This happens a lot.
When natural gas burns, it creates water.
Modern furnaces don’t send that vapor up the chimney, so instead it collects in a pan and runs to a drain line.
If you remove the access covers, you will find the drain line.
It is usually behind the lowest cover door.
Or trace it back from the drains you see outside your furnace.
You will probably see a black hose attached to some fittings with cheap hose clamps.
I remove one of these, and replace with a stainless screw drive hose clamp
Take it apart somewhere, near where it leaves the furnace, enough where you can get your mouth onto it to blow very forcefully back into it, several times.
You will hear water gurgling.
Backed up water might come trickling out.
If you are squeamish, you can adapt a piece of garden hose to it, and blow hard into that.
Or use compressed air, leaf blower, whatever, if you are a big wussy!
Sometimes, the flexible condensate hose is extending outside the furnace, and you don’t need to take anything apart, just blow right into that.
I have clients where I do this once a month, from October til April.
For all three of their furnaces.
Their hvac guys were charging a fortune in service calls for this, and they were still getting occasional nuisance flooding.
My crude method has worked perfectly for at least seven years.
I also rinse out the external pvc drain lines with hot bleach water, followed by a fairly gentle rinse with water through a hose attached to the water heater drain.
Once a month, year round.
The humidifiers and a/c coils also generate crud and slimy gook that plugs up these long drain lines that run to a sump basin.
