The constant Zone is a zone(area) that is always supplied air while the system is running. The purpose of the Constant is twofold. It mixes cool air (or hot air) into the air being returned to the indoor unit. The temperature sensor is in the return air. This sensor controls the amount of cooling or heating of the air flowing through the system. The constant also acts as a spill zone. e.g.if only one bedroom(zone ) is turned on and there is no constant zone the following will occur. The return air will be taking in air from a warm part of the house. The sensor will send a message to make more cold air. The system will try to send all of the air down one duct (because all the other ducts/zones are closed). The system will run at full capacity, yet not supply sufficient air to flow across the coil. This damages the Air Conditioning system and over cools the bedroom that is turned on. The constant zone is a very important part of the ducted air conditioning system design. It is located within a few metres of the return air grill.