Central heating ‘Sherlock’ needed to solve a mystery combi-boiler problem

[mtriff]

I have a (very) old, but good, Valliant combi-boiler (VCW 25/1 T3W), which has just started acting up.

In the morning (or evening), I turn the thermostat, it calls for heat, and the boiler fires up fine – no problem at all there. When the programmed temperature in the thermostat is reached, the boiler turns off fine. The problem occurs when the room temperature falls below the programmed temperature and the thermostat calls for heat again. The boiler pump starts running as expected, but the boiler does not fire up. I’ve waited for a minute or more, and no flames.

However, if I open up a hot water tap, the boiler fires up, and I can then turn off the hot water tap, and the boiler will remain fired up until the programmed room temperature is reached. The thermostat stops calling for heat, and the boiler shuts off.

My only other option is to turn the thermostat off and wait about 5-10 minutes. If I do this, when I turn the thermostat back on, the boiler fires up fine.

The final interesting fact is that this problem occurs almost exclusively in the morning. In the evening, the problem rarely occurs.

Other facts:

• Hot water works fine – no issue there.
• The pilot light works fine.
• It matters not whether the central heat is operated with the thermostat in manual or auto mode; the same problem occurs.
• I thought the problem might have been a temperamental thermostat, so I replaced the thermostat, but the problem persists.

Any ideas what is wrong?

 

[SimonJohns]

I had an issue like this before with an old vcw. Turned out in the end cheaper to replace boiler.

So do homework. Before spending £200-£400 on repairs.

You'll save on gas consumption too. Not the answer you wanted but prob the best one IMO

 

[Mbear]

Cut your losses mate, it must be 20 maybe more than 20 yrs old?

 

[Puddle]

Sounds like back main pcb faulty,however could check micro switched making correct contact in silver box on top of servo unit

Was a great boiler but as said,put your money towards a new boiler,this has more than paid for it's self,parts are hard to get now and expensive,you will find big savings on your gas bill also

imho

 

[mtriff]

Puddle,

Is a servo unit the same thing as a 'servo valve'?

And is a 'servo valve' the same thing as a 'diverter valve', or a 'differential valve'? The boiler manual refers to a differential valve, but not a servo valve. I'm hoping they are one in the same!

I show two screenshots below from the boiler manual. Which microswitch are you reffering to?

Screenshot 8.17 or 8.14.1.1?

Many thanks!

 

[jtsplumbing]

These boilers came out in the 70s you could get a service kit for these . it could be the wax stat at bottom of valve , part no 14-0106. but may not get them anymore, Boiler this age needs replaceing, Open Flue with P/Pilot that alone must be costing £100.00 per year, used to have loads of spaires for these boilers but not worth repairing anymore.