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Using the same dishwasher for both meat and milk vessels, even one after the other, can cause potential Kashrus concerns. Thus, initially, one is to have separate dishwashers for both meat and dairy utensils and have them connected to separate water pipes. Initially, even Pareve vessels should not be washed in the meat or dairy dishwasher. If one only has one dishwasher available, it should be designated for only one use [i.e. the more necessary] and have the other dishes washed by hand, and so is the custom of many. Nonetheless, from the letter of the law it is possible to use the same dishwasher for meat and dairy and Pareve vessels, under the conditions to be explained. One who moves into a home that contains a dishwasher must verify with the owner as to its status. If this is not possible, then it needs to be Koshered.
Time of need-Using the same dishwasher for meat and dairy vessels: One may not use the same dishwasher for washing both meat and dairy vessels at the same time, and one who does so possibly prohibits all the vessels that are in there. However, one may use the same dishwasher for washing meat and dairy vessels one after the other, if the following conditions are fulfilled: 1) The machine is taken apart and cleaned from any and all food residue. 2) The dish wracks are switched or cleaned very well before use. 3) One waits 24 hours from its last use of the opposite dishes of meat or milk. 4) One runs an empty cycle with hot water and soap prior to the current cycle. 5) The above process only helps if the dishwasher does not contain any porcelain or ceramic material inside, otherwise it cannot be Koshered to a different use. However, some Poskim are lenient to use the dishwasher even without Koshering it inbetween or waiting 24 hours [steps 3-5].
Bedieved: If one found a dairy vessel in his meat dishwasher which uses Yad Soledes water, then if one does not know for certain that the meat vessels were clean, the dairy vessel is forbidden. [However, the meat vessels remain permitted if one can assume that there were 60x against any dairy residue.] If, however, soap was used throughout the entire period of the hot washing, then many Poskim are lenient to permit all the vessels, as explained in Chapter 8 Halacha 30E. Possibly one may rely on this if he is unsure if the vessels were dirty with meat. Likewise, even if he is positive that there was meat and dairy residue there, some dishwashers may be defined as Iruiy Keli Rishon, and would contain an added leniency.
Koshering a dishwasher: A dishwasher that has become forbidden due to washing Issur, or meat and milk together, is to be Koshered in the following method if made of Kosherable materials [i.e. metal]: 1) The machine is taken apart and cleaned from any and all food residue. 2) The dish wracks are to be cleaned. 3) One waits 24 hours from its last use. 4) Run an empty cycle with hot water at the maximum possible temperature [i.e. Hagalah]. The above process only helps if the dishwasher does not contain any un-Kosherable materials, such as porcelain or ceramic material inside, otherwise it cannot be Koshered. Most dishwashers today do contain non-Kosherable materials.
 See Hakashrus 1:71-77; Hakashrus Behalacha p. 141; Article of Rav Yisrael Rozen of Tzomet published in Techumin 11:130; Igros Moshe O.C. 1:104; Y.D. 2:28-29; Y.D. 3:10-11, 13; O.C. 3:58; Beir Moshe 7 Kuntrus Elektri 60
 Beir Moshe 7 Kuntrus Elektri 60; Hakashrus ibid; Hakashrus Behalacha p. 141
 See Igros Moshe ibid and Rav Ovadia Yosef in coming footnotes permits using it one after the other; However, see article of Rav Rosen
 See Igros Moshe ibid; Hakashrus 1:73; Kashrus Bamitbach Hamoderni 2:1-6; 3:10-11
The reason: One who washes both meat and dairy dishes simultaneously in a dishwasher potentially prohibits everything there, as the leftover meat and dairy foods, and Ben Yomo status of the vessels, cause a forbidden mixture and cooking of tastes. [Dishwashers typically heat the water to 140 degrees Fahrenheit and the water is heated within the dishwasher itself. However, perhaps the spraying water can be viewed as Iruiy Keli Rishon and hence be permitted according to the Rama, and according to the final ruling, in a case of great loss, as explained in Chapter 8 Halacha 8. However, see article of Rav Rozen that many dishwashers have the status of a Keli Rishon as they have a heating element inside.] Furthermore, it possibly also transgresses the Biblical prohibition against cooking Basar Bechalav. [See Igros Moshe Y.D. 2:28-29; Hakashrus 1:73; Kashrus Bamitbach Hamoderni 2:1-6; 3:10-11]
