Worcester Combi boiler problem

shardman68

I have a Worcester 24i combi boiler (natural gas). When my central heating starts up with the timer control the gas lights but has a very weak flame that goes out after approx 5seconds. If i then turn the timer off and then turn on the hot water tap the boiler 'lights' up no problem with a strong flame and stays on until tap is turned off. If I then turn the central heating to 'on' whilst the tap is running the heating continues to run no problem and the radiators heat up no problem even after turning the hot tap off. I have replaced the timer and the air pressure switch as it was thought these were the cause and the water pressure is good. This problem was intermittent but has been happening everyday for past few weeks. Please can you help???

macman

On a combi, when you have a demand for hot water, the CH is not supposed to work. The diverter valve diverts all the flow to the DHW circuit, or should do. If you rran the hot tap for long enough, the CH circuit should go cold.
So I suspect the problem may be with the diverter valve.
But if you are changing the air pressure switch yourself on a gas appliance, you are presumably an RGI, so wouldn't you know where to look?

shardman68

Thank you for the quick reply

I did not change the air pressure switch or timer myself - gas safe engineer did that. I am just looking for a possible solution to my boiler problem without having to buy enough new parts to rebuild it completely

I did not explain myself very well. I turn the hot tap on which 'lights' the boiler and as it is lit i turn the CH to 'on'. i then turn the hot tap off and the boiler remains 'lit'. I then keep the CH on and so heats up the radiators. During the day the CH seems to switch on and off with the timer no problem - it is mainly first thing on a morning when it doesnt 'light' itself with the timer.

macman

So a possible faulty timer then. Is this a built in one, or a separate programmer? Make and model?
If it's an old-fashioned mechanical timer then it may be failing to click on at the first setting.
To rule this out, leave it on constant and controlled by the room 'stat. If it still fails to fire first thing, then it's not the timer.

shardman68

Thank you for the reply

I have replaced the timer last week with a new one (mechanical built into boiler) and still having same problem.

When the timer 'starts' there is a click and the pump etc can be heard running. The gas lights with a very weak flame and goes out in approx 5 seconds. This is generally first thing in a morning - during the day the timer turns CH on and off no problem.

I have no room stat controls. I just control boiler and room temp by having boiler on a very low setting.

macman

Then you need to get a RGI in to investigate.
That's a hopelessly inefficient way to control the system. A room 'stat costs from £15, a decent digital programmer (a Honeywell maybe) about £80. You could get that back in a season.

gas4you

Sounds like the minimum burner pressure has drifted out of range and is now too low.

The boiler will start up on minimum setting for ch then waits for a few seconds to check the flame is there, before coming on properly.

When you turn a hot tap on, the boiler fires up on maximum setting, so will stay alight.

Time for a local recommended RGI to have a look/service it.

shardman68

Thank you for the advise - min gas pressure sounds good and macman i really should get a room stat - thank you both:)