Worcester Bosch Greenstar 8000 System Boiler Issues

[marshian]

3 hours ago, EinTopaz said:

 

There is a separate much older WB CDI boiler upstairs that does the hot water for all the property and some rads up there too. It's a conventional type that one. And the flow pipe coming out of it (using the same measuring tools) is always within a degree of the LCD display on that boiler.... I only wish the one downstairs was as straight forward.

The boiler we're discussing here with the problems solely does the CH for the ground floor, as to not complicate the older system. 

 

Ahhh that makes perfect sense although two boilers mean two service bill/maintenance costs and I'd be fretting about which one was driving the bills but that's me all over

[EinTopaz]

14 hours ago, SimonD said:

So my suspicion is more leaning more towards the flow temp not being the same as the temp displayed.

Agreed. I've got a Senior Wb engineer coming out on Monday. What would reasonable next steps be? I believe he's bringing a new control board with him. And I was going to run him through all the symptoms. The last guy said there are zero problems with the boiler and it must be the system. But it really did feel like they were passing the buck. 

[EinTopaz]

13 hours ago, marshian said:

 

Ahhh that makes perfect sense although two boilers mean two service bill/maintenance costs and I'd be fretting about which one was driving the bills but that's me all over

I know, it was either that or replace the original upstaiirs one with something massive, like a commercial. it is a property on the larger side. 

[EinTopaz]

13 hours ago, JohnMo said:

Can I ask a basic question, how big is the house and what is the calculated heat loss.  For 36kW, it has to be pretty big, uninsulated and pretty drafty.  Is this the case?

 

Boiler are pretty simple in a lot respects.  They are trying to heat water, the bigger the boiler the higher the flow of water needs to be to move that heat away.

 

The boiler unless told otherwise runs flat out, throws as much heat as it can initially, then if it senses it can, it will modulate output down.  The idea is it runs as long as it can. Your boiler will modulate power output and circulation pump speed to do this.

 

Some boiler have a setting (bit like acceleration rate) where you can set the ramp rate, so how many degrees of heat are added per minute, this slows everything down and lets the system catch up.  But the WB8000 doesn't have this.

 

What does exist is I believe is Gradient Limitation (Temperature Blocking)

The boiler itself has an internal gradient limitation control:

• It monitors how quickly the flow temperature rises.

• If the rise is too fast (above a programmed limit), the boiler will temporarily pause the burner for ~2 minutes until it’s safe to continue

So you are possibly seeing this in your video.

 

Your circulation pump is a PWM, so it will try to maintain system dT, to within defined limits. To do this on Rising dT boiler increases pump speed, slows it for reducing dT. Your system seems to be fighting this you have what is described as an open system, big pipes, big radiators etc.

 

So unless a valve is closed, you have a blockage or you pump is failing (doubt that). Has the plumber installed something (gate valve or similar) to force a dT against the boilers wish and as such gagged the flow.

 

 

 

 

 

Hey @JohnMo It's a 1955 built property. Has cavity walls (75mm cavity) they're insulated, and it has a suspended subfloor which I insulated myself (awful job)...That said the house has been extended many times. Was originally a bungalow so a large single story footprint, it's now 3 stories. This boiler solely does the rads on the ground floor. For the rooms - Lounge, Hall, Bathroom, Dining Room, Study, Kitchen, Family Room, Utility room. Regarding the calcs it was done by the installer 2 years ago who's no longer contactable. 

Thats interesting to know about the gradient limitation. Regarding the raising rate of the boiler's flow temp. It's pretty quick and consistent, it doesn't seem to pause for anyminutes as it raises to 70 within 10mins. But as above my biggest concern is that the flow pipe and subsequent rads are no where near raising at the same rate / getting hot as quick. The video I put above of it going between the 20/80% output that is a new symptom. I suspect because we've put it to 80 target, im gonna drop that back down to 70 and see if it still does it. 

I've considered the blockage aspect too... Sorry if a silly question but what is the easiest way to check this? i've  tried all the rads on the front of the property TRV's turned down, leaving only rads 1-6 open, and then done the inverse, 1-6 closed and 7-15 open. Same symptoms of massivey differential of boiler temp to flow pipe temp. So if there is a blockage or gate valve... I surmised it must be close to the boiler before the 28mm splits into the 2 x 22mm parallel runs. If that makes sense? That said I had a plumber here last week down the sub floor inspecting the piping i assume he'd have mentioned if he seen a any sort of odd valves down there.

