The first thing you gotta realize, is that this isn't an idle game, nor is it Factorio. You shouldn't be letting it run without doing anything new.

What we need for sustainable heating is:

A way to make heating fuel reliably.
A way to see if the heating source has fuel in.
A way to check the amount of items in a storage.
A way to make new robots ourselves.
A central storage for robot fuel. (duh)
A way to paint the robots to differentiate them (optional).

First of all, the optional one is immediately accessible to us. I used white paint, as you can spot it immediately to see where the "refuelers" are.

As for the rest, we gotta follow the game's tutorial. Most of it is pretty self-explanatory, the game really tells you what it needs from you, and it's pretty easy to get some points. But where this guide comes in handy is where to spend them.

As you get points, you spend them to upgrade your programming language capabilities, pretty much.

1. Functions:

The MOST important part of the game, being able to put a bunch of commands inside a function, then invoke that function. This is like, a no-brainer.

The first thing I did when I got Functions was to create a "Go Recharge" function, which tells the robot to check its battery and go eat if necessary. I put the threshold to 50. Note that if you have a battery expansion, you might need to tell them to eat twice, for a full battery.

Inside a Function, you can go as crazy as you want, since you will then only invoke a single block to run its entire algorithm.

Anyway, yeah, functions numero uno.

2. Smart Find & Pick:

This is useful so that our refuelers don't pick from heating sources, but from storages. However, the reason we really need this is to get the next tier.

3. Checker Tools:

Is Empty and Wait are magical tools. See, when a robot Waits it doesn't waste any power (not that I've seen). And a bonfire is considered "empty" when there's no fuel in its queue, even if it's currently burning fuel.

We already have what we need for sustainable heating really, but I'm gonna tell you the next points too, for perfect sustainability.

4. Var Numbers:

The important tool that comes with this is to check the amount of items in a tile, and compare it with a number. So then, I found that the "golden number" for all items (apart from heating fuel, and robot fuel) is 20. So whenever someone is off to drop something, check if it has 20, and don't drop it if it does, instead wait 10 seconds and try again. There, no fuel spent, no materials spent.

5. Var Coordinates:

With this, we can input a coordinate, instead of a single number. Why would we need this? To modify our refueling function so that it only needs coordinates!

And with these we're done, we can make something completely sustainable! Yep, we only need 5 upgrades. And I think all of these cost 1 point each, so 5 points total.

It's quite easy to rack up those points by following the in-game tutorial.

So now we can have a refueller. The most efficient heating source is the Furnace, however to make new Furnaces we need access to copper (which is way up there). However, the logic for other bonfires (fire / battery) are the same.

What I've done personally is make 2 functions, one for all heating sources (depending on what the object the refueller has picked up) and one dedicated to furnaces. I named them "Drop Heat" and "Drop Furnace" accordingly.

But first things first, we will need a robot for EACH heating source for this - but don't worry, they won't be losing power, or constantly recharging.

By the time you unlock batteries, and the battery-related heater, you will have also unlocked a way to make robot bodies and robot heads.

Robot body: Battery + Steel
Robot head: RadioLamp + Steel
RadioLamp: Charged Plate + Glass

You can check the recipes from the items menu.

Drop Heat Function:

Input Variable
Click on variable number and select coordinates -> click on any tile (it will automatically get its coordinates from outside the function)

Is Empty
Connect the variable to that. If it's Full, or On Error, then Wait, and repeat the check. If it's Empty then

Drop To
Connect the variable to this too. And we're done! Just remember to connect this final thingie to the output.

So now any robot we want as a dedicated refueller, we can do:

Find and Pick item -> fire (for example) - remember to de-select bonfires from the menu -> Drop Heat -> Go Recharge -> To Idle

And then add a variable, connect it to the "Drop Heat" block, and click on the number of that variable, select coordinates and select the tile with the bonfire, or other heating source.

What will happen then?

The robot will go pick up the item required, go to the heating source, check if it has any fuel in its queue, and if it does, then it will wait (not spending energy) and check it again! When it finally has 0 fuel, it will add the fuel in its hand, check if it needs to eat anything and repeat the whole process.

