Lennox 14acx-060 5 ton AC low suction pressure issue

[hvacmatt123]

Good morning guys, want to run this by some of you phenomenal pros out there.
I'm dealing with a Lennox 5 ton resi split, issue is with low suction pressure. R410a, 80-90 ambient and can't get suction pressure above 105 psig. If I cover the condenser to drive the head up, it barely has any effect on the suction pressure.
15 deg system SH, 5-10 SC. Ensured airflow is good, even had to install a return air base to ensure 2000 cam capabilities. New coil and txv another tech put in. Everything seems ok, but the suction pressure, so with long run times it will sometimes ice up.
I've went over the specs for the unit, and on a 5 ton Lennox calls for 1-1/8" suction line. This unit is using a 7/8" suction. About a 35' run or so.
Will the undersized suction line prevent my pressure from getting to normal operating pressure?
Let me know if you need any more info. Thanks in advance guys

[AEW]

What's the RA, I had one last week where it was under no load, the suction wouldnt come up above 100PSIG.

Winter is coming...

[BNME8EZ]

Not a phenom pro but I will give my 2 cents.

I don't think that the 7/8" suction will cause that much of a problem, I may be wrong because of differences in equipment but I don't think Trane uses 1-1/8" anymore so 7/8" should work with other brands as well.

Air flow is good, how good? 1500 is ok, 1800 is good, 2000 is better, where are you at, how was it measured, what is your TESP, is this ECM or PSC?

You say you have a SC of 5-10, I have never seen a unit SC change much once it has run long enough to settle in, if you are at 5 you need to add refrigerant, if at 10 you may want to check the TXV even though it is new. One more thought, is the new coil a match to the condenser?

[vibration]

Suction pressure relies more on in door condition, like supply air temp, return air temp(wb), etc. Did you check those numbers?

[vibration]

What's the RA, I had one last week where it was under no load, the suction wouldnt come up above 100PSIG.

Winter is coming...

Tomorrow will feel like 40C.

[mrkelly]

5 ton's 'seem' to always run Low on Suction press and I have no 'idea' why but even back Long ago,as in the early '80s Always were consistently Low as in 60-65# on R-22 even on a 95° ODA day.
Long as you have 'some SH.15 is good imo,at the cond and you have a good split across the evap.It is what it is.
7/8" at 35' is nothing to 'worry' about. 3/4" would be another issue. 1 1/8" is for 50' and above and basically,was a 'recommended' stat for the Manuf's to CTA in the 'event' Of.

[HeatenCoolair]

What evap coil are you running . Lennox has a long list of txv swaps in there engineering handbooks for coils even when they are a match . Many of the counter people arent even aware of this list . Ive encountered something similar. on a brand new system about 5 years ago . Ended up needing to swap out txv and suction pressure shot up to 130 . Check out davenet if you have a log in .

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[hvacmatt123]

What evap coil are you running . Lennox has a long list of txv swaps in there engineering handbooks for coils even when they are a match . Many of the counter people arent even aware of this list . Ive encountered something similar. on a brand new system about 5 years ago . Ended up needing to swap out txv and suction pressure shot up to 130 . Check out davenet if you have a log in .

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Ya it's a good match, and doesn't require the txv change out you're talking about. I checked that right away, and you're right to say not many ppl know of that. I used to work for Lennox and match ups for tax credits was my thing for a while there.
Anyways guys, the original readings were from earlier in the year, that's why I couldn't remember the exact SC value. When I left last, I installed a freeze stat and low ambient fan cycling kit since this customer likes to run the ac when it's nearly winter, literally. When I went back the other morning, I checked the unit and it was actually running great! The call was for no cooling, but ended up being the furnace fuse had tripped. Don't know why, and hasn't had an issue since, chalked up as an anomaly. I guess the r/a values and low ambient conditions caused my low pressure issues.
I'm just glad it's all working great now.

[heatingman]

Good morning guys, want to run this by some of you phenomenal pros out there.
I'm dealing with a Lennox 5 ton resi split, issue is with low suction pressure. R410a, 80-90 ambient and can't get suction pressure above 105 psig. If I cover the condenser to drive the head up, it barely has any effect on the suction pressure.
15 deg system SH, 5-10 SC. Ensured airflow is good, even had to install a return air base to ensure 2000 cam capabilities. New coil and txv another tech put in. Everything seems ok, but the suction pressure, so with long run times it will sometimes ice up.
I've went over the specs for the unit, and on a 5 ton Lennox calls for 1-1/8" suction line. This unit is using a 7/8" suction. About a 35' run or so.
Will the undersized suction line prevent my pressure from getting to normal operating pressure?
Let me know if you need any more info. Thanks in advance guys

Yes undersized suction will absolutely cause lower then normal suction pressure.




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[heatingman]

Ah... I should have read the rest of the posts-

You got it working - leave it be.


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[d2305]

You have some heat during the day, but 62 (17) for a low looks comfortable. The low here yesterday was 79 (26).