how do I keep my st Bernard cool with no ac

Hi fans on him will work fine and make sure he has access to water. Fans will keep the air moving and keep him cool. Keep us posted.

Be very very careful!!!! I have had a St Bernard come into the vet office with a heat stroke just from a morning stroll. They have very thick coats and need somewhere cool. Keep on tile with wet towels and lots of fans. Lots of water and I mean buckets.

Years ago I heard an idea that I have always remembered.  In fact, it might have been someone on this forum who posted it.  

Fill a large pail (or Rubbermaid tub, etc.) with water and freeze it (this requires having ample space in your freezer!).  Put the frozen pail on the floor in front of a fan that's aimed over the top of the "ice pail" and it'll circulate some air that is a little cooler than the standing air in your house.    I have no idea if it'll work, but it's worth a try.  At the very least, as the ice melts in the pail, it'll serve as another water source for your dog to drink!