I had a guest who rented a two bedroom apartment from me for 3 people for one week. They stayed only two nights then left complaining there was no hot water. I had a problem with my hot water tank. My property manager called a repairman for a quick fix. He set up a device that one could light and place at the bottom of the hot water tank taking about one hour to heat the water. Thus I was able to provide hot water in this manner for my guests. My property manager came to the house twice a day to heat water for my guests. Did I fulfill my commitment for hot water or should my guest be refunded?
They had a problem and you fixed it best you could. If I were in their shoes, I would feel it was a bit of an inconvenience to only have hot water twice a day - but not enough to request a full refund.
I would maybe offer them a partial refund for the inconvenience.