Flushed three rads, added Fernox F3, still cold...

[SpecialK]

The pump may be new but if you have a large amount of crud in your system it could have clogged the impeller, which will reduce the flow to the boiler. You can also make sure the pump isolator valve are fully open.

You need to manually open your diverter valves (near pump) when draining and filling your system.

Does your boiler have bottom entry pipes? Could the AAV be faulty in the boiler?

 

[hsabbers]

You need to manually open your diverter valves (near pump) when draining and filling your system.

Does your boiler have bottom entry pipes? Could the AAV be faulty in the boiler?

Ah! Is that why draining was so slow? I'd forgotten all about the diverter valve manual switch. I need to drain it again in the next day or so to flush the Fernox F3, so will use this.

No idea at all about the AAV - again, beyond my ability/knowledge.