How are the BTU Designed values calculated for systems built on the Ductless Tab?

The IDU and the ODU BTU Designed values that display on the proposal PDF are calculated using the Design Criteria entered for the system, the nominal value of the ODU/IDUs, and the layout piping lengths. The app will make system BTU adjustments for Cooling and Heating Temperatures, Piping lengths, altitude, and in some cases the defrost factor.

The app uses the manufacturer's nominal heating and cooling values as the "starting point" for BTU adjustments.

Example: LG Nominal Test Conditions	Indoor	Outdoor
Cooling	80°F DB / 67°F WB	95°F DB / 75°F WB
Heating	70°F DB / 60°F WB	47°F DB / 43°F WB

To simplify this explanation, I built an LG LMU300HHV system with NO corrections. 

The Design considerations view allows you to see the BTU adjustments that have been made to the ODU and the IDUs.

First, the app calculates the available system BTU for the ODU. Temperature, altitude and defrost corrections are made to the ODU. ODU BTU designed is the maximum number of BTU the ODU can carry. In this example, the ODU BTU designed is 28,400 Cooling BTU.

Then the app makes piping length adjustments to the IDUs cooling value. This value is the IDU BTU Adjusted.

In this example, no adjustments were made to the IDU's cooling capacity.

Each unit can "produce" 7,000 BTU for a cooling system total of 28,000 BTU. The total cooling IDU value of 28,000 BTU is less than the ODU designed value (28,400 BTU). The IDU system total limits the capacity of the system. The total system capacity will be based on the available IDU BTUs. (28,000 Cooling BTUs)

7,000 is 25% of 28,000 (7,000/28,000)

25% of the IDU cooling system total (28,000) will be allocated to each IDU = 7,000 IDU Designed Cooling BTU

The IDU Heating Designed value is adjusted for this system. Each IDU unit can "produce" 8,100 BTUs for a heating system total of 32,400 heating BTUs. The ODU designed value (28,600 BTU) is less than the IDU heating system total. The ODU system total limits the capacity of the system. The total system capacity will be based on the available ODU heating BTUs (28,600 BTU).

8,100 is 25% of 32,400 (8,100/32,400)

25% of the ODU heating capacity (28,600) will be allocated to each IDU = 7,150 IDU Designed Heating BTU.