Generally speaking, the contemporary practice is to be very machmir when it comes to Pesach, above and beyond the letter of any law. I'm curious to know if there have been any relevant rulings or customs regarding using (clean, of course) seltzer-makers (such as the SodaStream) on Pesach, if they were used year-round in a chametz kitchen.
See answer to this related question regarding the kashrut of Sodastream for Pesach. According to OU, it is permissible for Pesach use. (I can't qualify it further as my web browser is blocking me from viewing the OU site, now. If someone wishes to edit my question to include what OU says, please do so.)
Sodastream recommends using a new bottle or one previously reserved for Pesach, which is what I do. This is esp. true if you used any their soda mixes or added flavorings to the soda after you pumped it. I don't as I'm a two cent's plain kinda guy, and that's my two cents worth on this topic ;-o
This page suggests that the matter is complicated and that there is halachic reason for worry unless specifics are well known. It cites a source for kashering seltzer equipment, and suggests there is reason that kashering it for Pesach is necessary:
Da’as Torah, addendum to Hilchos Teraifos ד"ה בזה"ז ** (Rabbi Shalom Mordechai Shwadron of Berezhan, Ukraine, sefer printed in 5651/1891?**) describes how to kasher seltzer equipment and justifies the need for hag’alah with a number of chametz concerns (i.e. b’en of chametz left in the equipment, chametz stored in the cylinders, and the equipment used to grind the raw materials) that appear unrelated to the question of vapors from a chametz reaction.
Soda Stream machines also do not need anything more than to be cleaned well.
Although use of the machine is iffy (at least for Ashkenazim), it seems that there is room for leniency on the matter of the CO2 itself:
If carbon dioxide is recovered from a chametz fermentation of bread, beer, or whisky, is it forbidden on Pesach? The earliest teshuvos on this topic are dated from when this industry was in its infancy, and all of the teshuvos assumed the carbon dioxide was in fact forbidden to the point that they did not even feel a need to explain the rationale for their ruling.3 In contrast, it is reported4 that when the question was posed to Rav Shlomo Zalman Auerbach, zt”l, approximately 30 years ago, he stated that the carbon dioxide is clearly permitted on Pesach. Why did the Poskim take positions that are so diametrically opposed to one another? We will see that a careful analysis of the relevant halachos shows that אלו ואלו דברי אלקים חיים , in fact both positions are correct!
Finally, although Sodastream's FAQ mysteriously states that "SodaStream seltzer is kosher for passover (click to see OUP certificate)," as of January, 2015, none of Sodastream's flavors appear to be Kosher for Pesach, and the certificate linked contains no evidence in support of the claim that the "seltzer"--whatever they mean by that--is kosher for Pesach.
Another rabbi won't drink Sodastream on Pesach for a completely different reason.
Whatever your reason, it seems that the answer for Ashkenazim is no--unless you go to special trouble to kasher your machine.
I asked my LOR about the Soda Stream and he said that the seltzer (unflavored) is usable on Pesach. However, If any of the flavors were added, then the bottles would not be acceptable. As a result, I got new bottles for Pesach and store them with the Pesach dishes. This was the psak of my rav and I did not ask for his reasoning. I think that it is probably because the CO2 is not derived from chametz, but you must CYLOR.
There is a discussion at Carbon Dioxide for Pesach which explains how CO2 generation was originally chametz, but nowadays is no longer a problem.
With this understanding, we can now address the question of whether carbon dioxide recovered from a chametz fermentation should be forbidden on Pesach. It turns out that the technology of recovering carbon dioxide has steadily progressed over the past 100 years. At first scientists figured out how to liquefy the carbon dioxide escaping from these reactions but had no effective method of purifying these vapors. As such, in those days, the liquid carbon dioxide likely did have a carryover of taste from the beer or whisky it was made from, and for that reason, the Poskim of that era concluded that the carbon dioxide was forbidden on Pesach.
However, in the past 50 years, the industry has developed 5 different methods of purifying the carbon dioxide6 – four while it is in the gaseous state, and a fifth after it is liquefied – to the point that nowadays carbon dioxide recovered from beer or whisky bears absolutely no taste of the original liquid it was created from. We can surmise that that this change of facts is why when the question was presented to Rav Auerbach, he ruled that it is surely permitted! Rav Auerbach appears to have held that a vapor/condensate only retains the status of the original liquid if it also retains the taste of that liquid, and since nowadays the carbon dioxide has absolutely no taste of the original beverage, it is not forbidden as chametz. [See the footnote for an alternate explanation of these halachos.]7
Of the kashrus agencies we conferred with, we found that many chose to take a strict approach and not allow carbon dioxide from chametz or even kitnios (but they took varying approaches to verifying the source of the carbon dioxide)8 but others came to the conclusion outlined above that one may be lenient nowadays.9 Due to the (commendable) strictness with which people traditionally treat questions of chametz, the cRc only certifies seltzer or soda after verifying that the carbon dioxide is not recovered from chametz (but does accept it from kitnios sources). While this may not be required on strict halachic grounds, it is within the spirit of חומרא דפסח to be machmir on this matter.
The following are some relevant facts about the current carbon dioxide market:
Nowadays, beer companies typically do collect the carbon dioxide vapors escaping from their fermenters for reuse in their products, and it is very rare for them to sell the carbon dioxide to others.
Most whisky and ethanol produced in the United States is kitnios, not chametz. Due to an overabundance of carbon dioxide byproduct, it is uncommon for a producer (e.g. a whisky or ethanol plant) to recover the carbon dioxide unless they have a specific buyer in mind. In fact, typically the carbon dioxide resellers are the ones who install the collection equipment in the producer of their choice, and then remain with that producer for many years. As such, although many new ethanol plants have opened up in the USA in recent years, it may take many years before the carbon dioxide produced in those plants is ever used in seltzer.