That's my problem, have to turn some good jobs down. I normally have a chat about the boiler fault. See if I fancy it, and then ask what boiler it is. Then say oh no, don't do,them or, parts are expensive ring the manufacturer.
Whilst its obviously less complex than plumbing, you will still have the same problems. The issue is that your employees won't care about your business in the same way that you will.
Learning to cope with that fact, and to realise that it didn't make them bad people was the hardest lesson I have learned in my business career. Now I understand that its my job to design systems and processes that allow and motivate ordinary people to do an extraordinary job.