Sounds like you have got a blockage in the boiler/pipework. Try chemically cleaning the system. Put it in and run for a few days.
Is the pipe work on the ground floor burried in the concrete?
Can you post photos of the set up you have
Went to a job recently where the pipework was burried. No way of draining muck out and struggled to flush because it was a one pipe system.
Customer then remembered there was a drain off in the garden for this purpose. Have a look around the out side of your buiding. His was hidden by a step!!