To prevent the fumes produced by the burning of the diesel fuel from accumulating indoors, these heaters must be properly vented to the outside of the building through a flue or chimney. This allows the fumes to be safely expelled outdoors rather than being released into the indoor air.

We are very good at separating the exhaust from the heat. We can extract about 90% of the heat energy, send the exhaust outside while keeping the heat inside. Basically there's a heat exchanger that gets heated up by the reaction, and air blows over it, getting heated up and the exhaust stays completely sealed off until it goes outside. The same thing happens to heat a gas car, it's just using waste heat from the engine.

The combustion changer takes in air from, and exhausts fumes to, outside of the room.

A heat exchanger transfers the combustion heat to the room air.

The heat exchanger circuit is hermetically isolated from the combustion chamber.

       ╔═══════════════════════════════════════════╗
       ║                                           ║
       ║                 Room                      ║
       ║                                           ║
       ║     ┌───────────────┐                     ║
───────╨─────┘ ┌───────────┐ │                     ║
> Air in       │           └─┴────────────         ║
───────╥─────┐ │           Hot room air out >      ║
       ║     │ │           ┌─┬────────────         ║
       ║     │ │           │ │                     ║
       ║     │ │           │ │                     ║
       ║     │ │           └─┴────────────         ║
       ║     │ │           Cold room air in <      ║
───────╨─────┘ │           ┌─┬────────────         ║
< Exhaust out  │   Heat    │ │                     ║
───────╥─────┐ │ exchanger │ │                     ║
       ║     │ └───────────┘ │                     ║
       ║     │   Combustion  │                     ║
       ║     │     chamber   │                     ║
       ║     └───────────────┘                     ║
       ║                                           ║
       ║                                           ║
       ╚═══════════════════════════════════════════╝

[deleted]

> The same thing happens to heat a gas car, it's just using waste heat from the engine.

Most cars do not use exhaust heat to heat the cabin. They use a water to air heat exchanger that extracts heat from the water cooling system. So, combustion heats water >> water heats air.

Yeah, that's because the engine can't handle that kind of heat. It's still the same principle of removing heat from combusted fuel. If there were no radiators in a car, the heat would stay in the exhaust and engine.

The point is combustion produces heat, and we can remove that heat without touching the fumes. It doesn't become exhaust until.we want to get rid of it

That is a very impressive ASCII picture. Might be more understandable as a linked picture though.