Hello @Dave126
They take host cancellation's seriously as it impacts on their reputation and the guest experience.
Have you read Airbnb's information on host cancellations on their Help Section?
They even tell you if you cancel you will be penalised, when you actually go to make a cancellation on a booking.
Their Help Section also tells you about how to cancel under it's extenuating circumstances - which a broken boiler could come under.
Unfortunately you have acted first, rather than following Airbnb's suggestions or giving them a call. You can call them and see if they will consider it under their extenuating circumstances (you will need evidence from someone like a heating engineer).
The 2nd post on the Community Help forum tells you all the ways of contacting Airbnb, or search on here, or use Google.