The Air Conditioning will not cool the 2nd floor

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.

Image of a AC system used in residential housing
Typical split AC system

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.

image of a partially frozen AC evaporator
Partially Frozen

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.

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