When a building has no cooling system, the software simulates a hypothetical system with the characteristics required by Table 150.1-A as if a cooling system were installed. The result is neither a penalty nor a credit.

When natural gas is not available, and all other eligibility criteria are met (see Chapter 4), a wood heating system is simulated as a hypothetical system with the characteristics required by Table 150.1-A or -B for a typical heating system. When all eligibility criteria are met, the backup system is not modeled; otherwise, see Section 8.5.3.

Buildings with multiple HVAC systems are treated as follows:

1. For buildings with more than one system type, equipment type, or fuel type, where the types do not serve the same floor area, model the building zone or floor area served by each unique type separately.
2. Supplemental heating may be ignored if (1) the capacity of the supplemental unit does not exceed 2 kilowatts (kW) or 7,000 British thermal units per hour, and (2) the supplemental unit is controlled by a time-limiting device that does not allow it to run for more than 30 minutes. (§150.1[c]6.)
   In a building with a central gas furnace and an appliance-rated gas fireplace, the furnace is the primary system, and the fireplace is the supplemental system. In this case, the controls for the fireplace would not need to meet the setback thermostat requirements (Exception to §110.2[c]).
3. For single family buildings served by more than one heating or cooling system, equipment type, or fuel type, model the least efficient system. For any areas served by electric resistance heat and another heating system (except for wood heating meeting all eligibility criteria), the electric resistance system is the least efficient system.

When there is more than one system meeting the heating or cooling load for the same space, all systems must meet all mandatory requirements of the Energy Standards.

When higher than minimum efficiency is modeled, a HERS Rater must verify the efficiency. This includes:

• Seasonal Energy Efficiency Ratio (SEER) or Seasonal Energy Efficiency Ratio 2 (SEER2)
• Energy Efficiency Ratio (EER) or Energy Efficiency Ratio 2 (EER2)
• Combined Energy Efficiency Ratio (CEER)
• Heating Seasonal Performance Factor (HSPF) or Heating Seasonal Performance Factor 2 (HSPF2)

The performance approach may be used to show compliance for alterations to existing buildings, additions, and existing + addition + alteration as discussed in Chapter 9. When existing conditions are unknown, the default assumptions in Table 8-1 must be used. The standard design for an altered component shall be the higher efficiency of existing conditions or the requirements stated in Table 8.1 Standard Design for an Altered Component For components not being altered, the standard design shall be based on the existing conditions. When the third-party verification option is specified as a requirement, all components proposed for alteration for which the additional credit is taken must be verified.