Ideal classic ff50 problem

I have a 20 year old ideal classic ff50 system boiler , serviced every year etc never had any problem as it’s a cast iron heater exchange and they are pretty bulletproof. Now it will heat the water no problem but when we switch to radiators the boiler will run for about 10 mins and then it cuts out . We thought we might need to replace but have been told if it heats the water there is no problem with the boiler .
It will heat the rads for about 10 mins and then stop , we think it could be a thermostat or circulating pump but it does heat all rads for little while . Also the towel rail in bathroom used to be piping hot and now just warm ... so we have hot water but radiators will not heat up fully . Could it be a thermostat or pump ,boiler serviced regular every year

 

very unlikely to be the boiler, it is a system problem

 

As above, you have a circulation issue. Boiler is heating the water adequately, but if it cant get around the system, then the system wont heat up.

 

Check the boiler thermostat is turned high enough. Even on low it will probably get your water hot enough, but not fully heat the radiators.

 

I suggest a circulation problem too.
Feel the two pipes coming from the top of the boiler - one may be considerably hotter than the other, indicating low flow.
Incidentally, you didn't say if the boiler came back on after a further delay, or whether you had to intervene to make it go again.

Check and rectify the pump problem (cleaning or replacement)