IS THE worcester BOSCH GREENSTAR SI COMPACT RIGHT FOR YOU?
This unit is available in two configurations: The 25Si Compact and the 30Si Compact. This is the first point that can trip people up when making purchasing decisions. For most units, the naming conventions follow the KW power to central heating and help identify the right size of unit to buy but.....
In this case, both versions are the same size, have the same ratings, and have the same central heating output of 24kW. The only real difference between them is their price and their hot water flow rate capability, with the 25Si rated at 8.9l/min and the 30Si rated at 10.7l/min.
It seems a bit pointless to have such a small difference between their largest and smallest units, but as a unit explicitly designed for small homes and limited uses, this is less of a flaw for the Greenstar SI than it would be for other models. It is likely to avoid potential extra gas runs by only producing the 30kw model.
These units have an overall ErP efficiency rating of A and come with various control options, including Worcester Bosch's EasyControl system, but neither model is compatible with solar systems as there is no storage and it cannot accept a pre-heated inlet.
The SI is an entry level Worcester Bosch boiler so if you are on a budget or buying for a rental perhaps but still want the Worcester Bosch brand and service then it may be right for you. It will comfortably heat a 1-3 bedroom house with one shower or bath and due to the "Compact" aspect is very versatile with regards to installation location, it will fit in a standard kitchen cupboard.
Build quality
Heat exchanger
Worcester Bosch boilers have Aluminium heat exchangers which are very good at thermal conduction and are cheap to produce compared to Stainless Steel, it also has far higher thermal expansion which will lead to movement over time and can have the issue of seals leaking. This problem is particularly evident in some Worcester Bosch RI heat only boilers that have been installed to less than spotless systems, the expansion and contraction breaks the top hex seal and causes a slow leak. Worcester Bosch are aware of the issue and have a fixed price repair available, as always with Worcester Bosh their after care is exceptional.
Aluminium is easier to form into complex shapes which makes it the ideal choice for heat exchangers with designs found in Worcester Bosch boilers, similar heat exchangers are found in some Ideal and Baxi boilers.
More care must be taken with the water quality of Aluminium heat exchangers, the table below shows the small window of acceptable pH when multiple metals are used. Worcester Bosch also do not recommend their boilers are filled with water softeners in the property as softened water reacts with Aluminium. If a softener is present it must be by-passed for the filling process or the system content should be monitored for pH etc.
Complexity of boiler
The Worcester Bosch SI compact has quite a complex build as you can see below, in order to keep the boiler small they have made some clever design implementations. The fan is positioned at high level above the heat exchanger and behind the air intake, the plate heat exchanger is behind the pump and condensate sump and the exhaust is a flat type in-front of the heat exchanger. These all contribute to a very compact boiler and it may look neat to a non-professional but to a servicing or repair engineer it is not what you want to see, and ideal (not Ideal) boiler would have the parts spaced out and easy to get to. Other than ease of access to parts there is the proximity to heat to be considered, although the components will be rated to be resistant against heat the long-term exposure will and does cause plastic parts to go brittle which can lead to failure.
Worcester Bosch SI Compact inside look
Controls options
Worcester bosch controls
Worcester Bosch really want you to use their proprietary controls, they use EMS BUS communication which is (or was) also used by Buderus. As far as I am aware there are no EMS controllers available in the UK other than the ones manufactured for or by Worcester Bosch but they have a good range to choose from. The wiring diagrams below show the mains voltage terminals which will be used for mains switching for 3rd party controls and the low voltage terminals for WB own controls. Like most Worcester boilers there is a very handy plug-in option, removing the blanking plate on the front of the pcb enables the installer to fit any number of basic and wireless controllers some of which are listed below.
The Worcester Bosch FW100 Weather Compensation controller plugs into the fascia reducing wiring time. It works with an external sensor to maintain a comfortable internal set point, there are preset curves for radiators and underfloor heating to enable easy set up by the installer. The FW100 is also compatible with Worcester Bosch Greenskies solar. The FW100 can report on faults and the installer will be able to access the fault history to aid in any breakdown visit or servicing.
This controller is weather optimised which directly modulates the boiler output and as such has and ErP class ii which adds +2% to the seasonally efficiency of the connected boiler.
The Worcester Bosch Comfort I RF plugs into the into the fascia of the boiler reducing wiring time. The I RF has a wireless thermostat which can be turned up and down as with a traditional wall unit and the controller and stat are paired out the box. The I RF can control both heating and hot water, has four on/off times per day for both and has the old fashioned on/off/auto/once configuration if desired. The I RF can report on faults and the installer will be able to access the fault history to aid in any breakdown visit or servicing.
This controller is load compensating which directly modulated the boiler output and as such has and ErP class v which adds +3% to the seasonally efficiency of the connected boiler.
The Wave is a smart thermostat developed and exclusive to Worcester Bosch boilers, it is capable of time and temperature control for both the heating and hot water and has smart device compatibility so you can turn the heating up or down wherever you are. The installation is simple although more complicated than with the plug-in controllers above, two wires connect to the EMP BUS on the ② terminals. Unlike the FW100 it does not utilise an outside sensor but instead (like Nest and others) uses localised weather from the internet.
