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One-line summary: Is there a solid-state version of the White Rodgers 90-113 Fan Control Center (Transformer and Relay Combination)?

EDIT to answer question: The thermostat draws 0.146A (3.5VA / 24V).

I recently upgraded to a smart thermostat. As part of the installation, I had to have an electrician run a C wire from my basement up to my thermostat. He installed a junction box in my basement which contains both a 24V transformer and an isolation relay. (White Rodgers 90-113, which comes with Honeywell R8222D1014. I'll attach photos below.) The setup is working fine; heat is being delivered perfectly.

The problem is: every time the thermostat kicks in, there's a loud click from the isolation relay. The relay/transformer are directly below our main living area, and the click is very audible and bothersome.

If there were no choice, we'd live with it. But my understanding is that solid-state relays are a silent and reliable alternative to the electromagnetic type which I have now. My question is: what's the easiest way for me to replace the existing relay with its solid-state counterpart? (And is it even possible/advisable to do so?)

Do I just replace the relay but leave the existing transformer in place? Or do I replace the entire assembly? Are relays designed to some standard form factor, so that they're interchangeable as long as the voltage specs match? Or will I need to find a particular size/shape relay to replace my existing one? Would something like this (Resideo RT850T-240/U) work, as a drop-in replacement?

In case it matters, it's a radiant hot-water system powered by an oil furnace. Thanks!

My transformer and relay

ron rothman
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    What sort of load is this relay turning on and off? – ThreePhaseEel Feb 25 '23 at 07:11
  • Also how much current does your smart thermostat need to operate? – Jasen Feb 25 '23 at 09:51
  • @ThreePhaseEel I'm not sure how to answer. Should I try to find my furnace's specs? Or the size of the circuit breaker which it's on? Sorry, not sure what info you need. – ron rothman Feb 25 '23 at 15:11
  • @Jasen Looks like it's 0.146A. I edited the question to include this. – ron rothman Feb 25 '23 at 15:12
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    The possible replacement you asked about ([RT850T-240/U](https://customer.resideo.com/resources/Techlit/TechLitDocuments/69-0000s/69-2813EF.pdf)) is rated for a _resistive_ load of up to 23A. It isn't clear what is being directly controlled by this relay and whether it presents a resistive or inductive load to the relay. – HABO Feb 25 '23 at 15:32
  • that won't work `Transformer ratings: 24 VAC / 1.2 VA / Class 2` - is too small, it needs about 3.5VA to run the thermostat and another 1 to run the relay. – Jasen Feb 25 '23 at 20:15
  • @ronrothman -- basically, assuming that this relay is controlling the heat (W) input to your furnace, can you post the wiring diagram of your furnace please? – ThreePhaseEel Feb 26 '23 at 02:17

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