Worcester 40CDi cutting in and out.

[Si 69]

Hi, Sorry for long post and I have searched to try to find the answer before posting.
We have a large house with a Worcester 40CDi classic which is cutting out then starting again in a cycle. It is currently running from 35° to 55° when it cuts out, cools down to 35° and starts again.
I can't see anything wrong with the pump which turns fine when the cover is off and is not noisy.
In addition, I've just had British Gas Homecare out for a repair and service. Of course the boiler worked fine for him (it had been off for several hours). After 10 minutes or so it cut out but that was at temps of 95° or more. BG man found nothing wrong with the boiler and suggested a power flush. (He only charged for the service though which is something!)
I told him I had watched it cutting in and out yesterday for CH and hot water and now he has left I am watching it cycle round and round.
Any ideas why it is doing this please?
Thanks

 

[ianmcd]

In addition, I've just had British Gas Homecare out for a repair and service. Of course the boiler worked fine for him (it had been off for several hours). After 10 minutes or so it cut out but that was at temps of 95° or more. BG man found nothing wrong with the boiler and suggested a power flush. (He only charged for the service though which is something!)

Never Never even dream about getting BG out to your boiler, if you really want to waste your money send it to me, check the condensate trap and run, sounds if it is restricted, if you want a proper result get Worcester out , either to service it, or a fixed price repair, BG are the worst route you can go down, they are an absolute rip off, with very few so called engineers that actually know anything

 

[Si 69]

Hi Ian,
Thanks very much. The trap was absolutely filthy and it's clean now.
Sadly the boiler is doing the same thing but it's going up to 66° before cutting out.
Any other good ideas?
Thanks again

 

[gazfen]

Plate or diverter

 

[Si 69]

Thanks Gaz,
Could you expand on that slightly for me please?

 

[gazfen]

If it’s a combi boiler and it’s overheating it’s usually a blocked plate, or the diverter is not moving. If it’s a heat only boiler then the main hex is blocked. Fan pressure test will tell that. Or main heat exchanger is blocked.

 

[Si 69]

Thanks
It's a heat only boiler. I'm not really sure that it's overheating as such as it only as it only gets to 50 or 60 degrees before it cuts out.
There's no kettling noises either.

 

[gazfen]

Diverter heads open fully. Thermistor pcb. Needs a decent engineer

 

[Si 69]

Ok, thanks.

 

[gazfen]

possible weak pump too

 

[Madrab]

Can I suggest you get a local independent experienced in diagnostic work in to service your boiler and check you system. There are a couple of things it could be but it needs proper checks performed to rule them out.

If the boiler is truly only getting to 65deg before cutting out then it's not overheating and there could a faulty sensor but if it was getting up to 95deg and doing it then something else is going on as far as circulation is concerned.

You may not know but when the condensate was cleaned was the drain connection to the trap from the main HEX checked by the BG chap, they can get blocked up too.

 

[Si 69]

Hi Madrab,
Thanks for your advice.
Yes, the connection above the trap is totally clear.
Assuming that the sensor readings are correct, I wonder if it's cutting off because it's warming up too quickly. I've had the Temperature blocking mode symbols top left and bottom right a couple of times that indicate this but not most times.
When starting from cold it operates l for about 10-15 minutes but then overheats and cuts out it the cools and restarts. After that it gets into a cycle of heating to 65 then cutting out and cooling and restarting. Before the trap was cleaned it was in a cycle of cutting out at 55.
My thought when this first happened is that the pump needs replacing but it has a Grundfos 25-55 180 which turns freely and isn't making any nasty noises.
What are the couple of things you think it could be and is the pump one of them?
Thanks again

 

[gazfen]

Start with the pump

 

[DP]

My thought when this first happened is that the pump needs replacing but it has a Grundfos 25-55 180 which turns freely and isn't making any nasty noises

Faulty pump would result in high temperature numbers

Second what Madrab says
Voice of experience

Have come across similar but that is RGI territory

 

[gazfen]

Fan compression test will tell him the hex is clean. Was gas rate ok on max rate.