CH with Microbore bypass loops and proper bypass valve

[John.g]

If your pump is now limited to 65% then its only a 3M pump, if it ran at full speed (6.8M) then you should get ~ 5.1LPM, still pretty hopeless, did you just hose through the rads individually or/and did you hose through the system with the manifolds and rads connected up?. Unlikely the problem but you might remove the TRV actuators from 3 or 4 rads and ensure the valve pins are fully out.
Have you a sealed system?., if so, and if you have a boiler pressure gauge note how much it rises (or falls) when you start the boiler/circ pump, if the pump is producing a 3M head then the boiler PG should rise by 0.3bar.

Even with a dT of 30C and assuming you have 18kw of installed rads then the boiler should still output ~ 12.7kw (67%). though 30C dT might be controlling the max boiler output.

 

[Rob1234]

It's a long story, but the system was power flushed before the new boiler was fitted - but now we know about the loops, so the flushing was most likely not as effective as it should have been, the large twin filters had a huge amount of build up on them twice, and the old boiler worked really well after the flushing, over night... New boiler fitted next day and converted to sealed and pressurised system, pump, tank and auto air vent all removed, ABV replaced a little later when we noticed it was stuck open a tiny bit, the three worst rads were taken off and flushed out with a hose - lots more black water from each of them...

The old boiler / pump was warming up the rooms pretty quickly - but now I get about 2 - 3C/hour temp rise like this, before today:

(Ignore the top line for the office - it's generally toasty in there!)

I have been removing the TRVs for testing and keeping the radiator with flow to try and flush it out more, hopefully they will improve move now the loops are not taking the flow away from the rads...

The temp rise before on the old boiler was far far steeper on the graphs...

 

[John.g]

Are you reasonably happy that there's no build up in your new boiler HX?, is the magnetic filter just that? or does it also collect sludge/dirt particles.
The boiler dT when heating the HW cylinder might tell you something if you take a few readings with cylinder temp ~ 50C and also before cut out at its setpoint, normally 60/65C.

 

[Rob1234]

Are you reasonably happy that there's no build up in your new boiler HX?, is the magnetic filter just that? or does it also collect sludge/dirt particles.
The boiler dT when heating the HW cylinder might tell you something if you take a few readings with cylinder temp ~ 50C and also before cut out at its setpoint, normally 60/65C.

Well the water heating works well, it seems - the filter is a MagnaClean Professional2, I'll see what it looks like a bit later after a day without the loops in place..

When they did the power flush, this is what the filters looked like twice:

It looked pretty bad to me - but I have no experience of what to expect...

 

[John.g]

I have no experience of these either, anyway next time the cylinder is heating on its own jot down the boiler flow&return temps.

 

[Rob1234]

I have no experience of these either, anyway next time the cylinder is heating on its own jot down the boiler flow&return temps.

When it was heating up this morning, I had 26C drop consistently as it heated up

 

[John.g]

That's a big dT with a cool/cold cylinder, would have expected less except there is a throttled in balancing gate valve on the coil return, there's none on mine and my dT starts at 8/10c when cold and just before cylinder cut out ~ 5/7C. I have a smart circ pump and its power output translates to a coil flowrate of ~ 12LPM which is confirmed by my 20kw oil fired boiler cycling times.. I'm just wondering if it's possibly a pump problem or some boiler protection (high dT) that's causing the high dT on CH as well. If you boiler % output is noted at the same time then the circ rate is easily calculatd but doesn't fix your problem.

 

[Rob1234]

Just a follow up for completeness...
I spoke to Viessmann tech support again and they suggested an engineer coming out to look at the boiler under warranty..

After some investigation, he was able to change the maximum pump speed from the 65% default limit on the 19kW boiler to 100% as per the 32kW model - and I now have radiators that are too hot to touch and much better warm up time in the house...

The setting was accessed via the Viessmann service assistant program on his laptop - but it may be possible to remotely configure the setting, he thought. It is not possible to change the setting from the boiler control panel.

He said it was unusual to need to do this, but the boiler control of the pump is geared towards it being eco-friendly and using the minimal amount of power required as per EEC regs - but clearly the pipe work here needed a far higher pump setting.

Thanks everyone for your help and advice with this