Worcester Bosch System Boiler - Best pump settings?

[snb1]

the option of A and B is not selectable. If your system is not getting all the rads hot and it is also new it could be that you still have some airlocks.
I would recommend that you change the pump speed to option 6 ( highest available). And then close all the rads. open 1 rad at a time with both valvesfully open to a couple of mins each to ensure that the air is pushed out if any. Then open all rad trvs' . on the lockshield side open 1 full turn for upstairs and 1.5turns for downstairs. This is a rough guide but will often work for most systems.

 

[Johntheo5]

I would think thats the percentage pump speed d3 is the minimum settable speed and defaults to 10%, d4 is the maximyum settabl speed and defaults to 100%, there might be another parameter that shows you the actual pump speed, (%).
I wouldn't try and overthink this too much but IMO its a very optimistic to expect a pump head of 2.5M to circulate sufficient water not only through the system but also through the boiler HEX, the head loss through this might be 2M at its design flow (normally designed for a dT of 20C, so in this case, maybe 35*860/60/20),25LPM or even less, this leaves (if 2M is the HEX head loss) a head of of 0.5M to circulate the 15 rads, not a hope IMO, I would just initially set the pump to its max head of 4.5M and see how it goes, its only a press of a button, you can allways step it back down or why not then or before try it on heat output, this might be a even higher head than 4.5M?.

 

[MNW67]

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.

It sounds like he might be talking about the option to have Priority Hot Water which means a different flow temperature for the cylinder and the radiators. Although you say yours is CH only. How do you get your hot water?

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

I would have thought if you set both parameters 3d3 and 3d4 to 100% the pump would follow curve A in your second photo. If both to 10% it would follow curve B. And then anything in between depending what mixture of numbers you choose. But I have just noticed @snb1 has said that they aren't selectable?

 

[MNW67]

but IMO its a very optimostic to expecy a pump head of 2.5M to circulate sufficient water not omly through the system but also through the boiler HEX, the head loss through this might be 2M at its design flow

I think these figures claim to be "residual head" i.e. the head left over after going through the HX. Some of them look a bit high if that is true.

 

[GeddyMortgage]

I’ve not needed to have the heating on since making this post so haven’t tried yet. But, I have set it to “5” so it looks as though the constant speeds are at least selectable.

The question regarding hot water - that’s a separate traditional boiler with an unvented cylinder, entirely separate to this one. This one solely does CH on the ground floor only. Long story, large footprint property.

 

[GeddyMortgage]

It sounds like he might be talking about the option to have Priority Hot Water which means a different flow temperature for the cylinder and the radiators. Although you say yours is CH only. How do you get your hot water?



I would have thought if you set both parameters 3d3 and 3d4 to 100% the pump would follow curve A in your second photo. If both to 10% it would follow curve B. And then anything in between depending what mixture of numbers you choose. But I have just noticed @snb1 has said that they aren't selectable?

So I understand the difference correct then, with a min and max at default…. It would modulate somewhere in that range relative to…. The output % of the burner? … essentially the harder the boiler is working, so too would the pump? Or nah?

 

[Johntheo5]

Yes, more or less, challenge is that if the heating demand reduced to 50%, the pump speed might (depending on the type of control) also reduce to ~ 50% but its head then is only 25%, so IF the original head was 4.5M then the head now is only 1.123M but there will probably be equilibrium somewhere before this, you boiler gives fantastic info, 1.b8 is the actual burner modulation and 1.c2 is the pump modulation, both in %, so note these at various times or during any changes. You can also what effect running in the various CP settings has on the pump modulation.

 

[MNW67]

So I understand the difference correct then, with a min and max at default…. It would modulate somewhere in that range relative to…. The output % of the burner? … essentially the harder the boiler is working, so too would the pump? Or nah?

That's how it is meant to work. I find Alpha write their manuals in plain English and this is how they explain it.

Proportional - The pump speed is controlled automatically in order to give a proportional pump head, the pump speed varies based on the heat output supplied by the burner - the greater the heat output the higher the pump speed.

But 10% sounds really low for a minimum, if that is the default minimum. If that is what is being shown as curve B in your photo then there's going to be hardly any head or flow. With some boilers the minimum pump speed is 60% or more. Maybe the pump in your boiler is really powerful. That boiler range does cover very big systems. The graph actually shows it giving 5.5m of residual head at 30 litres per minute. If that is true then that is a very powerful pump. You probably wouldn't want that running at full power on your system. But you could play with the maximum and minimum settings.

 

[snb1]

You can manually select pump speeds 1 to 6 but pump,speed A and B are only used in the auto function. Problem is to get pump pressure of A would require a free flowing primary circuit. My guess is that system will have 22mm primaries. If that is the case then the boiler would only ramp up to around 24kw max as that is the limit of 22mm pipework.

