Advanced Server-Side HVAC Auto-Switcher for Home Assistant
In Finnish folklore, the fox outsmarts larger, stronger animals not through brute force, but through patience, agility, and using the environment to its advantage. Your HVAC should do the same.
Most "smart" HVAC automations aren't actually smart, they are purely reactive and often fundamentally flawed. Standard thermostat logic usually falls into one of two extremes: keeping the unit off for as long as possible until absolutely necessary, or short-cycling constantly so the temperature never shifts. Both approaches are objectively terrible for your HVAC equipment and work directly against how modern systems (especially heat pumps) were designed to operate, which rely on long, predictable cycles. Järki Kettu Core is more than just an auto-switcher; it actively evaluates indoor physics against outdoor conditions to keep your thermostat in the optimal mode, enforcing healthy, efficient runtimes.
Created by Talvi Fox / FoxDen Software
- Dynamic Coast Zones: System rests comfortably between wide deadbands.
- Deep Soaks: Dynamically stretches the physical wall unit's setpoint to force the compressor to run long enough to actually condition the house.
- Heat Pump Optimized: Includes a "Setpoint Walker" option to prevent Heat Pumps from panicking and triggering expensive Aux/Emergency Heat strips.
- Graceful Degradation: The physical thermostat safely holds its last commanded state and onboard hardware safety logic if Home Assistant reboots or sensors drop offline.
Because Järki Kettu relies on wide deadbands and long runtimes to maximize efficiency, you have to break the habit of staring at your physical thermostat's display. The old "set it and forget it" mentality becomes "set a range, and don't look at the number" (unless, of course, there is an extreme error).
Your home's temperature will intentionally fluctuate within this "Coast Zone" depending on the active mode and outdoor weather. It might feel strange at first to see the temperature drift a degree or two, but this is the system executing deep, efficient thermal soaks.
Crucial rule: Do not set your Target Heat and Target Cool limits to the same number! You need to give the system a wide gap to coast in. It takes a few tries to find your personal "zone," and you really need to watch how your home reacts over a few days before you settle on your final temperatures. But once you dial it in, you'll find you are actually comfortable throughout the entire house instead of just the hallway.
Most residential smart thermostats are incredibly primitive - they react exclusively to air temperature using extremely tight "deadbands" (usually ±0.5°F). Järki Kettu fundamentally breaks this mold by mimicking the logic of a commercial Building Management System (BMS). Here is the actual building physics behind why this works, and why the big brands refuse to do it.
Air changes temperature very quickly, while your walls, floors, and furniture take much longer to absorb or release thermal energy. When a standard thermostat short-cycles, it mostly just conditions the air. The second the unit turns off, your physical structure transfers its stored thermal energy back into the room, causing the air temperature to bounce right back.
By enforcing wide deadbands and Deep Soaks, Järki Kettu forces the system to run long enough to condition the physical structure of the house. Once the thermal mass is properly soaked, the house can coast for much longer without the HVAC needing to turn back on.
Air conditioning is about removing heat and moisture. An AC evaporator coil needs to run for about 10-15 minutes just to get cold enough to start pulling water out of the air. If your thermostat short-cycles to keep the air exactly at 72°F, it rarely runs long enough to dehumidify. You end up with a house that is 72°F but 60% humidity, a cold, clammy swamp. Long cycles extract massive amounts of latent moisture.
Customer Psychology. People treat thermostats like a volume dial. If a homeowner sets their thermostat to 72°F, and the temperature naturally drifts to 74°F before the AC kicks on, they immediately call a technician and say their unit is broken.
To prevent customer support nightmares, smart thermostat brands use razor-thin deadbands to keep the number perfectly still on the screen. In doing so, they are actively ignoring the actual HVAC hardware manufacturer's engineering specs, which explicitly demand long, steady-state cycles for proper compressor oil return and electrical efficiency. They intentionally sacrifice your equipment's lifespan and your actual thermal comfort just to give you the psychological comfort of seeing a static number on a screen.
