Kelvin

Place an air system's unit and registers

Put a unit and its supply, return and exhaust registers on the plan, lay grilles out evenly, and read each outlet's throw without a verdict.

HVAC design · For owners, admins and engineers · Updated 7 Oct 2026

The Air distribution plan on Air Systems is the building plan with each air system's unit and registers drawn over it. Each register's airflow is worked out from the system's supply and the served spaces' loads, and the registers are what Ducts connects its ductwork to.

  1. 1

    Choose the system under Air system and the Level to draw on.

  2. 2

    Choose Unit and click to place the unit, where the system's supply ductwork will start.

  3. 3

    Choose Supply, pick a Register type, and click to place each supply register in its space. Return and Exhaust place those grilles.

  4. 4

    Or choose Lay out grilles… to place a space's grilles evenly in one go.

  5. 5

    Choose Save plan.

How each register's airflow is set

The system's supply airflow is shared among its served spaces in proportion to each space's peak sensible load. Within a space, a register set to Entered keeps the airflow you give it in Entered airflow, and the registers set to Space's share divide the rest equally. A return or exhaust register starts on Entered. Set it to Space's share to have it balance the space's supply.

A register outside every room, in a room that isn't a space, or in a space the system doesn't serve is listed under the plan with the reason, and has no airflow.

Laying out grilles

  1. 1

    Choose Lay out grilles… and pick the Space and the Supply type.

  2. 2

    Under How many supplies, give A count, or choose From a datasheet and enter the Most per outlet with the datasheet it came from. Kelvin divides the space's airflow by it and rounds up.

  3. 3

    Give the number of Returns, or leave it blank to keep the returns already there, then choose Place grilles.

Supplies are spread evenly over the space, and returns are kept as far from them as the room allows. Each grille takes an equal share of the space's airflow. This system's share supplies already in the space are replaced. It is an even layout, not a selection: check each outlet against its manufacturer's data.

Pressure drop and throw

  • Select a register to enter its Pressure drop at its airflow and From the datasheet. The drop is added to the duct run that ends at it, so it counts on the index run.
  • Under Throw, choose the outlet's Kc3 row with Outlet (for its Kc3) or type Its own Kc3, and give Its area and its source. Kelvin then shows the throw to 0.25, 0.5 and 0.75 m/s by ASHRAE Fundamentals Ch.20. The 0.25 m/s throw is reduced by 30 %, as Ch.20 directs for that zone.
  • The library's Kc3 values are the chapter's generic ones, given for example purposes only. The outlet's own constant from its manufacturer is the one to use.
  • The grille layout table gives each outlet's distance to the wall, half-spacing to its neighbour, characteristic length, throw to 0.25 m/s and the throw over that length.

Note

Throw is reported, never judged. The 2025 edition of ASHRAE Fundamentals Ch.20 no longer prints the ADPI selection guides that ratio was read against, so Kelvin holds no criterion for whether a throw suits its space. Kelvin also holds no register or diffuser catalogue.

Editing marks

Select a mark to edit it, drag it to move it, or right-click it for Rotate 90°, edit and delete. With a mark selected, Delete removes it and R rotates it. A mark belongs to the air system chosen when it was placed; other systems' marks show faint.

The plan is the same one Space Loads and Ducts draw on. Unsaved changes show on all three, and Save plan on any of them saves the whole plan.