This can be tested in a basic fashion by using a hollow tube fitted through a gap in the door. Seal the rest of the door opening completely with ceramic fibre and fit a balloon on the other end of the tube. A slight inflation of the balloon indicates slight positive pressure. Furnace pressure is controlled by the flue and flue exit size assuming the gas and air flows are at the most efficient settings. If burners have been undersized, more capacity is possible by increasing the flue height and the draw to allow greater furnace gas inputs.
Furnaces take longer to reach operating temperatures during the initial heat up phase.
Allow this time to slowly heat castables or furnace parts that may not have been fired past critical temperatures previously. Check the heat up rate recommended by the manufacturer to be safe and use a pilot or small torch as a low rate if necessary. Refer to specific instructions on setting up and adjusting burners. Nozzle mixing burners usually require special proportionators in the gas line for accurate high to low control of the flame although some furnaces require models that are able to modulate the gas only for better turn down and even temperatures. Premix burners have a gas adjusting valve under the aluminium cap located on the mixing chamber.