Low-Side Diagnosis on a Cold-Weather Heat Pump
A ductless mini-split serving a Kotzebue office runs in heating mode at 5°F outdoor ambient. The technician finds: suction line frost extending from the outdoor unit back toward the wall, indoor coil not delivering rated heat, suction pressure low, discharge pressure low, and subcooling low. The unit was charged by weight at install two years ago and has not been touched since. Line set is 25 ft, within the factory charge allowance. The outdoor coil is visibly clear of snow, but the unit sits on a wall bracket 18 in. above a snow-covered deck.
1. Distinguish between a low charge and a restriction/other fault using these readings, and state what additional measurement settles it.
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Low suction, low discharge, and low subcooling together are the classic signature of an undercharge (or a starving metering device), not a restriction. A restriction (e.g., plugged strainer or kinked line) typically shows a temperature drop across the restriction with normal-to-high subcooling upstream and low suction downstream, and often high discharge. Here everything downstream is low, pointing to insufficient refrigerant mass flow. The decisive additional measurement is superheat at the evaporator (or the manufacturer's target subcooling in heating mode) plus a temperature/pressure comparison across the reversing valve and metering device to rule out a bypassing reversing valve. Weigh-in verification with recovery and scale is the definitive check.
2. The frost pattern and the mounting location raise a second possibility. Explain.
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Frost running from the outdoor unit toward the wall in heating mode means the outdoor coil (the evaporator in heating) is operating below freezing and accumulating frost faster than defrost clears it - or defrost is failing. A unit bracketed 18 in. above a snow-covered deck can recirculate its own cold discharge and pull in snow-melt moisture, accelerating frost and confusing the defrost sensor. Check defrost termination sensor placement, defrost board operation, and outdoor fan cycling during defrost. Also confirm the unit is not buried by drifting snow and that clearance to the snow line is maintained. If defrost is functioning, the low charge itself lowers coil temperature and worsens frosting, so both findings may share the undercharge as root cause. Verify charge first, then re-evaluate defrost.