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View the thread, titled "Outer flow limited, unknown problem to solve" which is posted in Central Heating Forum on UK Plumbers Forums.

Hello,

recently our boiler stopped working properly, causing a lack of heat in our 14-radiator splitter in 3 storeys. having only one boiler at time it kind of working, only for one at time (and partially hot), depending on the priority in the system.

We contacted Valliant for assistance, and they replaced the PCB and pump, the pump was burned out. However, the issue persisted, with the engineers pointing to a problem within the heating system, even though it had been functioning correctly before the boiler failure.

A second technician reiterated the same diagnosis after flushing the system for 8 hours, there was still no improvement. They now intend to break through the floor to inspect the pipes' route. In the meantime, they applied some chemicals and left it to work for three weeks.

The boiler is a Valliant ecotecPlus 837 r1, the system is running 30 sec just to heat the water then it goes for 3, 4 minutes to S7 (pump heat overrun), where it drops the heat then starts again the ignites for a few sec etc...

Today, I removed two radiators on the ground floor and cleaned them. During this process, some dark liquid came out (nothing solid or viscous, only dark fluid). We tested the tubes, and the flow was strong and hot, but the return remained cold. we run the system opened with the radiator out and the in-water open to the boiler. It's likely that there's a blockage in the return, but my question is, after the flushing is still there? what could be causing this issue? and where and how to identify it.

I also have the magnaClener 2 mounted with the inverted flow, oppositive to the arrow, I believe it should not really matter, it is still on the outer before going back to the boiler.

it could be that the pump or the boiler still problematic? which test I could run? what do you suggest?

I really hope someone can help us

thanks in advance,
 

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they didn't,

I notice that the return is cold but then is going inside the secondary heat exchangers which is always hot even if we are not using hot water, then in the pump which it's get hot and out of the pump before the primary heat exchangers at almost the same temperature of the flow after the primary exchanger as immage,
 

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Please put the cover back on as you need to be qualified to work inside a boiler / gas appliance
 
done, but let's assume you didn't see that picture.
is it normal the primary exchange always hot and the boiler most 9f the time in S7?
 

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