I'm working from memory. Many, many years ago, I read one of Eric Browns books. I think it was The Fundamentals of Carpet Maintenance. I believe that he stated that his research supported everything that John has written above. But I also seem to recall that his research found that even if the water did hit a wool yarn at elevated temperatures, the exceedingly short time that the condition existed did not have any noticeable effect on the performance of the wool.
Going on from John's comments on pile burst, this is a well reported and documented phenomenon. It will happen with cooler water, but with the added heat being a catalyst, and especially at pressure, pile burst is, I feel, more likely to happen. This could happen with any heat set yarn, regardless of fibre or fibre blend. It is also of a more frequent occurence than in the past due to, I feel, that many carpets are now made down to a price instead of up to a standard. It is expensive to produce heat, so..........
Safe and happy cleaning:)
Ken