An unvented liquid propane–gas (LP) space heater, often used in animal housing, was modified to provide automated variable heat output. As manufactured, the heater had manually–adjusted output settings between 14.7 and 29.3 kW (50,000 and 100,000 Btu/h). The heater was modified by adding a stepper motor and speed reducer to the manual proportioning valve to provide continuous and automated heater output adjustment between 14.7 and 29.3 kW.

Experiments were conducted to compare animal occupied zone (AOZ) velocities, AOZ temperatures, and fuel consumption using on/off and automated variable control modes. These experiments were conducted using inside set point and ambient air temperature differences of 14 and 28C (25 and 50F). All experiments were conducted in the Air Dispersion Laboratory at Iowa State University.

Adding automated variable control reduced the AOZ temperature fluctuations from 5.3C (on/off) to 1.1C (9.5 to 2.0F). In the AOZ, the standard deviation across the entire test room was reduced from 1.4C (on/off) to 0.3C (2.6 to 0.5F). The AOZ airspeed fluctuations were reduced from 10 cm/s (on/off) to 2 cm/s (20 to 4 ft/min). No differences were found in fuel consumption. Results indicate a need for heaters capable of continuous adjustment to even lower energy output levels.