Every 2 years, since we burn a lot of logs to ease the fuel bill. We used to burn just oil when it was 12p per litre, then it went up annually to 18p, 26p, 30 odd etc - you know the rest. We didn't get it serviced annually then either, but now that oil is so pricey I think it's worth keeping up with the servicing to ensure efficiency. I'm thinking about replacing the old girl soon, as she is about 16 years old. First though I want to check out the bio fuel issues. I think I'll start a new thread regarding this.
K