What can cause an A/C to freeze up

I would call a third company. They should actually try find the leak, not just sell you a new system.

What brand is your A/C system (Lennox, Carrier, Trane, etc)? Find the local HVAC company that is an authorized dealer for that brand. They will be most familiar with your system.

Once you find out where the leak is, you can make an informed decision from there.

Common techniques an HVAC company would use to check for a leak:

1. Spray soapy water on the fittings and look for bubbles.
2. Use an Electronic Nose to "sniff" around all the fittings (it can smell the refrigerant).
3. Add a UV dye to the coolant, let the system run for a week or 2, then go over the system with a UV light (if you see the dye, there's your leak).

(Expect to pay for the leak testing. It can take a good hour or two to go over the whole system, and time is money.)

Once the leak is found, the HVAC company should be able to give you an estimate for the repair. If the leak is something simple like a fitting on a coolant line, the repair can be cheap/easy.

If the leak is in the coils or the compressor, expect the bill to be steep ($thousands). At that point, I would think about replacing the system.

Reasons to consider replacing:

• You fix this leak, but what else is going to go in the system? If the system has been running for a while without enough coolant, there is increased wear and tear on the compressor.
• R-22 systems will be getting progressively more and more expensive to service.
• Newer systems are way more efficient, so expect a lower electric bill. You should be able to calculate the "pay back" period of the new system (how many years of reduced electric bills will it take to offset the cost of the new unit).
• Check for Tax Credits and Rebates. The Energy Star Tax Credit expired at the end of 2011. However, you might be able to get a tax credit on your state or local taxes. Also check with your power company. My power company had a rebate of ~$300 for a high-efficiency AC.
• Newer features on the new system (variable speed fan, quieter scroll compressor, touch screen programmable thermostat, etc).

The condensate collection works by allowing the water to run down along the fins and collect in pans on either side of the 'A'. Upflow systems work a bit better because the air flow supplements the water's tendency to adhere to the fins, whereas downflow systems tend to encourage release. Typically, even downflow systems collect most of the condensate. One problem that can occur is dirt collects on the fins, causing the water's surface tension to break. Surface tension is what causes the water to run down along the fins instead of straight down.

Another problem, which is probably what you are seeing here, though the others may be contributory factors, is simply just too much humidity. With high humidity, so much water develops of the fins that there is too much mass to be supported by surface tension and the water simply drops straight down instead of running along the fins.

So part of the solution may be to mitigate the contributing factors. If you can cool with a lower air flow, that will help. Change filters frequently, which it sounds like you are doing. Reduce the humidity in the house. This is easier said than done. But short of installing whole house dehumidifiers, be conscious of lifestyle habits that generate moisture and try to minimize it, such as always using exhaust fans when showering and cooking. Clean surfaces with liquids during the cooler parts of the day. Try not to breathe too much :)

I don't think you will fully mitigate the dripping with such measures. Moisture happens, and it will end up on the ductwork. You should consider fashioning the portion of the duct on which the water falls into a wide shallow funnel to collect stray condensate droplets and pipe it off somewhere where it can be safely removed.