Unit lacks heat/defrost capacity.
1. Test the pilot solenoid and 3-way valve for proper operation.
2. Check refrigerant charge.
3. Perform compressor capacity test in cool mode.
4. Check the suction pressure regulator or throttling valve for proper operation.
5. Check the discharge pressure during heat or defrost. The discharge pressure should rise to 300 – 350 psig when suction pressure is 30 psig or higher for R-404A.
6. If the unit passes the pilot solenoid, 3-way valve and compressor tests, and the discharge pressure does not rise in the heat/defrost mode, replace the discharge pressure regulator.
Unit shuts down because of high discharge pressure.
1. Check unit pressures during cool mode operation and repair as required.
2. Run the unit in low speed heat and monitor discharge pressure. If discharge pressure rises quickly to an excessive level (in several seconds) there may be a restriction in the 3-way valve, discharge pressure regulator valve or the hot gas line at the evaporator inlet.
3. If discharge pressure rises slowly to an excessive level, check the suction pressure regulator or throttling valve for proper operation.