[mechanicalpulse]

Well that's certainly a take. Solid state relays using optoisolated MOSFETs have been around for fifty years. Mechanical relays are overkill for signal switching as in HVAC thermostats, IMHO, but you do you.

Anecdotally, I have a first generation Nest and haven't had a problem. Maybe some of the earlier hardware had fewer protection against misuse (e.g., with non-24VAC systems or otherwise incorrect installation), but that's generally the case with most new things.

[gnuplustoejam]

Sounds like something Nest engineers would have said.

It's not "signal switching", you see.

HVAC equipment is as old and varied as you can imagine, and there is higher current than you think running through those terminals, powering all sorts of nasties, oil burner relays, damper motors, crude AC contactors causing voltage spikes etc. HVAC low voltage power is as dirty as can be.

No one took this into account, they were more concerned with making the thermostat pretty.

[mechanicalpulse]

Nest is hardly the only thermostat out there using solid-state relays. Have you considered the possibility that they did take it into account and they deliberately chose to use SSRs instead of electromechanical relays? Have you considered the possibility that they were concerned about the impact that mechanical relays may have on the RF, especially if "there is higher current than you think running through those terminals"? Have you considered the possibility that they were worried about making the first one fatter than it already was?

In my heat pump, none of the thermostat wires directly control the contactors. They all run into a logic board that applies logic like time delays, temperature-controlled defrost cycling, and active protection lockouts for the compressor. I mean, there's a seven-segment LCD on the logic board for system troubleshooting. The air handler has a variable speed blower as well.

I understand that HVAC equipment varies wildly, but if you try to solve every possible problem or scenario and target every possible customer, you'll never make it to market.

I also understand that I am the target demographic.