Combi Boiler - Hot Off/On, Cutting Out/Overheating, Relights

[pleasehelpus]

Hi All,

Can we please have some advice on our problem? I have tried to read as much as i can on here before i made my first post.

We currently have boiler insurance with Domestic and General.

Recently our shower started to go cold and then hot again around 2 minutes a time. I noticed by looking at the boiler that it was cutting out, flashing and then re-igniting. This has gradually got worse and worse. Now on setting 6/10 water temp it cant run for over 10secs! The Radiators seem to be working perfectly...

We called out D&G for a repair and they sent out a guy who`s analsys was that the system was sludged up and the heat exchanger blocked. He took off the heat exchanger and cleaned it out with Cilit Bang!

When he had done this he refitted it and it was exactly the same. He said that it must be blocked and we are not covered under our policy.

He recommended a chemical flush that would cost around £320 and they cannot do anything else for us until we get this done....

After reading on this forum we took off the heat exchanger and used a chemical cleaner (small bottle S.Acid) and after it had turned blue, when we poured out the used liquid there was a medium amount of black sludge/bits in the bowl. We then repeated it again and this time there was hardly anything. We then hosed it and then cleaned it again with Fernox DS3 Scale Remover, this again didnt bring out any other dirt.

So.. we refitted it back after this mammoth clean, crossed our fingers and guess what..... everything is the same!!!! cuts off straight away

Even If it was cleaned out even slightly should it atleast last a bit longer before cutting out? The water DOES get very hot when its on the hot setting, would that mean that the heat exchanger IS working and not blocked?

Tomorrow we are having a temperature sensor delivered, do you think we are on the wrong path? Is there anything else we can please check

Boiler: Alpha CB28
When set to 10/10 water heat - lasts 28sec before going out and relighting
When set to 1/10 water heat - lasts about 5 secs
Heating Seems to be fine
Just before it cuts out, maybe 5secs before, the water goes cooler and then it cuts out.

We are a bit annoyed that we are having to pay for parts ourself when we pay for cover, but they are basically holding us to ransom untill we buy a chemical flush!!

Thankyou in advance to all !

 

[pleasehelpus]

Forgot to mention:
I know that both channels on the heat exchanger are not blocked as i put a hose pipe on all 4 pipe ends and the water flowed freely through and out of the other end

Tim

 

[BingoBongo]

who are d&g?

unlikely a powerfflish is the answer if its working on heating ok

 

[pleasehelpus]

who are d&g?

unlikely a powerfflish is the answer if its working on heating ok

D&G - Domestic & General

Thanks for the reply.
The engineer told me that the heating uses the upper exchanger so thats why the heating is ok

 

[namsag]

You say your SHOWER goes cold .
What about at the hot taps

 

[pleasehelpus]

You say your SHOWER goes cold .
What about at the hot taps

Well the shower was going cold as the boiler breilfy cuts out. The taps also go hot then cold

Thankyou for the reply. I am going to try and find a temperature sensor today!! but i hear there are more than one!

Am i right that this on circled below would be the first to change?

I have a digital mulitmeter here, is there a way i can measure and check that the sensors are faulty/ok before spending money replacing them?

 

[pleasehelpus]

UPDATE:
Changed the lower temp sensor and its exactly the same

BUT we have noticed that when we run the tap downstairs VERY SLIGHTLY and run the shower or the bath upstairs it does not cut out at all!
At last we can have a nice hot shower or bath again!

Regarding the temp sensor using a mulitmeter i measure 12K when cold and 2k when hot which is exactly what Alpha engineers state it should be.

Im now completely guessing this leads us towards the Diaphram or the divertor valve or swtich. (is this the flow switch?)
OR as the water is always heated up whenever the hot tap is turned on would this mean that this part of the boiler is working just fine?

I was told that i can remove the wire/plug from the divertor switch, this would simulate "hot water on", if the problem still occurs then i would then be able to illiminate all of the above parts as faulty?

My final question, the manual does not point out where any of the above is? Am i right thinking that it is the black switch below located on top of the diapham:

 

[davebgas]

does it actually go to lockout when you say it overheats?

 

[gas4you]

The ntc that I would suspect first off is the primary ntc, inside the combustion chamber. Anyway top right next to the overheat stat sensor, no 5 in the MI's

It is exactly the same sensor, but I've always found this is the one that goes.

 

[pleasehelpus]

does it actually go to lockout when you say it overheats?

Well actually your right, im guessing that it is overheating, i should say "cutting out" as it just cuts out and then relights, there are no error lights, it doesnt needs a RESET or anything like that..

The ntc that I would suspect first off is the primary ntc, inside the combustion chamber. Anyway top right next to the overheat stat sensor, no 5 in the MI's

It is exactly the same sensor, but I've always found this is the one that goes.

hi gas4you, ive never seen inside a combi before tuesday so excuse my stupidity but what is a ntc and an MI`s ?
am i right that you are talking about the other sensor at the top that is to do with the central heating?

If so, i guess i can swap the sensor with the one i just replaced (system will need emptying yet again!!)
As this sensor is in the gas area i think i better get an expert to change this one!!!!

The radiator/heating can run just fine for hours on its own, wouldnt this mean that this one is ok?

When the water temp is set to say 1 or 2 is it normal that the boiler shuts off and reignites all the time? or does the water temp get heated less some other way?

Thankyou to all who have helped so far!!!!!!![/img]

 

[steve1000]

Just wondering if you managed to solve this
cheers

 

[bengasman]

Bearing in mind that the post is 7 months old, I'd guess it is working now

 

[steve1000]

What i meant is what was the problem in the end.