The only trade-off to the Järki Kettu approach is that you basically have to accept the temperature is a range now. The exact fluctuation depends entirely on your home's thermal retention, in fact, this tool might help you realize you just need to address an insulation problem! However, human comfort isn't just about air temperature; it is heavily dictated by Mean Radiant Temperature (MRT) and humidity.
Because this system deeply soaks the physical walls and aggressively strips out absolute humidity, a room at 75°F under Järki Kettu logic will physically feel vastly more comfortable and breathable than a short-cycled room at 72°F under native thermostat logic. You lose the static number on the wall, but you gain profound, whole-house physical comfort and significantly prolonged equipment life.
You might have heard the old advice: "Just leave your thermostat at one temperature; it's cheaper than letting it fluctuate and forcing the system to catch up."
This isn't actually a myth, it's just an incomplete answer. It is absolutely true that massive, 10-degree temperature setbacks waste a ton of energy during the recovery phase (especially for heat pumps, which might panic and trigger expensive auxiliary heat). Holding a static temperature does keep your monthly electric bill predictable.
But you are trading your electric bill for the lifespan of a $15,000+ piece of hardware.
When you force a thermostat to hold a single, unyielding number, you are abusing the machine with constant 5-minute short-cycles. You are subjecting the compressor to massive start-up amperage spikes and terrible moisture-extraction efficiency. Järki Kettu takes this a step further by finding the perfect middle ground. It doesn't use massive, expensive setbacks, but it also doesn't hold a static number. By introducing a controlled Coast Zone of just a few degrees, it avoids the energy-wasting recovery spikes while also eliminating the hardware-destroying short-cycles. You take care of the equipment, and the equipment takes far better care of you.
After abruptly leaving the Ecobee ecosystem in search of something that crashed less, I bought into the Honeywell ecosystem and was immediately disappointed to find out they barely supported their flagship T9 thermostat. Frustrated, I pulled the unit into Home Assistant to try and fix it myself, but eventually hit a wall and shelved the project.
Months later, a coworker showed off their own Home Assistant setup, which completely reignited my motivation. I spent the next six months straight coding my ideas through pure trial and error. I built the fan controller from scratch and literally lived through a freezing winter of bugs, waking up to the system stuck in the wrong modes, until I finally ironed out the math.
Because no good deed goes unpunished, my outdoor HVAC unit recently got crushed by a falling tree. It hasn't been replaced yet, but it is somehow still limping along. I am fully convinced that the only reason it hasn't completely died is because this exact logic increased its efficiency and stopped it from running itself to death!
I have to give a massive shoutout to everyone who patiently listened to me ramble about boring thermal physics that I barely understood at the time, as well as the AI tools that helped me untangle the math and build this out.
What finally motivated me to clean this project up and push it online was getting validation from actual professionals. An HVAC installer took a look at the system and pointed out that I had successfully solved a notoriously difficult floor-temperature split issue using what is effectively "virtual zoning." Shortly after, an in-law who literally teaches HVAC saw the dashboard and instantly recognized that the backend was actively manipulating real building science.
But you don't just have to take my (or their) word for it. I literally live with this automation running my home every single day.
I built this for me, and it will keep running my house regardless of whether anyone else downloads it. But if you are tired of your thermostat short-cycling and want to see if this logic works for your home too, welcome to the Den.
If you run a smart home YouTube channel, a blog, or just love staring at Grafana dashboards, this project was built for you.
Don't just take my word for it, prove the physics with your own data. I highly encourage you to set up History Stats to track your compressor's average cycle time and total daily runtime for a week before installing Järki Kettu. Then, compare it to the data 30 days later.
You will see the erratic short-cycling disappear entirely, replaced by massive, highly efficient, deep-soaking runs. If you make a video, write a blog post, or just want to show off your Grafana charts comparing the "Before and After," please let me know! I love seeing this logic optimized for different environments and hardware.
