Correct heat output of radiators for the size of boiler.

[jimbobc2012]

Hi this is my first post, hopefully the question isn't too obvious?

I moved into a house in December and after about 3 weeks the boiler developed a fault.

It is as Worcester 28Si II, once it has been on for a couple of hours and the radiators are hot, the boiler shuts off and the fault light flashes (approx once per second), the manual says it is either boiler overheat or ignition lockout. After about an hour the boiler can be reset and runs again, until it stops with the same problem.

I have measured the size of all radiators and looking at various catalogues, I have worked out we have total radiators of approx 6.9KW, plus whatever heat losses the pipework will have.

Having looked at the manual for the boiler, it says the minimum heat output for the boiler is 8.1KW. Is it be possible that even at it's lowest output, the boiler is a bit too big for the job and is overheating?

It does provide hot water without any problems, so I would guess the gas valve and fan etc are working ok.

Hopefully someone could help answer this.

Many thanks

James.

 

[D_Hailsham]

Having looked at the manual for the boiler, it says the minimum heat output for the boiler is 8.1KW. Is it be possible that even at it's lowest output, the boiler is a bit too big for the job and is overheating?

If the required heat is less than the minimum the boiler can supply, the temperature of the water returning to the boiler will be higher than expected, so the flow temperature will rise quickly. The boiler should then turn off for a few minutes, until the water temperature has dropped, and then start up again. This is called "cycling".

All systems do this as it it is impossible to exactly size a boiler to the heating requirement. This is particularly true in milder weather, when you will require less heat.

The problem will not be due to an oversize boiler.

When was the boiler last serviced?

The listed possibilities for the fault light flashing once a second are:

No gas or low gas pressure.
Weak spark or spark not across elecrodes.
Gas valve leaking or failed.
Low water pressure or boiler dry.
Pump fail.
Primary sensor position.
Flame sensor fail or connector.
Overheat stat tripped or disconnected or faulty.
Blocked burner.
Blocked heat exchanger.
Faulty harness.
Board fault.

Time to call in a Gas Safe registered engineer!

 

[dcawkwell]

Most likely cause the boiler has overheated caused by a faulty
thermistor.

Size of radiators doesn't matter as boiler switches off once
they are at temperature.

 

[Agile]

Most likely cause the boiler has overheated caused by a faulty
thermistor.

That is certainly one possibility!

But not a common one my experience on that model.

Regardless, it needs a competent engineer to diagnose at the boiler.

Tony

 

[jimbobc2012]

Many thanks to everyone who posted an answer, I should have mentioned that it's a rented property, an engineer has been once however I wasn't here so I don't know what he did.

It seems the "book of excuses" has been opened though: Apparently the boiler has had problems in the past. The circuit board was replaced a few years ago by Worcester. Then a man from British Gas came and said "the pilot light could blow out on rainy days" (which seems odd as it doesn't have one), etc etc, there is a definite whiff of BS in the air!!

Basically it's the same old story, nobody wants to part with any money. However thanks again to everyone for the advice and info.