Worcester Bosch System Boiler - Best pump settings?

[GeddyMortgage]

Hi all,

We had a 35kw Worcester Bosch Greenstar 8000 life system boiler installed a few months back. It's CH only and powering 15 rads of about 30kw or so at T50. All the rads and piping are all new too. Basically a full new system.

We're facing a problem where we cant get all the rads hot, had them balanced 3 times and no matter what there's always a small few that aren't heating up right.

On doing a bit of digging online I saw one of the things to look at in this case would be pump speed. With it being a system boiler its internal and controlled via the boiler itself. I took a quick look at the manual and wondered if based on the number and kilowattage of radiators mentioned above, what setting should this be put to? (it's currently on default of 250mbar). Wondering if it'd be better to simply up it, or try option 0 which appears to then work via a curve thats proportional to the thermal output (which seems way more complicated for a DIY'er like me!)

pics of settings below

thanks all look forward to hearing your thoughts and helping me solve this tepid house..!


Geddy

 

[Johntheo5]

You could try that but might be best to change it to 5 at 350mb or 6 at 400mb, the existing setting at 250mb, 2.5M is far too low IMO to service 15 rads, a setting of 350mb will increae the total flow by sqroot of 350/250, 18.3% and a 400mb by sqroot of 400/250, 26.5%.

If the menu shows the actual pump speed in %, you might note that before and after any changes.

 

[GeddyMortgage]

You could try that but might be best to change it to 5 at 350mb or 6 at 400mb, the existing setting at 250mb, 2.5M is far too low IMO to service 15 rads, a setting of 350mb will increae the total flow by sqroot of 350/250, 18.3% and a 400mb by sqroot of 400/250, 26.5%.

Thanks John. That’s really helpful.

Probably a silly question - but do you need to re-balance once the pump rate is increased?

I should’ve probably also mentioned all the 15 rads are type 22 double panel double convector. Mostly large, some medium, no small.

 

[Johntheo5]

You may have to, see how it goes.

 

[MNW67]

Maybe do it one step at a time. Try setting 4 first?

 

[vulcancontinental]

Maybe do it one step at a time. Try setting 4 first?

yes

 

[GeddyMortgage]

Sure but that’s what was behind my question above about re balancing

If we try setting 4 and it’s still not much better. Do we try setting 5? Or try balancing again at setting 4?

 

[Johntheo5]

I would just try it on the 3 settings, if no improvement to the cold rads move up one setting, setting 5 at 3.5M should result in a improvement, if setting 6 at 4.0M gets them very hot then suggest backing off to setting 5, 3.5M and try a bit of balancing. I have 10 rads, 8 with TRVs, I have the pump set to proportional mode 4.6M which get all rads hot without balancing, as the TRVs throttle down then the pump speed/head fall to ~ 2.5/2.8M. Total rad output ~ 15kw.

 

[GeddyMortgage]

I see, that’s good to know Johntheo5 thanks.

Is it generally better practice to use proportional mode? Is that just essentially loading the pump down as the burner modulates down too?

 

[Johntheo5]

PP mode is generally recommended for rad systems but CP mode can sometimes work just as well if not better, it depends on the pipework etc, PP mode reacts to the rad flowrate and head and is not linked to boiler modulation even though the boiler will modulate based on the required heat load, as the heat load reduces the boiler return temperature and flow temperature rise, the boiler modulates down to keep the flow temperature at its target value. CP mode reacts as well but it just keeps the pump head at its set value so some extra savings with PP mode, PP mode also helps to prevent noisy TRV operation as they are open more (less throttling) with reduced head to give the same required flow rate with less/no noise.

 

[MNW67]

Is it generally better practice to use proportional mode? Is that just essentially loading the pump down as the burner modulates down too?

PP mode reacts to the rad flowrate and head and is not linked to boiler modulation even though the boiler will modulate based on the required heat load,

I think you are talking about different things here.

The boiler pump is a modulating pump. In what you call proportional mode, the pump speed depends on the power output of the boiler.

A Proportional Pressure (PP) pump mode is different. This mode is usually found on external pumps, rather than those inside a boiler.

 

[Johntheo5]

This boiler circ pump doesn't have PP control, I was talking about PP mode in general, the pump above will modulate based on the boiler thermal output, if on setting 0, it will still modulate in all the other settings to maintain whatever head setpoint is choosen.

 

[GeddyMortgage]

Yeah, so the difference between those two bits is where im hazy. What's the benefit on going setting 0 on this one? and under what circumstances is it preferable to use that? vs a constant pressure of the other settings?

 

[MNW67]

Here is a post from someone who has done a lot of these. I can't remember any other relevant posts, but I am still relatively new.

Worcester Bosch are unusual in giving the option of Constant Pressure. Maybe there is something about their heat exchanger design which makes this work well for them. Usually combi/system boilers just use a pump with variable speed linked to the boiler output, with sometimes the option to set the pump to automatically vary the speed to keep the dt at 20C.

There are theories about which modes are better for different system designs. But really there are so many variables, it is probably just a case of experimenting with all the settings.

Have you noticed the settings 3-d3 and 3-d4 in your photo?

The installer should have done all this for you!

Many installers set the pump on modulation and this just causes issues. Set the pump speed to maximum ( fixed not modulating). re check system balancing of radiators ( fully close all lockshield valves and then open 1.5 turns downstairs and 1.0 turns upstairs). make sure the auto bypass valve is set at around 4 or fully closed. If the system is trying to heat the hot water at the same time the radiators then you will encounter problems. Always set system to heat the hot water for at least 30mins before the heating comes on.
If any radiators struggle to heat up then it could still be trapped air the system ( to get this out close all rads off apart from 1. fully open both lockshield valve and trv and run until nice and hot. then move on to each rad in turn. once all have been cleared of any air you can re balance the system. It only takes a small amount of air to stop a system circulating the water around certain sections.
I have assumed that trv pins on all your rads operate correctly

 

[GeddyMortgage]

I see, won’t comment on the installer too much but he did say most options on the boiler are only relevant if I have an external weather controller outside the house. Which sounded bizarre so I didn’t opt for it.

I have seen 3d3 and 4 yes. But wasn’t sure what the percentage was of…. 100% is equal to what? Setting 6 @ 400mbar?