CH-89-20-1 -- Effect of Furnace Size on Morning Pickup Time PDF

Thermostat setback at night is widely used during the heating season to achieve energy savings. The time required in the morning to recover from overnight temperature setback is an important characteristic of a heating plant.

Various performance aspects of residential space conditioning systems are being assessed in tests at two unoccupied research houses in the Washington, D.C. area. Tests were conducte during the 1987-1988 heating season using four commercially available gas-fired and electric resistance furnaces sized 20% to 103% over the design heat loss for the houses. The thermostat setpoint in the houses was 72°F (22.2°C) and an eight-hour night setback of 12°F( 6.7°C) was implemented. The presence of occupants was simulated by use of heat and moisture generators, lighting, and scheduled operation of appliances. The research houses are moderately furnished to approximate conditions of occupancy for a family of three.

At an average indoor-outdoor temperature difference (delta T) of 50°F during the setback period, the morning pickup time was two hours for a gas furnace oversized by 37% and and 0.9 hours for a gas furnace oversized by 103%. Morning pickup times were linearly correlated with a wide range of delta T for each furnace. Test results indicated that the morning pickup times for the furnaces tested were longer than those previously published in the ASHRAE Handbook. Results are presented and discussed as they relate to furnace sizing for acceptable pickup times.