The condensate is about as acidic as red wine, initially. But as you evaporate the water out of it in a humidifier the acidity increases quite rapidly.
Other negatives: Humidifiers can degrade the indoor air quality, and even rot the structural elements of your house, independently of the acidity of the water used.
As a general rule humidifiers are a bad idea- a band-aid solution for excessive outdoor air infiltration or overventilation. If a house is over-humidified in a cooler climate such as NY moisture builds up in the walls, increasing the mold spore counts in the spring when temperatures rise to the point that promotes rapid mold growth. If using a humidifier in a NY location keep it to no more than 35% during the coldest weeks of winter, and under 40% even into April.
In most homes it's better to apply the humidifier money on blower-door and infrared imaging directed air sealing, which will increase wintertime indoor humidity levels to a more comfortable level. If the interior relative humidity climbs over 35%, running some mechanical ventilation (say, a bathroom fan) can bring it back down. The savings on the heating bill from the tighter house will be many times more than the cost of the additonal fan power to keep the humidity down.