TRUST ME this is the most efficient way I've found. I have tried having a refueller for all bonfires, in a cycle, but all this does is have the robot run around without needing to, and spend energy in the process. Having a dedicated refueller for each bonfire is not only efficient, but actually quite cheap, especially if you streamline robot making.

It might seem counter-productive (have 10 robots for 10 bonfires??) but it's because of the game's mechanics. See, we need to reduce the time spent moving as much as possible, to avoid energy loss, so yeah. That's the way.

If you do this, you won't have to worry again for a heat source not having fuel, or having too much fuel.

DO NOT WASTE any wood you get, until you can make more of it.
ONLY SPEND copper for Furnaces, until you have access to the constant supply of it.

So this is were Furnaces come in handy, and why I prefer them. Because they burn wood, which is something you can make reliably. They take longer to burn it, they warm up "very cold" places, and they heat up a large area too.

But to make wood, you have to first unfreeze that Sulphur Spring (that the tutorial guides you towards). Once you do that, it acts as a permanent heating source of its own, so the next step is to plant the seeds, water them (you can do so manually at the start) and you get 5 wood from each!

Then you gotta first unfreeze copper, before you unfreeze the brewery. With copper we can make new furnaces, and we can start replacing those bonfires.

I can't tell you how much this will help you.

Before you unlock the "Organics" upgrade, you can streamline watering the plants easily by utilizing the "On Error" output. See, if you tell a robot to go drop water to a plant, it will go there, and attempt to drop water. If the plant is currently growing, or is fully grown, this command will return an error. So you can have a robot attempting to water 4 seeds (4 are more than enough for this stage) by using the "On Error" output, and if it reaches the last plant and still has an error, wait and repeat the whole process.

Then you can make a robot that simply searches for wood and puts it in storage. Remember to remove the "bonfire" from the smart find and pick.

And there, you have sustainable wood for all your Furnaces now. Nothing is gonna get cold ever again.

Remember how I said at the start that this game isn't an idle game, nor is it Factorio? That's because, at least currently, the resources are limited.

To get wood, you need water. To get water, you need fire+ice. To get fire, you need 2 stone (spark) + coal. Stone, Coal and Ice are all limited resources. They are, in essence, a TON of them, maybe more than you'll need really, but they are, objectively, limited.

So never let the game run in the background, ever. Always pause. During pausing you can still combine, build stuff, even give stuff to the HQ, and program your robots. You can build an entire new colony or streamline entire production chains in a single "tick". This is invaluable, as you won't be wasting resources in the process.

ALWAYS REPORT BUGS!! You might think this is a waste of time, but the devs need to know how many people have had the same bug to prioritize it. Even if it's a small bug, there might be some conflict in the game's code that the devs need to know about!

After this point in the game, it's quite easy to progress, since you can make new robots for refuelling, new Furnaces, and sustainable fuel. After this it's up to you what you wanna do, but my advice is to streamline every new resource you get (always up to 20 items! 100-200 for fuel and robot fuel).

Make use of the large robots! What they do is that they swap storages mainly. So if you want two recharge stations let's say, you can have the large robot pick up an empty fuel storage, and swap it with the base one. In my opinion, you should always swap, never "drop", so that the robots don't drop anything on the ground. The large robot can check the secondary storage (say, close to the sulphur spring) if it's empty (or has less than 10 fuel) pick it up, go swap it for the main one, and bring the main one back and drop it.

The next obstacle is the weeds, but once you're done with sufficiently heating up everything, this is gonna be a safe trial & error really.

Oh and another thing. They added ale sprinklers. You cannot imagine how valuable this is. The primordial ale gives you +2 energy/second for a little time, so you can put it in a passage where a ton of robots pass through, and spend even less time refuelling the robots. With the "is empty" command, you can do it the same way as with Furnaces.

Anyway, I'll stop this guide here as we've covered what this guide is about.

Have fun!