Question: My kitchen sink often gets clogged up. Can I use a plunger to unblock it on Shabbos?
Answer: The Gemara (Kesubos 60a) writes that one can’t normally dislodge debris from a gutter on Shabbos. If the blockage would potentially cause them a financial loss, one may step on the debris to unblock the gutter (See Shulchan Aruch OC 336:9).
The Mishna Berura (336:47) writes that while one can step on the debris to help the water flow, it is always forbidden to remove the debris.
Based on this, R’ Moshe Feinstein (Igros Moshe OC 4:40:9) writes that if the sink is totally blocked then it is considered to be broken and unblocking the broken pipe would be similar to creating a new one. One can ask a non-Jew to unblock it for them if necessary, though shouldn’t unblock it by themselves. If it happens regularly, however, the sink is not considered to be broken, and one may unblock it with a plunger (See Yabia Omer 5:33).
Other poskim take a more lenient view.
R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 5:75) writes that ideally one should ask a non-Jew to unblock it for them. If absolutely necessary, one can unblock the sink oneself as clogged up sinks can be so repellent, graf shel re’i (See Shemiras Shabbos Kehilchasa 12:n50).
R' Moshe Stern (Be’er Moshe 1:29) writes that there is a big difference between unclogging a sink or toilet with a plunger and clearing a blocked gutter. A plunger merely dislodges a temporary blockage, whereas a blocked gutter may have a solid accumulation of soil that requires a new hole. Thus, one may use a plunger, especially in time of need.In conclusion, even R’ Moshe Feinstein would allow one to unblock a sink that gets clogged up regularly.