Product review (1 stars): Measure Your Old Pad
This pad does NOT fit the Aprilaire 550. The pad is almost an inch and a half too short. I wouldn't even turn the water on, I've had issues with the paper model quickly deteriorating and leaking water on the floor, in the furnace and ruining the air filter. It's original equipment replacements for me from now on. This is nothing but cheap "will fit" junk.