@Monica1317 Happy to see that the issue is resolved. The policy is a little bit vague. On the one hand, the Terms of Service state:
7.1.2 Once a Guest requests a booking of your Listing, you may not request that the Guest pays a higher price than in the booking request.
So while the host can require payment for optional services, or changes to the booking (such as number of guests), he can't raise the room rate. The room rate by default includes all the items on the Amenities checklist, and if any of those come at an extra charge it has to be stated in the listing.
The loophole here is that the Amenities checklist does not have "Heated Pool" as an option - only "Pool." So while he's obligated to provide you access to a functioning pool, he is allowed to say after booking that heating the pool is an additional paid service. Not a good move from the customer-service point of view, but technically allowed.
@Susan151 makes a fair point that muddies the water a bit. The text of the listing description is also relevant to the host's obligations, so if the listing explicitly said the pool is heated year-round, that's the condition it should be delivered in. Otherwise, I'm not so sure. A listing can say that the home has central heating, but that doesn't necessarily oblige the host to turn it on in July or let the guests leave it running all day.