During The Arks scenario in Frostpunk, it’s your job to make sure that The Seedling Arks stay warm. You will need to keep the temperature above chilly for prolonged periods of time.
If the Seedling Arks get too cold, you’ll fail the scenario. The way that you complete this scenario is by using Automatons. You’ll need to build a Factory first, by finding the factory plans first. Then you can craft Automatons which will gather resources and work on machinery 24 hours a day.
Of course, you will need to gather lots of Coal, in order to keep the generator running. You can assign automatons to work on Coal machines, so that the generator never needs to be switched off. Meanwhile, you can search into Coal Optimisation and Automaton Efficiency, which will greatly increase the amount of coal gathered.
Keeping the Arks Warm
The first, and easiest way to warm up the Arks in Frostpunk is by researching heaters. You can do this by building a workshop and hiring some workers. In the workshop, you can research heaters straight away. These allow you to heat up most buildings that hold workers.
You can turn on a heater by selecting the Ark, and pressing the heating button towards the top of the interface on the left. Heaters won’t really need to be activated until it becomes slightly cold. Just keep an eye of the temperature of buildings by clicking the temperature icon at the top of the screen.

Later on, you will need to research Improved Heaters and Advanced Heaters to increase the heat that it outputted. Research Heater Efficiency Upgrade to also make heaters require less coal.
Another way of keeping Arks warm is using Steam Hubs. These are built on streets, and allow you to heat up an area around them. Build them near Arks to increase the heat supplied to them.

The amount of heat let out by Steam Hubs entirely depends on what level your generator is sitting at. You can also research heater efficiency too later on, to reduce the amount of coal used.