Anyone who is using oil solely to heat their hot water should take pause & fully evaluate their situation. Factor in all the maintenance costs, fuel costs, liability potentials (oil leaks are very bad things), tank upkeep & replacements, space requirements, insurance issues, etc. etc., and you may be farther ahead getting rid of it all together & replacing with an electric water tank for DHW, & a few electric baseboards for backup heat if needed. That said & back to the topic, my old oil/wood combo unit that had a Riello burner was only serviced twice in the 17 years it was in use. It was always kept hot, burned regularly (way too regularly) making DHW in the off-heating season, but 'only' went through a couple hundred gallons a year (90% of that for off-heating DHW). Had no issues with burner performance - that Riello worked like a top. It is now a memory though and I am off the oil.