Worcester Bosch Greenstar Boiler / CH issue - SOLVED

1 Jan 2010
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United Kingdom
Hi folks,

Apologies in advance if I use the incorrect terminology in describing the problem.

First, some History.

The current system is a WB Greenstar 8300iW 30 S (or maybe 35 S) condensing boiler. No cold-water header tank in loft, not a combi-boiler (we have a hot water tank in the airing cupboard). This runs a total of 13 radiators over 3 floors. Top floor is a separate zone (with its own programmer and wall-mounted thermostat). A second zone (for the middle and basement floor) has a thermostat that is wall-mounted in the hallway on the middle floor. Boiler is in the basement. For future reference I'll refer to the floors as floor 0 (basement), floor 1 (main living floor) and floor 2 (bedrooms at top of house).

This was piped up and confgured as above by British Gas around 25 years ago when we installed an Ideal Mimimiser boiler (what a dog that turned out to be). With no changes to the pipework or zoning (as far as I remember), it was replaced by a Vaillant Ecotec which worked for the next 11 years faultlessly between 2009-2020 until a single and catastrophic heat-exchanger failure in May 2020 killed it.

In May 2020 (mid-pandemic) the Ecotec failed and we had the Worcester Bosch Greenstar fitted. Apart from some fancy new pipework around the boiler, no changes were made to the system configuration (as far as I know).

We have used the system since the BG install of 1997 as follows:

We have separate temperarure and time settings for floors 0 & 1 - controlled by programmer/timer and wall thermostat in the hallway (floor 1). Floor 2 has its own time and temperature settings controlled by a separate timer/programmer and wall thermostat.

We could have the heating on on floors 0 & 1 and off on floor 2, on on floor 2 and off on floors 0 & 1 or all three floors being heated.

This is how our heating has worked from 1997 until mid-October 2021 when something has happened.

Due to the mild spring / summer / sutumn this year, the heating (on all floors) has basically been off since March. We have had domestic hot water (provided by the bolier) throughout with no issues.

Come to the first week of October 2021 and whilst floors 0 and 1 work as expected and we continue to get hot water as normal, floor 2 is experiencing issues.

To explain (in the following, I'm referring to floor 2 only unless otherwise stated).

Yesterday morning, both bathroom radiatiors were cold. Checked timer & stat - both calling for heat. Checked boiler - cold. Bled both bathroom radiators (although no water came out). Very puzzled. Boiler was reporting LoPr so water pressure was brought up to above 1bar.

Bled all radiators on floor 2.

We are out all day. Fitst thing I did when we came home (and before putting the heating on for floors 0 and 1) was put the heating on for floor 2.

Success - radiatiors became very toasty to the touch (both bathroom radiatios needed a lot of bleeding).

This morning - same scenario. Both programmer and stat on floor 2 calling for heat but boiler is off Went into the L.A Service Function and 1-A1 current status is 203 meaning a"bolier is in standby, no heat energy demand".

Currently, whilst writing this on floor 2, my feet are cold and the radiators cold. Programmer and thermostat are on (stat is set to 26C) but rads on floors 0 and 1 are hot.

Is this a faulty sensor?? Is this a faulty 3-way valve that may be sticking?? Is this some odd setting on the boiler that hasn't kicked in before??

Why do I have to keep (around six times in the last few days) bleeding the bathroom radiators and also topping up the boiler pressure when there is NO SIGN of a leak anywhere?

I'm totally baffled.

Everything worked up as expected up to the long lay-off (March - October) but now something is screwy!

Any thoughts / help / advice gratefully accepted.

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Good morning Dave, and thanks for all the information. It sounds like you have 2 issues, failing motorised valves, be it the 2 port for top floor and 3 port (if this is your setup) for middle and bottom floors and hot water. The 2nd issue sounds like it’s a leak in the system somewhere or perhaps a failed expansion vessel/pressure relief valve. For this I recommend you checking the copper pipe outside that comes from your boiler for signs of water dripping. For the former, try turning on a demand for hot water, see if any rads heat up.
Many thanks for the swift reply Chris.

A quick check outside there is only a large diameter drain (for the condensate?) and a small pipe that is capped off with some sort of domed valve (??). No water under this copper pipe. :(

The boiler was reporting LoPr 0.6 bar again, so again topped off.

All upstairs (floor 2), middle floor (floor 1) and basement (floor 0) bled (floors 0 & 1 - non needed bleeding).

