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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