Just to update.
Theory proved correct
I replaced the CP50 pump for a Grunfos UPS3 15-50/65 as it looked like the best performing pump that would fit onto the same fittings.
The difference was chalk and cheese.
The house is a big old house and now the temperature, as displayed on the boiler...
Thanks for replying. That thread talks about a Glowworm Ultracom 38hxi boiler and a pump starting much bigger. Did you post the right thread?
I do accept that the new pump I've bought may still not be large enough. But it will bolt straight onto the existing fittings. The other pumps will need...
The whole system was power flushed on installation continuously for a couple of days whilst they re-sited the boiler. The performance was the same for the Valiant from day one and it didn't seem to offer improvement from the old oil boiler. So I doubt it's a blockage in the boiler.
When we moved into this property we had an oil fired boiler with a Grunfos pump. It was incredibly inefficient and was getting through £300 of oil a month. We changed it out for the Valiant 428 and our heating bills certainly took a tumble.
But I was still not impressed with the time it took to...