Natural gas central heating in our house. We've had to use it for half an hour a few times because the new house is badly insulated - single glazing on a north facing bedroom is a bad idea - and we haven't had the time or the free cash flow (still waiting to sell other house!) to fix it.

Since I'm here all the time we try to keep the occupied areas - all on the north facing side of the house - at about 15-17C or so.