Here is another one from the Mi Yodeya files. This is the long version of my answer here.
QUESTION: Many offices have a water dispenser (with hot water spout) or a hot water tap next to a sink. Is this hot tap Kosher? Assume for a moment that someone uses the tap to dispense hot water into a cup of non-Kosher instant soup or non-Kosher instant hot chocolate. Does the steam from the cup make the tap non-Kosher? – Seth J
ANSWER: You may use the water from the tap.
The following is the basis for my answer:
The Rema rules:
אסור לערות מכלי שיש בו שומן כשר לנר דולק שיש בו חלב או שומן איסור ובדיעבד אין לחוש.
It is forbidden to pour from a utensil that contains kosher fats into a lit candle that contains non-kosher fats, but after the fact one should not be concerned about it.
At first glance, the Rema seems to be dealing with precisely your case, and it would seem that the correct answer is that you are not allowed to use the water from the tap. However, there are a few key points which need to be explored before saying so.
1. The contents of your cup are kosher. Your concern is that the last person’s non-kosher food rendered the tap water non-kosher. The Rema said that “after the fact one should not be concerned about it.” This seems to be a case of after the fact.
2. The specifics of the Rema’s case are that the item you are pouring from is kosher while the item you are pouring into is not. Also, the item you are pouring from is cold while the item you are pouring into is hot. These facts are not present in your case, so the question really is how do these facts play into the Rema’s ruling?
TO ANSWER POINT #1 whether this is called lechatchila or bedi’eved: Just prior to this Halacha there is another Halacha which Shulchan Aruch states is only lechatchila, but after the fact one should not be concerned. The Shach explains that it doesn’t mean that once it happens, lechatchila one shouldn’t eat it. Ruling that way would by definition introduce an after-the-fact dimension of prohibition, which we are trying to avoid because of the terminology employed there – bedi’eved muttar. Rather, says the Shach, it means that we should take any precautionary measures we would take would this be a potential after-the-fact issue.
If we should apply this to our case, the correct ruling would be that it is forbidden to do the act of pouring, but that once done there is nothing forbidden after-the-fact; the previously kosher stuff may still be consumed lechatchila, and certainly no utensils become non-kosher due to this act. Therefore – and this is touching slightly on point #2 – since your whole question is whether you can drink the water from the tap after the other person used it for non-kosher stuff, your question essentially is whether there is any issur retained after the fact. Based on the above understanding of lechatchila and bedi’eved which I think is correctly applied here, this is a situation of bedi’eved, and as the Rema said, “after the fact one should not be concerned about it.”
POINT #2 IS A BIT more complex. For this we need to explore the basis of the Rema’s ruling.
The Taz writes that the reason for this Halacha is that the ‘nitzuk’ – the ‘flow’ between the two items connects them and they can therefore render each other non-kosher. The question is where in the world does this come from? Things don’t become non-kosher because of a ‘connection.’ Who cares if in some legalistic sense these two things are connected? All we care about is whether there is a transfer of flavor! And it’s not like we can simply push this explanation aside, because in the Darkei Moshe where the Rema cites his source (the Mordechai) he explicitly brings the reason of nitzuk!
I think that the answer is as follows. Of course, nitzuk per se isn’t really a reason to make something non-kosher. It is really just a concept borrowed from the laws of ritual impurity. However, we know that there are very specific rules about when we assume transference of non-kosher flavor. One of those rules is tata’ah gavar – the bottom wins. Meaning, we begin with a premise that generally speaking flavor is only transferred with heat. What if there are two items on top of each other and only one item is hot? Here’s where this rule comes into play; we say the bottom always wins. If the bottom is hot, we say that flavor transfers from one to the other, but if the bottom is cold, we say it does not. All this is only when the two items are actually touching, for that is generally the only way flavor can transfer. I believe that nitzuk, as it pertains to us here, is just a fancy way of saying that the flow between the two things makes us regard them as touching, and thereby subject to the rule of tata’ah gavar. In other words now that we view the entire nitzuk as part of the thing it is flowing from and not a separate entity, we can say as we always do that if the bottom is hot it will cause flavor to transfer not just within the flow and the bottom, but also within the top – since the nitzuk is part of the top, and there are simply two things here which are touching each other.
If this explanation is correct then it would be entirely inappropriate to compare the Rema’s case to the hot tap case – because there is a crucial difference. In the Rema’s case, the bottom is the hot candle. Our case is exactly the opposite: the contents of your cup are normally cold, while only the top – the water – is hot! Therefore, if we go by the simple rules of tata’ah gavar we are forced to say that the last person’s non-kosher stuff never transferred any flavor into the water! Ergo, the answer to point #2 is that some of the minutiae of the Rema’s case are very relevant, and they significantly differ from ours.
I must note that the Shach appears to understand the Rema differently. He writes that the reasoning is that the steam rises from the fats in the candle and into the items on top. Although I think that it is difficult to fit this into the Rema because it basically gives no reason for the sources of the Halacha to mention nitzuk; yet even if we should concede to the Shach, the rules of tata’ah gavar still apply. Steam is not better than actual hot liquid, which can only transfer flavor when the bottom is hot and not when it is poured onto something cold.
IN CONCLUSION, a hot water tap and the like that were previously used for non-kosher items may still be used lechatchila. Similarly, the same hot water tap may be used lechatchila for both meat and milk.
 Se’if Katan 5
 Ad loc.
 Tur YD 105:5
 Chullin §719
 You may ask - but there is a rule that even when the bottom is cold one must still peel off the outer layer of the other item just in case it did absorb a drop! Why doesn’t the water become non-kosher – at least that much? The answer is that the ‘peeling’ would take place on the bottom layer of the top item, and that bottom layer is part of the flow, so we still don’t have to worry about the water in the tap. Furthermore it is questionable if there is even a requirement to peel by liquids (that you obviously cannot peel), and even if there is, everyone agrees in these cases that if there is 60 times the amount one theoretically needs to peel then the whole thing is kosher.
 See also the Pri Megadim who learns that the Shach and Taz agree, and he also clearly understands that if the bottom is cold then there is nothing to be concerned about.