Who is correct?

superfittertech

unless it's ancient that Carrier RTU doesn't have a cap tube it has multiple pistons in the evap header. more than likely one or more are plugged or the liquid line screen (it's not a dryer) is plugged up. The easiest way to tell is to remove the belt off the blower or disconnect power to the evaporator motor if it's direct drive and then run the compressor. You should have equal frost on all of the orifice tubes. The ones that are not are restricted or if the liq line screen is plugged it will have a temperature drop across it. If it's the orifices replace the whole evaporator. It's a real pain to replace the orifice tube header

superfittertech

also reason other tech installed head pressure control is to raise the head pressure high enough to raise the suction pressure to prevent freeze up. Probably running about 300 or more head pressure

DirectComfort

A super heat and sub cool reading would have told you every thing that was wrong with the refrigerant side of the system. You can not properly diagnose a problem like this with out a sub cool reading. The sub cool reading would have told you if it was under charged, or it was a restriction.

hvac2010

Thank you for your advise. I will check the subcooling next time.

user2561

This is the dead give away. Diagnose why the compressor has been replaced so many times. It is never good to simply replace the compressor....that is the symptom of a bigger problem.

1)Check air flow for proper static pressure
2)Check that there are no "hidden" dryers that others techs may have installed or left behind.
3)Check the temperature of each capillary tube while in operation. They should all be relatively close. If one or more are way off..well, there's your answer.
4)If all of them are icing up and you are positive that the correct charge has been added and you have eliminated air flow issues and other possible causes...then the manifold for the cap tubes is the culprit.
5)If the unit is utilizing a compressor with manual service valves make sure they are not either broke or partially closed/opened.
6)Make sure the blower wheel is turning in the right direction.
7)Make sure the condenser fan prop or motor have not been incorrectly installed or sized by prior techs.
8)Depending on the ambient temperature during which the unit is cooling you do need a fan cycle switch however the setting you described would be incorrect. Most manufacturers will tell you what subcooling they are trying to achieve of the high side. The switch should be set accordingly. For example if this is a hair salon in a bustling mall or unit for a restaurant with a large heat load in the kitchen, they will be using cooling during the winter months. When ambient temperature get too cold and the equipment is trying to cool without a fan cycling switch the evaporator and metering device will ice up. The symptoms would be very similar to an undercharged situation even if the equipment was properly charged.


To help in properly troubleshooting it is very helpful to have dianostic sheet where you can add data and look at it collectively. This helps make sure that you are looking at all possible causes and allows you to have a comparitive look.