going back to the oversized furnace short cycling...couldn't he just lower the btu output through the gas valve.
manufacturers recommend 2-4 in wc, but 2 would work in that situation...right?
Not on something with in-shot burners, 2 stage and variable furnaces slow down the inducer to match the lower gas flow. On a natural draft furnace underfiring would just cause heat exchanger to run cooler and possibly rust from condensation unless you slowed the blower too. On an in-shot reducing gas flow might cause ignition and flame issues from the incorrect air-fuel mixture. On an 80% you might be able to get away with disabling a couple burners, but i wouldn't try it on a 90, not only could the different expansion of the hot and cold cells crack something, but the air would dilute the flue gas possibly reducing the amount of condensation in the secondary HX reducing the efficiency of the furnace.
Originally Posted by J-thetechwizard