Why does my wall thermostat go off and blank the screen?

The Problem

• Symptom stated is likely a clogged drain from the central AC coils in the attic (it's called the evaporator coil condensate drain)
• The drainpipe from the evaporator coil in your attic unit will have a small reservoir with a float switch in homes newer than 15 years old. when the drain clogs and water backs up and fills the reservoir, the float switch will open the 24-volt circuit powering the thermostat; the thermostat will go blank on many models.
• When a slow drain empties that pipe, the reservoir float switch will return to the unclogged (drained) state and return power to the thermostat; the thermostat then powers up and works normally.
• This drain and float switch described above are in the PRIMARY drain routed to your home plumbing drain: often to a sink drain or similar. The large pan below your AC unit is an EMERGENCY drain to catch drained water when both the primary drain AND float switch described above fail to drain or detect a problem in the drain. This emergency drain pan should be dry if this float circuit is stopping the AC unit due to the clogged primary drain. The primary drain needs to be unclogged, or water damage can to your home can occur if you rely solely on the emergency drain (which usually drains to a pipe dripping outside from the eave of your house. Simply disabling the float switch and relying on the emergency drain is unwise.

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The Solution

1. Check that the sinks etc in the vicinity below and near the AC are draining properly after running a few gallons of water through the sink. You will often find a slow drain in the home that, if unclogged with a household drain opener, wll suffice to drain the AC unit.
2. Use similar drain cleaner to clear the AC drain pipe. There is usually an open pipe above or near the float switch to pour in a small amount of the drain cleaner. Avoid getting that strong product on the float assembly else it may be damaged.
3. Plugging the open pipes (including those to the inside of the AC unit, and blasting with compressed air may clear the drainpipe. Don't try this until the pipe has completely drained and any drain cleaner has been flushed from the system. Those chemicals cause skin burns.
4. Once the pipe is draining freely, begin an annual early Spring routine of treating the drain with 1 cup of a 50-50 solution of vinegar and water to protect the drain from algae: the likely original cause of you clogged drain.

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