Best CH pump to control flow rate

[JonathanM]

I have a choice to replace my regular boiler setup, with either another regular boiler, or a system boiler.

With the crazy cost of gas, I'm very keen to get the best modulation and condensing possible out of the new boiler.

Currently, we have a British Gas Multihead G pump, which seems identical to a later version of the Grundfos UPS 15-60. It has three fixed speed settings, and we currently use it on number two. It is a large house with sixteen radiators, mainly double convectors.

I'm looking at the Worcester Bosch 8000 Life 30KW, which modulates down to 3KW.

Instinctively, I feel that the system boiler, with its internal pump, will do a better job of managing the flow rate, to maximise modulation and condensing. Because it's integrated into the boiler controls. But it really just is my gut feeling.

I would be grateful for any thoughts.

 

[CBW]

16 radiators I doubt that 3KW will be enough. Could be wrong, as I haven’t delved much into radiator output and boiler modulation.

 

[JonathanM]

These days, I usually only run the downstairs!

And I never have it very warm.

I'm hoping to run the flow at 50. I reckon this will take 3.5KW downstairs.

 

[Johntheo5]

Its great to get a boiler that modulates to 3kw, The Viessmann Vitodens W200 can modulate between 1.9 and 35kw, a ratio of 1:18.4, fantastic.
You will get far better control IMO with a external pump, the internal pumps are generally set to a fixed level but sometimes may be set in the users menu, if a combi or a system boiler then the pump will be internal?.

 

[JonathanM]

I've been reading, in a bit more detail, through some of the manuals for system boilers. On most system boilers, it looks like the pump speed is being constantly varied by the boiler depending on the firing rate, and it will often go down as low as 10% of the full speed. It's amazing what they can do these days! That seems a pretty smart way of making sure the flow is kept as low as possible, to keep the flow/return differential as large as possible.