Boiler Temperature Rising and Falling Rapidly

[coldguyfromwales]

I'm having problems with the CH for my Worcester Bosch Greenstar 42CDi combi boiler. The boiler is basically repeating the same routine:

1) The flame lights, and the temperature display rises rapidly (around 1 degree per second) until it hits around 60 degrees or more.
2) The flame then goes out, and the display drops to below 50 before the flame lights again and repeats the cycle.

The hot water is still fine, but I'm just having this problem with the CH. Yesterday it was showing an E9 error, which means:

The display shows alternatively the CH flow temperature and this code. The gradient limitation is active. The primary temperature is rising too fast and the burner has switched off for 2 minutes.

According to the manual, these are the suggested check/solutions:

- Sealed system (pressure between 1 and 2 bar
- Open vented system (enough water in the feed and expansion tank)
- Pump seized or sticking
- Harness disconnected or broken between flue safety stat or overheat stat
- Harness connections from both safety stats to the PCB
- PCB board defective

Any ideas of what to look at? Is this a job for a completely inexperienced guy like me, or a professional?

 

[muggles]

Any one of the above, or more likely the boiler is waaaay to powerful for the system it's connected to

 

[coldguyfromwales]

The boilers has been working well since I moved in, and it's been in place for the last 2 years. So I'd guess that it's not a case of it being too powerful.

Out of all of those things, is there anything that someone with zero experience with boilers could easily look in to before getting a professional in to look at it?

Any tips would be hugely welcomed.

 

[Dan Robinson]

How big is the house? 40kW is a maahhooosive output.

 

[David937]

The CH output is rated at 30kw for the 42CDI combi.

 

[Dan Robinson]

Still mahoosive.

 

[coldguyfromwales]

How big is the house? 40kW is a maahhooosive output.

Slightly bigger than average I'd imagine. Detached, 3 bedrooms. Not a castle, but probably about as big as you'd imagine an average detached 3 bedroom house to be.

 

[Dan Robinson]

So about half the size the boiler was designed to cope with.

Not saying that it isn't one of the problems listed in the manual.