Systems & Airflow

These issues come from system configuration — specifically supply temperatures and whether a system has been set up at all.

Load-determined Supply is negative, or cooling airflow stays at the minimum

Select a space in System Design > Air Balancing. The sidebar shows Load-determined Supply and Required Minimum Supply.

Load-determined Supply is the airflow the space’s sensible load needs. It goes negative when the supply temperature is above the cooling setpoint. It is zero when the two are equal.

The design supply airflow itself is never negative. HVAKR sizes a space to the largest of three values: its load-determined supply, its required minimum supply, and its outside air requirement. A zero or negative load-determined value is therefore discarded. The space falls back to its code or ventilation minimum.

The symptom is cooling airflow that stays at the minimum and does not respond to the load. It is not a negative CFM.

Causes:

Supply temperature too high. The final cooling supply temperature (at the central or terminal unit) is at or above the indoor cooling setpoint. ΔT is then zero or negative, so the load-determined supply is zero or negative.
Missing supply temperature. The coil has no target temperature. HVAKR skips load sizing for the space and reports Supply air temperature not specified for central (<system>) unit! (or terminal (<zone>)).
Swapped values. Heating and cooling inputs entered in the wrong fields.

To fix, check the cooling supply temperature in the system configuration and set it below the indoor cooling setpoint. See System Configuration. If it’s still unresolved, share the project with HVAKR support for a closer look.

A system’s cooling-coil outputs read N/A

This happens when a system has no enabled cooling coil. Neither its terminal unit nor its central unit has a cooling coil turned on.

HVAKR then has no coil target temperature to size against. It reports No enabled coil on terminal (<zone>) or central (<system>) unit!. Load-determined Supply reads zero, and the coil-dependent outputs are unavailable.

On the Loads report, the affected cards are the per-mode unit cards. They are Central mode Data at a system scope and Terminal mode Data at a zone scope — for example, Central Cooling Data - AHU-1.

It does not affect the underlying raw load calculation. If you’re only after a theoretical load calculation, missing coil outputs do not mean your envelope or space loads are wrong.

Until the system has an enabled cooling coil, expect:

Per-mode unit cards reading N/A
Coil-related outputs (coil loads and psychrometrics) unavailable
A Load-determined Supply of zero, so airflows fall back to the code or ventilation minimum

Enable a cooling coil on the terminal or central unit to populate these outputs — see System Configuration.

Average Space Setpoint reads N/A

HVAKR reports two differently weighted average setpoints. Check which one you are reading before you compare them:

Average Space Setpoint on the Loads report’s Project Cooling Data and Project Heating Data cards is weighted by each space’s required supply airflow for that load condition.
The Floor-Area Weighted Average Space Setpoint point on the Configuration psychrometric chart is weighted by floor area. It also covers only the spaces the selected unit serves.

The two values differ whenever airflow per square foot is not uniform.

A space contributes to the Loads report value only when both conditions are true:

Its space type sets the setpoint for that load condition.
Its required supply airflow for that condition is above zero. Code and ventilation minimums count, so a fully configured system is not a prerequisite.

The value reads N/A when no space contributes. For cooling, every weight is zero until the project has a computed cooling peak, so the cooling value stays N/A until cooling loads calculate.

To populate a real value:

Assign a space type that sets the cooling and heating setpoints to every space.
Give each space a supply airflow requirement, from its load or from a code or ventilation minimum.
Let cooling loads calculate, which the cooling value needs for its weights.

The setpoint then displays correctly (e.g., 72°F).