coolige,
If you are losing 20* in two hours, that's actually not that bad. 600 gallons times 8.33 pounds per gallon comes to about 5000 pounds. 5000 pounds times 20 degrees is 100,000 btu's. 100,000 btu's divided by 2 hours equals 50,000 btu's/hour. 50,000 btu's times 24 hours/day comes to 1,200,000 btu's/day. At 80% efficiency, you'll get about 5,000 btu's out of a pound of wood transferred to water. 1,200,000 btu's divided by 5,000 btu's comes up to 240 pounds of wood/day. If i remember, seasoned oak(@ 20%mc) is close to 4000 lbs. 4,000 pounds divided by 240 pounds/day comes out to about 17 days/cord. I don't know how many sq.ft. your home is, but if it's something over 3000 sq.ft., you're not doing that bad if you are in an area where the climate is colder.
Bill brought up a great point that hit home with me this morning when I cleaned mine out. Mine was opened up front and rear, pump unplugged, almost nothing in primary burn chamber for about 15-20 minutes, and with a 19* ambient temperature, I lost 3 degrees, which really surprised me. If 5 of your 20 degrees were lost to the atmosphere in the while you were cleaning(in other words-your home only took the temperature down 15*), that would cut your wood consumption down to 180 lbs./day-which means the same cord would last over 22 days. I would suspect you could have feasibly lost even more than 5* in 2 hours to the atmosphere, which would lower your daily consumption even more.
I'm wondering if the reason yours is cycling more frequently than mine and Bill's might be because your mechanical aqua stat is set too low. Mine is set at 205* and I've never had an issue. For some reason, from the factory it was set at 180*, so I bumped it up before I ever built a fire.