From: Rabbi Moshe Sofer
To: Rabbi Binyamin Wolf Hamburg
The Chasam Sofer mounts a comprehensive defense of the widespread practice of constructing mikvaos with rainwater channeled through copper and wooden pipes, supplemented by hot water from a fixed boiler. He argues that the rabbinic decree against bathhouses applies only to entirely hot drawn water, that drawn water added to a valid forty-se'ah mikvah is rendered valid by the majority view, and that channeling through pipes constitutes hemshachah. He concludes that these mikvaos are valid, while acknowledging that a stringent position is praiseworthy.
Source: Shu"t Chasam Sofer YD §214