This controller is weather compensated which directly modulated the boiler output and as such has and ErP class vi which adds +4% to the seasonally efficiency of the connected boiler.
third party controls
As WB use their own control communication bus it is not possible to use OpenTherm controls, it is possible to use basic controllers from Honeywell or Drayton to switch the heating on and off according to time and temperature settings. Although any standard or "Smart" controller can be compatible with basic mains switching like the Nest or Hive only the Tado controller offers modulation of the heating output and is truly "Smart". The SI has (like most boilers nowadays) an outside sensor option, number 4 on the low voltage terminal block allows any controller to be used and a weather compensated primary flow, the SI also has an optional external frost thermostat to protect the boiler if it is installed in an unheated space.
WORCESTER BOSCH ACCESSORIES
Worcester bosch primary filter
Worcester offers multiple filters in metal and plastic, several sizes are available.
If the boiler is installed by a Worcester Approved Installer and one of their filters is installed then the maximum warranty can be applied.
worcester bosch flue options
- Standard 60/100mm horizontal flue with plume kit if necessary
- Larger 80/125mm flue for long runs
- Vertical flue for roof installations
Worcester bosch keyless filling link
Worcester have long been criticised for the old filling key, most other boilers have either a lever or knob style filling loop but Worcester have for a very long time incorporated a plastic access key and knob. This is very difficuly for the end user to grasp and often got lost.
The new Keyless Filling Link is quick and easy to install and reduces the potential callouts for low pressure, it is approved by WRAS as it incorporates a check valve.
Size and weight
Just how compact is the Worcester Bosch SI? Well the dry weight of 32.9kg is fairly low average compared to most combi boilers, the smaller size makes it an easier lift though. Worcester call it a one man lift and certainly most installers I know would be fine getting this on the wall.
The external dimensions of the SI are much lower than the average, with a width of only 390mm and 280mm depth there are few boilers in that range. If you are looking for a small boiler but still want to stick with the industry leading manufacturer then this ticks your boxes.
These clearances must be observed during and following installation, the smaller dimensions are the distance any object can be from the appliance and the larger are servicing clearances.
*Minimum clearance to removable door
** Minimum clearance for servicing
*** Height required for either 60/100 or 80/125 flue
WOrcester SI Compact Output
The SI Compact is available in two outputs 25kw and 30kw and can run on LPG if needed.
Heating output
The maximum output of the heating is 24-25kw depending on flow temperature required, this is the same for both 25 and 30kw models so the only reason to opt for the larger unit is for hot water production.
Most boilers also give a minimum heat output, Worcester have not given a minimum on the data within the manual for the SI which makes me think it is not impressive. If we find out we'll update this area.
Hot water output
The hot water is rated properly unlike the heating, a minimum of 7.15kw input gives a modulation of 3.5 which is fine considering the entry level status.
The maximum flow rate is 9 litres per minute from the 25kw and 11l/m from the 30kw model. Both are based at a 40°c rise which is more than other brands allow for.
MISCELLANEOUS performance info
The residual head of the Worcester SI Compact is surprisingly low, only 2m is available after heat exchanger loss, this may mean that long runs or complicated primaries will be too much for the pump. This boiler is really designed for the smaller properties but 2m residual is very low.
The maximum electrical consumption is 98w at full rate and 1w on stand-by.
The maximum condensate discharge rate is 2.5l/h.
RELIABILITY of the Worcester bosch si compact
warranty
The SI has (at time of writing) a standard 5 year warranty, this can be upgraded to 10 years if installed by a WAI with a WB filter.
Worcester Bosch are very good at after sales care, if there is a problem they are one of the best in the business at looking after their installers and end users.
Ease of service
The compact nature of the SI is the main selling point but this comes out some cost, the proximity of all components to each other mean that servicing will be longer than with many other brands and models. Take a look at the position of the fan or gas valve and think about how easy it would be to get a hand or spanner around them, experienced Worcester engineers may be a little quicker but for most engineers the layout of this type of boiler is unfavourable to say the least.
Final verdict
The Worcester SI Compact Combination boiler is far from the "best" boiler on the market, but that isn't what Worcester are going for and probably isn't what the buyer is expecting. It is very small and fits into a kitchen cupboard (which nowadays is not very common to find) and has a respectable hot water and heating output.
in the "compact market" the Greenstar SI sits comfortably as other compact boilers include offerings from Ideal and Glow-Worm which are not as renowned as anything by WB.
To be clear, this doesn't mean I think the Greenstar SI is a bad unit. On the contrary, I suspect it's a perfectly good unit that generally performs about the same way as the rest of Worcester Bosch's lineup, and they have a lot of outstanding reviews from real customers.
In truth there are very few "bad" boilers available, if this is installed to a clean system and regularly maintained it will perform perfectly well in a small to medium property. Don't expect anything special, what you see is what you get.
