Gas and Oil Home Heating Furnaces - Water Leak Under Furnace When Below 0°

I have a strange problem when the outside temp gets below zero. My furnace sits on about a 3" concrete pad that appears to have been poured after the basement floor was in. When the outside temperature gets really cold, water will seep THROUGH the porous sections of the concrete pad and onto my basement floor. So far, nobody can explain this and I'm not sure if I can simply seal the pad or if sealing will just cause water pressure build-up and heaving somewhere. Trust me, there are absolutely no leaks from my furnace or ductwork. It is bone dry everywhere except where the pad and floor meet. Any ideas what the heck is going on?

When it is below 0, your furnace will run more, obviously. Could someone have drilled a hole in the floor and stuck the condensate tube in the floor or pad?

Forgive my ignorance, but is the Ø1/2" tube that runs out of the duct work directly above the actual furnace the condencate tube? That runs to a floor drain, but when I checked the tube it was dry. Of course, there was a small puddle on the floor and the tube was dry. Kind of makes me wonder.... It does not appear that anything drains into the concrete pad though and thats what bothers me about this.

Usually these condensate tubes are connetcted to the PVC exhaust pipe on high efficiency units. I really can't tell you. Maybe someone else can weigh in on this.

You still have water appearing on the slab?

Thats where the mystery lies. The water is not appearing on the raised slab that the furnace sits on, but appears on the basement floor with no sign of dripping or water running from the furnace, down the edges of the slab and onto the concrete floor......

Have you opened up the bottom blower door to see if any water is down on the bottom of the furnace? Where is your consdensate water going?: Is it being pumped away?

Could be a coincidence that it isn't even furnace related, also. What are the weather conditions outside, where yo live? Also you could have some break in underslab drain or pressure line.

I found the problem last night! As I was sitting there staring at everything through a couple heating cycles and muttering to myself, I caught something out of the corner of my eye. A small drip of water was running down the AprilAire drain tube and onto the floor. That explains why there was no water on TOP of the slab and it met the floor near where the slab and floor meet. The drips then wept around the crack making it look like it was coming from the crack! Hallelujah! Easy fix!

Thanks for your help!

Glad SOMEbody posted back that something was successful.