Hoping that someone can shed a little light on my problem or that this may help someone in a similar position. To give the background;

1 - During this recent cold snap, my boiler, a Worcester 19/24 CBi failed to light on the timer. The water pump and the fan ran up. The boiler ignited but failed to catch. (Indicated that the electrode is ok.) It then repeated the process several times with a continued failure to ignite until it goes to the safety lock out mode. (Happy that the safety features work!) There were the Blue flame and ignition spark visible through the sight glass but the flame dies instead of continuing to burn across the whole burner. (Indicated that the gas supply was ok?)

I thought it might be a minimum temperature cut out on the board so a hairdryer was used to blow hot air over the base of the boiler and PCB to warm it slightly, when the boiler is reset after ~3 mins it lit first time. Inspection of the board showed some lifted tracks and discolouration around some components. (Possible damaged components adding weight to my thermal cut out idea?)

2 - When the boiler was lit, the only temperature control seems to come from the overheat thermostat shutting the boiler off and not the wall thermostat in the hallway. The Wall thermostat does shut the boiler off but the room is not getting hot enough for this to act. The Drayton RT1 has also been replaced by an RT8. (This was the first thing done before the boiler was played with and clicks out at about 12oC.)

The pipe work and central heating pump seem VERY hot to touch when everything is up and running.

3. I have fitted a new thermister, Overheat thermostat and Board. (Excellent service from keeptheheaton.com I would recommend them!) The central heating system now fires up on timer correctly first time but still shows the same temperature control issue. So some progress. My questions are;

A. Within what temperature range should my boiler be heating i.e. how hot should my pipework be as I can't find any reference to this question on the net?

B. What is the probable cause and how can I fix this problem?

C. There is a lot of gurgling and knocking when the system fires up so is it linked to trapped air in the system? Radiators have been bled though and don't seem too bad.

Any advice would be very much appreciated and I'm hoping that there are some helpful people out there or this helps you out a bit.

Thanks in advance

Rob Cooke, January 2010

An Update and Thanks to those of you who have responded or viewed my issue.

Over the summer I had a good look at the system and found the following;

Replacement board and components working ok and boiler fires up first time on timer. (Good)
Still running hot (Thermometer on radiator is measuring ~100oC) (Not good)
No Leaks (Good)
When running, the system was pumping water over and back into the header tank (Not Good)
3 inches of red oxide sludge in header tank (Not good)

Great advice for the treatment of the system from Charles so many thanks on that one. I tied up the ball valve as suggested and drained enough water out of the system to allow the sludge to be removed. This was a really messy job. I then refilled the header tank and added some Sentinel X400. Several sessions of circulating at max pressure pump the cleaner through and I have more sludge settled in the header tank and (hopefully) not in the system.

The system seems to be running cooler and not overheating. So it looks like the problem was sludge causing overheating.

The plan is to drain the whole system, clean out the sludge again then refill and top up with system sentinel inhibitor. I'll let you know...

Rob Cooke, September 2010

First of all you should find out if you have a closed system or an open system. If you have an open system you will have an expansion tank (in the roof probably) if you have a closed system you will have an expansion vessel located somewhere with a pressure gauge on it.

You should look for any leaks. If you cannot find any I would turn off the mains stopcock to your expansion tank, if you do not have one then tie up your ball valve so that fresh water cannot top up your tank if the water level drops over a period of a day or so you will know you have a hidden leak somewhere. You need to check the water levels when the system is cold so that readings are not changed by expansion of hot water.

Next you should check your pump speed is it running too fast. This may cause water to flow into your expansion tank causing the water to oxygenate.

The reason for all the above checks is because it sounds as if the heat exchanger has a build up of limescale which stops the heat being efficiently dispersed into the water circulating in the system causing it to overheat and cut out.

If you run your heating system at the lowest setting on the boiler you will probably not hear the noises that you mention this may help to confirm the above diagnosis.

To fix the problem you could use 1 can of Everad descaler and sludge dispersant in your system for 2 weeks or 2 cans for 24 hours and then flush the system and refill, also add a corrosion inhibitor.

You may find that in using the descaler that whilst you fix the boiler any weaknesses in the system may be exposed and you have to fix a few leaks. You should also check all your radiator valves to check the glands are not leaking.

I hope that this may help

Charles, March 2010

Hi
Can you please provide the link to the PDF document that you recommend.

many thanks

playman, February 2010

try the bottom of page 238 on (home heating systems) for worcester boiler faults for engineers a good nights read download it in pdf format.



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wirecutterman, January 2010