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My goal is to use a bunch of sensors to control my HVAC system. I’m thinking of getting a Nabu Casa Yellow and a bunch of zigbee temp sensors. My house is big and the HVAC system is not “balanced” as the engineers would say. Thus one room can be uncomfortable when the thermostat in the central hallway is happy. So I want to write programs (automations?) so I can change the set point depending on the outside temp, temp in various rooms, the return air duct, etc. Maybe even a sunshine sensor.

I am a retired software engineer, 40 years of systems programming and a couple of years of embedded systems. But I don’t have a good overview of how to find what I want.

I have no interest in a “smart thermostat” that can be “programmed” with times when you go to work, I want to do the programming. I want my set of temperature sensors to be analyzed every X minutes and run thru a state machine or algorithm to adjust the set-point, turn on the fan, etc.

First question: how do I search/google for a normal 4 wire thermostat that I can completely control with the HA system?
How do I decide how to power the various temperature sensors, I assume one can use CR batteries, or a low voltage DC power brick or maybe even POE? Should the sensors use zigbee or maybe the Ethernet POE setup?
And what is a sane way to test/prototype this?

I’m not worried about minor optimizations, the Nabu Casa Yellow’s CM4 has to be serious overkill for my needs. But I do wonder if my whole approach is wrong.

Thanks
Pat

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This Index includes how to search the forum: The Home Assistant Cookbook - Index.

It also contains other information you may find useful.

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Welcome to the forum loony bin! :slight_smile: Great you have that background, but in HA you can throw that out of the window. :rofl: :rofl: :rofl: Just kidding! :laughing:

That’s hopefully why you are here, this is literally the job description for HA. The learning curve might be steep in the beginning, but after a while you’ll love it.

The imho best way to search for something, is the other way around. You look for the two or three “perfect” devices for your use case. Then take the model and brand into the search here in this forum. This will instantly give you an overview, if something isn’t working, how hard the integration is, all the necessary stuff.
The thing is, HA has over 2000 integrations for different devices, chances are high, HA is able to work with it. If you look the other way round, you’ll get hit with the motherload of information.

Another advantage is, that way you’ll get your next questions answered by the information you find. If you look for a temp sensor, you’ll likely stumble upon the Aqara sensors. They are cheap and reliable, and work with Zigbee. Looking for information about them here in this very forum, you’ll find, that these have a battery life around one year, are Zigbee, and easy to setup. If you like them, you take it to the search here, and you’ll find, you need a Zigbee dongle or something like it. Your initial search with the temp sensor will much likely already have presented some information about working Zigbee dongles or where to find that information.

On a sidenote: this forum can be overwhelming, and interesting topics tend to be hundreds of posts. The search (top right) can be restricted to the topic you’re actually reading in. :slight_smile:

Fortunately with HA there is no wrong approach. You will grow into it, and in no time, you automatically scan the reviews at Amazon for someone who already tried that device with HA. You’ll be surprised! :laughing:

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Thanks, but my search problem is too many hits. And I was thinking more in general. I’d like to buy a nice thermostat/HVAC control, but searching for “programmable thermostat” or even “zigbee programmable thermostat” returns zillion of hits/products, and nearly all of them are just “set the A/C two degrees warmer when you leave for work” which is not at all what I want to buy.

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I don’t think you’ll find a thermostat that allows you direct control of the relays, if that’s what you’re after. The best you could do is get a zigbee or zwave thermostat (because by definition those allows local control so you don’t have to worry about what features are available via some cloud API) and then you can control the setpoints to force the outputs to be on or off.

Common choices are the Honeywell T6 Pro Z-Wave or using a Ecobee with the HomeKit integration which gets you local control.

If you really want direct control of relays, you need to go down the DIY route. ESPHome would probably be the easiest path; you can take some inspiration from this site: ESP8266 WiFi Smart Thermostat Powered by ESPHome and Home Assistant :: Joe Stump

You can just google “esp relay board” and find one with the number of relays that you require and that also takes 24VAC (or whatever power you have available). You will need to feed a common wire up to your thermostat if you don’t already have one.

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Welcome! This is definitely in Home Assistants wheelhouse.

For the use case as you’ve described it, I don’t think you need to get down to control of the actual relays. A zwave (or similar controllable) thermostat should be fine. You’re just going to want to change its set-points to make it do what you want. E.g., if your bedroom is too cool, set the set-point of the hallway thermostat to be 80 to force the heater on until the bedroom is the desired temp, then adjust the set-point again to turn it off.

This will definitely take some tweaking of the automations to handle all the edge cases, but the thermostat itself can be anything that you can programmatically control.

