I hear all the time that the condenser unit should run a saturated condensing temperature which is at least 20 ( newest models) or 30 ( older models ) degrees higher than the out door temperature. Now, If I have my pressure gauges connected to the port valves at the condenser unit, I'm reading the pressure of the refrigerant after being sub-cooled, in most cases a good sub-cool value is 10 degrees. it could go up to 15 degrees. ( saturaded condensing temperature - liquid line temperature)
My question is that: Using this pressure ( head/high/ or liquid line ) then converted to degrees I should have a difference of about 20 ( newest ) or 30 ( oldest ) degrees. Am I , right? Then , I could determine whether the out door coil is dirty or not.
Novice. Thanks for any help you can give me.
Usually the 20F-30F rule will get you in the right area as far as head pressures go. It only takes a few minutes to spray the outdoor coil with a hose.
High head pressures may also be caused by a excessive load, high ambient, a refrigerant overcharge or a liquid line (hot gas line) restriction.
On some of the older units when I find a high superheat and low subcooling and I suspect the unit is undercharged, the first thing I do is clean the outdoor coil. A plugged outdoor coil can cause the head pressure to increase. A plugged outdoor coil can also cause the suction pressure to increase. After I wash out the outdoor coil and let it dry, I notice the head pressure drops and the suction pressure drops. Now, in some cases the suction saturation temp has fallen below 32F and the indoor coil will begin to freeze. That's why it's always important to make sure that outdoor coil is clean, before you add refrigerant. Alot of people get fooled by a dirty condenser coil.
For example, before I clean the outdoor coil my pressures are 70 psig low and 200 high. After I wash the outdoor coil and let it dry, the low side can drop below 60 psig and the high side can drop to 150 psig. Now it is clear the unit is undercharged.
outside air temp plus seer = condensing temp
6-7 seer plus 30
8-9seer plus 25
10-11 seer plus 20
12-13 seer plus 15
14-16 seer plus 10
so lets say that the outside air is 80 degrees and your working on a r-22 system 10 seer
80 plus 20 = 100 condensing temperature