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     several times on ac systems with cap tubes or piston metering devices I need to add freon to raise the evap pressure  to 60#. but in so doing I get too low a superheat reading. normally I expect the suction line to be slightly cool to the touch. but when the suction line gets very cold I think its because of the freon I added in order to raise the suction pressure to a point high enough to keep the evaporator from freezing. the system will work this way but I don't like that cold suction line.                                   any suggestions of insights?




  • JStarJStar Member Posts: 2,752

    Why do you try to get 60# suction on every system? It should be different for every system depending on a number of factors.
  • TechmanTechman Member Posts: 2,144

    Having a lower than 60psi is a" it depends " situation. A rule of thumb here might help,so,what is the outdoor air temp? Not lower than 70-72*.What is the indoor air air temp? again ,not lower than 70-72-74* Is this your 1st time on this unit?Oversized? Dirty evap,filters,blower wheel?Duct sizing wrong?Supply registers not open?And checking the charge is better done by the subcooling,not the superheat.
  • TechmanTechman Member Posts: 2,144

    Got your e-mail. I TOTALLY agree with you about Profressor Silberstein' book "Pressure Enthalpy without tears" very good but can't say I grasped all of it .LOL LOL. Again,a rule of thumb, on cool days you can get fairly close pressure readings. On a 50-60* I place a plastic tarp around the cond coil to raise the head press press up to 95* condensing. [r-22 is about 182psi] and then charge the low side upto 60psi or so ,depending on the other rule's of thumb.It's close ,but better than waiting for all of the units  for a warmer day. It's kind of like a motormaster head press control.
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