Boiler switching off - cross leak??

[Unregistered]

Since October last year we have had a problem where the boiler goes to red light (turns off) whilst trying to heat the house. The problem was very intermittent from once a week to now at least once a day if not more. It also seems to go to red light when running the taps, for example whilst taking a shower it starts off fine then the water goes ice cold and have had to turn on the boiler again as its turned off, just this evening, the heating was running fine until I turned on the cold tap in the bathroom to brush my teeth and it turned off the boiler. On occasion we have been completely unable to get any hot water unless we have the heating running at the same time.
We had a gas safe engineer look at the boiler and told us it all looked fine, he then diagnosed that he suspected a cross leak behind the shower which was letting cold water back into the system and shutting off the boiler. I haven't been able to find any information on this online. When we moved into the flat the shower always pretty much had one temperature so a plumber suggested fitting a new temperature valve, in case cold water was flowing back into the system this way. This has been done but has not fixed the problem.

The entire flat is underfloor heated, but has anyone heard of a cross leak turning off a boiler? Or has anyone got any idea why this might be happening? Many thanks

 

[SmokeyJ]

You'll be told to register and repost. Was told any 'unregistered' person can post replies to thread and then it can get confusing with who's the original poster. Thought I'd let you know as may save you some time with your replies.

 

[jbeb]

Thanks, registered

 

[Riley]

Can you isolate the cold feed to the shower? If you can, do that and see if the problem persists. With a new shower I'd assume it won't make a difference as new shower likely has non return valves fitted

 

[Sparkgap]

I take it's a combi. What make/model and is there an error code? If there's a 'cross leak' from cold to hot on the shower I'd think it would happen either all the time or just when you used the shower, not when ordinary cold taps are opened. Could it be turning on cold water drops the cold pressure to the boiler and it's going out on low pressure?

 

[jbeb]

Yes it's a Sabre 25 HE combi boiler, there is no fault code that I know of and the pressure gauge stays at around 1.5
really the water turning it off is a secondary issue as the boiler shuts down by itself without any taps being on, so what would that suggest? That there is a constant leak so when the heating is on it is leaking cold water back into the system? The boiler also makes a frantic clicking noise like its struggling to keep the pilot light on. This morning alone it has gone to red light 5 times whilst the heating has been running without any taps on. We also didn't have any hot water for 2 1/2 hours this morning even with the heating on.
I just want to be sure it's definitely a leak before the landlord rips out the entire shower to get to the pipes.

 

[Sparkgap]

Just had a look through the users instructions and it only mentions a lockout for the red LED. When was it last serviced? If it's started getting worse could be either something external (low gas pressure or a number of things) or possibly something inside the boiler starting to give up the ghost. The clicking would indicate flame failure and attempting to relight so looking at the gas supply might be in order (thinks: more probable as the boiler would be drawing most gas on combi function and less if modulating down on heating.)