smart thermostats with internal relays

[g-and-h_electric]

As the electrician for an hvac shop I have been running into incompatibility issues with "add a wire " kits and smart thermostats.

The kits the shop sends out with the install crew are from IO hvac controls. Their instructions (yes I read them) clearly state that their product is incompatible with thermostats using triacs to control the hvac equipment. I have rewired several systems with 8 conductor wire, and these were easy ones (fish wire from furnace and make no new holes). Sooner or later my luck will run out.

I know the Honeywell T6 PRO wifi has relays. Does anyone know for sure if any others do? I know for sure ECOBEE uses triacs. Lacking that Are there any known "add a wire" kits that will work with a non relay smart stat?


Howard

 

[Hv&Lv]

You’re exactly right about the IO HVAC Controls “add-a-wire” pieces—they explicitly require a thermostat with mechanical relay outputs (not solid-state/triac).
Which smart thermostats have relay outputs (OK with the IO kit)?
Honeywell/Resideo T6 Pro Smart (TH6320WF/TH6220WF) – uses mechanical relays (Resideo’s pro install guide labels the heat/cool outputs as relays). This is a safe match with the IO kit.

• Emerson Sensi family (ST55 / Sensi Touch / Sensi Lite) – commonly reported as relay-driven (audible click, works fine with traditional relay equipment). Emerson’s docs don’t spell “relay” out, but field use and wiring guidance match mechanical-relay behavior. If you want a “no-doubt” pairing with the IO kit, stick to the T6 Pro above.

Which popular smart stats use solid-state (triac) switching (NOT OK with the IO kit)?
Google Nest – Google’s own pro guide: “Nest Thermostats use solid state switching instead of relays.” (i.e., triacs/SSR).

• ecobee – ecobee powers the stat with a C or their PEK (not “power stealing”), and the outputs are silent solid-state (widely reported in the trade). If you must use ecobee, don’t use the IO kit—use ecobee’s PEK or a different wire-add solution.

“Add-a-wire” options that do work with solid-state (triac) stats
These don’t depend on the stat having relay contacts:
Add-A-Wire (ACC0410) – classic 4-to-5 (or 5-to-6) wire adapter; installs at the air handler/board. Works with 24 VAC systems broadly, regardless of stat output type.

• FAST-STAT (Model 1000/3000 series) – creates one or two extra conductors over existing cable; marketed as compatible with “all 24 VAC” systems (i.e., independent of the stat’s internal switching).
• Honeywell/Resideo WireSaver (THP9045A1023) – only for Honeywell stats that have a K terminal (e.g., T5/T6). It splits K back into Y + G at the furnace. Since it’s a paired module/terminal scheme, it’s not sensitive to triac vs relay.
• ecobee PEK – only for ecobee thermostats; combines Y + G to free a C. Not a universal add-a-wire, but the right fix when you must use ecobee on 4-wire runs.

Practical playbook (what I’d do in the field)

If you’re using the IO kit: pair it with a relay-based stat (Resideo T6 Pro Smart is the sure bet). If the customer insists on Nest/ecobee (solid-state): skip the IO kit and use Venstar Add-A-Wire or FAST-STAT; for ecobee specifically, the PEK is often simplest.

1. When in doubt or on tricky multi-stage/HP jobs: pulling new 18/8 is still the gold standard (as you’ve been doing). If that’s not possible, you can also isolate triac outputs with external relays (e.g., a small 24 VAC control relay per call) to present clean dry contacts to devices that require relays—handy for oddball interactions. (General best-practice tip; choose a relay with a coil draw that won’t upset the triac output.)

I’m not that smart with HVAC, here is an AI answer. I have Sensi myself.
I’m really curious if all this is correct in your expert opinion.