From: Rabbi Moshe Sofer
To: Rabbi Ber
The correspondent asks whether one may heat a mikveh on Yom Tov for women requiring immersion from niddah. The Chasam Sofer personally refrains from ruling leniently but acknowledges that communities relying on the Noda BiYehuda and Pri Megadim need not be corrected.
Source: Shu"t Chasam Sofer OC §148