Worcester 28i junior combi. Goes cold after a while.

[arnoldboy]

Hi

I have a Worcester 28i junkior combi boiler. The problem seems to happen when running a bath (so my wife says) and I have noticede this when washing up: The water gets hot, as normal and all is good, but then all of a sudden (after a few mins) runs cold again, just like normal cold ambient temperature water. Then when I turn noff the tap and on again it gets hot again.

I believe this may (due to reading several posts/books etc) be scale build up in the boiler's heat exchanger, but the water temperature is not set above 65 degrees when scale tends to form, also the boiler does not seem noisy which scale usually does?

Are there any other reasons why this is happening and is it possible for me to be able to fix it myself or de-scale perhaps if it is the cause? I would of course like to prove this is the fault before trying to rectify it!

I was thinking maybe the diaphragm in the diverter valve or the flow switch may be faulty/wearing out but then the basin tap, pillar type the same as the bath, seem fine!

Thanks for reading my post and I welcome any help.

 

[gaspro1981]

is this the greenstar he or se model
how old is the boiler roughly

 

[fitz1]

min gas pressure is not set to correct setting on gas valve possibly . get an engineer to test it.

 

[arnoldboy]

Boiler is not greenstar, not a condensing type either. I'd say about 8years old? I have lived here for 3 months and have noticed it. The boiler has gas cooker have landlord certificated dated recently.

 

[steved101]

Gas pressure being set low would make water temp generally not very hot surely?, but OP said it was hot as normal and then just goes cold.
Could be scale? Is it a hard water area? IS there a reducer fitted before the boiler? The diaphragm may be failing possibly inside the diverter valve or the flow sensor faulty....?

 

[gaspro1981]

get your landlord to sort it out its his appliance

 

[fitz1]

as temp increases boiler cant modulate down enough , so cuts out.

 

[fitz1]

you will probably find that if you leave the tap running it will get hot again once the temp has cleared the boiler

 

[arnoldboy]

It is my house, I bought it and the landlord cert's were left here along with the manual. I believe the house was owned and rented out before that's why the service/landlord certificates, as of the law I believe.

I am stumped!

Fitz yes that does happen, exactly that does, after a min or more it gets hot again. Is this scale and the boiler stat reaching temp and the cutting out (but wouldn't that shut down the boiler) or just allow cold through? or is it the gas pressure as you said??

 

[gaspro1981]

this boiler does not have a diaphragm or dv it doesn't sound like a problem with the flow sensor but could be a faulty dhw thermistor/sensor

 

[fitz1]

min gas pressure setting on the gas valve. you need a rgi to test and set.

 

[fitz1]

i bet your cold main is weak pressure.

 

[arnoldboy]

the cold main is very high, i'd say 4-5 bar, very very high! why would this make a difference though??

if the boiler has no diaphragm, how does it work then! is it a wax capsule type?

if a faulty thermistor, will i need to drain down the boiler then? or is it 1 that just clips over the pipe in this boiler??

 

[steved101]

I suspect no lights on the boiler (fault lights) show up when this happens?

 

[fitz1]

in that case it could be scaled heatx or thermistor.