Friday, July 8, 2011

Suction Pressure too low on Carrier 48SS?

Refrigerant undercharge of the system will  cause the suction pressure to be abnormally low.  The head pressure will be low as well.   Check the superheat and subcooling (refer to Refrigeration and charging).  If  the condenser coils and fan are okay but the head pressure and suction are low and  the superheat is high and the subcooling is low
, then you likely have an undercharge  of refrigerant.  Restricted indoor airflow will also cause low suction  pressure.  Check the fan speed to make sure it is on the higher speed for cooling and thus  will be likely to be on a lower speed for heating.  Check the cleanliness of the evaporator coils and blower wheel.  If either or both is  dirty, you will have a major restriction of the airflow and thus you will have a lower  suction pressure because not enough heat is being added to the refrigerant from a correct  amount of air as it passes over the cooling coils.  Make sure all the supply air grills are open, or at least mostly open.  Make certain that  the return air grill and opening is clear.  Check the filter.  Be aware that the so-called permanent and "free of allergy" type filters  will restrict a lot even when perfectly clean.  Don't ever pull the filter out and set it  aside until you can buy a new one.  The unit will get very dirty in a hurry.  LESS LIKELY CAUSES:                <Crushed ducts are not real common, but do occur.   It is even a possibility that insulation inside the ducts has become unglued and is  blocking the flow of the air inside a section of the ducts.  Installation booklets  have been left inside the evaporator cabinet or under it and caused restricted airflow.  Restriction of the refrigerant flow can also be the cause and there  are many places this can occur.  Look for tubes at the end of one or more of the capillary tubes not sweating.  If  capillary tubes become restricted with carbon, enamel or foreign objects, that tube  or tubes will not allow its coil tubes downstream of it in the evaporator coil to  cool well and they will not sweat like the others.  Sometimes a thermostatic expansion valve (see Refrigerants and Charging) will  not flow enough refrigerant. 
drawing of TXV remote bulb on suction lineLook for the REMOTE BULB and make sure it is secured to the suction line well by CLAMPS and the surface of the suction line under it is reasonably clean. Try removing it from the cool suction line by loosening the clamps and dangle it there in the hot closet or attic for a while and check the gauge pressures at the condensing unit. If the suction pressure is now higher and the subcooling and superheat become more normal, then you likely have a problem with the TXV.
Look also for "kinked" refrigerant lines and check the filter/dryers to make  sure they are not restricting the refrigerant flow.  The downstream end of the  drier will be noticeably  cooler if there is a restriction inside it.     picture of filter/drier
Click this link for manual of Carrier 48SS:---,sx-12si.pdf