It is generally forbidden to allow noisy appliances to operate on Shabbat. This is true whether the appliance was in operation before Shabbat began or whether it is activated on Shabbat itself by means of a timer. This is known as the prohibition of avsha milta and is a violation of the requirement to ensure that the peaceful atmosphere of Shabbat is not disturbed with unnecessary noise. The issue of avsha milta often affects the permissibility of using timers to operate household appliances and other electrical devices on Shabbat.

The origins of avsha milta, and, by extension, the permissibility of allowing timers to operate household appliances on Shabbat, are found in the Talmud. The Talmud records a dispute regarding the permissibility of turning on a wheat grinder shortly before Shabbat, and allowing the grinder to continue operating throughout Shabbat, thereby completing the grinding on Shabbat itself.[1] According to one opinion, it is perfectly permissible to do so, as no one is actually violating Shabbat. According to the other opinion, however, the noise produced by the grinder is a zilzul, a disgrace to the honor owed Shabbat, and is therefore forbidden. It is also argued that allowing the grinder to operate should be forbidden due to the concern that onlookers, not knowing that the grinder was turned on before Shabbat began, might falsely suspect the owner of the grinder of violating Shabbat.

Sefardic authorities generally follow the lenient opinion and permit one to operate household appliances on Shabbat that make noise. This is true whether the appliance was in operation before Shabbat began or whether it is activated on Shabbat itself by means of a timer.[2] It follows, therefore, that Sefardim are permitted to turn on a washing machine shortly before Shabbat begins, despite the noise that it makes, and allow the washing cycle to be completed on Shabbat itself. The same holds true regarding a clothes dryer that is full of wet clothes that were placed inside the machine before Shabbat.[3] In fact, it is often permitted to wear such clothes on Shabbat once they are dry and the dryer has gone off.[4] In contrast, however, most Ashkenazic authorities generally follow the strict opinion and forbid one to allow appliances that make noise to operate on Shabbat.[5]

Nevertheless, most Ashkenazic authorities rule leniently regarding avsha milta in a case of need. This is especially true with today's high quality household appliances that make very little noise and often cannot even be heard in an adjacent room. Indeed, noise that is not loud enough to be heard in an adjacent room is generally not subject to the concerns of avsha milta or zilzul Shabbat.[6] As such, it is permitted to make use of appliances whose noise level is minimal and cannot be heard beyond the confines of the room it is in. Furthermore, with such nearly-silent appliances, there is no concern that neighbors might suspect one of violating Shabbat, either.[7]

As mentioned, even those who generally forbid the use of appliances on Shabbat due to the concerns of avsha milta and zilzul Shabbat, often permit their use in a case of need or loss. For example, Rav Nachum Rabinovich was asked whether a soldier who returns home on Friday shortly before Shabbat, and must return to his base almost immediately after Shabbat ends, is permitted to do his laundry if the washing machine will have to operate even after Shabbat begins. He ruled that it is permissible to do so, as the inability to do one's laundry while at home is considered to be a "great loss." He also notes that the Gemara calls the inability to wear clean clothes "distressing."[8] This is also the ruling of Rav Ovadia Yosef for Ashkenazi Jews.[9]

As noted above, for the purposes of avsha milta and zilzul Shabbat, there is generally no difference between appliances that were in operation before Shabbat began, and those that are turned on by means of a timer on Shabbat itself. That being said, it would be remiss not to cite the view of Rav Moshe Feinstein who was well known for his nearly unconditional opposition to the use of timers on Shabbat.[10] He holds that any appliance that one wishes to operate on Shabbat must already be on before Shabbat begins. He argues that using a timer is comparable to asking a non-Jew to perform a forbidden Shabbat activity. Just as it is forbidden to ask a non-Jew to perform a forbidden Shabbat activity, it is likewise forbidden to "ask" a timer to perform a forbidden activity, as well. Rav Feinstein does, however, permit the use of timers to operate household lighting. This is because it was common in Europe to ask non-Jews to turn lights on and off as needed. Therefore, there is no reason to be stringent when there appears to be a pre-existing precedent. It is also reported that Rav Feinstein allowed operating air conditioners by means of a timer, as well.

Nevertheless, common custom is to permit the use of timers on Shabbat to operate electrical appliances of all kinds, especially those that don’t make much noise.[11] It is also permitted to operate appliances that make noise, even if the noise can be heard by others, if it is unlikely to arouse suspicion that one is violating Shabbat. This would include alarm clocks, clocks that chime every hour, or an answering machine that is activated when the phone rings. It is well known that such devices operate automatically without human intervention, and that no Shabbat desecration is taking place.[12]

[1] Shabbat 18a.

[2] OC 252:5.

[3] Yechave Daat 3:18.

[4] Beitza 27a, Rashi s.v. "elah"; Shemirat Shabbat K'hilchata115:18; Tiltulei Shabbat, Responsa #36 from Rav Moshe Feinstein cited in Rav Dovid Ribiat, The 39 Melachos, Melaben, Footnote 121. See Avnei Derech 7:55 for more. It must be noted that many appliances nowadays have digital displays that will go on/off or change when opening or closing appliance. Also, newer clothes dryers often have interior bulb that goes on when opening the dryer. It would be forbidden to open or close such appliances on Shabbat. 

[5] Rema, OC 252:5.

[6] Igrot Moshe, OC 4:70.

[7] Yechave Daat 3:18; Yashiv Yitzchak 8:7.

[8] Siach Nachum 15. See Nedarim 80b.

[9] Yechave Daat 3:18.

[10] Igrot Moshe, OC 4:60. See also Rivevot Efraim 3:248.

[11] Chazon Ish 38:2-4; Minchat Shlomo 1:66; Chelkat Yaakov 71; Tzitz Eliezer 1:20:9; Yechave Daat 2:57; Yabia Omer 3:17.

[12] Rama, OC 252:5; Shulchan Shlomo 252:14.