We live in northern Michigan & purchased a 3 story house this past summer that is 3200 sq feet. It has has a high effifiency furnace with an Aprilaire 550 whole house humidifier. I turned on the water supply for the humifier in late November & replaced the filter for the humidifier because of scale build-up. I set the humidistat control and verified the water flows through the filter correctly. I also switched from the summer to the winter position so the air goes through the humidifier filter. The water flows through the unit, but our humidity is still very low (about 18-20 percent at most) so we purchase & continually run two additional large console humidifiers in the house - one on the 2nd floor and one the 3rd. After 4 weeks I shut off the water supply & turned off the humidistat for the Aprilaire 550 unit as we seemed to be removing large amounts of water from the air - we were pumping gallons & gallons of water through the system with no results of benefit. We kept the other humidifiers filled & running, but I found the furnace is pulling 12 to 15 gallons of water out of the air every day like the AC is running during the middle of summer in high humid conditions! Why is it taking all the humidity back out? Are we suppose to shut something off for the furnace to stop condesing and removing moisture or what can we do to keep the moisture in the air as our efforts are currently non-productive and the dry air is causing some real problems? Thank you

Condensing-type furnaces produce water (condensate, actually) as a by-product of combustion, when the combustion efficiency exceeds 90%. This does not mean that the furnace is "dehumidifying" the indoor air.

Condensing-type furnaces produce water (condensate, actually) as a by-product of combustion, when the combustion efficiency exceeds 90%. This does not mean that the furnace is "dehumidifying" the indoor air.I can understand the furnace will have some moisture draining out due to the heating, but when we are registering 12 to 15 gallons of water every day going down the drain and this is with the humidifier water supply shut off - this appears to be more than just a product of combustion. The two whole house humidifiers put out a maximum of 8.5 gallons per 24 hours so it appears we are putting moisture in the air, just to have it removed when going through the furnace and put down the drain. We have significant static problems including occasional sparks because of how dry the air is. Another visible problem is our house is 100 years old with large amounts of carved woodwork and wood flooring that is shrinking and even cracking due to moisture loss. I also keep antique historical papers & books in the library, but the drop in humidity is causing the paper to dry and become brittle. The cooling system did an excellent job of maintaining the humidity levels during the summer in extracting the extra moisture, but now it is far too dry and I need to know what can be done to help maintain a better humidity level during the winter with the equipment we have - between the Aprilaire 550 whole house unit and the 2 console humidifiers I should be able to keep our home properly humidified at around 30-35 percent consistently. I hope my explanation is understandable and hope someone can help us solve this issue. Thank you.

As KC said that a 90% furnace is draining water from the burners. I would look in to bring in fresh air to furnace room(only if your furnace is not a two pipe PVC).Next try to winter rise your home stop cold air leaking in from windows,doors exe.
Honywell make a true steam humidifier (9gal or 12gal I think that is how many gal. it puts in to air a day)with these you can get all the watter to the air with out wasting any down the drain.