The Garn system design manual warns against installing a hot water coil in systems serviced by high efficiency condensing furnace. Doing so will void the furnace warranty and UL listing of the furnace and create the potential for flue damage and a building fire. This is not referring to coil install on the return side but a warning against installing at all on a condensing gas furnace. Anyone know the reason behind this? I know I've seen hot water coils installed on the supply side above the A/C A-coil on condensing LP furnaces plenty of times, usually supplied by a Central boiler OWB. The manual has a separate warning against installing the hot water coil on the return side mentioning the blower motor and furnace electronics will not be happy .