Good luck with the work. If you know HVAC then apt maintenance should not be an issue. Some electrical and plumbing would be about as tough as it might get.
But seeing how you would not be competing with your employer I would see what it takes to get licensed to do HVAC business in you area and begin selling tune ups and service. If you find full time HVAC service or install work your boss will not like you competing with him.
Tom