While the central air unit is on, water is leaking

Check the drain it must be plugged. Check the evaporator coils for exessive ice build up on the coils, you may have a dirty filter and not getting enough air flow to keep ice from forming on the coils. The unit should have a drain built in it under the evaporator coils inside the unit. If it is a central air the evaporator is usually above the furnace. This drain pan may be rusty and leaking as well. You may have to unscrew a panel or two depending on how your unit is installed to get to the drain pan.
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Hello, sounds like a clogged condensate line or a faulty condensate pump if the unit has one. Cleaning the drain line should fix the problem.

How old it your Aprilaire 700?
Eventually they all need maintenance, cleaning or replacing dirty or inoperative parts. The bottom Water tray has a drain tube that needs to be cleaned or replaced every fall as it will plug up with mineral deposits washed down from the unit. Adding a water filter to the inlet line (if you don't already have one) will reduce mineral scale build up and reduce the need for cleaning and maintenance. Some water filters may bypass uinfiltered water when very dirty. If thats OK, then the solenoid valves could be dirty, worn and now leaking all the time (not shutting off at end of operation cycle) These valves, 2 usually 1 Low volt operated 24 VDC supplied by thermostat and or humidistat when caling for heat, and 1 High volt 120 or 240 VAC suppiied by furnace fan delay switch, All Gas furnaces have a fan delay to allow the heat exchanger to warm up so the furnace does not blow cold air at the at the start of every cycle of heating. Both have to br powered on to allow water flow.
Click on the link below and read the Aprilaire Manual maintenance page. This Manual covers several models, so disregard parts not pertaining to your model number.
Remember to turn of electrical power to unit, furnace, air handler if a split unit, and don't attempt anything your not exceeding your experience level.
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http://www.aprilaire.com/themes/aa/en/manuals/AA-Humidifier-Owners.pdf

No. The installers did not properly plumb you hose. You have a high efficient furnace which is really nice. However, this furnace takes out moister from the air when heating, causing condensation. I would look at the installation manual and see if they installed them properly. They might of installed the wrong unit for you home. Most Goodman furnaces can only be installed up-flow meaning in the closet or horizontal installed meaning in the attic. Look at the installation manual to see if they installed the right unit for your home. www.fastacservice.com

yes you must get the clog cleared. you nee to clear the 3/4" pvc drain

you need to have a drip line that take away the water when the ac is on so the water does not goes into the furnace ,also check for leaky line that could causing water to leak back

All A/C units drain water and need a clear path to drain water.
If window unit the unit MUST lean to the outside so the condensation water can drain that direction and insure the drain hole is clear of debree.
If an indoor central a/c unit the drain must be clogged the unit is leaning away from drain or the pipe going to the drain is clogged somewhere as it exits house. Cant drain if doesn't have a clear path to do it's job. Doubt it's a problem with the brand.

Install new water panels yearly. On the new panel make sure that the painted dot on the edge of the panel is on the top when you install it.
Always clean the distribution tray and the frame. Make sure the notch on the edge of the distribution tray faces away from the furnace. Make sure the humidifier was installed level. Keep the drain tube opening clear. Insure there is no blockages in the drain tube.
Good luck
HVAC Teacher

This is not a lot to go on.

I assume since you called it a furnace, you have hot air heat and central air conditioning. If so, the only way a furnace/AC unit can generate water is by condensing moisture from the air. This is common to all air conditioning and removes humidity from your home. If so, this water, called condensate, must drain. The condensate is collected at the AC cooling coil that is typically located on top of a vertical upflow furnace or downstream of a horizontal furnace. If the drain is clogged, it will overflow and create a mess in and around the furnace. If this is your problem, cleaning and clearing debris may solve the issue. I suggest starting with the easiest accessable part of the drain path. You may need to open the panels enclosing the AC coil and clean the small pans at the bottom of the coil and clear openings from above that look similar to a laundry tub drain. If you get that far and find rust holes in the pan, repairs are needed. If you have an electric condensate pump that may also be the cuprit and require cleaning. Check and clear all drain lines from the coil connection to the final dicharge - sump pit or outside.

Change the furnace filter. Make sure you use a cheap one when air conditioning, as the better ones restrict airflow when they filter out so much stuff and then plug up, not allowing enough air to pass over the cooling coil in the furnace's air plenum. Turn the A/C off for a few hours (with only the furnace fan on if possible) to melt the ice off the cooling unit inside the furnace.