My home thermostat has relays to switch the heat and A/C. They are very loud. Is there any reason why this couldn't have used MOSFETs for the switching?

 

hi,
Have you considered suitably rated SSR's Solid State Relays.?
I assume your A/C etc is AC powered.
E
https://www.google.com/search?client=firefox-b-d&q=ac+ssr+relay

 

I'll have to check. While the heat and A/C are AC-powered, I think the switching is low voltage (12V) DC.

 

him,
Many SSR types can be controlled with DC 3V thru 35V.
E
https://www.amazon.co.uk/slp/solid-state-relay/rm48vgswvcob3g9

 

The relay provides electrical isolation from the AC and heat units. A MOSFET would not. A solid state relay using an optocoupler and MOSFETs provides isolation. The relay is probably switching a control signal (typically 24VAC) and not the AC power directly.

Edit: Corrected "24VDC" to "24VAC"

 

I'll have to check. While the heat and A/C are AC-powered, I think the switching is low voltage (12V) DC.

Many control systems are low-voltage AC, e.g, 24V. My home system is.

 

Yep, I just found it. 24VAC.

 

So a small SSR should work as a replacement.

What are the ratings of the mechanical relays (control voltage and contact current)?

 

Here's the circuit board. 2.4V relays? Dunno why there's five. And I guess the ULN2003A is to drive them?

 

Those relays are loud?
BTW, according to the spec, they are dual coil latch/unlatching types (LT).
DPDT
Max.

 

You still see relay usage in modern designs so Home Thermostat design must include some pretty old school HVAC system low voltage AC requirements. Little latching relays are certainly not cheaper than mosfets so using latching relays provides something worth the extra cost.

 

Those relays are loud?
BTW, according to the spec, they are dual coil latch/unlatching types (LT).
DPDT
Max.

I can hear them click from anywhere in the house, even the basement.

 

Are you sure it's the relays that you can hear? Maybe it's something else?
Have you ruled out Rent a Ghost?
I wish my ears were as good as yours.
Good luck.

 

I can hear them click from anywhere in the house, even the basement.

Some EDM sound-damping inside the enclosure might work.
https://multimedia.3m.com/mws/media/1693093O/3m-engineered-damping-materials-edm1000-series-tds.pdf

 

True, but I'l still like to either replace the relays or build my own thermostat using SSR's.

 

The mounting for the board may be serving as a sound box, amplifying the relay clicks. I’d consider replacing the mounting screws with a little longer screw and use rubber isolating washers between the control board and mounting box.

 

True, but I'l still like to either replace the relays or build my own thermostat using SSR's.

Sure but it's not a trivial matter.
http://ritetemp-thermostats.com/Professional reference guide 21dec07.pdf

Mechanical relays will always

I love the sound of relays in the morning.

 

True, but I'l still like to either replace the relays or build my own thermostat using SSR's.

Those latching relays use pulses to activate and deactivate. Going to get a lot more involved using a SSR.
SG

 

I'll have to scope it to see what it's actually doing.

 

My electric stove arc's real bad when the elements turn on, jeez I need to pull it apart and see if the cheap little relays are melting the solder, sounds horrible, but barely notice any clicking


So what about the circuits in hotair stations, that use an zero-crossing AC opto-coupler, to trigger a SCR/triac ? They use 5V logic iirc, to run huge current things like BA16 iirc, at like 15 or 25Amps.