A combustion system model, HFLAME, was developed and validated in the 1970’s to simulate the seasonal energy performance of furnace and boiler systems. We describe here its basic features and its use to analyze the influence of cycling frequency.

Cyclic operation of combustion furnaces and boilers is a standard technique for matching output to heat demand, and more recently, for shedding load as part of energy management. However, there seems to be a lot of confusion about achievable fuel or cost savings with combustion systems as a result of increased cycling frequency.

This is interpreted to result from 1) Failing to distinguish clearly between operating modes at constant output and those modes intended to reduce the output (and comfort) via duty cycling, and 2) A range of field tests indicating both positive and negative savings results for the constant output mode.

All the savings we obtained with the HFLAME mathematical model were small and within a ±4% range. We found that fuel and cost savings vs. increased cycling rate were negative for conventional FWA systems with atmospheric burners, standing pilot and cycling circulating fans or pumps, operating to provide equal comfort. We obtained positive savings only with “Low Off-Period Loss Systems”, in the 1-4% range, depending on cycling frequency change, off-period loss, fan/pump operation or heat exchanger design (flue enthalpy flow profile symmetry).

To be meaningful, the above type savings and those resulting from load shedding should, in our view, only be quoted in relation to the associated equipment and/or to the fraction of energy delivery/energy demand, and/or comfort level.