Help, my boiler keeps firing in and out for hot water

[Simon75]

My boiler keeps firing in and out constantly when we try and use the hot water. It will light for approx 4 or 5 seconds at best and then cut out, then relight, then cut out and so on......

The boiler also bangs whilst doing this, like there is air in the pipes or something. It is a combi.

Would appreciate any help, central heating is fine and no banging, air type noises.


Simon

 

[Blooflame]

Need very much to know the model and make. The first thing i would be checking is the Gas pressures (always do the basics first). This is something a qualified guy would have to do mate. With the scant details it would be foolish to offer any concrete advice...I wait with baited breath mate

 

[Simon75]

Hi Blooflame, i did have it serviced recently (2months ago) and funnily enough i did take some notice when the fella put his test gauge on the burner? anyway, he said there was 21 mbar and if i remember rightly 8mbar working pressure.

The model is a Vokera compact he25 combination boiler.

hope this helps, my misses is tearing her hair out!!

 

[Blooflame]

Firstly with the boiler off bleed the rads upstairs, then repressurise the system to between 1 & 1.5 bar
Then go outside and check that the condensate is not fdrozen, has this only started with cold weather? Anyway if frozen, pour hot boiling water over the pipe (white). If it isn't insulated then get some.
The standing pressure means little (around 24ish). The gas valve inlet should be at 23 - 19mb when the boiler is on. The outlet should be between 3..5 - 9.4 mb. This means the lower bracket of 3.5 must be adjusted to, as the boiler has to modulate down when hot. This is more pronounced with DHW than CH. If it isn't then the sensors are saying 'modulate down' but the gas valve can't move down beyond the 8 mb you mentioned as it hasn't been set correctly. (by the way, sensors can't really talk!!)
After that the sensors have to be considered (don't have resistance readings I'm afraid). If it only happens with water then the sensors would have to be the answer.
Where in the country r u?

 

[Simon75]

I'm in Lincolnshire near Skegness, probably a bit too far for you.

I rang the chap who serviced the boiler that is now living in London (he proffessed from the start not to be any good at boiler repairs) and said that he had set the minimum and maximum pressures fro CH and DHW.

Have had a flick through the instruction book, would that be the DHW sensor that might be Kaput??

Should have mentioned that i had a radiator added in my downstairs toilet and kitchen by a plumber, so the system was drained down but an inhibitor was added - sentinal yellow label - could this have caused the problem? Me being me didn't think it would have an effect?

 

[mrgas]

Please dont play with your gas pressures.
It wont help.
I suspect your plate heat exchanger is blocked with crud from your heating system.
Read the faqs about blocked plate heat exchangers then if you feel you can do it, have a go. if not call a pro!
As i remember the compact isnt the hardest to do, however ive been at it nearly 30 years.

 

[Blooflame]

All that has been said is valid, and should be checked first, then, as mrgas says check the temps in / out of plate to plate. Things must be checked before replacing parts

 

[mrgas]

Blooflame
I know you are only trying to help, but....
Please dont give out advise on working with gas in this forum

 

[Simon75]

Thanks for advice guys.

mrgas..... I don't think that Blooflame mentioned at any point for me to check the gas pressures myself and i feel most responsible people wouldn't just give it a go.

I'll have a read on the heat exchangers - presume it's the secondary that i have to look at. Whats the best way to check temperatures of the in and outs on the heat exchanger - any way a multimeter can do it (only ask because my dads an electrician)

 

[mrgas]

Yes its the secondry heat exchanger or the plate heat exchanger or the domestic heat exchanger -all the same thing with different names. Looks like a can of sardines at the bottom of boiler!
The heating side of the plate will be blocked not allowing enough flow through it too loose enough heat so the boiler quickly reaches 82 Deg C and shuts down. After a few seconds it cools sufficienty to start the whole process again.
I wouldnt bother measuring temperatures of pipes other than carefully feeling the flow pipe which will be really hot and the return pipe which will be hot but not really hot.
The plate heat exchanger doesnt need changing it just needs a good clean out.
You will find info on this in the FAQs

 

[Blooflame]

Blooflame
I know you are only trying to help, but....
Please dont give out advise on working with gas in this forum

At no time did I advise anyone to do anything. I advised on what may be required, so your 'Gas watch' tone is not appreciated thank you..
Your tone is patronising and pretty much uncalled for. Think I'll drop off this thread and let you have it all.

 

[Simon75]

I got your back Bloo, i appreciate any advice but i understand that you were ensuring everything was checked beforehand as you never know!

And you did mention in your first reply that the work (gas pressures) would need to be done by someone qualified

Thanks again fellas!