[ajross]

> Moist air heats better than dry air

This is sort of true, but only because the density of humid air is lower, and thus the heat capacity (though IIRC the specific heat of gaseous water is actually quite a bit higher than N2/O2, which takes back a lot of that benefit). There is no meaningful change in heat conductivity given that the medium is a (highly convective) gas, obviously.

The overwhelming majority of the energy used to heat your "living space" doesn't go into the air anyway, it's used to heat the walls and floors and furniture which make up the bulk of your home's mass. Being able to heat the air around your candle rapidly doesn't change anything, because that air will just cool down when it hits the ceiling.

[fullstop]

Dry 18°C air _feels_ significantly colder than humid 18°C air. Doesn't humid air also retain the heat longer than dry air?

As for the air cooling down when it hits the ceiling, this is true.. for a period of time. In an ideal situation you have a significant amount of insulation on the other side of the ceiling which inhibits thermal loss.

In my case, I have a heat pump and electric resistance backup. A few years ago I added a whole-house humidifier, which made my home feel significantly warmer at the same set-point and reduced the amount of electricity that I used each month under similar weather conditions.