Floorboards loose from when central heating was installed

[totalitarian]

I've just bought a house and the previous owner had a plumber fit a new boiler. As a result he cut out some of the floorboards to make way for the central heating pipework.

Dotted around the top floor are sections of the floorboards which feel like they will collapse through at any moment. How should I fix this? Some of the loose sections are quite small, 8" sq

 

[ree]

Typically, you have to lift the loose boards, and see how or if they have been supported.
Short lengths of wood known as nailers or bearers can be nailed to the joists or across the joist bay.
Screw the boards back into these nailers, or the joist edges, after making sure that you are avoiding any pipes or cables.

 

[totalitarian]

Thanks. The boards run under the walls, how can I lift them?

 

[ree]

How did the plumber lift them?

 

[totalitarian]

They didn't. They just cut portions out to make way for the pipes and then relaid them with a screw here and there

 

[ree]

Simply lift/unscrew the pieces of board that the plumbers opened up, and then re-set them as described above.

When pipes and joist bays are exposed, why not post pics for more detailed help?