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First of all apologies if this is reoeated, I have a hot water system heated by an old rayburn royal via a flow and return to the hot water cylinder upstairs, there is no drain cock on the hot water cylinder but a drain cock on the side of the rayburn. The problem...every few days the hot water cylinder makes terrible noises ..cracks and bangs as if it were about to explode and I immediateley turn on the hot water tap which at firs runs erratically and the water is badly disclooured as if it has disturbed sediment from the bottom of the system if the hot water is all drwan off the system behaves ok for a couple of days and the repeats the problem, on the hot water cylinder the connectors for the flow and return are only six inches apart, Am I right in believing that the cylinder has no coil inside (heat exchanger) just an entry and exit for the flow and return I say this as I can see no way of filling the rayburn back boiler and associated pipes wirth waster other than through the hot water cylinder? Is it possible I have managed to get air into the back boiler sysem if so what do I do or is it another problem, PLEASE HELP

Well, the noise you describe is the water boiling. This theory is backed up by the discoloured water which is caused by the water in the tank being stirred up by the steam.

I would guess you have a circulation problem which is preventing the hot water leaving the Rayburn's boiler. Maybe the pipes are scaled up?

I seem to remember a problem similar to this many, many, MANY years ago when the feed pipes inside the Rayburn's boiler tank had corroded away, causing the gravity circulation to stop.

This is only an idea though, so don't strip the Rayburn simply on this post.

Rayburns chuck out a lot of heat and generally there is a radiator on the circuit too to help dissipate the heat. Worth checking that your is turned on.
The hot water heating is indirect, so if you are getting sediment coming out of the tap there surely has to be a break in the heating coil in the tank. In this case you will need a new indirect hot water cylinder.
Either way you have some work ahead of you ! Good Luck

Rayburns chuck out a lot of heat and generally there is a radiator on the circuit too to help dissipate the heat. Worth checking that your is turned on.
The hot water heating is indirect, so if you are getting sediment coming out of the tap there surely has to be a break in the heating coil in the tank. In this case you will need a new indirect hot water cylinder.
Either way you have some work ahead of you ! Good Luck

Thanks for the replies, I managed to download an old manual for my rayburn which dates back to the 60s, the system works with a direct hot water cylinder and it states not to fit a radiator although it does recomend a heated towel rail run of the flow and return pipes, at present I have the rayburn turned off (it was converted to oil some years ago fom M/F) I have concluded with your help and others that there is some type of restriction in the back boiler system which requires flushing out, I am unsure if I can do this, any hints? also the drain cock on the pipe next to the back boiler looks as if it has not been used for decades, I have asked a couple of local plumbers for a price to flush the back boiler and I am waiting for a response although one suggested £350 which I thought criminal as a new boiler only costs £100 and the replacing is a simple slide out/slide in which the manufacturers state is a 15 minute job, anyhow if you can steer me in the right direction to flush the system (would I use a chemical?) I would be very gratefull,....