Combi Boiler Problems

[Benno]

Hello All,
Our old combi boiler seems to be getting worse in it's old age (about 10 years ), at the moment we do not have running hot water down stairs, the water flows from the hot tap but the boiler fails to heat it, it just seems to light up and then go out (pilote light does stay on) and repeats this as long as the down stairs hot tap is on. If we put the heating on then we get constant hot water from the tap. It also makes a lot of popping noises after use which as we live in a hard water area I guess is a build up of lime scale.
Would flushing and cleaning the system be a good idea as I believe it has never been done ever !
Cold water seems to have good pressure,
cheers
Benno

 

[baxpoti]

Give us a clue to make and model of the boiler and you may get some helpful answers!

 

[Benno]

...off the top of my head I think it is a Glow Worm 75

 

[John_D]

when the hot water tap is opened a diaphram flexes and triggers the boiler to fire up. this then circulates boiler water through the domestic hot water heat exchanger and heat transfers to the tap water.

If you get poor flow from the hot tap, expect scale problems, typically boiler fires up, overheats cuts out tap water cold-very hot-cool-very hot. and slow flow. This doesn't sound like your complaint.

when the heating is already on the heat exchanger is also hot so the tap water comes hot.
hence diaphram failed.

 

[Paul Barker]

If it's a swiftflow its bi flow heat exchanger.

Maybe max burner pressure set too high, (noises and failure to stay on for hot water) domestic flow switch stuck (left of heat exchanger ball bearing pushed up by water flow operates reed switch.)

You get hot water when heating is on because heat exchanger already hot, from heating, which modulates down so max burner pressure being set too high doesn't cause it to cut out.

Just a guess. Could be anything.

 

[Benno]

many thanks for the replys,
I would like to give the system a de-scale but have never attempted this before. We have only suffered a couple of leaks in the system but these have only been at the boiler end and caused by botched repairs to a couple of joints (a plumber we wouldn't use again !) so I guess fernox heavy duty restorer will do the job ?
Can anyone supply detailed instructions on how to drain/flush the system, I know that this is hard having not seen our system but there must be a standard way of doing this.
I guess most of you are cringing at the thought but I really would like to do this myself,
cheers
Benno

 

[croydoncorgi]

Why do you think it needs 'de-scaling'? (as opposed to 'desludging' or some other process involving a flushing machine. )

And I'd be VERY cautious descaling a boiler with any history of leaks.