Worcster Si or CDi, that is the question

[pg325]

I'm going to replace my current WB 240 and have been recommended the Greenstar Si or Greenstar CDi.

My choice of either 25/30 Si or 27/30 CDi, what I want to know (apart from the obvious DHW and BTU) is what is the main difference with these boilers.

Both seem to offer independent DHW temp, DHW flow is the same (ish), any ideas.

 

[bengasman]

CDI is better build quality. SI is a tarted up version of the budget boiler i-model.

 

[ServiceNumpty]

The cdi is manf in Germany, different pump, heat exchanger, diagnostics, sturdier, generic hydroblock as the si/i series. Easier to service than the si/i series, both will be a pain in ass to repair though.

 

[boilerdoktor]

the cdi is more efficient!..both easy to work on should you need to( you probably wont)

 

[andycox]

As my learned friend said CDi is better and cheaper to run, last longer
GET ONE!

3+ people or 3 beds = 30Kw
small place fit 24 is not worth buying CDi go Si cheaper

 

[ianniann]

Fancier modulating pump. The major marketing points they use to distinguish the two ranges are that the CDi has a "modulating pump" and is "fulliy condensing in hot water mode".

The modulating pump is true and certainly uses less electricity than the non-modulating pump but it is still a pretty hungry pump. Better to have it than not though. You may also notice higher noise from the Si pump, especially when TRVs are shutting down.

The thing about condensing in hot water mode is mostly a steaming bag of marketing ... errr ... stuff. Both boilers condense, not fully not not zero, in both heating and hot water modes. Details depend on the settings and operating conditions. The CDi has a larger heat exchanger which helps to extract condensing energy even when the output temperature is above the dewpoint. The difference isn't huge (1%-2% overall?), but again it is better to have it than not.

The CDi can also be fitted with a wider range of controls, including more sophisticated modulating controls and weather compensation. This really should be the biggest selling point in terms of features, although the CDi is generally more robust (and a lot heavier!).