Control theory is a bitch
this post was submitted on 19 Mar 2025
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If you want your climate control to be automated, you need to automate it.
I used to be a building automation technician for commercial buildings. Simple temperature control is standard for them and easily achievable on your own with consumer hardware, but you need to know what you want your system to do and then do your research on the hardware and software necessary to achieve it.
You may be interested in something like HomeAssistant, where you set it up on your own, or you might want an installation company to come in and set up something like Control4 for you.
You could also go super simple and run a wire from the unit to a thermostat or temperature relay. Then you just set up the temperatures you want it to turn on and off at.
That absolutely 100% exists. In commercial buildings, this is the standard. Nobody has to go to the BMS (Building Management System) and tell it to switch to cooling or heating. It's all automatic. Even higher end residential thermostats do it. A couple of years ago I had my gas furnace replaced with a heat pump/electric heat system and it automatically switches between heat and cool as I've programmed it.
Hell, you can get systems that are sophisticated enough that every individual room in your house has an independent thermostat that automatically maintains a different temperature in each room. Of course, that'll require a separate VAV for each room, which isn't common in residential buildings.
The primary reason you don't see "smarter" systems in homes more often is because they're more expensive. Most people leave the fan on their HVAC system on auto and just switch between hot and cold manually as needed. And for most people, that's only a few times per year. Most people switch it to cool sometime in the Spring, then leave it there until the Fall when they switch back to heat.
A more complicated system is just paying more money for something you don't really need.
Higher trim cars have radiation sensors to help account for solar heat gain.
Honeywell has individual room sensors available that you can average or prioritize.
I think the real reason you don't see them- the average person is too stupid to understand them and set them up.
Less about people being too stupid and more about people just not seeing the point. The average person touches the AC/Heat switch only a few times a year.
I personally love the days where it's cool enough at night to open the windows and get the house temperature down. Means the AC the next day doesn't have to do nearly as much (and thus my electric bill isn't as high).
They can, they're called smart thermostats. They need to know the outside and inside temp and honestly the easiest way to do that is to just connect them to the internet. Plus they're even better when they know it's going to be cold all day vs cold for half the day, then the sun comes out and is really hot the second half.
Most thermostats do not have an external thermometer or internet connection.
They achieve comfort by setting a desired min/max temperature that ignores outside temp because that has nothing to do with indoor comfort.
I have an Ecobee that is set to heat to 68°F and cool to 78°F in the daytime. The only time I care about the outside temperature is when I could open a window to save money.
There are a lot of smart engineers that design those.
If you have an external temperature measurement it helps. But it can't see the difference between a cloudy day and a sunny day.
If you connect it to an online weather service that helps, but it's still hard for it to know how much the sun heats your home due to big south facing windows for example.
This is something AI could be useful for. Feed it forecast, inside, outside temp and let it figure out how to balance those against each other for your home. I'm not sure there are many systems like this on the market yet.
You don't need to go to that level of complication.
Two sensors in combination, one that detects current heat input one that detects absorbed heat. These modules would be placed about the outer walls.
Then calculate how much heat is going to radiate into the building the rest of the day.
And it can compensate.
We don't need to be more than a fraction of a degree off and a system like that would be amply accurate.
What kind of sensor do you suggest for measuring that? I'm genuinely curious why they are not industry standard.
My guess is price and/or robustness. The RTCs we use are cheap and durable.
My thermostat has a hot and cold setting that will heat when cold and cool when hot- I can also set it to a mode based on outside temp or time of day. You might just need a smarter thermostat.
I do exactly what you’re wanting with a smart thermostat integrated with Home Assistant. Because my old, horrible apartment is poorly insulated I have everything automated based on indoor/outdoor temps.
Why do you care about outdoor temps for your indoor setting?
If the outside temp is hotter there is no need to heat and if it is cooler there is no need to cool, because the house will even out with the temp.
It is slightly more complicated because of sunlight heating the building, but there is no need to spend energy changing the temp a few degrees when it will happen due to changes in temps outside. Like there isn't a need to run the AC in the evening shortly before dark if it is only a couple degrees too warm inside because it will cool off on uts own pretty quickly.
But that's why thermostats have a range and turn on/off cool/heat as needed?
If you were gone for a day with everything off then it would make sense to not overshoot. But when you are home it's not like it goes from 30-60F instantly outside. As the outside gradually changes, so does the amount of heat or cooling on the inside.
With a thermostat, smart or dumb, you set a target temperature and a time. With a dumb thermostat it waits till that time and then activates. With a smart thermostat it should learn how long it takes to heat or cool to that target temperature in certain conditions and then aims to hit the target at that point.
So if you got up at 8am and wanted it 20c with a dumb thermostat you got to work out when it needs to go on in order to hit that as no heating system is instant on something the size of a house, with a smart thermostat with learning you do not need to do that at all, just set it for 8am.
