Wats wrong with our boiler ??

Question:we have a combie boiler. The piolet light keeps cuting out when we try and run the hot water but doesnt cut out when we put the heating on. If we leave the heating on for a while and then want some hot water the pilot light works fine. What could be the problem its coming up to summer and we dnt want to keep putting the heating on.

Answers:
wats a combie boiler? and wats a piolet light