Ice on fan coils

a) Abnormal gas pressure:

ice usually develop when suction gas pressure has dropped to ~ 40psi to 45psi (of an r22 system). As such, it 
could mean that your system has a problem of gas leak. Special attention must be taken when working on an Inverter system. Because an inverter air conditioner regulates its compressor rotational speed, it will affect the head and suction pressure. If it spins too fast, the suction pressure drops too low and thus creating the same results similar to gas leak in a non inverter system. I will cover more in details in troubleshooting an Inverter system in later sections.

ice on fan coil

b) a faulty compressor magnetic switch; if icing problem is as severe as to causing the entire fan coil to become a block of 
ice, then it is most likely caused by a faulty magnetic switch that doesn’t cut out when you power off your indoor air conditioner; meaning, the outdoor condenser is still running and continuously supply the refrigerant to your indoor aircon blower.

Since you have powered off the aircon and the indoor fan is not turning, the supplied cold refrigerant is not exhausted, the fan coil can be so cold that it turns into a block of ice. If you are using a Fujitsu non inverter or certain model of carrier (it’s the same machine), 
this is a common problem.

block of ice on fan coil
block of ice on fan coil filter lifted

c) a choked fan coil can cause ice forming on the fan coil. When the fan coil is choked, 2 things happens: First, the liquefied gas has difficulty fully evaporating and that will cause the suction pressure to drop by 5-10psi. Secondly, when 
the fan coil is choked, the fan coil is in fact colder than normal as the coldness is not dissipated well enough.

DO NOT top up the gas upon seeing ice! As mentioned above, ice can be caused by other factors too. If ice is not caused by gas leak, topping up gas will screw up your compressor big time. You may like to read this article i wrote previously, things technician do to sabotage your aircon.

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