Propane

Propane is the fuel where the marketing phrase does the most damage. 'Indoor safe' on a box means the heater carries an oxygen depletion sensor and a tip-over cutoff — it does not mean the appliance can run in a sealed room, and every manual says so in a box that most owners skip.

Two things separate propane from kerosene in practice. The first is water: burning propane produces roughly 1.6 kg of water vapour per kilogram of fuel, all of it released into the room, which is why streaming windows and mould in wall corners are the classic symptom of an under-ventilated propane heater long before anyone worries about carbon monoxide. The second is the cylinder. Most serious domestic propane incidents involve storage rather than combustion, and the rule that a 20 lb barbecue cylinder never comes indoors is the one that is genuinely non-negotiable.

Propane vapour is also heavier than air, so a leak pools at floor level in basements, pits and floor voids rather than dispersing upward. That single physical fact drives most of the storage regulations in this area.