From: Rabbi Moshe Sofer
To: Rabbi Meir
Rabbi Meir permits heating a mikveh on the second day of Yom Tov for a delicate woman unable to immerse in cold water. The Chasam Sofer validates this as a legitimate emergency ruling for Yom Tov Sheini, but refuses to endorse it as an ongoing leniency.
Source: Shu"t Chasam Sofer OC §146