Figure 1 plots the furnace run time (minutes/hour) for a 1,200-head, wean-to-finish room a few days after the pigs were placed into the facility. In addition to three, 250,000-Btu direct-fired propane furnaces, the pens were equipped with propane-fired infrared brooders located in the sleeping zone of each pen.

In the morning, after every furnace run cycle, the variable-speed fans on Stage 1 minimum ventilation increased their speed, effectively increasing the ventilation rate to the room. During this period, the controller was set to turn the furnaces off when air temperature was 1°F below the set point, with the minimum-speed fans increasing speed whenever air temperature was 0.1°F above set point.

At noon, I reset the controller to have the furnaces turn off at 1.5°F below the set point. For the remainder of the day, the run time of the direct-fired propane furnaces was 0 minutes.

The net result of changing the furnace off temperature setting by 0.5°F in this facility was a savings of $4.50/furnace/day in propane when propane was priced at $1.20/gal.

In general, furnaces should be set to turn off at 2°F below the set point temperature of a facility. If the variable-speed fans still occasionally operate after the furnaces cycle on/off, this should be increased. Variable-speed fans should never increase speed following a furnace heat cycle, as variable-speed fan increases are designed to remove heat, causing the furnace to cycle on/off sooner.