Why This Zone System Kept Freezing the Coil

Comments · 111

• LK

  @LKN-HVAC · 3 months ago · pinned

  Some home owners need to build walk in coolers for their homes! 🤣🤣🤣 Zone system isn&apos;t helping with bypass allowing entering air in the evaporator coil to be colder than 66 degrees with no discharge sensor to cycle the compressor off. The ambient when it drops will also drop the your suction even more and when people are trying to run cooling when ambient is in the 50&apos;s you need a low ambient control or install a air side economizer 😂. Whenever I had a customer that wanted to turn their home into a walk-in cooler I would upsize the indoor coil by 1/2 ton you can gain about 10 PSI higher suction pressure so saturation stays above freezing. As we discussed before we try to run fans slower for higher latent removal but these customers need 450 CFM per ton not 350! You can always install an automatic freeze stat on the coil to cut Y circuit if it ever does reach below 32 Justin. Here is some food for thought....format sucks but shows the differences in a 3 ton coil vs a 3.5 ton coil on 3 ton system vs more air flow......Use your thermal camera on coil to make sure all circuits are feeding too!<br><br>| Scenario / setup | RA temp (°F) | Airflow (CFM/ton) | Total CFM | Suction (psig) – est. | Evap SST (°F) – est. | Freeze risk | Notes tied to your current readings | <br>| --- | --- | --- | --- | --- | --- | --- | --- | <br>| <b>*Your current condition – just thawed*</b> | <b>*70 → 66*</b> | ~350–400 | 1050–1200 | <b>*105 psig*</b> (measured) | <b>*≈33°F*</b> (from PT chart) | <b>*Borderline / high*</b> | At 70°F RA you’re ~33°F SST; as RA drops toward 66°F, SST will tend to slide closer to **31–32°F**, right at freezing. | <br>| Baseline – matched, low airflow (3T/3T) | 66 | 350–400 | 1050–1200 | <b>*≈100–105 psig*</b> | <b>*≈31–33°F*</b> | <b>*High*</b> | Low CFM + 66°F RA drives SST into the <b>*30–32°F*</b> band → repeat freeze‑up likely. | <br>| <b>*Option 1 – increase airflow (3T/3T)*</b> | 66 | <b>*450*</b> | <b>*1350*</b> | <b>*≈112–120 psig*</b> | <b>*≈36–40°F*</b> | <b>*Moderate–low*</b> | Raising CFM pulls SST up into mid‑30s/around 40°F, giving ~5–8°F margin above freezing. | <br>| <b>*Option 2 – larger evap coil (3T/3.5T)*</b> | 66 | ~400 | ~1200 | <b>*≈118–125 psig*</b> | <b>*≈38–42°F*</b> | <b>*Low*</b> | Oversized coil warms saturation; at 66°F RA coil typically sits upper‑30s to low‑40s°F. | <br>| Option 2A – larger coil + higher airflow | 66 | <b>*450*</b> | <b>*1350*</b> | <b>*≈119–131 psig*</b> | <b>*≈40–45°F*</b> | <b>*Very low*</b> | Best margin: SST well above 32°F even with “meat‑locker” 66°F setpoint. |<br><br>Modern air‑conditioning systems are engineered according to ASHRAE and energy‑code design standards, which specify indoor cooling conditions of 75°F and 50% relative humidity. Your system’s coil temperature, airflow, refrigerant charge, and moisture‑removal capacity are all built around this national design point. When a thermostat is set to 66°F, the system is forced to operate far outside its engineered range. This causes the evaporator coil to run below dew point, stay wet longer, and lose its ability to remove moisture. The result is excess humidity inside walls, ceilings, and insulation, which can lead to mold growth, structural damage, and premature equipment failure. For these reasons, manufacturers and industry standards do not support or warranty operation below design temperature. Keeping your thermostat at 75°F ensures proper dehumidification, protects your home, and allows your system to operate the way it was designed.<br><br>Enjoy your weekend!<br><br>Stay safe!

  5

• SH

  @shelshele · 2 months ago

  It’s awesome that you took the time to explain everything to him, and you did it in such a manner that even i understand it too. Great job Justin !

  1

• JI

  @jimbaxter4097 · 2 months ago

  You have the most wonderful personality a you attitude toward customers is outstanding. Thanks for being you.

  2

• LA

  @Lancedsanders · 2 months ago

  That little dog liked you. He stood right by your feet while you were at the counter with his parents.

• AG

  @Ageehvac · 2 weeks ago

  I like that impact driver, I have the exact same one...

• TE

  @terrencecescon102 · 2 months ago

  Really good explanation for a saavy homeowner.

• PE

  @petermapleback5034 · 3 months ago

  You should have told him that the system is not designed to run at 66 degrees return air, when outside air is 60 degrees. Fresh air is a gift.

  7

• JI

  @jimmylanders2175 · 3 months ago

  It has to be airflow. &nbsp;I appeared that the refrigeration side looked spot on.

  2

• ST

  @starlite528 · 3 months ago

  Gotta catch &apos;em all!🤣

• TH

  @The_Gas_Man_Canada · 2 months ago

  Highly recommend installing a &nbsp;fan cycling switch to keep discharge cycling from 90 f to 110 f. that be the best option.

• DA

  @davetherave-o3c · 1 month ago

  Some poor soul from one of those mounted trophies, are haunting the HVAC unit....

• XI

  @xINTENSORx · 1 month ago (edited)

  Our new-build home is about a year old, and I have this exact system installed. I experienced a freezing issue as well. An HVAC technician visited, installed a second return air duct, and resolved the matter. I believe my problem was clearly a lack of airflow.