Yes, some use triacs for switching.
I’ll ask about the Honeywell stat mentioned in the OP.
ETA: My coworker says they use relays and will click.
You have to be careful with solid state relays. Unlike a mechanical relay, a solid state relay is never fully on and never fully off. It’s often close enough that it doesn’t matter, but you do have to worry about things like leakage current that you don’t need to worry about with a mechanical relay. For example, if you have a very low current device, the leakage current from the solid state relay in its “off” state might be enough to power the device, leading to the unwanted result that when the relay is “on” then the device is on and when the relay is “off” the device is still on. Sometimes the fix is as simple as adding a loading resistor to the output, but the point is, a solid state relay isn’t always a drop-in replacement for a mechanical relay.
In a furnace thermostat, if the furnace’s controller has a high impedance input, the leakage current from a solid state relay might be high enough to trigger the controller into thinking the thermostat has switched on when it hasn’t.
So based on this, I think it is safe to assume that all thermostats will use an electromechanical relay, and will click (with varying levels of audibility).
The original thermostat design is a bi-metalic strip and some form of over-center spring that provides the action. The over centre spring provides the holding force, holding the contacts on or off, and the hysteresis needed to stop a simple on/off control from constantly cycling (and wrecking the device.)
Add a simple screw mechanism to bias the position of the spring, and you have an adjustable thermostat.
(Run a little current through the spring to heat it when in the on state and you have a device that can provide a pre-settable on/off proportional control.)
Over centre springs are the key to many switching devices. Micro-switches are basically a packaged switch with an over centre spring. There are also relays that use them for force amplification, or building a set/reset relay.
I would bet that most clicks you hear are the contacts shutting under the effort supplied by an over centre spring. Hence the clear click.
In days of yore thermostats were simply two terminal devices that had a bimetalic strip style thermostat, and they went click. Lots of people still use them. Add a common wire, and you have the ability to power your thermostat, and then you can add any amount of smarts. But the furnace it talks to may well still be old and dumb.
Sound insulating is not too hard. As above, compliant mounting to isolate the housing from the wall is important. Damping the housing is next. My bet would be to use the adhesive damping pads sold for damping car panels when installing good quality in-car audio. Basically felt full of half set gunk (used to be bitumen) you stick to the inside of the doors. Cut a bit up and apply to the inside of the thermostat’s cover. Assuming there is room. It isn’t the thing going click you hear directly, but the sound transmitted to the radiating surfaces it is attached to.
So is it worth paying a bit of money to have it replaced to the model in the OP? Probably won’t be any worse right?
It looks to be pretty spiffy and capable. What I do notice is that it is rated to 100,000 cycles. Which would suggest to me that there is still a relay inside. But it could well be near silent. One would hope so.
Agree with what you said. Though most HVAC systems for residential use perform all the control stuff using 24 VAC and contactors/relays, thus loads have low impedances. The thermostat simply routes 24 VAC to the coils of three contactors based on temperature and user settings:
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The thermostat will send 24 VAC to the coil of the contactor for the furnace’s blower motor when it wants the furnace to blow air around.
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The thermostat will send 24 VAC to the coil of the contactor for the heater when it wants the furnace to supply hot air.
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The thermostat will send 24 VAC to the coil of the contactor for the outdoor air conditioner compressor when it wants the furnace to supply cold air.
To switch on/off the 24 VAC to the coils of these contactors, most thermostats use small mechanical relays or reed relays, or some kind of bimetallic-contact mechanism as described by @Francis_Vaughan. They’re cheap, simple, efficient, and reliable. But yea, they have an audible click. I suppose triacs could be used, which are completely silent. But they’re more difficult to drive, less reliable, and might not be “fail safe.” More significantly, though, the voltage across them is around 1 V when they’re “on.” This means the coils of the contactors will receive 23 VAC instead of 24 VAC, and it means the triacs will get hot when they’re on. So not only does it mean the triacs are less efficient than a mechanical relay or bimetallic-contact mechanism (and thus “less green”), but the thermostat’s circuit board and housing must be designed to deal with the heat they produce.
Just a general note, but solid-state switches / solid-state relays using SCRs, triacs, or MOSFETs tend to be used when something needs to be cycled on and off with a very short period (on the order of a few seconds or milliseconds). Examples include an oven heater modulated on-off using PWM, motor speed controller, and a light dimmer circuit using a phase-controlled proportional controller. When something needs to stay on for a fairly long period of time (minutes, hours), designers will usually opt for a mechanical relay instead.
FWIW, the replacement thermostat you’re being offered appears to have remote temperature sensor capability, so … even if the new t’stat was still too loud … you’d likely have the option to ‘insulate’ it, and use a remote temp sensor to preserve its operation (see post #18).
I have my doubts if the Honeywell could be insulated enough to be sufficiently quiet in the bedroom. The Honeywells I replaced had a click so loud I could hear it throughout the house. I could hear the click of the upstairs thermostat when I was downstairs watching TV. I’m thinking it would take quite a bit of soundproofing and be a hassle to deal with. Instead, I would recommend getting your own thermostat that is actually quiet so you don’t have to jump through all those hoops. The Lux thermostat I linked above is only $60 and is very quiet.
As an experiment, you could try soundproofing the Siemens thermostat you have now and see how well that mitigates the click. If you are happy with the results, then maybe the Honeywell with remote thermostat would work. But keep in mind that you would have to take the soundproofing on and off every time you needed to change the thermostat.
I have two Honeywell TH8321R1001 thermostats with remote temp sensors. They are absolutely positively silent when switching on or off the HVAC.