Air Conditioning Icing Up
Seeing frost or solid ice on the copper pipework running to the outdoor unit, or on the indoor coil itself, understandably worries people, particularly in the middle of summer. It's caused by refrigerant getting too cold as it evaporates, which happens when either not enough warm air is passing over the coil, or when there isn't enough refrigerant in the system to begin with.
The first thing to do is switch the unit off and let it defrost naturally rather than continuing to run it, since running a system with ice built up can eventually cause liquid refrigerant to reach the compressor and damage it.
What causes icing, in order of likelihood
- Blocked filters or dirty coil restricting airflow across the indoor unit
- Low refrigerant charge from a leak, dropping evaporation temperature below normal
- Running the system in cooling mode at very low outdoor temperatures without a low-ambient kit
- Faulty indoor fan motor not moving enough air across the coil
- A stuck or faulty expansion valve metering refrigerant incorrectly
Immediate steps you can take
- Switch the unit off completely and set the fan to run alone if possible to help it defrost
- Do not attempt to chip or scrape ice off the coil or pipework, as this can damage the fins or pipe
- Once defrosted, check and wash the filters before trying the unit again
- Place towels under the indoor unit, as defrosting ice produces a surprising amount of water
Why repeat icing needs proper diagnosis
If icing clears with a filter wash and doesn't return, that's usually the whole story. If it comes back within days or weeks despite clean filters, the cause is more likely refrigerant charge, the expansion valve, or the fan motor, and identifying which one requires pressure and temperature readings taken with the system running, which needs an engineer's gauges and experience to interpret correctly.
What we do on a call-out
We let the system fully defrost if it hasn't already, then run it under supervision while taking superheat and subcooling readings, which tell us fairly precisely whether the issue is airflow, charge, or the expansion valve. We'll also check the fan motor's speed and current draw, since a struggling motor is an easy cause to miss without direct measurement.
Stopping it recurring
Regular filter washing solves the majority of icing complaints on its own. Beyond that, an annual service that checks refrigerant charge against baseline figures catches slow leaks before they cause icing, and in turn before they cause a full loss of cooling.
FAQs
Common questions
Is it safe to keep running the unit while it's icing up?
No, we'd recommend switching it off. Continuing to run it can allow liquid refrigerant to reach the compressor and cause lasting damage.
How long does it take for a unit to defrost naturally?
This varies with how much ice has built up, but a few hours with the unit off is typically enough before it can be safely restarted.
Does icing mean I definitely have a refrigerant leak?
Not necessarily. Restricted airflow from dirty filters is at least as common a cause, and it's worth ruling that out first.
Can icing happen in winter too?
Yes, particularly on systems used for cooling at low outdoor temperatures without the right low-ambient controls fitted.
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