Guide - control a BACnet output at a chosen priority

Goal: you control a BACnet output from Home Assistant at the priority you choose, and you know how to hand control back to the controller.

Before you start

The device is already added (see Getting started).
The output is a switch (Binary Output or commandable Binary Value) or a number.
Read Safety on a live site and try this on one object first.

Steps

Go to Settings → Devices & Services → BACnet IP. Each BACnet device is its own entry, titled with the device name. Open the entry of your device.
Under Configuration, the Write Priority entity is not listed yet, because it is disabled by default. On the device page, open the "+1 entity not shown" link, open the Write Priority entity and switch on Enabled. Home Assistant reloads the integration after a short delay.
Use the dropdown to select a priority. The levels are 8 (Manual Operator), 9, 12, 13, 14, 15 and 16 (default, lowest). The selected priority is kept after a restart and applies to the writes of all entities of the device. Switching on, switching off and setting a number now write at that priority.
To write once at another priority, without changing the Write Priority, use Developer tools → Actions and run bacnet.write_value on the entity. Binary objects take 0 (inactive) or 1 (active):
```
action: bacnet.write_value
target:
  entity_id: switch.ahu_1_supply_fan
data:
  value: 1
  priority: 11
```
To release the command, run bacnet.relinquish at the same priority. After the release, lower priorities or the Relinquish Default of the object take over.
```
action: bacnet.relinquish
target:
  entity_id: switch.ahu_1_supply_fan
data:
  priority: 11
```

Off is not released

Switching off writes inactive (0) at the same priority. That commands the output off; it does not release it. Only bacnet.relinquish hands control back.

If it doesn't work

Home Assistant shows an error on a write: the device rejected it. Check the log, and check the bacnet_commandable attribute of the entity.
The write has no effect: a higher priority (a lower number) holds a value. Priorities 1 to 15, including a Manual Operator command at 8, win over 16. Write at a lower number on purpose, or release the other command.
You cannot find Write Priority: it is disabled by default. Enable it as described in step 2.

More detail: How writes work, bacnet.write_value and bacnet.relinquish.