In newer homes, excess water from condensation goes right into a nearby floor drain. But many older homes don’t have a floor drain next to the furnace. So furnace installers mount a condensate pump right on the furnace and route the drain line to a far-off sink or floor drain. If that pump fails, the water overflows the pump and spills onto the floor. That doesn’t necessarily mean the pump is bad; the problem could be just algae buildup in the pump’s check valve.
So start your diagnosis by unplugging the pump. Disconnect the drain line and empty the water into a bucket. Then remove the check valve and plug in the pump (Photo 1). If the pump doesn’t work, buy a new one (about $60 from a home center or online HVAC store) and swap out the old one. However, if the pump works, you’ve got a stuck check valve.
Try cleaning the valve by soaking it in warm, soapy water. Then flush it. Clean out any remaining crud with compressed air and test it (Photo 2). If you can’t remove all the crud or the valve is still stuck, replace it with a new valve (about $10 from the pump manufacturer’s parts department). The furnace or A/C will continue to drain while you’re waiting for the new part to arrive, so jury-rig a bucket system (Photo 3). Clean any algae buildup from inside the pump with soapy water and a brush before installing the new valve. Then install the new valve and test. To prevent algae clogs, place algae reduction tablets (such as Pan Tablets No. AC-912) in the pump reservoir.