drain and disconnect a backbiler

I'm sorry, I couldn't help it, a backBILER?

There should be a supply somewhere - usually from a second small water tank in the attic.

Is this a gas or solid fuel back biler, sorry, boiler?

Owain

Your not going to fit a log burner with a boiler?

There will be a supply to the gravity loop somewhere and a vent pipe. Most likely T'd off at the cylinder and disappearing into the loft to a small header tank. If there is central heating that will be part of this circuit as well.

Craked, sorry cracked, chimney breast sounds a bit serious from the structural point of view. There are a lot bricks (ie weight) in a chimney stack...

You need to check if you have a Primatic hot water cylinder.

It is not clear if the back boiler just heats the water cylinder or does more e.g. heat a radiator or two.

If it is a Primatic AFAIK you will have to drain the hot water tank.

If it is not, then you will just have to drain the header tank.

Either way, there will be a cold water feed somewhere to a header tank. You will need to turn off the cold water feed or tie up the ball valve in the header tank to stop the tank filling, and then drain the system.

After that you can cap off the flow and return from the back boiler, preferably as close to the hot water tank as possible (to avoid buried and concealed old bits of pipe still connected to the water supply).

I assume you have an immersion heater in the tank to heat the water.

I must say that I can't see a logical connection between the chimney breast cracking and the need to remove a back boiler and fit a log burner.

The two seem completely unrelated.

Regards

Dave R

Decommisioning a back boiler when the fireplace remains in use has some hidden dangers, which can result in the back boiler exploding some time later (a woman was killed when this happened 5 years ago).

I have a vague recollection that specific measures are required, such as ensuring it's completely emptied of water and filled with dry sand. I think you need to find a detailed description of the process, or bring in someone experienced in doing this properly.

Emptied, filled with sand, and holes drilled to stop potentially explosive pressure build-up.

The HSE document I just posted a link to says they should be removed completely.

Ideally, yes. But if removal has to wait (lack of time and/or cash), it can be made safe for the short term by filling and drilling.