The second question I have is why do the coil temps vary so much with different equipment brands. I just but a 4 Ton 12 Seer unit in my home. The home already had the ductwork installed so I just retrofitted it. I used a 4 Ton 12 Seer Luxaire condenser with a 5 Ton Air handler and coil. I have been instructed that this will gain me a little in the effeciency. I did not however install the TXV valve I used the recommended pin instead. I understand that this hurts the Seer a bit. It seems the system has a hard time keeping up in my home. The freon is boiling at about 44F (74Lb) which seems high. (High Pressure on the Suction side) and the High pressure seems low at 175 lb of pressure. I matched the pin however it seems I may need a pin with smaller oriface. Is this a product of larger coil or of the 12 Seer of the unit. Or is my unit over charged. I added 29oz which was what the manual said for the lineset length as well as the coil installed.
First of all, I agree with jultzya, one OEM's chart does not apply to all equipment out there.
Nailing a charge on a piston system without an OEM's chart isn't a breeze but it's not all that difficult, either. By using a combination of factors, such as superheat, subcooling, delta T across indoor and outdoor coils, etc., you can get very close to the exact charge for a piston system, which incidentally is a critical charge system.
Notice I included subcooling as one of the parameters to observe while charging. Yes, you do charge by superheat with a piston system, but knowing subcooling helps you to know how well your condenser is working. Knowing the indoor and outdoor delta T's after a charge is adjusted will help you know how well the system is performing. Knowing the superheat will tell you if the evaporator is being overfed, starved, or fed properly.
In your case, if you live in a humid climate, your 5 ton evap on a 4 ton condenser may give you a sensible heat removal advantage but a dehumidification penalty. Especially with a piston on the system. Pistons do not regulate superheat well, so there are times your system, properly charged, will run a starved evap under high heat loads and a flooded evap under low heat loads. This condition can be aggravated by a mismatched coil and/or an incorrect piston orifice size.
My recommendation to you is if you wish to keep this coil size, put a TXV on the system. You'll get superior superheat control, higher efficiency, better dehumidifying. You will have to shift your thinking from charging this sytem by superheat to charging by subcooling once you go with a TXV.
If you wish to keep the system as is, tweak with the charge until you get good delta T's, superheat, subcooling, etc. Make notes of all readings when you obtain good performance from the system, such as ambient temp, amp draw, all delta T's, subcooling, etc.
If no matter what you do this system still runs with its tongue hanging out, your piston orifice may be oversized. This would show up on your superheat reading as low superheat under normal operating conditions (flooded evap).
Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.
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