2-stage Gas Furnace
High-efficiency 2-stage furnaces have been promoted using the promise of lower operating costs for gas and electricity, thanks in part to the brushless DC variable speed blower motor. 2-stage gas furnaces are simply gas furnaces with 2-"step" gas valves that operate at two distinct heat input rates. One example control strategy used responds to a call for heat by firing at a Low heat input first and if the call is not satisfied within a set time-frame the gas valve increases to the Hi heat input rate. Control logic determines the lag time between Low and Hi input based on a number of system conditions. The Low firing rate can be 50-75% of the Hi firing rate (e.g. 20 000btu/hr-40 000btu/hr or 50 000-75000btu/hr).
Combined with a variable speed blower, the manufacturers claim that this system will increase comfort, reduce noise and save money on operating costs. Electricity savings were expected due to the BLDC (brushless DC motor, aka ECM) blower and improved gas conversion efficiency were expected as well. The purpose of this project was to use the CCHT to shed some light of the "real" performance of a two stage system in comparison to a single stage system in order to enlighten Canadians, utilities and HVAC contractors. The purpose was not only to identify the potential benefits in comfort and savings but also to determine the source of any savings (e.g.. better efficiency, BLDC motors, etc.).
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