I have a 1.5 inch PVC pipe coming out from under my house. Today it started putting out a good amount of clean cold water every 15 minutes or so, all at once, not in a drip or trickle. Instinct would make me think clogged ac condensate line but this house only has a furnace. What other appliance or system might cause this weird problem?

Hi SF

Only other thing I can think of is a SUMP PUMP? Have you had rain lately?

I'm with Mark on this. A 1 1/2" pipe is a discharge line for high volume. AC condensate line are 3/4" and drain by gravity,
You either had a lot of rain or your setting on top of a raising water table. Look in your basement for a sump pump. Good luck, Tom

X3