>>> 2022-06-22 thermostats

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2022-06-23 18:30:08

Let's discuss the humble thermostat. You probably have one in your house, and it probably connects to a set of wires. If you've ever replaced your thermostat, you've probably found those wires a little irritating due to the lack of well standardized nomenclature for identifying them. This is particularly clear in the new generation of smart thermostats which attempt to be "consumer-friendly" to install, and thus must have sort of complex install wizards (InstallShield (R) for Thermostats) just to generate your hookup instructions. So what's up with that?

Your house is full of a bunch of 120VAC wiring. Well, that's assuming you live in the United States, and to be fair US residential wiring is typically 240v split phase, so you have both 240v and 120v wiring, depending on how you count. The idea of this split phase thing, if you're not familiar, is that the utility delivers to your house 240VRMS AC with a neutral wire that is at a potential halfway between the other two pairs. We could label this -120V, 0V, and +120V, which while "0V" is always arbitrary makes some sense since neutral is bonded to ground. These are all of course VRMS, which in this context is Volts Root Mean Square, not Virtual Richard M. Stallman (which is a piece of software that chastises you for being complicit in your own subjugation). Since AC implies a voltage that changes constantly, there are a few ways to measure, and VRMS is conventional. 120VRMS is about 170V peak to zero, or 340V peak to peak. We call it 120V because, well, that 170V only exists briefly at the two peaks of the waveform. 120V is a more useful number for actual power calculations, although AC power calculations can always become a bit complicated because the phase relationship of potential and current can vary (this is called power factor). This is all basically an irrelevant tangent, the point I want to make is that we all understand that residential electrical wiring is 120VAC or 240VAC depending on how you look at it [1]. But after all that, what if I told you that it is also conventional for residential electrical systems to have a low-voltage AC supply?

Well, it's true, but in sort of a limited sense and with a lot of variations. Almost all homes have at least one small transformer mounted on the side of a junction box in a basement or closet that produces 12-24VAC. There are two standard residential applications of low-voltage AC: the first is the doorbell, which typically uses 16VAC although 12VAC and 24VAC doorbells also exist. The second is the HVAC control circuit, which is nearly always 24VAC. Most of the time these have two separate transformers but you can use one for both purposes, although I'm not sure that it's wise or code compliant.

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