I am not a licensed HVAC technician. I'm a Systems Administrator. But if you think about it, a house is just a giant server chassis, and your HVAC unit is a massive cooling fan. Standard smart thermostats run on logic that feels like a BIOS from 1995: "It got hot. Turn on fan."
As a sysadmin, I look at the world through telemetry, feedback loops, root cause analysis, and uptime efficiency. When a thermostat short-cycles, an HVAC tech might see a hardware quirk; I see a flawed logic loop and poor resource allocation. I didn't approach this like a mechanic fixing a machine; I approached it like an IT engineer optimizing a data center. The result is a mathematically sound, telemetry-driven orchestration engine that treats your home's thermal dynamics with the exact same respect as a production server environment.
This module is designed to work with any smart thermostat exposed to Home Assistant as a standard climate entity that allows setting HVAC modes (heat/cool/off) and target temperatures. The logic lives entirely inside Home Assistant and is not locked into any specific ecosystem (tested and refined primarily on central ducted 24V platforms like the Honeywell T9 and Venstar T7900).
A Note on the Honeywell T9 vs. Venstar T7900:
This project originally started specifically to work around the Honeywell T9's biggest annoyances, and 99% of the foundational code was built and tested against it. However, I ended up transitioning to the Venstar ColorTouch (T7900) platform earlier than expected.
This does not invalidate the T9 work! The core logic remains identical. However, please be aware that all future updates will be focused on the Venstar T7900 platform and generic open-API thermostats. This absolutely will not bar the T9 (or any other restricted thermostat) from working, but depending on the thermostat's internal logic, future updates may require you to rely on some of my less desirable, brute-force workarounds to get the same results.
Recommended Sensors:
To feed the logic engine, I personally use Shelly ZB e-ink displays for indoor metrics, and standard non-e-ink Shelly sensors for the outdoors. You can use whichever brands make sense for your environment, as long as they pipe reliably into Home Assistant!
- Ensure you have your thermostat and temperature sensors integrated into Home Assistant. (Fully supports native Home Assistant HVAC integrations. I use the Homekit Integration for immediate local-push updates).
- Download
jarki_kettu_core.yamland place it in your Home Assistantpackages/directory. - Open the file and perform a Find & Replace for:
climate.YOUR_THERMOSTATsensor.YOUR_ROOM_SENSOR_1(and2)sensor.YOUR_OUTDOOR_TEMP_SENSOR(Recommended local Shelly/Zigbee sensor)weather.YOUR_LOCAL_WEATHER(Optional cloud weather alternative)
- Download
jarki_kettu_core_ui_dashboard.yaml, open your Home Assistant Dashboard, add a "Manual" Lovelace card, and paste the code. - Important: On your physical wall thermostat, delete all native schedules/timers and set the fan to "Circulate" (if available). Let Järki Kettu drive!
Järki Kettu Core is just the foundation. Over on my Patreon (FoxDen Software), I offer structured tiers to help you take your system to the next level.
(Want just one specific feature? You can grab individual modules a-la-carte from my Ko-fi Shop!)
The definitive upgrade for your smart home. This tier gives you access to my entire suite of premium standalone modules, allowing you to build out a complete, professional-grade HVAC ecosystem:
- 💧 Kuivatta (Moisture Control): Advanced psychrometric math (Absolute Humidity & Dewpoint) to optimally control a dehumidifier via a smart relay, preventing short-cycles and fighting your AC.
- 🌬️ Kierrättää (Scavenger Engine): Intelligent fan equalization logic. Detects thermal anomalies (like a hot gaming PC or a sun-baked upstairs) and runs the HVAC fan to distribute the heat, delaying expensive compressor cycles.
- 📊 Vertailukohta PRO (Envelope Analytics): Upgrades the free benchmark engine to track physical Heat Loss Rate, Thermal Pressure (Stack Effect), and Room Imbalance Penalties.
- 🛡️ Kilpi (Environmental Security): Adds window-open proxy failsafes, cross-breeze weather alerts, and AQI-driven particulate scrubbing.