As no system is working in a vacuum how hot or cool it is outside, even how sunny it is, has a big impact on how quickly your system heats or cools. Being able to measure and compensate for the outside temperature means the actual start time can be adjusted for you. This can save significant amount of cash.
As an example, lets say the outside temperature was going to be -10c 6am till 8am and you wanted it 20c by 8am. Doing it with a dumb thermostat you would either have to live with an under or overshoot on temperature. Say next day its 2C, now you need to adjust your overshoot again. With a smart thermostat I do not need to do that at all.
Sure, you can just live with the under/overshoot, but its better for your bills and better for the environment not to.
Yes, this entire post is in search of a thermostat and it's hilarious. Bizzaro.
There are experiencing hysteresis!
Mine is amazing, I set it and no matter what the temperature is outside it stays the same inside!
For the reason I explained:
Because my old, horrible apartment is poorly insulated I have everything automated based on indoor/outdoor temps.
thermostats don't run off what the temperature is outside.
usually, they have set points- heat comes on if the temperature goes below x, cooling/air conditioning comes on if it goes above y.
Does your system have a physical outside sensor for temperature? or is it pulling that information off the internet? your immediate location and how you're situated on your property (plus things like how well your home is insulated, the color, how many windows you have... etc,) has more to do with how hot it gets inside. Or, for example, if you get a lot of sunlight coming in, it can heat up the house with it being horribly cold outside.
Regardless, if it's not staying comfortable, look for a "heat and cool" option and then adjust the set points to within a more comfortable range. keep in mind that will cause energy costs to go up as it's now actively heating or cooling more. (This time of year plays merry hell with HVAC systems,)
Tbf, they are just annoyed they have to do something with the HVAC, like press a button or a switch.
Wait until they find out you have to change a filter
If your willing to get into the weeds, most HVACs are very basic, and you can easily make your own thermostat with a cheap relay board. That plus a microcontroller and some code (and/or homeassistant) and you can make it as smart as you like.
There are smarter thermostats, but they are usually more expensive.
If you do microcontroller, look up PID regulator functions. Stabilizes anything measurable with super basic math.
Most HVACs thermostats have a target temperature range and can toggle heating and cooling to stay within that range - usually with some minimum range logic to prevent constant swapping.
This is also a classic modern house problem. In areas with wild temperature sways like what you're describing traditional house building would include hacks to capture and retain the cool air in the morning and shed as much excess heat as possible during the day.
When we lived in Puerto de la Santa Maria we'd throw open windows to create a cross breeze in the mid afternoon and keep the air flowing (assuming it didn't get too cold) into mid-morning at which point we'd seal off the east facing windows and draw curtains over the windows and doors to keep the heat out - swapping to full enclosure as it got to just before lunch time - then opening east facing windows and leaving the west shut util it was time to start the cycle again.
It was unbearably hot in direct sunlight during the hot months but the house stayed cool - especially with thermally conductive tiles and stone being the primary exposed surfaces. As long as you properly cycled the house you could keep it comfortable and it was easy to adjust by airing it slightly less or letting more heat in.
Wooden houses with huge sliding glass doors in direct sunlight that have no shades or curtains can't do this, however.
My friends family has a shore house we've gone to a few times. It's an old house, built before a/c was a thing, and still doesn't have any. We throw open some windows and the house stays pretty comfortable, it's warm but not at all unbearable even when the temps are in the 80s, 90s, occasionally even over 100 (fahrenheit of course)
It does help that it's at the shore so there's basically always a nice breeze.
I just set my ecobee to “auto”; the only possible inconvenience is the required gap between heating setpoint and cooling setpoint so it doesn’t constantly flap back and forth as a result over overshooting the setpoint. I have the gap set to 3 degrees and tend to shift it up and down seasonally (e.g. heating setpoint at 72 and cooling setpoint at 75 in the winter, heating 69 and cooling 72 in the summer)
Insolation and air infiltration based on wind conditions likely affect loss as much as outside temp, and the thermostat doesn’t know that (it could presumably learn to relate time of day, cloud cover, day of year (solar angle), wind conditions with that - but it would take 2 years to learn and its response would be opaque - most homeowners want bang-bang control since they understand it.
Ecobee thermostats have an automatic feature that does that. I only use the dedicated heat/cool settings since there aren’t wild shifts like that in my area.
My Nest thermostat does this. And I use Nest Sensors to adjust to temps according to time and location in the house.
I tried that once but it wasn’t for me. If it’s been cold out and is suddenly warm, I’ll bask in the heat or open windows.
Actually I’m a bit annoyed that my car does that. I set the car thermostat to heat to a specific temperature but then it also cools to that temperature. I haven’t figured out if there’s a way to set it to heat (or cool) only
I may be a Luddite, but I never use the Auto setting for the cars ventilation system. I also don't want A/C on with heating. If it's cool outside, I'd rather run the fan harder with no A/C.