Modeling DX systems with set cfm/ton constraints

Barbara, Appendix G is not penalizing you as you describe. If your baseline DX systems work out to less than 350 CFM/ton than the baseline supply air CFM has not been calculated correctly. Based on a 20F design delta T, required by Appendix G, the supply air CFM should be approx. 555.5 SCFM/ton which is higher than the 350/400 CFM/ton you describe. I would confirm that you are performing the sizing runs correctly. The proper method for doing a sizing run for an Appendix G baseline energy model is: 1. Set all internal load and envelope parameters to the baseline conditions. 2. Set design day criteria to the required ASHRAE conditions specified in Appendix G. Remember that programs like eQUEST require the creation of special heating and cooling design days otherwise the peak heating and cooling is based on the weather file. 3. Simulate the model and find the peak heating and cooling loads for each system. 4. Increase the peak heating loads by 25% and the peak cooling loads by 15%. 5. Calulate the supply air flow rate for each system using a 20F delta-T. 6. Enter in the calculated heating and cooling capacities and the supply air flow rates for each baseline system. 7. Simulate the final baseline HVAC energy model. If your energy modeling program is not doing this correctly you may have to do it manually to get the correct answers.

I did. That formula applies to any sensible load whether its a load on a coil or a load in a space. Appendix G assumes that ventilation loads are handled by the system prior to calculating the baseline supply airflow rate. That is why it specifically says you use a supply air temperature to room air temperature delta-T of 20F, it assumes a 55F SAT and a 75F room temp setpoint. So you are correct that if you add in the ventilation load the CFM/ton of a specific unit would be less than the 555 CFM/ton I mention. The baseline does not assume you are using a "real world" piece of equipment.