whole house fan with high low switch

[last cave]

I have a whole house fan that uses a high low switch for control. I'd like to connect it to home assistant. The switch has three positions, low, off, high. This wires to a box on the fan. Most setups include timer to turn off the fan, but I don't have that.
I'd like to add a relay to control a fan through home assistant. The switch only needs to close the two contacts, but for safety I think it cannot be allowed to close both in the high and low position at the same time.
I am looking for ideas of what I can use to control this safely.

[floral fractal]

Do you know what it's driving? I assume it's low voltage because of the design but it'd be good to check that

[keen compass]

It looks like regular romex to me which I’d think would be ran for higher voltage things

[rustic dome]

Do you have access to a different control box at the whole house fan? You’ll need to power a relay regardless

One or the other would need an interlock between the relays. Esphome has an option for that

Assuming it’s an AC motor and this is the only on/off location, the middle terminal is probably hot and the other two connect to different windings on the motor

[jovial wing]

I think you could probably do it with a shelly 2pm. but you would want to have it set up in such a way there is an interlock to prevent both outputs being on at the same time. I am not sure if it can do that by default or is something you would have to customise a bit (I don't have a 2pm on hand currently to check). You would also need a neutral connection to power it up. and i am not sure if you have one tucked in the back there or otherwise have one close by.

[last cave]

I believe when I tested it It was at line level. I don't think that there is a neutral in that box, but I'll go to the attic to connect this if need be.
I've been looking at Shelley for a while, but I have never used one. I am concerned about programming it to only allow one switch to be on at a time. I would rather this be done by hardware so no one can mess it up.
A little concerned. A little concerned. Ensuring that there is only one switch on. I also want to make sure that there is still a switch that can control the fan.

[rustic dome]

If you’re ok with diy, you can get interlock relay modules or put together an appropriate circuit yourself. The key will be handling the physical switch and electronic controller in a sensible way. You could turn the physical switch into A/B inputs into an esp32, then have the esp drive the interlock relay inputs.

Or a software interlock esphome, which you can usually flash onto a Shelly iirc. I don’t know if the Shelly firmware supports something like that (two switch inputs controlling two interlocked outputs)

Amount of current needed for the whole house fan also leads to the question of a separate, isolated relay or even contactor

[floral fractal]

I would use a pair of relays, low/high connected to NO/NC contacts, and the input of that relay is connected to the output of another.

2nd relay is off - system is off
2nd relay on, 1st off - low
Both on - High

However you drive those relays (shelly, esphome, zigbee etc), it's safe so long as that's the layout

[main mesa]

Tip: to find relays with NO/NC connections, search for your protocol of choice (zwave/zigbee/wifi) plus “relay” plus “dry contact”.

[last cave]

This is a very sensible option and makes sense to me. What I'm unable to do is find normal open / normal closed relay. It does not Look like Shelly sells one. I'm not advanced enough to piece together an ESP home device. In my general search I found one option but I don't think the power is high enough. My preference would be something that uses matter, over wifi or thread. Does anyone have a link to a device that might work?

[floral fractal]

Electrically you could get everything you need (other than power supply) for an esphome build with something like this if you wanted to experiment https://www.aliexpress.com/item/1005005808811274.html
There are some more off the shelf options as well like https://ewelinkstore.com/product/smartwise-5v-32v-wi-fi-bluetooth-2-gang-smart-relay-switch-no-nc-dry-contact-momentary-interlock-ewelink-compatible-plastic-case/?v=e3fb3fca2abf

But yeah sadly it's a fairly niche use, most smart relays do not expose NC

[chrome kestrel]

I've done something similar with a Zooz relay unit driving a larger mechanical SPDT relay that is actually controlling the load. With a DT (double throw) you handle the interlock issue. Zooz stuff is safety listed (ETL) which is important for insurance reasons. Even though soild state relays can be configured for NC or NO operation, they can fail unsafe. A mechanical DT relay is far less likely to fail in a way that would close both switches. The Zooz stuff has been rock solid for me and I'm a big fan.

I just realized you need to be able to turn it off as well, so that will change my suggestion a bit. heh

A Zen52 (double relay) driving two mechanical relays. First relay is power, second would be speed select. Zen52 is $30USD
https://www.thesmartesthouse.com/products/zooz-700-series-z-wave-plus-double-relay-zen52
Two relays and sockets will be $20 + shipping. (UL Listed)
https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/electro-mechanical_relays/781-1c-120a

BTW, am I wrong to be focused on safety listings? From what I understand if an unlisted device causes or is involved in anyway in an electrical fire my insurance is off the hook.

[rustic dome]

Household wiring is not something I’m willing to skimp on safety listings to save a couple bucks

[floral fractal]

It depends where you live - where I am so long as it's SELV (safe extra low voltage, <24v dc) nobody cares, and the relays switching mains need to be properly rated, which those blue ones (srd 5v sl c normally) are. The only thing you them need to worry about it is the the power supply.

[floral fractal]

Fwiw I've never heard of a ssr failing with both contacts powered. They will normally failed closed (why the "on/off" relay should be wired to NO)
EMRs like that one linked will normally fail closed as well, after either contact welding or return spring failure.

[chrome kestrel]

It's a super low-risk, TBH. I had one horror story from someone I worked with related to an Omron SSR in a machine that failed in both modes. It didn't actually result in a catastrophe, but could have and was told to me as a "be careful with SSRs in certain situations" kind of way. Anyway, I just have a probably irrational belief that EMRs are ever so slightly safer due to the mechanical separation and near-impossibility of them failing in both modes (short of a complete melt down situation).

[floral fractal]

I mean if you want to do it properly you need to start including safety relays with position feedback signals and stuff - and there EMRs have an advantage over ssr

[last cave]

Thank you for everyone's help.
I think I am going to get a Shelly 2PM Gen 4. Looking more into it, This model allows only one output to be turned on at a time, in cover mode. This is designed for garage doors lines that go up and down with one output going up and one output going down. The only concern is that there's some reports that it will turn off the relay if it's been running for a certain amount of time, since in theory for a door or something it reach a limit.
My next steps are to go into the attic and check that the wiring is what I expect, and to measure the current to make sure these units be sufficient. Unfortunately I'm in a sling right now and can't get into the attic. Surgery is giving me plenty of time to look up new ideas, but then I can't execute them until I recover.
Thank you for all the help it's been useful. I'll update this thread when I finally get a chance to do the work.

[last cave]

Failure!

I set everything up and got it partly working. We got sparks and the fan would sputter on high.

Time for an electrician.

Does anyone know someone who can deal with automation and electrical? (I'm in Massachusetts.) Our regular electrician does not understand smart switches.