ERV systems, from what I understand, are there to circulate fresh air into a well sealed home - not sure you’d want to eliminate that function by turning it off. I think a bigger/better humidifier in your piano room might be a better option.

If you are in a cold climate, your ERV is bringing in outdoor air that has very little humidity. You need to reduce this outside air intake or increase your humidification. You should be at 40% to 50% for wooden musical instruments.

Candace, my piano is in the basement, which is rather large.

Fresh air ventilation and control of relative humidity are separate--but, often, as in the case of an ERV-- related functions. Don't compromise ventilation in an effort to regulate relative humidity.

I'm not an advocate of humidifying living spaces, but if your piano needs a higher relative humidity, a room humidifier is the ticket.

Here is an interesting video of humidifiers and pianos.

https://m.youtube.com/watch?v=MQKpfwxTgv4

Very interesting, thanks. A few salient points - according to the several piano manufacturers, the humidity should be around 50%, ranging from a minimum of 30% and a maximum of 70%, and that the humidity should not be changed "quickly". I don't know what quickly is - 10% per hour, 10% per day? My little humidifier hasn't gotten the humidity up to 30% yet, but it consumes a gallon of water (its capacity) in about 8 hours. I did a RH calculation, converting the outdoor humidity of 70% at 27°, to the indoor temperature, 73° and got 12%, so the humidifier is doing some good. On the other hand, my piano, which I got new in 1975, has lived all its life in a non-climate-controlled environment in NY state, so I don't know how much effort I should do to do better. Of course, I could "throw in the towel" and get an electronic piano, but I'm not ready to do that.

It is difficult to change the indoor humidity quickly using a small humidifier. The quickest way is for a big change in outdoor humidity and then open every window in the house. I am a little surprised your basement has humidity below 30%. That would normally be regarded as a good thing for those like myself who have houses with basements.

Changes in humidity has an affect on the tuning. I would think having it tuned on a day when the humidity is near the middle of the basement's range would be best.

The piano in the video is a Bosendorfer grand piano. I can't tell which model is in the video, but a new one starts at $125,000. I would trust the recommendations about the humidity ranges. The Venta humidifier gets a lot of good reviews on Amazon. It is worth investigating.

If it's been as it is for 45 years, I wouldn't spend another moment of time thinking about the piano.

Perhaps more relevant is that the indoor humidity you cite is arguably less than healthy for you. In your neck of the woods, nothing short of a whole-house system is likely to do the job for you.

I have no experience at all with a basement (have never lived in a house with one) but I've read so many accounts over the years of people having to deal with high humidity and dampness in basements, it's interesting and I guess a good thing that yours is as dry as the rest of the house.

Last week, as part of the preventative maintenance for the ERV, we had to turn it off and clean out the filters and other things. I took that opportunity to leave it off and see if that had any effect on the humidity. After a week, nada - humidity still stays about 25%. So, I'm going to follow Elmer's suggestion.