How do I determine the CR for this trap involving repeated activation?

Question

Can anyone please tell me how to determine the CR for a trap that has a duration of 10 rounds?

It is a repeated steam jet trap that emits 2 steam jets every round and each steam jet deals 2 damage + 1d4 subdual damage (fire damage?). damage can be avoided completely by making a successful reflex save at a DC of 14 - x where x is the number of rounds that have elapsed since the trap went off.

Some context for the trap: a spa / bath house on top of a rich mineral deposit is being terrorized by 2 thoqqua, when the players enter one of the baths for investigation the thoqqua have burrowed from an adjacent pool (situated slightly higher) to this one and attack the players. then water starts pouring through the 2 smoldering holes they left behind creating the 2 steam jet traps. I should probably also mention this will slowly fill the bath with very very hot water, so the floorspace available for melee decreases slowly..

Answer 1

The jets aren't a trap if the thoqquas made them by burrowing right now

While I'm challenging the question's frame a bit here, if the rogue couldn't've made a Search check to find the steam jets beforehand, and no matter the rogue's Disable Device check result, he wouldn't've been able to disable the thoqquas, the steam jets aren't really a trap. Instead, the steam jets are a feature of the location in which you've opted to stage the encounter, the presence of the thoqqua-created steam only making the encounter more interesting and difficult, not, for example, adding the equivalent of a creature or trap to the encounter. (Take a look at Location on page 50 of the Dungeon Master's Guide.)

To compute XP for this situation by the book, consult Modifying XP Awards and Encounter Levels on page 39 of the Dungeon Master's Guide. Then, rather than assigning the steam jets their own Challenge Rating, consider adjusting the XP reward for the encounter as a whole. Fortunately, the DM can individualize this XP adjustment according to the PCs: having to make 10 increasingly easy Reflex saving throws in a rapidly shrinking arena would, for example, barely merit a +10% XP reward for a party of rogues but might double the XP reward for a party of fighters wearing plate armor who've specialized in charging! However, for the typical mixed group that may find this encounter significantly more difficult because of the steam jets, an adjustment of 1½× XP seems reasonable.

Answer 2

I presume the players can move out of the room freely?

If so, as long as those 2 streams are directionally stable (and not spreading hot water all over the place), only 1 reflex save seems in order here.

Only when characters are bull-rushed, should they make another (or be submerged). Also, I don't think the duration is a factor on CR per sé, as long as damage can easily be circumvented (by not stepping in the bath) after the first round / reflex save.

Note: steam can hamper vision (concealment), which COULD (DM-discretion) have an effect on reflex saves.