I have 7 wire system and would like to know how to properly wire

xyz · June 19, 2025, 8:48pm

Hello for fair warning I’m new to doing DIY projects, but I have background in IT(sysadmin). I do have experience in building customer PC’s and I do know how to solder.

I’m very interested in building and installing a HestiaPi for my A/C unit(US) in order to control it and not be stuck with the mess that is in the consumer market for “smart” thermostats. My primary concern would be how to properly wire my current A/C as my thermostat has 7 wires instead of 5 going to it.

System currently has an C, R(c)w/ wire jumper to R(h), Y, G, S2, W/E and O/B wire. I do know based on my A/C units documentation that the S2 is a sensor wire and not inherently needed to run the system. I’m pretty sure it’s not supported to begin with, but I figured it wouldn’t hurt to include. I do know the O/B wire is very likely needed. I have confirmed my unit is providing 28V AC on the C wire. So I’m pretty sure I can build a unit that uses my A/C’s provided power for the HestiaPi.

Any advice on if my system is supported and/or how to properly wire it would be greatly appreciated.

jrtaylor71 · June 23, 2025, 2:37pm

I would use this project instead.

The HestiaPi software uses openHAB and uses a non supported version. The remote connector will no longer connect because of the outdated version. There has been talk about updating but the pi-zero cannot run the newer versions of openHAB.

hestia_hacker · June 23, 2025, 3:59pm

No matter what theormstat you use, you’ll need to understand the wiring. Here’s my best guess at what they are:

Common
R(c) hot wire for heating
R(h) hot wire for cooling
Y fan
G cooling ; or possibly heating/cooling if O/B is needed
S2 sensor, apparently
W/E w = heating ; E = emergency/electric heating
O/B reportedly a “reversing valve for Heat Pump systems”

This mainly comes from the diagram here: Wall installation ONE · HestiaPi/hestia-touch-openhab Wiki · GitHub

And the additional ones I filled in from Honeywell’s support page: https://www.honeywellhome.com/us/en/support/how-do-i-wire-my-thermostat/

It’s always good to check and verify these things though. If you jumper the R to Y, the fan should turn on, R to G for the compressor for cooling, and so forth.

If you can turn on the heating and cooling with W and H (respectively), then you should not need O/B. You will also be supported by the HestiaPi out of the box.

If you can only get heating or cooling, but not both, then you’ll need the O/B wire and the HestiaPi will not work out of the box. It’ll require updating some rules on how to do the controls.

If I was a gambler, I’d say you’ll probably be supported out of the box. Having two R wires implies there are separate heating and cooling systems. But please check and make sure.

As for which open source thermostat project to use, I can say that I am still supporting the software. The latest image uses a supported version of Debian, but what @jrtaylor71 says about OpenHAB being old and not upgradable on the original Pi Zero boards is true. I don’t know anything about a “remote connector” but I can say that I have connected remote temperature sensors to the current HestiaPi.

Here’s the plan for software updates:

split the front end (web browser) and back end (OpenHAB)
upgrade to Debian 12

People with the Pi Zero 2 (which is what all new units are built with) should be able to run both on the thermostat itself. Or they can to run the back end on another machine (a newer Pi, a server, etc.).

People with the original Pi Zero will have to run the back end on another machine if they want to upgradem if they don’t want to set up another server, they can just not upgrade. That’s always an option.

As to the question of when I’ll get to these updates, probably pretty soon, actually. I’ve been getting some other projects to a good stopping point which has freed up some of my time. I’m not going to promise to have it done by a certain date, because life happens, but I’m much more optimistic than I normally am when trying to figure out when I can squeeze in some updates.

xyz · June 24, 2025, 1:01pm

Hello and thank you for the response!

So need to quickly clarify the wiring. The system only has a R wire going to my current Thermostat’s R(c), but then has a jumper on the R(c) and R(h) ports on the thermostat. So I’m under the impression O/B will likely be needed for the system to operate properly. My apologies on not making the OP clearer on that.

So looking at the diagram what would be the proper way to wire with a O/B wire? (Apologies I don’t see it referenced and would rather not damage something) After that I’m under the impression I will need to modify configuration files for this properly work. I have no qualms with doing this.

