US 5-wire (O/B) setup

Apologies as this question may be a bit off topic:

Unlike the 4-wire setups that use Y for cooling, my system uses Y for the heat pump compressor and the additional O/B wire to move the reversing valve to cooling when it is energized.

Since the relays only have 4 positions I need to short the compressor wire (Y) to cooling (O/B) when I want cooling and manually move Y to heating (W) when I need heat.

I feel that jumping Y to both and using something like an inline diode would solve my issue but didn’t know if there was a more elegant solution?

My last post contained some misinformation so let me correct and sort of answer myself. After inspecting the unit more and doing a little research I found that my wiring is actually exactly like this unit: Understanding and Wiring Heat Pump Thermostats with AUX & Em. Heat! Terminals, Colors, Functions! - YouTube

Since W is actually emergency heat not primary heat then I believe only Y needs to be powered for heat. So the wiring for that would look like this:

This wiring configuration seems to be common in my area, specifically hvacs with 1-stage Lennox heat pumps.

I haven’t gotten my HestiaPi unit in yet but I will report back with my findings in case it helps anyone else out reading this in the future.

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Hi jay, have you had a chance to give that a shot? I have a similar setup in my apartment (O, G, Y, and R wires only), but I’d like to confirm that it would work before I buy one of these if I can. For what it’s worth, I think the diode idea will work as long as the breakdown voltage is higher than 24V. An alternative I can think of would be to modify the firmware to make the “unused” W wire act like the “O” wire instead, though that’s obviously not ideal either.

After making my post I realized I couldn’t use a diode because it’s AC current and the only way to switch AC current one way is with another relay. You can’t fit an additional relay AND a buck converter (to step down the 24VAC down to 5VDC) inside the hestiapi enclosure with everything else that’s in there (I tried).

I talked to a friend last week who has mocked up a new PCB with support for 6 or 7 wires and powers the pi off the common wire. This replaces the hestiapi Pi hat and mounts inside the enclosure the same way. Once I print it and test it works I’ll share the design but it may be at least a few weeks before I have a chance to do that. I know there are others that don’t want to wait for the crowdsupply campaign.

I had to put things on pause for my newborn son but I have resumed work and posted a new topic in the customization section for anyone following this conversation.


Congratulations for the baby! My biggest, always-evolving “project” :slight_smile:
I will try to make time and study your design.
A big thank you from the open source community!!!

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