Frozen evaporators are a common problem with air conditioning systems. A typical evaporator coil will run at 40°F/4°C, if the coil temperature drops to 32°F/0°C, the coil will start to freeze. This reduces the efficiency of the system and can damage the equipment. The evaporator can freeze and then thaw, when the compressor cycles off, so there may be no indication of a problem until the day you really need it.
When the load increases on an air conditioning system on hot summer days, it is not uncommon for the compressors to run continuously. If the coil is below freezing and with no off cycle, the ice will build up on the coil as shown here.
On split AC systems, you may not be able to see the evaporator coil but you can see the copper piping. If ice is forming on the suction line, it means the coil is likely to freeze. If the line is insulated, then pull back a little bit of the insulation to expose the pipe and see if frost forms.
The most common cause of a coil freezing is low air flow. As the ice covers the coil, the air flow is restricted and accelerates the freezing process. Other common causes are refrigerant leaks, low ambient temperature, undersized liquid lines and dirty filters.
Anti-ice controls are common on large air conditioning system and can be installed on any air conditioning system to shut off the compressor when ice is detected. This is a safety device to protect the equipment under extreme conditions when something has gone wrong. There are contractors that will use the anti-ice control to mask a problem with an installation and can go undetected for years. When the coil is freezing the unit will use a lot more energy, drip water outside of the drain pan and shorten the life of the equipment. If the AC coil is freezing up, there is a problem that needs to be corrected.
In a two level home, hot air will rise to the highest point making the upper floors warmer in the summer than the rest of the house. The upper rooms are the often the furthest point away from the air supply which requires larger ducts to compensate for the distance and if the duct work is not properly sealed, any air loss if noticed first at the point farthest away from the fan. In some cases, the builder in trying to avoid having bulkheads on the 1st floor will squeeze the duct work into a wall cavity which can choke of the air supply.
If there is access to the main duct work, have a look at the joints and branch connections for duct sealant or foil tape to reduce air loss. Sealing up these connections can really improve air flow at the end of the long duct runs. If the duct work is new, then it is fairly easy to tape the joints with aluminum foil tape or apply a duct sealant. If it is an existing system then you need to clean the duct of the joints first before apply sealant. Cloth duct tape work too, but does not stand up well when used on heating systems.
There may be balancing dampers on the branch line of the ducts which can be used to reduce air flow to the lower floors and force more air to the upper floors. Be careful not to close off too much air or the AC will freeze up. The supply air grill is the rooms can be used to adjust the air as well. The cheap plastic floor registers have little slider to control the air flow that slide open or close. These can be difficult to try and balance air flow with but will work as an On/Off control. A better floor grill will have an opposed blade damper below the diffuser that can be used to reduce air flow.
Poor return air flow is another common problem. Make sure the is a clean pleated style air filter installed and do not use 1” wide high efficiency filters, these just choke off the air supply. A 4” wide high efficiency filter works great by having a much larger surface area that cleans the air and still allows proper air flow.
If you can remove the filter while the fan is running, try to slide the filter out a few inches and let go. If it gets sucked back into the filter rack it can indicate a strong negative pressure in the return air duct.
Another common is issue freezing of the evaporator coil. A partially frozen coil will choke of the air flow. Leave the AC off for a few hours with just the fan running and then turn the AC on. Let it run for 15 minutes and check the temperature of the air supply. It should be above 50°F or 10°C, any lower and the evaporator coil can form ice. The ice can slowly build up and reduce air flow.
These are a few of simple steps that DIYer can do but never attempt to modify the control, electrical, piping, sheet metal or refrigerant of any AC system, call a qualified technician for service.
HVAC, Heating, Ventilation, Air Conditioning & Refrigeration