low suction pressure lennox heat pump
I have a Lennox 410A heat pump system, model XP19-036-230-01 (2-stage, 3 ton), with indoor unit CBX32MV-036-230-03 and Signature thermostat 81M28. The only problem I can find is a low suction pressure which is 15-20 psi less than normal at design ambient. Other measurements seem OK. Here's an example: Ambient outdoor temp 105F yesterday; in Y2 (stage 2) mode, suction pressure 133 psi (normal 153), liquid line pressure 420 psi (416 normal), subcooling 6 degrees and approach 9 degrees (both normal), split condensor temp 13F, split evap coil temp 19.5F. Only 0.3F drop over entire liquid line temp. The numbers are similar for Y1 (stage 1), that is, the suction pressure is 135 (normal 157), again a 22F difference. Indoor coil (including TXV) just replaced with new (old coil had a leak). Line sets are 75 feet in length, 1/2" OD high side and 7/8" low side (I mention this because the Lennox Refrigerant Piping Design and Fabrication Guidelines on pg 18 show 3/4" OD to be the optimum pipe size for 3 ton, 410A, even though the Lennox Installation Instructions on pg 6 recommends 7/8" for the vapor line). Do I have a problem? Is this evaporator starvation? Any help will be incredibly appreciated.