Fireplace void filled or not?

harry091

Hi,

So I have a semi detached house with a fireplace with a back boiler.

Similar to this
https / lh3.googleusercontent.com/proxy/MWvQ2Wz7i51S1I2_eOZ5w25QF6wJfK6RnHgRvjt38Krn2DB8IOW7WH4gLGLTOs7ZgM41MVvTdOKalQdGcTyzkUjRpoXE5IHOAVt9tddjnOoCNORUXhVfif8Ulmfnw7DIC_6gVjAzFZmZPzNIqIfO0UZs

On the left is a hatch for back boiler piping controls etc
The middle is the fireplace and chimney
The right is what I'm unsure about. It's either an empty void or filled. Does anyone know the best way how to check? Or is drilling a hole the best option?

Upstairs above this room the width is just the chimney i.e. the left (back-boiler section) and right side don't continue after ground floor.

I lifted the floorboards and its "capped" with a cement block. The whole chimney breast is cement block construction. I'm thinking this would be an empty void but not sure.

House was built in early 90s.

Reason I ask is I would like to run some ethernet cables and if I can drill this capped block, drop them into this void & drill out bottom of wall it would save chasing out the wall.

Thanks for any information. Harry

Calahonda52

Is the RHS solid or hollow when you tap it?
Depending on the plumbing, the flow or return could be in the RHS void