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"Zone Forced Air - Discharge Air Sensor (DAS)"

14 Feb 2020

Summary: most zoned forced air systems have an discharge air sensor (DAS) installed in the duct work just above the furnace blower but before any zone duct work dampers. This sensor is used by the zone control board. On most systems, it simply monitors the temperature of cooled air and if air gets too cold, shuts down compressor to avoid the coils in the blower unit from freezing. On other systems, the DAS is used by the zone control board control to provide more flexibility in the control of the blower unit, outside compressor and heating unit.



Background: Zoned forced air systems use a discharge air system (DAS) to monitor the temperature of the air coming out the top of the blower unit. Air coming out the blower unit can either be heated or cooled. In most cases, the DAS simply monitors cooled air and if the air temperature falls too low, the zone control board shuts down the outside compressor to keep the AC coils in the blower unit from freezing. Why air would get so cold coming out of the blower could be because the fluid/gas/Freon in the AC system is low or perhaps too many vents in one zone are closed or perhaps the return grills are blocked by dirty filters or, or, or.....  The point being, a frozen coil can damage your outside compressor and thus the auto-shutoff via the DAS.

In a Lennox zoned forced air system, the DAS is also used to allow for some limited control of other functions, such as staging of outside compressor, shutting down natural gas furnace if temperature gets too high, etc.

Most HVAC technicians simply think of the DAS as a frozen coil monitor but if you have a Lennox system, this is not entirely true and you might need to educate your tech on how the zone control board uses the DAS.

Below is an image of my DAS installed in my duct work. Although not clear in image, it is well above the coil set in the blower unit but before any zone dampers. A DAS is a simple, no polarity, 2 wire probe. In the case of the Lennox system, it is not possible to read any voltage on the problem electrical connections as the zone control board pulses this probe periodically and pulse is of low voltage.

Replace DAS: DAS probe do go bad. In the case of the Lennox system, the zone control board will display an error code indicating the DAS has failed. If you get this message on whatever system you have, first check that the wires from the zone control board to the DAS have not come loose.

Depending on your system, new DAS should be available from multiple sources but you must get the exact model number of old installed DAS.

How you remove old DAS and install new depends on your specific HVAC configuration but in my case, probed slips into a mounting plate and is tightened to the plate by provided nuts.



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