If you have an oil fired boiler with a tank-less coil (for hot water), your boiler must maintain a high boiler temperature to produce a minimal amount of hot water for domestic use such as showering and cleaning. Oil fired boilers with tank less coils are constructed of either cast iron or steel and contain between 12 and 40 gallons of water. That is allot of water to heat, even in the summer time. We suggest a Cold start boiler (does not need to maintain a high temperature), Low mass (low water content 4 – 7 gallons) and multi pass which means that the heat produced from the oil burner will pass through the heat exchanger (cast iron sections) multiple times. A tri-pass boiler will have about 7 feet of heat exchange verses a conventional pin type boiler which has about 18″ of heat exchange. This results in a much lower exiting flue temp of about 350 degrees verses 500 or higher from a conventional boiler. That is a big difference. The cold start boiler we recommend is a Biasi boiler.