Disaster recovery and virtualisation

[toggledbits]

@rogero Here's a version that latches on once tripped and resets itself when the thermostat stops calling for heat.

In any case, this was a fun distraction. And really, this is minimal mucking. I know it looks daunting, but probably the worst you can do if you screw it up is (a) it doesn't work right, or (b) you blow the cheap 3A fuse on the 24V circuit in the unit (just replace it). If you've blown a thermostat with the resistor version, you've already done worse and more expensive than you are likely to do with this.

You can do this simple logic in an ESP32 or ESP8266 quite easily. The problem you always have with such things is getting them powered. You usually end up with a wall wart that's prone to failure. And since the micros are 3.3V volt with low current handling on the I/O pins, you need an interface to drive a relay (more parts or a board that carries both the I/O interface and relay), and you need some kind of enclosure to mount the pieces in, connectors to bring the wires to, etc. IMO, it quickly becomes more complicated than the relay alone. But for sure, you have to what's comfortable for you.

[RogerO]

I like that design. Actually don't even need to muck around inside the HP. Can intercept the thermostat wires where the come into the furnace closet. need to measure the pipe diameter in the HP and see about getting one of these. http://senasys.com/product/2570l211

Thanks,
Roger

[toggledbits]

They sell that temperature switch on Amazon.

[Elcid]

Hi, would it not be simpler to add a second external thermostat on the return loop, which mimics ( connects to same wires) the external stats functions. So if loop goes below set point it switches to aux heat

[toggledbits]

Depends on what you mean by simple. Since he's measuring the temperature of a water loop, it would require a temperature sensor external to the thermostat on (attached to) that plumbing, so right there, you've reduced the number of available thermostats for the job to those that support external sensors, and that remaining set is likely a bit costly. And you still have to get 24VAC power to that thermostat. Still wiring, really all the same wiring, in fact. An expensive sensor and relay with a front panel, is all a thermostat is, so on net, I think you're paying a lot for a UI you don't need.

[therealdb]

I don't know in US, but here in Europe mechanical thermostats are quite common and very cheap (I'd say around 20/30 Euros). They usually support external contacts, as in a refrigerator, and are like the picture I'm attaching:

[Elcid]

There are plenty of 24v thermostats with remote sensors here in UK (about £30). The OP wanted "Trying to determine the best platform for a set it and forget it black box that works with as close to zero maintenance as possible."!

So my thinking is thermostat in tandem with external sensor, A little wiring job done. You could also probably use a shelly 1 with temp sensor addon if you want feedback.

[akbooer]

@elcid said in Looking for ideas on how to implement an automation.:

You could also probably use a shelly 1 with temp sensor addon if you want feedback.

Now you're talking!

[RogerO]

Appreciate all the discussion. I like Patrick's third diagram, however, looking at discs available I haven't found any with a 5/8 inch diameter connection. Additionally, they are not super accurate according to reviews on Amazon. I might use an ESP32 board with a DS18B20 probe and a DPDT relay in place of the snap switch unless I find one available in my search.

[RogerO]

So, I have looked into the snap disk thermostats that are shaped to connect to a pipe and found, for the one that meets my specs, that the minimum order is 100 units. Additionally, comments in the interweb indicates that they are not all that accurate. Have been thinking about my design and have come up with this.

I plan to connect a 24VAC to 5VDC step down buck converter to the Venstar common and Venstar Y wires as shown in Patrick's second drawing. Using that to power an ESP32 board connected to a DS18B20 sensor and a relay controlled by GPIO on ESP32.

Logic on this design is pretty simple: Monitor loop temp and switch via relay to aux heat source when temp falls to trigger temp.
Remain in AUX mode until thermostat shuts off and powers off ESP32.
Next time furnace starts, relay defaults to Heat Pump mode until temp drops to threshold again.

Questions for those of you with HVAC and ESP32's:

1. Will the 24V AC powering the thermostat supply enough power to run the ESP32?
2. Are the ESP32's able to withstand multiple power cycles without getting corrupted?

Input's to my design are welcome.

[toggledbits]

The typical fuse on 24VAC control circuits in residential HVAC systems in North America (lot of qualifiers there, sheesh) is 3A, so you're good. I don't think the power cycling will be a problem for the ESP, but YMMV on your buck converter.

[RogerO]

Thanks, at 4 for $15, I can replace them cheap.