For extreme power users and developers who want it all. This premium tier grants access to my fully sanitized, monolithic production configuration. This is the exact 2,000+ line backend YAML and Lovelace Dashboard code that runs my own home. It is designed as a raw, unfiltered masterclass in advanced Jinja templating, trigger-based sensors, and massive-scale smart home architecture. If you want to see exactly how a high-end, automated ecosystem is built and interconnected from the ground up, this is it.
So you've seen the Fox Den, and now you want to talk about it. Every project needs a crazy tier. If you are actually wild enough to purchase this, I will personally get on a 1-on-1 call with you. We will talk through exactly how I implemented this system, review your specific home's floor plan and HVAC setup, and I will make custom architecture recommendations for your environment. I don't expect anyone to ever click this, but if you do, we're going to nerd out and build something awesome together. (A friend joked that I should add a $10,000 "You Get Nothing" tier, but I figured I should at least offer a consulting call!)
To manage the sheer scale of a massive smart home architecture, I utilize AI coding assistants as a development co-pilot. While AI helps accelerate the writing of heavy YAML and complex Jinja templates, the core logic, psychrometric math, and fallback mechanisms are meticulously designed, rigorously tested, and actively lived-in by me. This isn't raw AI output; it's a handcrafted ecosystem, supercharged by modern developer tools.
A personal note: I fully understand how important your HVAC system is. It is arguably the most critical and expensive piece of hardware in your home. Please do not proceed with this project if you do not fully understand what you are getting into. These systems cost tens of thousands of dollars, and if something goes wrong, I will not be there to help you fix it.
This configuration controls physical, high-voltage HVAC equipment. It is provided "AS-IS", without warranty of any kind. Local electrical and building codes vary everywhere; if you choose to install any supporting equipment of any voltage rating, you do so entirely at your own risk and without my recommendation. Proceed with extreme caution. You are solely responsible for ensuring your hardware failsafes and compressor delays are active at the equipment level. This code also assumes your system is operating within manufacturer specifications; equipment running out of specification will produce unexpected results.
The "Bug" Disclaimer: Just a final bit of CYA (Cover Your Ass). I am surprised you read down this far, but I think it needs to be said: I did not write Home Assistant. While we all know Home Assistant is incredibly reliable, it is not perfect, and I am not a full-time software engineer. Bugs will happen. I will do my absolute best to patch obvious issues when I find them or when they are reported, but this is a one-man passion project and I can only help so much. If you've read this far, you clearly care about your system, and I'm glad you're here.
Host Availability & Coast State: Because Järki Kettu commands an off state during Coast Zones to prevent short-cycling, if your Home Assistant host suffers an unrecovered power outage or hardware failure while coasting, the thermostat will remain in off until Home Assistant boots back up, you manually select a mode at the physical wall, or the thermostat's native hardware freeze protection threshold is reached. Always ensure your wall thermostat's minimum safety/freeze limits remain enabled in its installer menu.
Trademark Notice: Home Assistant is a registered trademark of the Open Home Foundation. All product and company names (including Home Assistant, Ecobee, Google Nest, Honeywell, Venstar, and Shelly) are trademarks™ or registered® trademarks of their respective holders. Mention of them in this repository is strictly for platform compatibility, identification, and comparative purposes, and does not imply any affiliation with or endorsement by them.
PolyForm Noncommercial License 1.0.0
Provided for personal, non-commercial use only. You may modify and run this in your own home for free, but commercial redistribution, bundling, or paid client deployment (e.g., smart home installers or contractors charging clients to configure or install this logic) is strictly prohibited under the base license.
💼 Commercial / Installer Licensing: If you are a professional smart home integrator or HVAC technician wishing to deploy Järki Kettu in client builds for profit, you must purchase an active Commercial License Exemption via Patreon. This tier grants the legal right to use the code in commercial client environments. Note: Commercial tiers are for licensing rights only. They do not include technical support, an SLA, or liability assumption. You assume all risks for your client deployments.