low suction pressure lennox heat pump

[dr air]

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.

[cjpwalker]

I have found Lennox's "normal operating pressures" chart to be extremely unrealistic.

There is no way to know if you are starving your evaporator without a superheat reading, but based on your pressures, subcooling, and split I would say it sure looks good so far. Is the house keeping cool?

[dr air]

I have found Lennox's "normal operating pressures" chart to be extremely unrealistic.

There is no way to know if you are starving your evaporator without a superheat reading, but based on your pressures, subcooling, and split I would say it sure looks good so far. Is the house keeping cool?

Thanks for your help. Yes, the house is cooling fine. Sorry that I forgot that superheat--- which was 27F (Y1 or Y2 mode). I wonder whether that SH is a bit high, as well.

[dr air]

Had original installer come out to assess my concern. Everything checked out OK. They suggested that the lower pressure may be due to the longer run of 75 feet. The slightly higher SH taken at the compressor (rather SH at the evaporator) was also due to the long run through two hot attics and a vaulted ceiling. Makes sense to me. I'm happy.

[jtrammel]

Low side gauge out of calibration?

[Bsmith816]

You don't have any problems. You can't go by what it says your pressures should be. If you are within 1 degree of your target subcool. And your temp split is within 3 degrees, your system is running perfect. Then your pressures are what they should be. That chart is for ideal perfect conditions

[vstech]

if the chart was listing wetbulb numbers, you would likely be exactly where it says you should be. drybulb numbers are based on a indoor RH of 50 usually, so if you are higher or lower, the pressures are going to higher or lower. yeah, 75 feet of lineset through an attic will certainly effect the numbers. I wonder why they used such large lineset...

[dr air]

if the chart was listing wetbulb numbers, you would likely be exactly where it says you should be. drybulb numbers are based on a indoor RH of 50 usually, so if you are higher or lower, the pressures are going to higher or lower. yeah, 75 feet of lineset through an attic will certainly effect the numbers. I wonder why they used such large lineset...

1. The Lennox manual doesn't specify, but I would assume dry bulb when it says, "Outdoor Coil Entering Air Temperature"
2. I put in the long line-set 15 or so years ago, in order to move the outdoor unit to the opposite side of the house and make use of the driveway!

[dr air]

You don't have any problems. You can't go by what it says your pressures should be. If you are within 1 degree of your target subcool. And your temp split is within 3 degrees, your system is running perfect. Then your pressures are what they should be. That chart is for ideal perfect conditions

Thanks much for the reply.
A month ago I set the SC to EXACT spec of 6F (outdoor temp was 115F). Recently I had the installer come out to check the unit and the SC was 10F (using my Fluke meter with thermocouple, SC was 8F--- don't know who's equipment was better!); outdoor temp 100F and quite humid (just had rained). Tech said all was fine, forget the charts. The difference in SC, he said, was due to ambient temp changes between the two days which will change SC. So, one day I'm at target SC, off 2-4F on another!
One question: Not sure what you mean by temp split within 3F?
Thanks again.