Given the number of times I have to keep increasing the pressure of the water in the boiler, I suspect a leak somewhere.

I'll now keep a meticulous recored from here on of both the pressure being reported by the boiler and the time.

Seems like a leak somewhere but there is absolutely (so far) no evidence ANYWHERE.

The expansion vessel is inside the Greenstar's casing - is that correct??

Kind regards

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Yes vessel inside the casing - although you could top the pressure up, put a demand on and see if the pressure rises. Likely to be a leak, as your system is not that old.
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Trying to post some pictures - but the have to be on a picture-hosting site - such as Flickr or imgur? You can’t upload direct from your pc? If so, I’ll have to upload them first.

No, you can upload them on here, should be able to select upload a file.
Bottom right hand corner "upload a file" then simply add your pics, the small copper pipe is not capped off that is a WB PRV, tie a freezer bag or balloon or something over the end and see if any water gathers in the bag, also set your boiler pressure when cold to 1.5 Bar, then put the heating on and go back after it has been on for about half an hour and check the pressure guage and see what it is now reading, if it has increased up to around 3 Bar you have an expansion problem
  • Thanks
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Okay - an update.

Been out of the house all afternoon from mid-day and just got in at 7pm.

Went down to see what the boiler is saying - LoPr 0.5 bar

There a few drops of water on a pipe underneath the boiler, but looking upwards at the 'innards' there's no sign of any water.

Anyhow - to the pictures (thanks to Ian pointing out where the 'Upload a File' button is.


^ ^ The boiler in the basement ^ ^


^ ^ Pipework under the boiler (view 1) ^ ^


^ ^ Pipework under the boiler (view 2) ^ ^


^ ^ Boiler 'innards' (which are dry)^ ^


^ ^ External Vent and Condensate Pipe ^ ^


^ ^ Close-up of Vent (Pressure-Relief Valve?), Condensate Pipe and Trace Heater ^ ^


^ ^ The end of the Condensate Pipe ^ ^

These pictures should be viewed referring to post #3 this thread. They were taken this morning.

@ianmcd Two questions.

1) I presume WB PRV means Worcester Bosch Pressure Relief Valve?

It wasn't fitted when the new boiler went in - this dates back to the British Gas installation of the Ideam Minimiser way back in 1997.

2) Is it not dangerous to over-pressure the system? The boiler yells at me to stop above 1 bar. If the cold pressure was increased to 1.5 bar as suggested and the heating fired up - what could be the possible negative consequences of doing that?

I took a couple more pictures - showing the drips under the boiler. The floor underneath the boiler has a damp patch but no 'water' as such. The inside of the boiler - as far as I can see and feel - appears to be dry.

Boiler is reporting a new error code as well as LoPr 0.5 bar, namely '2965' which apparently means 'Flow Temperature Too High'.


^ ^ Low Pressure 0.5 bar Error Message ^ ^


^ ^ Pipe underneath boiler showing water on the surface ^ ^


^ ^ Close-up of said pipe ^ ^

Installation Manual says that 2965 'Flow Temperature Too High' means '1. Check the flow through the appliance and the system is filled. 2. Check for airlocks in the system and purge if required. 3. Check the isolation valves'.

This leads on to 3 questions!!

1) How can I check the flow through the appliance and that the system is filled?

2) How do I check for airlocks and purge if needed?

3) What do I need to check about the isolation valves??

In the meantime I'll see what putting some more water into the system does. Currently the upstairs zone is calling for heat (set to 26C and programmer/timer is ON) yet boiler isn't working and showing the 2965 fault code.

Thanks for all the help & advice so far.

Much appreciated.

The boiler has lost pressure, hence why you’re probably getting the additional error message. Remove the insulation above the pipe that has the water on it, this might reveal something, although there’s a drain off with some green on it that looks suspect.
An update.

I just wondered what would happen if I turned off the call for heat for floor 2 (reduced thermostat from 26C to 14C) and increased the thermostat for floors 0 and 1 - depite the boiler saying '2965'.

Boiler started up and after the reported temperature dropped steadily until about 30C when the ignition icon came on and the temperature started to rise.

The rads on floor 0 and floor 1 are warming up.

I just don't understand why a call for heat from the stat on floor 2 is being ignored. That doesn't (to me) make any kind of sense.

Will cut away that insulation now and see if I can spot anything. Also have a look at the 'innards' to see if I can see anything dripping.

