Have you ever had an outside air temperature sensor fail, causing a building’s automation to go nuts? Instead of overriding the entire building, why not just override the sensor? Usually with building automation systems you have to explicitly program this functionality. This is an area where BACnet shines: the protocol was designed by ASHRAE with servicing parts in mind!

Override BACnet Input Objects

Normally you can’t override BACnet input objects, since they are read only. However, they all have an “out of service” property that will allow it. Accessing the feature will vary depending on what automation system you’re using. You should be able to access the “out of service” property of a point in its property sheet or detail page. When you mark the point as “out of service” it will accept override commands. This will allow you to set it to something reasonable while you source a replacement part. Be sure to release the “out of service” property when you have replaced the part, and your override will automatically be replaced with the actual sensor reading.

Gotchas

Like always with BACnet and even other protocols, it’s possible the manufacture of your control system did not take the standard seriously when implementing it and using this feature will do nothing, or it will throw “write access denied” errors at you when you try. Sadly, there’s no way around that. If they don’t want to implement the protocol, they don’t have to. If you’re using a BACnet gateway to translate one protocol to another, you can likely disregard this post and most other posts about BACnet as gateways typically implement a bare minimum of the protocol.