As is common with regard to Pesach, the halacha is particularly strict on this matter, and the practice that many of us have is even more stringent than the classical sources indicate.
During the course of the year, the almost universal practice is to use the same stovetop grates without even cleaning them between milchig and fleishig use. This surprising leniency is based on one or more of the following possibilities: 1) Even if the grates have absorbed taste from spilled milk and meat (and are thus treif), there is no transfer from the grates to the pots that sit on them. In general, there is no transfer from one utensil to another when there is no food or liquid in between them.[1] Although when there is spillage, there is liquid in between the two, the Chavat Da’at[2] rules that only the presence of a significant amount of liquid can facilitate the transfer. This does not occur on grates, as excess liquid quickly runs off. 2) The fire on the stove serves as a means of kashering.[3] 3) The substance that falls on a grate is expected to be burnt up before it can affect the grate.[4]
We might expect that one could likewise use the same grates without problem on Pesach. Yet, the Rama[5] writes that a chatzuva (tripod, which people used like our grates) requires libun (the form of kashering that employs extreme dry heat) for Pesach.
There are various attempts to explain why there is a stringency specifically for Pesach. One is that as opposed to year-long prohibitions, where only a discernable influx of forbidden food taste renders food non-kosher, on Pesach even the smallest amount of absorbed material renders it not kosher for Pesach.[6] However, poskim seem to posit that not even trace quantities of taste particles are transferred from one pot to another.
A different possibility is the claim that it is more likely that one will put matza directly on the grate, as opposed to milchig or fleishig food during the year. The Rama’s source, the Mahari Weil,[7] seems to justify the need for libun for Pesach on different grounds. He notes that chametz is a particularly severe prohibition, one which we are not used to avoiding. This, then, is the most authoritative difference,[8] which is also one that is used to explain many Pesach stringencies.[9]
Because this kashering is a stringency, the Mishna Berura[10] says that it is sufficient to do the easier form of libun, known as libun kal, a level of heat that most modern ovens are presumed to reach at their maximum setting within half an hour. He also rules that if one put a pot on a grate that was used during the year and had not been kashered, it would not become chametzdik.
The suggested way of kashering is described by Rav Shimon Eider as follows.[11] Clean the grates and then put all of the burners on high for 15 minutes with a sheet of metal covering them so that they reach a very high temperature throughout. It is also possible to put them into an oven on high for around half an hour. If one puts them through a cycle in a self-cleaning oven (assuming this is safe for them), any imaginable halachic doubt will be removed.
Many poskim rule that one can either kasher the grates or clean them of any residue on the outside and then cover them.[12] There should be no need to both kasher and cover the grates.[13] However, our holy nation is at its most stringent mode on Pesach, and many fine Jews cover the grates with aluminum foil after kashering. (Rav Sheinberg is among those who cite this as the standard practice.) Although we certainly do not mandate this, we do not scoff at the idea either.
If only from the perspective of time, it is likely worthwhile for those who can to purchase replacement grates for Pesach and spare themselves significant time and work over many years of Pesach preparations.