Some additional pix and info ..


^ ^ The floor underneath the boiler (the darker red area is damp but not wet) ^ ^
Easier to see in 'real life'.


^ ^ The 2965 Error ^ ^


^ ^ Pipework under the boiler after insulation rmoved ^ ^

Note the drop of water at the bottom of the white condensate pipe and on the copper pipe below.

No evidence of any water anywhere else.


^ ^ Close-up of condensate pipe and water drop ^ ^


^ ^ Close-up of drop of water on the lower copper pipe ^ ^

The green colour seems to indicate that this has been 'wet' for a while ?


^ ^ Innards (1) - eveything looks dry ^ ^


^ ^ Innards (2) - everyhing looks dry ^ ^


^ ^ Innards (3) - everything looks dry ^ ^

With the main room thermostat on floor 1 calling for heat, the boiler is working and the radiators are getting toasty.

With the room thermostat on floor 2 calling for heat and that on floor 1 off - nothing.

Ok, so the leak was the condensate pipe, which needs attention. Again, given the age of the boiler, I doubt it’s a failed main heat exchanger. Likely something on the 2nd floor.
Hi Chris,

There is a motorised valve for the heating right next to the hot water tank which. In the past it clicks and the whirrs shut (?) but I don’t know if it is a 2-way valve or 3-way. As far as I can remember, BG back in ‘97 only fitted one motorised valve. There aren’t any on floor 1 for sure and I don’t think there are any down with the boiler on floor 0.

Sunday morning, 07:30.

Turned on floor 2 heating (thermostst set to 23C and programmer/timer on).

Checked boiler - LoPr 0.6. No evidence of water ANYWHERE. Pressure topped up to 1.2 bar.

Bathroom started off at 17.7C and rose, albeit reluctantly, to 18.4C in half an hour.

Radiator under window warm-ish all over, bottom of towel radiator only the bottom third was warm-ish.

Towel radiator needed bleeding.

Checked boiler for error - got a 203 meaning a"bolier is in standby, no heat energy demand".
(this was despite programmer/stat on floor 2 calling for heat)

Nothing seemingly getting any better on floor 2.

Turned thermostat up to 18C for floors 0 & 1 (henceforth I'll call this zone 1) and rads got nice and warm - hot to the touch.

There are severeal things that are puzzling me.

1) there is evidence of some (probably small) leakage (more like weeping) from the boiler and/or pipework but nothing huge - and no real evidence either. Can't seem to pinpoint this.

2) the hot water is working as normal - no issues.

3) the basement and main floor (what I'm calling Zone 1) works as expected.

4) the top floor (floor 2 / zone 2) is the one where heat isn't getting to - or at least it starts to get to this floor and then for some reason stops

5 the constant, sometimes several times a day, bleeding of one or both bathroom radiators. Sometimes the rad produces air when you open the valve
and sometimes NOTHING comes out - no air, no water.

These two radiators - mainly the 'ladder-style CH-heated towel rail' - seem to catch the air in the system since BG installed the boiler back in 1997 and frequently need bleeding (maybe once as a couple of months or so) - but not near as much as currently.

6. the fact that both programmer & thermostat can be calling for heat on floor 2 (Zone 2) and NOTHING HAPPENS.

So, given the above I'm thinking

a) there is a small leak somewhere - probably not inside/adound the boiler - and the evidence has yet to appear
Not sure why there is a damp 'witness mark' on the floor underneath the boiler nor a single drop of water on the bottom of the condensate pipe.

Most likely a valve at the end of a radiator that has been knocked?

b) Floor 2 / Zone 2 programmer/thermostat not working correctly

c) A sensor (somewhere) not giving a correct reading

d) A problem with a sticking 3-way or 2-way valve

e) A problem with the boiler of some sort (pcb??)

f) A general "wiring problem" either between the Zone 2 programmer (the thermostat is wireless) or inside the boiler somewhere

g) An airlock or a 'plug' of sludge somewhere that is preventing the proper circulation of the hot water.

Given that the CH system wasn't used for around six months (March-September) this year because it was relatively mild this would explain (2), (3) and (4) above but point (6) defeats me.

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It does sound a little odd, and slightly confusing.
  • Does zone 1 work with low pressure or does it only work when you top up?
  • Zone 2 could just be a stuck zone valve
  • Was any of the old system, eg tank kept?

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