CH with Microbore bypass loops and proper bypass valve

[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