 

[Johntheo5]

That's how it is meant to work. I find Alpha write their manuals in plain English and this is how they explain it.



But 10% sounds really low for a minimum, if that is the default minimum. If that is what is being shown as curve B in your photo then there's going to be hardly any head or flow. With some boilers the minimum pump speed is 60% or more. Maybe the pump in your boiler is really powerful. That boiler range does cover very big systems. The graph actually shows it giving 5.5m of residual head at 30 litres per minute. If that is true then that is a very powerful pump. You probably wouldn't want that running at full power on your system. But you could play with the maximum and minimum settings.

Where are you seeing mention of residual head?, the 30LPM at 5.5M looks very much like a typical 8M pump curve, ie, a 8M Dab Evosta 3 will flow 30LPM at a 5.3M head.
Residual pump head is just "pump head" at any given flowrate?, remaining head is probably the one that gives the head after passing through the HEX??.

 

[MNW67]

Where are you seeing mention of residual head?, the 30LPM at 5.5M looks very much like a typical 8M pump curve, ie, a 8M Dab Evosta 3 will flow 30LPM at a 5.3M head.
Residual pump head is just "pump head" at any given flowrate?, remaining head is probably the one that gives the head after passing through the HEX??.

On the second photo in the first post it says H is Residual Head.

Residual head is the same as remaining head. "Residual" means what's left remaining, as in "residual estate". It is the head that is still left over after passing through the heat exchanger. That's why they only show it for combi boilers and system boilers.

This is how vulcancontinental used the phrase in another post:

"Stainless heat exchangers can be very restrictive meaning the residual head of the pump is too low for system circulation necessitating an additional pump and closely spaced tees or low loss headers to maintain adequate flow rate"

I know it is a Grundfos UPM3 pump. I believe it most likely is this one.

Boiler manuals can be wrong though. That is a major caveat!!!

https://product-selection.grundfos....00?pumpsystemid=2296989012&tab=variant-curves

 

[GeddyMortgage]

You can manually select pump speeds 1 to 6 but pump,speed A and B are only used in the auto function. Problem is to get pump pressure of A would require a free flowing primary circuit. My guess is that system will have 22mm primaries. If that is the case then the boiler would only ramp up to around 24kw max as that is the limit of 22mm pipework.

Oh I see, by primaries if you mean the main flow and returns directly from / to the boiler. They’re 28mm for the first 2 meters, then split into 2 parallel runs of 22mm (front of house run) and (Rear of house run) then down to 15mm for the last meter or so to each rad.

 

[Johntheo5]

Yes, I see that now but am still of the belief that it only shows the pump head, the pump head that you show above is exactly the same as the one in the UPM3 attachment, it couldn't possibly be the head available after "the" HEX, as no HEX is mentioned, IMO its the generated pump head, if you stuck a valve on that pump discharge and throttled it to give a flow rate of 30LPM, 1.8m3/hr then the pump head is 5.8M, I think remaining head is the word but of course if the boiler makers just gave us the HEX head loss at any given (normally the design) flowrate and the pump curves then no more confusion.

 

[GeddyMortgage]

Yes, more or less, challenge is that if the heating demand reduced to 50%, the pump speed might (depending on the type of control) also reduce to ~ 50% but its head then is only 25%, so IF the original head was 4.5M then the head now is only 1.123M but there will probably be equilibrium somewhere before this, you boiler gives fantastic info, 1.b8 is the actual burner modulation and 1.c2 is the pump modulation, both in %, so note these at various times or during any changes. You can also what effect running in the various CP settings has on the pump modulation.

Now that makes me wonder if this is related.

I’ve noticed this boiler gets to target flow of 70c in about 25 mins. But when it gets there, the burner goes out for about 1minute during which time the flow temp rapidly goes down to maybe 50 or so. Then the burner comes back on at a much reduced output then gradually creeps back to target temp again over the next half hour, then stays there.

I’m assuming that’s due to the constant pump still running when the burners gone out for that minute. So I’m guessing option 1 would change that?

Should it be even doing that though? It’s always struck me as quite inefficient that it does that. Why cant it just stay at temp and modulate down?

 

[MNW67]

Yes, I see that now but am still of the belief that it only shows the pump head, the pump head that you show above is exactly the same as the one in the UPM3 attachment, it couldn't possibly be the head available after "the" HEX, as no HEX is mentioned, IMO its the generated pump head, if you stuck a valve on that pump discharge and throttled it to give a flow rate of 30LPM, 1.8m3/hr then the pump head is 5.8M, I think remaining head is the word but of course if the boiler makers just gave us the HEX head loss at any given (normally the design) flowrate and the pump curves then no more confusion.

I am sure I am right about the meaning of residual head. But I don't trust the info. on this in the Worcester Bosch manual. I think you are correct that the graph is simply showing normal pump head. Probably for a standard UPM3 rather than the more powerful version I posted.