Tech Talk - Reverse Cycle Air Conditioners - Defrost control this Winter

June 29, 2017

As you can know it’s cold outside in Winter! But this is where the bulk of your reverse cycle air conditioner lives. As the outside temperature plunges to 0 and below, your air conditioner has to deal with these freezing conditions. Reverse cycle air conditioner units has built in functionality to deal with this called defrost. This week we will delve into the how and why of reverse cycle air conditioning defrost control.

Defrost?…..So how does it work

Reverse cycle air conditioning units do two things well, heating and cooling. As the name suggests, the unit will reverse direction of the refrigerant flow to either heat inside or cool inside. Whatever is happening inside, the opposite will always happen outside. If your air conditioning system is cooling inside, it will heat outside and vise versa. When heating inside, the outdoor will cool. Herein lies the problem with freezing outdoor temperatures in Winter...as the unit tries to cool outside it will start to freeze. The outdoor unit refrigerant coil will reach temperatures of -10 deg C to -20 Deg C which freezes the moisture in the air causing a buildup of frost and ice.

To cope with Mr. Freeze, your reverse cycle air conditioning unit will actively monitor the outdoor ambient temperature as well as the temperature of the outdoor coil. As the temperature of the coil becomes too cold for the designed conditions, the unit will initiate a defrost cycle. As we mentioned earlier, your reverse cycle air conditioning unit can heat and can also reverse its mode and cool. The easiest and most common way for your unit to defrost is to swap from heating to cooling. This effectively heats the outdoor unit coil which in turn, cools inside.

What happens if my unit doesn’t defrost

Defrost is quite important to ensure correct unit operation. Without the defrost operation, the outdoor unit will form a layer of ice over the coil and slowly reduce performance of the system. As the layer of ice grows, the amount of air over the outdoor unit will decrease. As the airflow decreases, the amount of heat the unit can pull from the outside air is reduced. This then reduces the amount of heat your air conditioning unit can pump inside.

The ice build up can become large and dense enough to cause damage to other parts of the air conditioning unit. It can get in the way of the outdoor fan motor, causing it to break. We have seen these pieces of fan blade hit the coil and cause the refrigerant to leak. Damage to the coil like this is difficult and expensive to repair.

But I don’t want it to cool inside...right?

So your air conditioning unit is now cooling in the midst of Winter...now what? Luckily the manufacturers of most air conditioning units realise that you don’t want this. They have designed reverse cycle air conditioning units to minimise this cold draught by stopping the indoor fan from running, thus no cold air inside during the defrost cycle.

It still blows cold air

If your air conditioning unit is blowing cold air during the defrost cycle, then you may have an issue with the indoor coil temperature sensor (this sensor realises the indoor coil is cold on heating mode and tells the fan to stop). Alternatively your air conditioning unit might be heating but the air is only luke warm. This could mean the defrost operation is not working. The back of the outdoor unit could have a layer of ice covering the coil. This stops airflow to the unit which will greatly affect the ability to heat your home.

Check the fan setting on your controller

Some air conditioning units will constantly blow the indoor fan motor, even on the defrost (cooling mode). Ensure that your air conditioning fan setting is not set to ‘CONSTANT’, this will keep the fan blowing air even on defrost. Ensure the fan is set to ‘AUTO’ setting for best results when set to heating.

Fan running during ‘heating dead band’

Dead band is the point between when the unit reaches the set temperature and stops heating; and when the room temperature lowers far enough that the unit decides to heat again. This can be anywhere from 1 - 4 deg C depending on the unit setup. The fan should stop or change to ultra low during the dead band to stop cold draughts.

Check the back of your outdoor unit

If your air conditioning unit is freezing up on the outdoor unit coil, this could mean the defrost cycle is not operating correctly. This will generally mean a service call to your local technician specialising in the brand air conditioner you have. In the meantime you can switch off the unit and let the ice melt or bucket some warm water over the back of the unit to melt the ice faster. DO NOT try and break the ice with sharp objects as it could cause damage the unit and cause refrigerant to leak.

Some units require settings to be changed

In some newer air conditioning units, there are settings within the programming that need to be set for the fan to be off during heating ‘dead band’ or ‘defrost mode’. You will need to research individual brands to find what settings are required. These are generally printed in the controller or indoor unit manuals published online.

So what’s with the water around the outdoor unit

This is pretty normal during the cold Winter months. As mentioned earlier, the outdoor unit both cools and dehumidifies. This causes water to condense on the outdoor coil (think a cold drink just pulled from the fridge on a hot Summer day) and drip to the ground. During the defrost operation, the unit will drop a lot of water as the ice melts. Some units will be drained to a downpipe or garden bed. Having constant water near the side of your home can be a problem. This moisture can cause salt damp and encourage white ants. If you require the water to be drained, a drain pan and drain pipe can be installed under the unit which will drain the water away.

In a nutshell

  • Defrost is normal and helps keep your air conditioning unit heating in Winter
  • The indoor fan should stop (or turn to ultra low fan speed) during defrost
  • Some settings may need to be changed to stop the fan on defrost
  • Make sure the indoor fan setting is set to AUTO not constant
  • Water from the outdoor unit is normal and can be drained away with a drain connected to the unit or a drain pan under the unit

Need help?

T&K Airpower have been working to keep Adelaide homes warm in Winter for the last 20 years. We specialise in Temperzone, Hitachi and Mitsubishi Electric air conditioning units and know how they tick. If you have problems with your Temperzone, Hitachi or Mitsubishi Electric air conditioning blowing cold air or not defrosting, give us a call and we will be more than happy to help.

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