Same principle just with a different efficiency number. I doubt any but a fraction of combi boilers are 98% efficient: most people run them too hot, so they are

Same principle just with a different efficiency number. I doubt any but a fraction of combi boilers are 98% efficient: most people run them too hot, so they aren't able to recover sufficient energy to reach that goal. But in any case, you now have to account for installing and maintaining the gas network, refining the oil into gas, shipping the oil to the refinery, extracting the oil in the first place, exploring for the oil in the first place, and so on. It's far more efficient to burn that fuel to move existing energy from one place (outside) to another (inside)... you can turn 1J of fuel into 4J of heating if you're lucky!