To limit the probability that, where multiple HVAC systems serve a single space, the energy model calculation method will not accurately account for details of the proposed house with regard to the priority of order of operation of heating and, where installed, cooling appliances, which could lead to inaccuracy of the model, which could lead to overestimation of the energy used by the reference building or underestimation of the energy used by the proposed building, which could lead to excessive use of energy, which could lead to an unacceptable effect on the environment.