Other opinions: Some write that those who are lenient to wash both meat and dairy vessels in the dishwasher simultaneously have upon whom to rely. [Rav Yitzchak Yosef in Otzer Dinim Laisha Ulabat 30:15; Hakashrus 1 footnote 137 in name of Rav Ovadia Yosef, although I could not find his source in the Sefer Hakashrus Behalacha which he references to, and on the contrary, that Sefer writes in the name of Rav Ovadia that one may use it one after the other, and makes no mention of using it simultaneously. Nevertheless, in essence there is some truth to the statement, as according to the Michaber 95:4, if a spoiling agent is added to the water, meat and dairy dishes may be washed simultaneously in a Keli Rishon. Furthermore, perhaps one can judge a dishwasher as Iruiy Keli Rishon rather than Keli Rishon, as the water is sprayed. Vetzaruch Iyun, as possibly there are coils within the actual washer which heat the water, and perhaps the cycle which sprays clean hot water without soap to clean the soap off the vessels, loses the above leniency of a spoiling agent even according to the Michaber. See article of Rav Rosen who explains that these Rabbanom who gave their lenient rulings were unaware of how the machine works, and that in today’s machines it is judged as an actual Keli Rishon, as they contain coils inside, and the food does not become Pagum due to the soap, and there is no 60x when the water drains out, and other facts that the Rabbanom were unaware of today.]
 Based on different Teshuvos of Igros Moshe ibid, as compiled in article of Rav Rosen; Hakashrus and Kashrus Kehalacha ibid
 Rav Moshe in Igros Moshe Y.D. 2:28 writes to have separate dishwracks for meat and milk
 Hakashrus 1:74 as such materials are not Kosherable; See Igros Moshe O.C. 3:58 regarding Pesach that it is not Kosherable, however Rav Moshe never made this distinction when using from meat to milk
 See Igros Moshe ibid in various Teshuvos on the basis that it is Nat Bar Nat Diheteira [the vessels to the water to the dishwasher] as there is usually 60x versus the residue.
 See Yad Avraham ibid
 The reason: As there is no Safek here, as we know for sure it was washed with meat dishes in Yad Soledes water in a Keli Rishon. [Dishwashers typically heat the water to 140 degrees Fahrenheit and the water is heated within the dishwasher itself. However, perhaps the spraying water can be viewed as Iruiy Keli Rishon and hence be permitted according to the Rama, and according to the final ruling, in a case of great loss, as explained in Halacha 8. However, see article of Rav Rosen who explains that these Rabbanom who gave their lenient rulings were unaware of how the machine works, and that in today’s machines it is judged as an actual Keli Rishon, as they contain coils inside, and the food does not become Pagum due to the soap, and there is no 60x when the water drains out, and other facts that the Rabbanom were unaware of today.]
 Now although a Stam vessel that is being washed is dirty, as explained in Shach 95:1, nevertheless here if one does not know for sure the vessel is dirty one may be lenient as perhaps the leftover residue became spoiled due to the soap or other spoiling agent. Nevertheless, there are Poskim who in truth learn that the vessel is only permitted if one knows for certain that it was clean. [Yad Avraham]
 Igros Moshe Y.D. 3:58; Hakashrus 1:77
 Igros Moshe ibid
 See Igros Moshe ibid that initially the water is to reach boiling point although if not possible, it suffices if it reaches a hotter temperature than usual; See Igros Moshe 3:29 that his son, Reb David, suggested that there is no need for running a cycle of hot water for Hagalah, and it suffices for 24 hours to pass, as it is similar to the allowance to Kosher non-Ben Yomo vessels in less than 60x water. Rav Moshe negated this by saying that this is only required when it comes to Koshering, however, when it comes to washing dishes and other food related matters, the Sages decree non-Ben Yomo versus Ben Yomo. [Brought in Hakashrus 1 footnote 145]
 Igros Moshe O.C. 3:58; Hakashrus 1:74 as such materials are not Kosherable; Koveitz Mibeis Levi 1:30