I have a 11 year old, 2.5 ton trane package unit at my home. the tech added refrig 3 weeks ago because the unit was freezing up. coils still freezing up. air coming through vents only getting down to 70f. tech came back last friday. trane said there must be something (trash) in lines. tech evacuated system, purged lines and recharged. air from vent only gets down to 74f now. compressor seems to be working fine. i just thawed coils with water. they were frozen completely.

Had error code 91 on Trane XL19i thermostat. Heat season on duel fuel system. Repair tech said board is bad ($963) and transducer ($1000). Unit is 3 ton 12 years old. Does it make sense to repair or put in a new unit?

If the unit is running in cold weather than there is ice building up on the coils located outside. This is normal in that the unit is running in heat pump mode heating inside and cooling outside, After the unit detects the ice build up it briefly reverses the cycle heating the outside coils melting(defrosting) the ice which you observe as water on the pad.

The video gives you an example of the defrost cycle!
2012 Trane XL15i Heat Pump Defrost Cycle Steam Show

It sounds like you are low on charge. I think you have a thermostatic expansion valve and you need to check the subcooling, you should have 12 to 15 degrees at 95. At 95 degrees your head should be about 280# for a 12 or 13 SEER unit or higher depending on the effeciency. At 78 to 80 degrees indoor depending on the humidity, that is normal. The valve will open to try to fully feed your indoor coil and it sounds like your head pressure is to low to force enough refrigerant thru the coil.

First off - the 'rule of thumb' is 600' per ton of Air Conditioning. In other words your old unit is a 2 ton unit. So - 2 tons x 600' = 1200'. As you can see if you install the 2.5 ton unit - you will be installing a AC that 'could' cool a 1500 sq ft house (2.5 x 600' =1500 sq ft.). Slightly more than what you need; and the 3.5 ton unit is 'way to big,' (3.5 x 600' = 2100 sq ft.).

Note: fyi - many in the AC business will sometimes refer to tonnage in btu's, i.e. 1 ton = 12000 btu - hence a '2 ton unit' can also be referred to as a 24000 btu unit and vice versa.

So... from the above - you can easily see that "2 tons" of Air conditioning is what is required to cool the 'average' home of 1100 sq ft. "roughly speaking."

Note: it is always best to have a professional 'size' your cooling/heating needs.

One of your questions was could you 'mix tonnage?'

The answer is 'usually you don't mix the tonnage of your outside/inside units.' However, professionals sometimes do (mix the tonnage) in certain situations, and installing a 2.5 ton outside unit with an existing 2 ton inside unit is often done, however, there are some 'tech issues' here and - I would "again" recommend that you call a Service Tech to help you with the sizing/mixing of your cooling/heating needs.

hope this has helped

You have a problem with the defrost system. There 3 components... heater, thermostat, and timer or adaptive defrost control depending how old your refrig is. Check heater and thermostat( located on the evapotator coils) with an ohm meter. The heater will have some resistance and the thermostat will be closed(must be cold). If these check good replace timer or adaptive control.

This can be caused by the drain hole below freezer coil being blocked by ice. You MUST remove all ice in drain under freezer coil. Use turkey baster to squirt hot water into hole and use hairdryer to defrost. Possible defrost thermostat is defective. Unit never goes into defrost, ice forms on coil and in floor of freezer. After defrosting may work OK for a week or two but gradually gets warmer because ice is blocking ports where cool air gets to refrig. May also have water in refrig box. Can also be caused by defective defrost timer.

First of all, clean your outside coils so that you are assured of good cooling air. The compressors have an overtemp device that is usually internal to protect them against heat. If you have an air compressor use it to blow out the coils good. If the compressor will start, the capacitor is good so it sounds like a cooling problem with the compressor

the tons is the power of the unit, so sure. if you were happy with the power of your old unit, get the same. if you felt it was never strong enough to cool your house, get the next size up. newer units have the same power, but use less energy.

That is not correct. The system is not mis-matched. They just put a heavy condensor outside to get more air volume for your house. The failure of the valve is not to common of a failure but can and will fail. The problem with Palm Springs is the weather you get there, From very hot to very windy with sand storms to freezing rain and sometimes snow depending on where you are. What is the tech wanting to cahrge to replace the valve? Also this is something that should have been looked at closey and examined before the home was bought and that should have been known by the realator

That is a fairly large difference. It is usually ok to have the inside evap. coil and blower up to 1 ton larger than the outside condenser. That will make the unit slightly more efficient as well as less likely to freeze up on low airflow situations. It is not recommended to install a new condenser on an old evap coil. There has been a lot of changes to the design of the coils in the last little while. For example a 10 year old 2 ton coil may only have 3 cubic feet of volume but a new 2 ton coil may have 4 cubit feet of volume.

There are many factors that may have infulenced the decision on what size condenser to install. Many of which can only be done by visiting the home and doing alot of work, checking the duct sizing bioth supply and return, inspecting the insulation and windows of the home etc. etc. Most of the time that never gets done. You can blame the contractor for not doing a complete check, but at the same time you can blame the customer because many contractors that are that good loose the job to a cheaper bid that did not no any of the research. It is a catch 22 for everyone involved.

There is ALOT more to sizing equipment that many people think, sadly that also includes many HVAC contractors. Way too many people use "rule of thumbs" or flat out "guess".

Sorry for the rant but your queston can only be answered by a good well educated HVAC contractor visiting your home. That type of a contractor is getting hard to find these days in such a price competetive world.