Ravenheat LS80 hot water cyling on/off

[rad121]

Hi
Is it possible to add a sludge treatment to a boiler without getting a professional in.
I have just replaced a hot water thermistor/relay to my LS80 combi boiler as I was getting hot/cold/hot/cold water, before eventually the boiler would stay on and give hot water constantly It would sometimes do this upto six or seven times before coming on for good.
After doing a bit of web research I have replaced the old thermistor, but when I took it out the end piece was coated in a black substance.
Although the boiler is now much better with the new thermistor, and 7/10 times will fire and stay on straight away for hot water, it's not 100% fixed.
I've done some research and found it could merely be the sludge affecting the thermistor. Now I'm also think it could be down to flow rate, as it seems to work better from some taps than others???
And so, back to my original question. How do you add sludge treatment without a costly call out to a professional?
Any other thoughts greatly received!

 

[ChrisR]

Do a search oin cleaning the secondary heat exchanger. DIYable if you're clever.
Intermittent firing could be absolutely anything though

 

[glasgowgas]

isolate 1 rad from system

drain down rad

remove 1/2" plug at top of rad

pour in 1 litre of sentinel x800

open rad to system and vent

run on ch and hw mode for 6 hours

dump complete system water, refill ,vent,run,dump system water

remove 1/2" plug pour in 1 litre of sentinel x100

fill and vent system ,run boiler as usual

about as much as a diyer could do i suppose