Section 658-111-5 - USE OF PRIORITY CONTROLS FOR MULTIPLE UNITS1. A priority control for installation of an oil-fired warm air furnace and an oil-fired hot water heater when the existing chimney flue is not of adequate size to accommodate both appliances is allowed. The chimney must be lined. This is required when replacing an existing furnace or water heater when the chimney is not properly sized for both appliances. 02-658 C.M.R. ch. 111, § 5