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Welcome to the community. One thing to keep in mind is that a Zigbee network with only end devices (temperature sensors) will probably give you issues. To get a solid Zigbee mesh you would need routers (mains powered devices). Zigbee buyer's guide

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Nobody seems to have mentioned the docs, so if you haven’t found them yet…

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There are others here in this forum who have done what I think you are trying to do.

I searched for “hvac control” and found some interesting reading.

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This. A commercial thermostat, even a networked programmable one, has the opposite goal of what you have. It is intended to simplify/obfuscate the process of HVAC control; you want to expose and control every detail. For your use case I’d recommend looking into ESPHome as the technology, coupled with the HVAC Control search results detailed above.

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Questions to the 1st question: Would you like to show what your existing 4-wire thermostat look like? Also could you help clarify when you say “complete HVAC control”…? For example, does that mean you want to switch on and off of the relays of each of those wires…? Or does that mean you want controls regarding when to turn on/off heat / cool / fan, and you want controls regarding the heat/cool setpoints…? Do you plan to do overshoot / tolerance in HA or from the thermostat? Do you plan to do heat/cool harvesting using HA automation or from the thermostat? How do you plan to handle humidity?
(Responses above this one had assumptions, so after your clarifications, some of those might become less relevant in your use case, but still be interesting for other use cases.)

Also, if the HA server dies or become disconnected or out of service, do you still plan to have a functional thermostat so that someone else in your household could still cool the house to 70F and not 74F…? (I realized that I assume you are in the US. So please help clarify this also while you are at it.)

If you plan to populate multiple temperature / humidity sensors around the house, those sensors tend to be low power and as such POE would not worth the hassle - I would recommend sensors using low powered wireless protocol (zigbee, z-wave, bluetooth) with CR2032 or CR2050 batteries.

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This may be helpful as well:

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quote: I don’t think you’ll find a thermostat that allows you direct control of the relays, if that’s what you’re after.

I don’t think I want that. I just want the HA logic to be able to change/set the temp set-points, and tell the thermostat to turn on the fan.
Might be nice to be able to switch the “cool-off-heat” switch, as sometimes
in the Spring and Fall, I have to manually do that now.

Quote: You look for the two or three “perfect” devices for your use case.

I actually don’t care about perfect. I’ll be happy with good fit. The perfect is the enemy of the good and all that.

Quote: You’ll likely stumble upon the Aqara sensors. They are cheap and reliable, and work with Zigbee. You’ll find, that these have a battery life around one year, are Zigbee, and easy to setup.

I checked them, they do look like they will fit my needs for room temperature. But they don’t offer (or I can’t find) sensors that are suitable to be outdoor temp. And I don’t see one that is suitable for the return air duct temp.

Quote: One thing to keep in mind is that a Zigbee network with only end devices (temperature sensors) will probably give you issues. To get a solid Zigbee mesh you would need routers (mains powered devices)

I have a bunch, five or six Eero 6E Pro wifi-mesh “routers” and the Eero marketing copy claims they do Zigbee. If I can figure out how to make Zigbee actually work with the Eeros, I should have plenty of powered mesh.

Quote: Would you like to show what your existing 4-wire thermostat look like? Also could you help clarify when you say “complete HVAC control”…?

Not sure I see why my current thermostat is important. I expect to replace it with a suitable Zigbee connected, HA controlled device. Picking the new thermostat is one of the first steps in this project.

My thermostat, and nearly every non-automated ones that I’ve ever seen have two switches and a way to set the “set point”. One switch, auto-on, sets the fan to “auto” so that the fan comes on when the furnace or A/C is running, or it can be set to “on” that blows air around. The other switch has “cool-off-heat” which controls whether the setpoint controls the A/C or furnace. Then it has a setpoint. Might be set with a slider or buttons, but you can set the temperature where the furnace or A/C comes on.

My “complete control” means that the HA automation or script can tell the thermostat to change the cool-off-heat and auto-on switches, and set the setpoint.

I do not plan any fancy humidity control, like re-heat which is when the A/C cools the air below the setpoint and then heats it up so that the relative humidity can be controlled. Its a hot air furnace, so it has a plumbed-in humidifier. A future enhancement might try to manage the
humidifier, but its been working for 50+ years, so its unclear if a change is needed.

Quote: Also, if the HA server dies or become disconnected or out of service, do you still plan to have a functional thermostat so that someone else in your household could still cool the house to 70F and not 74F…?

My idea is that the thermostat is nominally in control, providing redundancy if the HA is down. It still has its normal control, so a visitor could just change the set-points the normal way. Neither the A/C nor the furnace will work without 120V or 240v power.

Yes, I’m in the US so we use dumb Fahrenheit and inches instead of sane units.

Getting a year or two out of a normal coin cell CR2032 or CR2050 is sufficient. No need for anything fancier than that.

Looks like “generic thermostat” is a big step in the right direction.

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I’m using this, that has some more possibilities :wink: