HI XXXXXXXX,
The schnauzer breed is prone to hypothyroidism which could account for the hunger and they are also prone to bladder stones which may account for the excessive drinking. I would have the dog looked at for these. They are also prone to diabetes which would show an increase in thirst, however yo may see a decrease in weight despite a good appetite.
The thyroid produces a hormone called thyroxine which controls many body tissues. If this hormone is out of whack even a little you will start to see symptoms. Depending on how off the hormone is may determine how many symptoms you see. http://www.petplace.com/dogs/hypothyroidism-in-dogs/page1.aspx
Bladder stones can show symptoms of increased thirst, increase in urination, blood in the urine and straining to urinate. However some dogs show almost no signs at all. Bringing in a urine sample for testing can tell you if there is a problem. It may even be a urinary tract infection.
http://www.petplace.com/dogs/urolithiasis-in-dogs/page1.aspx
Diabetes http://www.petplace.com/dogs/diabetes-in-dogs/page1.aspx
dog training consultant
24 years training & behavior,pet first aide, Therapy dog eval., PA Animal Response Team