CH with Microbore bypass loops and proper bypass valve

[Rob1234]

If the boiler is at 26% output then only running at 4.94kw and a flowrate of 3.37LPM!! If that is correct then something seriously amiss. Is the DHW cylinder heat up time OK.

Yes - Cyl seems to heat up fine - all 22mm between boiler and cyl, predictably...

It's got sludge removing stuff in the system at the moment, which is still building up on the filter magnet to maybe 2 - 3 mm thick bumps in a day or 2, - I'm hoping this starts to clear more out now the flow is going round the rads instead of the bypass loop stupidity...

Before the boiler change, I had a grundfos pump set at maximum rate and that did heat up the rads OK (old glowworm 18HXi, looked ready to die at any moment)

Th new Vitodens has a more powerful pump internally - but the boiler never seems to set it above 65% speed, making it less powerful than my old one - and there's no minimum speed setting option, only a max speed setting for the pump, which I had already set to 100% before I found the bypass loops... The boiler pump is a fancy PWM controlled version, so no switch or buttons to play with, unfortunately.

I was wondering about getting the old pump put back in place...

 

[Rob1234]

10mm gets blocked very easily sounds like you need to start looking at maybe a blockage at the inserts

Hi Shaun, yes, the plumbers who fitted the boiler really don't like the push-fit and I can see why, having seen the little holes in the inserts... Unfortunately cheapskate Persimmons pay installers by the number of houses they complete, so quick and dirty is very much what you get. I was really hopeful that blanking off the loops was going to fix it all, but still some work required it seems... I'll keep the system pumping through the worst rad in the daytime for another couple of days and see how much more gritty gunk comes out I think..

Thanks for your help and suggestions - Screwfix today was a real bonus

 

[ShaunCorbs]

The boiler controls the pump to manage a difference of 20dc flow and return

 

[ShaunCorbs]

Hi Shaun, yes, the plumbers who fitted the boiler really don't like the push-fit and I can see why, having seen the little holes in the inserts... Unfortunately cheapskate Persimmons pay installers by the number of houses they complete, so quick and dirty is very much what you get. I was really hopeful that blanking off the loops was going to fix it all, but still some work required it seems... I'll keep the system pumping through the worst rad in the daytime for another couple of days and see how much more gritty gunk comes out I think..

Thanks for your help and suggestions - Screwfix today was a real bonus

They will be straight runs so worse comes to worst remove from the manifold clean the inserts if blocked and do the same at the rads and all should be good

Easy way to spot them if any rad is very slow to heat / doesn’t

 

[Rob1234]

They will be straight runs so worse comes to worst remove from the manifold clean the inserts if blocked and do the same at the rads and all should be good

Easy way to spot them if any rad is very slow to heat / doesn’t

The boiler only goes to 60% even with 30C drop when I put more rads on - I feels like it's trying to be too efficient and if there was more flow, the boiler could ramp up more, like the old one did.

The pipe run to the worst rad is far from straight or sensible, the rads all have a pair of elbows on each pipe at the hole in the plasterboard and swap to copper microbore from the hole to the rad itself, so loads of opportunities for blockages, but the flow at the valves when the rads were off for hosing through seemed pretty strong.

 

[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