Combi CH - Hot supply pipe, cold radiators

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Ideal 30KW Combi Boiler, 22mm supply, 10mm to 10 rads. Two Story. Rad not serviced since new 4 years ago. "Engineers' refused to remove surrounding cabinet. I did it in 4 minutes. We moved in last April. All rads seem to work ok, when we turned 'em on late Oct they all got warm but not hot and just got worse until now where most are cold,
Copper to vinyl tee feeding 6 branch manifold then to copper going up stairs. Manifold upstairs must be under floor somewhere.
Boiler cycles - pipe almost too hot to touch. Vinyl pipe warm, 4 out of 6 branches cold. The 2 rads are getting around 25° at the top half. Copper going upstairs very hot. Two rads at 25°.
Plumbers here in Whitby won't power flush systems with 10mm lines.
I removed and flushed 3 rads and ran water through the lines. Rad were way dirty, did 'em right side up, upside down and sideways tapping with rubber mallet, lines just had a little brown then clear. Opened the manifolds - no black and a little brown,
Put the 3 back in. Turned all rads off, filled each rad separately until water vented. Pressurized to 2 atm, bled all the rads (none had air) and no improvement! Checked the pump vent partly opened - it is.
No noise or gurgling.

What else can I do? I'll flush out the rest of the rads next week. I would think the 3 that I flushed would be up to temp.
 
Gas Engineer after servicing boiler - "The 10mm lines have to be changed to 15mm, nothing can be done until then."

Gas Engineer/plumber - "The 10mm lines have to be changed to 15mm, nothing can be done until then."

Power Flusher - "The 10mm lines have to be changed to 15mm, can't flush until then."

Problem solved by me turning the CH Flow Valve full on.

I'm just a renter who moved in May '23 from the USA. All I knew was forced air systems.

The valve was mostly off so that kept some of the lines warm. Couldn't have been good for the pump.

On studying the manual I saw that all valves were supposed to be vertical.

On doing so, in 15 minutes all 10 rads were too hot to keep hands on for more than a few seconds.

Simple solution.
 

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