Thank you for continuing to support this! We need more open source out in the world. I will go through with getting a Pi Zero 2 then. I looked at the other project and it’s a bit too complicated for me to add to my plate at the moment and doesn’t appear to be at the same place Hestia is as well if I’m properly understanding it.

How would backend deployments work? Guessing via a docker container to avoid any weird dependency issues.

hestia_hacker · June 24, 2025, 9:38pm

Ah, thanks for the clarification. I read that as there being two wires and them being jumpered together.

Full disclosure: I haven’t personally worked with a heat pump HVAC system. However, from what I am reading, it looks very similar to traditional HVAC controls.

First, we will test everything to verify it works as expected. You don’t need to disconnect your thermostat for this. Here are the tests:

Use a wire or paperclip or something to connect R + Y. The fan should turn on. The compressor should not.
Connect R + Y and R + G at the same time. The fan should turn on, the compressor should turn on, and it will likely blow cold air. There are some manufacturers who reverse their controls so it blows hot by default, so if that happens, don’t panic, just make a note of it
Connect R + Y, R + G, and R + O/B at the same time. The fan should turn on, the compressor should turn on, and it will likely blow hot air. Unless the last test blew hot air, then this should blow cold.
Connect R + Y and R + E at the same time. The fan should turn on, the compressor should not, and it should blow hot air. If the emergency heat hasn’t been used in a while, it may smell like dust is burning. It likely is, and that’s normal.

If any of those tests don’t give you an expected result, stop testing and report back here. If everything goes as expected, then the HestiaPi can work for you with some modifications to the rules.

Here’s my revised list of what each wire controls now that I believe you have a heat pump:

C Common
R hot wire
Y ~~fan~~ (Edit: compressor, likely cooling by default)
G ~~compressor~~ (Edit: Fan)
S2 outdoor temperature sensor
E emergency/electric heating
O/B reversing valve for Heat Pump systems

And from now on, I’m going to refer to the iFixIt post on what the wires are for a thermostat. They do a very good job at being concise!

The upcoming split

As to the questions of how deployments would work for a split system (thermostat front end and server back end), I was planning on the main solution to be a Raspberry Pi image. This doesn’t require anyone to have a traditional server or know how to use docker, LXC, any VM hypervisors or anything else. Just plug it in, tell the front end how to connect to the server, and voilá, it should work.

I also plan to cater to people who do have some kind of server infrastructure or want to set one up for this. This gets a little tricky becsuse I don’t want to force people to use a particular solution (e.g. Docker) against their will. That means I can’t do what many projects do: publish a Docker image and tell people to either use Docker or you are on your own.

My plan is to use Ansible for this, which is the same tool that builds the Raspberry Pi images today. So the same code can be used for whatever technology people want to use:

Point it at a physical server
or a VM on Proxmox, Hyper-V, ESXi, or whatever
or spin up a Docker container and point it there
or an LXC container
or… well you get the idea

I don’t have to do extra work configuring additional technologies, and users get something they can use no matter which technology they like. It seems like everyone wins.

Once I get a little further down this path, I’ll make a new thread about this.

xyz · July 9, 2025, 2:00am

Just wanted to provide quick update.

Thank you for the response. Life got busy and I was not able to get to my A/C unit until today.

The fan unfortunately did not start the fan when I did R+Y when I attempted to jump the system.

I double checked the wiring diagram that came with the fan coil unit… 99% sure that should have worked, but I’m not entirely sure why it didn’t. I’ve attached a snip of the diagram if that will be of help. This is from the documentation that came with it and not exactly as it’s wired on the thermostat as it has the added S2 and the wire on the thermostat is labeled W/E. Though thinking the heat pump has it’s own wiring/documentation, but haven’t had the much success finding that.

I believe that it actually turned on the compressor/heat pump. I’m going to do some more testing this weekend when I have some more free time jump wires to confirm. I’m thinking R+G will turn the fan on.

HestiaPi · July 9, 2025, 5:53am

According to the documentation

I believe you’ve mixed around G and Y.
G is fan and Y is cooling. Some systems expect the fan to be turned on first (connecting R+G) before starting cooling (R+Y) or heating (R+W). Try that and let us know

hestia_hacker · July 9, 2025, 7:23pm

Oops. Yeah. It’s funny, I was looking at that exact same diagram when I came up with that mapping and somehow I still got them switched around. Thanks for the correction.