An air handler or air handling unit (AHU) is the device that circulates air as part of an HVAC system. It is a complex mechanism that comprises of blowers, heating and cooling elements, filters, and dampers. The air handler is a critical part of the HVAC system and Experts on Just Answer have answered questions on all aspects of air handler troubleshooting, problems and repairs, below.
The best way to answer this question is to list out the pros and cons of variable speed air handlers. The pros first. A variable speed unit will consume less electricity because they usually run at below top speed whereas single speed unit always run at 100%. Variable speed units ramp up slowly so there is no sudden blasts of air when you start the system. Variable speed air handlers are much quieter. They also remove more humidity from the air, and they deliver air better. The only con is that if there are duct leaks, the losses will be increased.
Case Details: Doesn't work in either manual or auto, however, it does work in the heating mode
The fan relay appears to be bad. This is a common problem with this make. It is actually not a relay, though that is what they call it. It is a sequencer. This will need to be changed. The reason the air handler works in the heating mode is that the unit is fed from the normally closed contacts on the blower time delay relay from the heat sequencer. The heat sequencer should be located next to the relay and it would be a good idea to replace it at the same time.
Case Details: When the outside temperature rises, the unit begins to function properly but never warms the house.
The most likely cause of the problem is that the unit is not defrosting properly. Without proper defrosting the unit cannot deliver heat effectively and the electric backup comes on. Another reason for inadequate heating could be that a defective sequencer may be affecting the operation of the heat strips. Rheem had a problem with the defrost boards for that model and the demand was so high dealers were often out of stock. The replacement boards now available are much better may need to replace the defrost board.
Open the blower door and check if the 5 amp fuse is blown and replace it if need be. If the fuse is not the issue find the R, W, G, T, O and C terminal on the control board. Jump between R and G and see if the fan starts. If it does take a voltmeter and place one lead on R and one on C and see if you have 24 volts. If so, it looks like you have a thermostat problem – jumping R and G bypassed the thermostat and the fan started. Pull the front cover of the thermostat off and jump between the R and G terminals and then R and W. If the unit starts then your thermostat probably needs to be replaced.
As far a matching goes you can have larger indoor unit but not an outdoor one. For example, a 3.5 ton indoor unit will work fine with a 3 ton outdoor unit, but not the reverse. However, this does not apply in your case as both the units are 3 tons.
An improperly functioning air handler can affect the efficiency with which an HVAC unit operates and one that is nonfunctional can cause the HVAC system to stop working completely. When air handler troubleshooting it is best to follow the instruction given in the owner’s manual. If this does not sort out the air handler problems it is advisable to get the advice and help of experts to resolve the issues