I fixed a post from last week on mitzvos lav l’henos nitnu because I had reversed the opinions involved. The Ran holds that mitzvos lav l’henos means only that the spiritual benefit of doing a mitzvah does not count as a hana’ah. A neder not to listen to shofar is chal because there is no physical pleasure involved in hearning shofar, only the spiritual benefit of the kiyum mitzvah which is not a hana’ah. However, if someone took a neder not to fulfill the mitzvah of onah with his wife, according to Ran the mitzvah cannot be performed because there is physical hana’ah involved in addition to the spiritual hana’ah of fulfilling the mitzvah. Rashba offers a different reason why a neder on onah is chal: a person is not bound to remain married to the particular woman he took the neder on. If not for that reason, even physical hana’ah that goes with a mitzvah is permitted because mitzvos lav l’henos nitnu.
The point of disagreement seems to be: According to the Ran, the cheftza of hana’ah is prohibited, regardless of how it is attained; according to the Rashba, only a ma’aseh hana’ah is prohibited, i.e. an act a person engages in for the sake of pleasure, but any act engaged in l’shem mitzvah is permitted.
So why according to the Rashba does the gemara (R”H 28) prohibit someone who took a neder from a spring to be toivel on a hot day? Even the physical hana’ah of cooling off should be permitted because the act of tevilah is a mitzvah? Some explain that concern is for the droplets of water that remain cooling the person even after they emerge from the spring and are done with tevilah. But I think R’ Shternbruch’s answer in Moadim u’Zmanim is more satisfying. The Rashba set out his position in the case of onah, where the mitzvah by definition is to give hana’ah. In the case of tevilah, where the mitzvah is not to receive hana’ah from the water but the act of dunking, perhaps the Rashba would agree with the Ran.