Why does my 4 year old Optima boiler not heat the house to the required thermostat temp of 25 deg C any more, even though the boiler is constantly running.
In the past the bioler easily coped with any demand, and the house was heated amply with the radiators set to 2-3, now they are set to 5-6 and the inside house temp does not exceed 15-16 deg C?

The usual case of failure to heat is a water temperature problem. Many times a sensor/control circuit board or other control device is not allowing the boiler to run at the desired water temperature necessary to heat your home properly. This in turn drops the boiler output water temperature and causes the home not to heat as it did once before. There are many other items that can also cause this problem but I would check the water discharge temperature first then check the return water temp and see what the differential is.

If you know what you are doing you can make an adjustment but more than likely it would be best to contact the company that installed your unit for service especially if it has not been serviced this year.

Thanks for your prompt answer, think it would be best to call in an expert now as this seems a bit beyond my radator bleeding skills!! Thanks again.

No problem hope it all works out.