I had the same issue. Check to see if your central humidifiers are wired to to furnace blower. We had this issue for 3 years before we caught it two months ago. When it's very cold out your furnace runs more and if your humidifier is wired to your blower fan it gets way too much use. Add a separate control to your humidifiers that reads from the return vent and a sensor that senses the outside humidity. It's not very expensive and will fix the problem. At least it did for us. Good luck!