T027_Z
Supply Temp Limit Exceeded to Bypass
Status
This is a non-critical fault. The bypass damper will close and may result in a blower cutback fault. System is sending too much heating or cooling back into the return via the bypass damper.
Description
A bypass damper can be used to send excess air, between the maximum airflow to a zone or zones and the minimum capacity of the system. When too much air is being bypassed it can threaten system reliability.
Cause
Furnaces and air conditioners are designed to send conditioned air into the space, they are not meant to just circulate air through the coil or heat exchanger over and over. There may be a false expectation at play here with tiny zones and a system unable to move enough air into the zones.
Solution
Run a duct measurement to determine zone sizes in the service menu. Then check the airflow limits in the zone settings screen of the main thermostat. Do NOT use unlimited as this will only complicate the problem. Set the smallest zones to high. If the problem persists, the technician may have to set mechanical minimum damper positions on other zones to take some of the excess air when the smallest of zones is running. It's either that or divide up the zoning more realistically.
For Example, lets say you have a 4 ton system that is single stage. The airflow requirements to allow the system to run may be 1400 CFM. And lets say its broken into 3 zones, two of which can allow 1000 CFM by themselves, but the smallest zone could only allow 300 CFM. In order for the system to run with just this zone calling, 1100 CFM would have to go elsewhere. Allowing it to all go back into the return will surely result in the leaving air temp sensor exceeding it's limit. If the sensor weren't there, surely the furnace limit would trip or the coil would freeze. It may be best to look to the other zones to not go completely closed by setting a mechanical minimum position.
