I know temp control is a big factor when home brewing and I have found that keeping the temp where I need it during fermenting is difficult. I have seen temp control devices that help heat up the fermenter but do they also cool it down if need be (not that I have run into high temp fermentations yet)
Not sure where in the world you live
@nvillone so my answer may be US-based on some assumptions. People all over the world do this however so you can too.
You sort of need to start with the proper terminology. You first have a controller. This is responsible for sensing temperature and causing devices to turn on and off to maintain a temperature in the controlled mass (be that air, water, whatever.) You have a system to either add or remove heat (you never really cool, physics tells us that) or both. When you add this to homebrewing you typically have a chamber of some sorts (a small refrigerator is an awesome choice, I have a couple of keg forts I use for this. The fridge provides a nice confined area in which to control temperature, and is also of course gives you that heat removal. It depends on your needs however. Some folks never need cooling or very little. If you have a nice cool place and/or are doing only ales, something like
Son of a Fermentation Chiller can work.
When you hear (read) me talk about a controller though, you may think "thermostat." A thermostat is a sort of controller but not all controllers are thermostats. A controller, especially for our purposes, should be smarter. A thermostat generally turns on when the temp is a few degrees above or below it's setpoint, then the inverse happens on the other side. So, if it turns on at 68 for a setpoint of 70, then back off again at 72, you have a comfortable house but a 4-degree swing on your beer which is not a good thing for your yeast.
A controller can be a lot smarter than that. Let me use the analogy of a car since nearly everyone has driven. To maintain 70 MPH (again, I'm American so I apologize if you are not) you could keep your foot in one place till the speed dropped, or you could see a hill coming up and gradually apply more power so you maintain that speed without dropping. The steeper the hill gets, the more you press on the accelerator, and everyone behind you is happy. Now the top of the hill is coming, you begin to let off and as you head downhill you use less and less accelerator to make sure you don't come flying off the hill like TJ Hooker on a Saturday rerun. In this "circuit", you are the controller. You provide proportional, integral and derivative changes to the accelerator based on your desired speed and the environment, along with an understanding of the power of your car, to maintain a nice 70 MPH.
That proportional, integral and derivative control "loop"
is called PID and is everywhere you look in the world. It used to be the realm of pneumatics (ever walk past a commercial thermostat and hear it hissing?) but things have almost all gone digital.
At the bottom end people use room thermostats. The problem with this is 1) the wide swing in most thermostats and 2) you are sensing the air around your beer, not the beer itself. Your little yeasties generate heat while they do their thing, and fermenting beer can be several degrees warmer than the ambient temp. As long as you are going to so "something" you may as well do it well. A lot of people kid themselves by saying "I don't need anything that expensive" or "I don't need anything that complicated" but done correctly the results are far worth a couple extra bucks and a few hours. Spend well, spend once, cry once. I don't know how many dollars I've wasted in my live "saving money." Solutions like the STC1000 or Inkbird do sort of control things in a nice compact package, and are fairly easy to implement, but my personal opinion is that if you are going to do it, do it well.
So hopefully by now you are with me that if you are going to do this you 1) want
control not just a thermostat and 2) you want to control the
beer not the chamber it is in. Are you with me? It's easily possible Here's a screen shot of
@CadiBrewer's fermenting beer. You can see it's maintaining the beer temp (yellow/amber/straw-colored line) within 0.1°F which is pretty good in anyone's book:
Brewers have had hair-brained ideas to do this since they have been brewing. Some worked, some did not. When technology meets brewing however, wonderful things happen. As far as I know, Elco Jacobs in 2012-ish was the first to look at a pile of parts on his bench and an old fridge and decide "I'm going to do this right!" He created "
BrewPi", an amalgamation of "brewing" and "Raspberry Pi" (a small single board computer) to which he added an
Arduino Uno which is a small single board digital controller. For around $175 US back then you could buy a system that would yield these results. Amazing. He's improved it ever since and BrewPi 3 is far more capable than ever before.
He did another amazing thing though - the project was made open-source. Around 2014 or so he decided to abandon the Arduino in favor of more capable controllers because Brewers being the geeks they are, wanted more. Also in 2014,
@FuzzeWuzze started
the mega thread on making your own BrewPi "for cheap." It's gone through a lot of iterations since then, and there have been forks like
BrewPiLess,
Fuscus,
Fermentrack and others. The original BrewPi (called "legacy") continued to be available on Elco's GitHub, but eventually it became harder and harder to make things work because technology moves on. Supporting software was no longer available since the Raspberry Pi's operating system no longer used some older, less secure, less mature packages. Someone resurrected BrewPi and now
BrewPi Remix is available which is as faithful a representation of the original as possible while still adding some functionality and arguably easier to implement than it's ever been.
So ... what's it cost? The answer is "it depends." Folks have "stuff" laying around so you never know. Assuming you have or can lay your hands on an old fridge, you add the cost of a Raspberry Pi (if you don't have one) which can be as low as $10 for the Raspberry Pi Zero W if you go to your local Microcenter or similar venue. Add an Arduino which can be under $10 for a Chinese clone, some miscellaneous wiring, a couple probes ... it's not that expensive. Or you can go the route of one of the other solutions which use ESP8266 controllers - I think ultimately the cost will be similar. You will be able to keep your yeasties VERY happy, brew beers you never considered previously. and it's just plain cool!