@James193 I think an appropriate credit for a missing/defective amenity would be proportionate to the degree of inconvenience and discomfort caused. The heated pool is featured pretty prominently in your listing, so if the heating issue made it unusable for the whole stay it might merit a credit of 20-25% of the room rate.
Of course, that's assuming that the guests left the home in decent condition with no damage and did not demand a full refund - otherwise, I'd suggest offering only an apology.