I’m thinking it’s basically just a voltmeter across a switch, right? I want something simple to see when the furnace is on, when the AC is on, etc.
Not exactly. And with many of the things you mentioned they’re high amperage or multi phase current - absolutely not ‘just a voltmeter’ typically clamp-style inductive load sensors.
You MUST know the range of the device you’re sampling to find the right things so specifically what do you want to monitor… (the more detailed the better, and your geography matters because power and RF requirements vary by locale.)
The clamp is one thing and if you can (and do not mind) soldering then you could connect any device to the power switch of the device and notice when it comes alive.
You didn’t define “cheap”… if 15€ is cheap for you then you have devices like shelly PM series, which can monitor power consumption (they exist without relay, too, only power monitoring —> One example).
I doubt you’ll get anything much cheaper than this. You can use it “as is” or you can replace firmware with esphome. Gen3 is somewhat more complicated, but it can be done.
I monitor my boiler’s burner for runtime (on/off) was a relay powered off the input terminals of the burner. I’ve mentioned it here before, including this thread just the other day.
Once you have the relay, you have a few options you can connect to the contacts on the relay. A door sensor has a reed switch inside which you can replace with a wire to the contacts. Or connect the contacts to the GPIO pins on an ESP device running ESPHome.
A CT clamp and ESP device is probably a more elegant solution, but a bit more fiddly to set up.
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If you are really just looking at HVAC items, get a connected thermostat.
I use these optocouplers to monitor my Heating and hotwater controller. Connects to an ESP8266 as part of my ‘smart’ controller.
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