My old Aprilaire 550 has seen better days and it seems to make more sense to replace it vs repair. It has never really quite kept up even though it ran 24/7 with continuous fan. Probably because of the 60 recessed can lights in the house.
I did a little math comparing water use and included the increased sewer charges for running the 400, 500, and 600 Aprilaire's. My sewer charges are based on how much water is used during the winter quarter, so flow-through designs cost me extra for sewer all four quarters.
Based on the math and past billings my 550 cost me about $161 a year in water ($61) and sewer ($100) charges. Not to mention the cost to heat 140+ Gallons per day. OUCH!
The 600 would use about $81 a year. It flows 3 GPH vs 6 GPH for the 500/550.
The 400 would use about $19 if it used all of it's 17 Gallon a day output capacity, and no hot water.
The 400 will require more maint. because of needing an extra pad per year, and added complexity, but from a water use standpoint is the clear winner.
Flushing from 12-24,000 gallons of water down the drain over 6 months just to keep the pad clean seems like a real waste.