggestions?
Everyone is over thinking the hell out of this. Almost all thermostats use a cheap NTC resistor, just change the resistance so the hardware thinks its at the whacky temperature. Or you could figure out the bias voltage the system uses through the NTC then apply your own external voltage to trick the system.
HVAC-R tech here.
Not sure what you mean about simulating high temps on the thermostat. If you want to trick the thermostat into seeing a higher temp than it is actually at then you would need to find the temp sensor on the thermostat (usually a thermistor) and replace it with something where you can manually control the input like a potentiometer if there was a thermistor there.
If you’re talking about simulating calls from your thermostat to your hvac system, then you can usually do that with just some jumper wires if your hvac system has a built in transformer (almost all new systems do). You just remove the thermostat and jumper the hot wire (R or Rc) to whatever call you want to make.
Edit: I should probably note that if you accidentally jumper anything to ground or to common then you will likely trip the breaker or blow the fuse on your systems transformer. If you do that then you’ll need to find that transformer (usually in the airhandler, assuming a standard residential split system) and reset the breaker or replace that fuse before your system will work again.
I used to work as a PQI contractor at Nest, and we actually had test setups like this in the office that were just a circuit breadboard mounted on a plate behind the thermostat. The thermostat doesn’t really “communicate” with the HVAC control system at all (all it does is just send ~3V along the circuits based on its current mode), so as long as you have the circuits routed properly on your board, the thermostat will think it’s connected to a real system.
The stock firmware doesn’t let you go above a certain temp (like 80F or something, I forget the exact limit). But if your custom firmware allows it, the only thing that would realistically happen is that it just runs a 3V circuit to what it thinks is the heater, infinitely, since nothing is actually causing the ambient temperature to raise at all. The ambient temp reading comes from a sensor on the thermostat, itself.
At those temps I can take a pretty good guess as to what it would do:
Melt.
That’s right around the melting point of some types of lead free solder.
Yeah I want to simulate the temperature readings. No blow torches or extreme heat preferred.
Any particular reason? Just for funsies?
Cuz I mean, if you really want to measure oven temps you can get thermometer probes that can withstand that kind of heat to make sure your food is at temp. No simulation required.
Of course if it’s just an engineering project to learn more about building the device that isn’t gonna help lol
It would be fun to torch a google device while laughing psychotically.
If you’re just simulating the temperature there’s pretty much just three options:
- The thermostat will simply command the HVAC system to maximum cooling, just like with any temperature well above the setpoint.
- The thermostat will treat that value as a faulty sensor, report “error faulty sensor”, and not do much in particular.
- The thermostat will crash due to an overflow of some sort as its software was never tested with inputs like that.
Personally, I’d go with, “error faulty sensor” as the most likely outcome.
(Edit: you can stimulate the temperature by setting your house on fire. Better to just simulate it)
I do want to stimulating the temperature. Also interesting theory, it would be cool to put it to the test. But I need some way to simulate a thermistor. Also, The testing rig I want to build could be a cool demonstration piece for how a thermostat works. I might donate it to some tech school when I’m done with it. Time will tell
thermistor is just a resistor that has resistance varying with temperature. often that relationship is linear over a reasonable range, like with Pt100, and you can probably get away with using potentiometer instead. that said you have to figure out what you have there, because there are many options
You need a variable resistor with digital control.
It contains lots of plastic, that will melt. Also, electronics are designed to work up to 80°C or so, then there is some safety margin, and then they will also just cease to function correctly.
I don’t think anyone designed it to do that, becsuse 220C is like 100C above critical point of some common refrigerants (evaporation/condensation stops being a thing at this point and heat can be transferred only from hot to colder and not very efficiently)
