Radiators get hot then run cold, Hot water fine

[chubbiechecker]

Hi all hopefully someone can help me

I have an Ideal Classic Slimline FF350 boiler that is approx 10 years old and seem to have problems with the central heating but not the hotwater.

I have tested the power supply to the pump, threeway valve, thermostat, controller and boiler and all is working as it should.

I have also checked that the pumps impeller is running and it is. I can hear the three way valve adjusting for when you have just hot water on/off, just heating on/off or both on. I have checked the radiators for air but all were fine. The expansion tank is not running low so the ballcock valve is ok.

I have performed individual testing of the central heating and hotwater by doing the following.

before doing either I set the boiler stat to max number 6 was on 4

To test hot water I turned off the central heating and made sure room stat was set to lowest temp. Turned up the cylinder stat to max and fired up the water on the controller. The three way valve wirs and the boiler lights up and the pump starts running. The pipes either side of the pump get very hot and water heats up nicely.

waited for the boiler to cool right down

To test Central heating I turned off the water on the controller and set the cylinder stat to lowest temp. Turned up the room stat to max (30) and fired up the central heating. The three way valve adjusts fine the pump runs and the boiler ignites. The pipes either side of the pump get hot but not very hot. The radiators start to heat up all round the house fine.

but strange results from this test

The boiler seens to run fine but after about 8 - 10 mins the boiler cuts out and the pump then just pumps what hot water there is in the boiler around the system which then results in the pump just pumping cold water round the raditors. The boiler does not seem to to want to reignite to heat the water anymore.

I reset all thermostats and then tested the hot water and central heating. The hot water heats fine and the raditors initially get very hot but then run luke warm at best. pump runs fine and three way valve adjusts fine.

does anyone have an idea what could be at fault i feel that it lies within the boiler itself. maybe boiler thermostat/ PCB fault?

 

[twgas]

From cold does the pipe to the cylinder, from the 3 port valve, get hot if you run the CH only?
When doing your tests did you leave the system alone until the boiler refired or did you not leave it long enough?
Were the pipes at the boiler hot, warm or cold when you finished running your tests?

 

[chubbiechecker]

Hi Mike

Thanks for your reply

I can confirm that the pipe from the three way valve to the cylinder stays cold when the CH is on.

I waited and waited for the system to fire back up and it appeared to only do this when stone cold again as though it was going through the first phase again.

The pipe from the boiler initialy gets slightly hot but then goes cold and I can confirm that the boiler is also off when that happens.

All the above is just when the CH is on. When the hot water is just on the pipes get roasting hot from the boiler and the boiler appears to function normal.

This issue certainly has me scratching my head.