Central heating problem

[Grasshopper1979]

I have an alpha cb24 combi boiler.

My problem is the boiler is knocking off on high temp after running for a few minutes. The upstairs rads are getting hot but the downstairs are only luke warm.

I've recently changed the primary temp sensor, drained system and fitted 4 trvs. When I started it up I was struggling for flow so removed flow and return isolation valves to find flow pretty blocked up. I've cleaned these out and it's a little better (the rads are heating up a bit now) but the flow still doesn't seem fast enough to take the hot water out of the boiler fast enough.

Here's how it's set at the moment:

1. Pump set to medium speed, I've tried max and min, no difference.

2. Upstairs lock shield valves opened 1/4 of a turn, tried 1/2 turn, no better.

3. Downstairs lock shield valves fully open, tried 50% open, no better.

4. Trvs all set to 5 (max).

5. All air bled out of all rads and pump.

Please can anyone give me any suggestions of things to try next, I'm at a bit of a brick wall.

Many thanks in advance.

Mike.

Edit: for info, don't know if it's relevant, when I drained radiators from downstairs, the upstairs empties completely but the downstairs do not, when I crack the bleed valves on any of the rads downstairs, water comes out and the flow to drain from my drain point doesn't increase. Am I missing something?

 

[Slugbabydotcom]

Your pump should always be on its highest setting.
It's possible that you still have airlocks. Close off all rads apart from one and work your way around the system opening one and closing one behind it until they all get hot in turn.
If you still have problems after that you could be looking at needing a powerflush or perhaps a pump.

 

[Dan Robinson]

Your pump should always be on its highest setting

Twaddle.

 

[Grasshopper1979]

Ok thanks.

Do you think it's worth adding some flush stuff for a few days? Do you think this will help the problem?