1. Keep existing heating system and replace central AC with an ASHP

For the existing heating system: under the "Input" screen in the HVAC field, add an HVAC system.

Select equipment type = heating only, furnace/boiler/etc. that matches the current heating system. If you are keeping the existing heating system, select upgrade action = Keep an existing system as-is.

If the home has an existing cooling system, add a second HVAC system with equipment type = cooling only, central AC with standalone ducts/room AC/etc. that matches the current cooling system if applicable. Select upgrade action = Remove a system permanently. This is necessary to ensure proper utility calibration of the current mechanicals in the home.

To add a new ASHP, add a third HVAC system and select the proper ASHP system under "both heating and cooling", whether ductless or ducted. The Upgrade Action will be, "Install a new, non-existing system".

At this point, it is a judgement call to determine how much of the heating load will be met with the ASHP versus the existing furnace. For example, in climate zone 3A like northern Georgia, it might be reasonable for the ASHP to take on 90% of the heating load; whereas in climate zone 7 in the Colorado mountains, it might be closer to 50%. The percentage of the heating load that is not met by the ASHP should be met by the furnace. If the ASHP meets 90% of the heating load, the heating load of the existing gas furnace should change from 100% as the base load percentage to 10% of the 'improved' load percentage.

The HVAC section of the inputs screen should look roughly like this for proper modeling of an existing heating system with an ASHP replacing a central cooling system:

2. Fully replace existing heating system and central AC with a hybrid gas/heat pump heating system

Complete replacement is a bit easier than keeping an existing system. The general principles remain the same.

Under the "Input" screen in the HVAC field, add an HVAC system. Under "System Equipment Type" dropdown, choose furnace/central AC (shared ducts). Under upgrade action, choose "Remove a system permanently".

For the ASHP, add an HVAC system. System equipment type = Central Heat Pump (shared ducts), and Upgrade action = Install a new non-existing system. Enter the available information for the new system.

For the supplemental heating system, add a third HVAC system. System equipment type = heating only/furnace with standalone ducts. Upgrade action will be, "Install a new non-existing system". Choose the correct fuel type and enter the remaining available information. As with the other scenario, it will be a judgement call for how much of the heating load the new ASHP will take on. If the ASHP is taking on 50% of the heating load, the furnace will take on the remaining 50%.

If fully replacing a conventional split furnace and AC with an ASHP with fossil backup, the inputs screen should look roughly like this: