The issue became apparent during the Winter, following the installation of our Nest back in 2019.
The heating engineer also flushed the system and installed a new pump at the same time.
On odd occasions during the Winter months each year since then we have a situation where all bar the radiators at the top level of our house refuse to heat up. This can coincide with cold weather but not exclusively. This January we decided to call in expert help and contacted the company that services our oil fired boiler. For the next three months they tried various means to get to the bottom of the problem. This has inevitably ramped up costs.
Also relevent is the fact that the same fault occurs when refilling the system, having been drained for pipework modification, etc.
It was decided to disconnect the Nest and install a simple 24 hour timer to try and see if it is the control or system is as fault. Since then the heating has run normally. So our plan is to have the Nest tested independently to see if there is a fault. But apparently that's not possible.
