Heating a coil pressurized with nitrogen in itself is no problem. However! If the coil is plugged with rubber plugs on the connections, you could wind up with a high velocity rubber plug being shot out of the coil when the furnace heats up that nitrogen. This happened to me with an oil furnace I fired up to keep us warm during the installation of a new HVAC system. When that plug blew out of the coil, it about scared me half to deathIs the new coil sealed with nitrogen pressure in it?