 

 

[JohnMo]

2 minutes ago, EinTopaz said:

inspecting the piping i assume he'd have mentioned if he seen a any sort of odd valves down there.

Take some photos of the boiler and any piping especially any valves.

 

Our last house was 3 storey sandstone house almost no insulation and drafty - it was 6 bedrooms etc, we had a 34kW and it never struggled even in the depths of winter in Scotland.

 

From heat geek website 

if you have an older Victorian or pre-war building with little upgrading — say, single glazing and minimal loft insulation — you should expect 95–110 W/m² (of total floor area, not just ground floor).

 

If the same property has upgraded to double glazing and some loft insulation, this drops to 65–85 W/m². Further upgrades such as cavity insulation can reduce it to 40–65 W/m².

 

Pre-2006 builds or recent full renovations typically range from 30–50 W/m².

 

So look at you total floor area and multiply by watts to get something close to your heat loss.

[marshian]

11 minutes ago, EinTopaz said:

Hey @JohnMo It's a 1955 built property. Has cavity walls (75mm cavity) they're insulated, and it has a suspended subfloor which I insulated myself (awful job)...That said the house has been extended many times. Was originally a bungalow so a large single story footprint, it's now 3 stories. This boiler solely does the rads on the ground floor. For the rooms - Lounge, Hall, Bathroom, Dining Room, Study, Kitchen, Family Room, Utility room. Regarding the calcs it was done by the installer 2 years ago who's no longer contactable. 

 

I can see how you ended up with it - many gas engineers oversize boilers "To be on the safe size" it's how I ended up with a 24kW boiler for a house heat loss at the time of 6kW but with T11 rads and a flow temp of 60 to 80 deg (no weather comp so flow temps were manually adjusted) the boiler cycling was manageable.

 

My issues with that boiler started when I changed the majority of rads from T11's to T22's then dropped the flow temp to 50 and it was a bit sh1t but I lived with it.

 

(I subsequently installed 75 mm PIR under the suspended ground floor (horrible job) - increased the depth of loft insulation and resolved a few uncontrolled ventilation issues - so was then at ~4 kWh heat loss and the boiler was unable to manage the house temps without cycling excessively 

[EinTopaz]

48 minutes ago, marshian said:

 

I can see how you ended up with it - many gas engineers oversize boilers "To be on the safe size" it's how I ended up with a 24kW boiler for a house heat loss at the time of 6kW but with T11 rads and a flow temp of 60 to 80 deg (no weather comp so flow temps were manually adjusted) the boiler cycling was manageable.

 

My issues with that boiler started when I changed the majority of rads from T11's to T22's then dropped the flow temp to 50 and it was a bit sh1t but I lived with it.

 

(I subsequently installed 75 mm PIR under the suspended ground floor (horrible job) - increased the depth of loft insulation and resolved a few uncontrolled ventilation issues - so was then at ~4 kWh heat loss and the boiler was unable to manage the house temps without cycling excessively 

Yeah, well to sanity check I did do a quick tote up of the kw/btu of the rads fitted. I worked it out at around 27kw or so @ t50..... Now whether that is overkill for the house, or indeed whether the 22mm pipework is enough to accomodate for that kilowattage if im running the boiler higher than 70'c (despite the WB guy saying the target temp set on the boiler doesnt mean the system will go that warm.... bizarre) 

[JohnMo]

10 minutes ago, EinTopaz said:

Yeah, well to sanity check I did do a quick tote up of the kw/btu of the rads fitted. I worked it out at around 27kw or so @ t50..... Now whether that is overkill for the house, or indeed whether the 22mm pipework is enough to accomodate for that kilowattage if im running the boiler higher than 70'c (despite the WB guy saying the target temp set on the boiler doesnt mean the system will go that warm.... bizarre) 

That's ok but not really how it works. You house has a defined heat loss. The radiators are there as a heat transfer mechanism to replenish the heat lost. The boiler replenishes the radiator.

 

So you start from room heat loss, you specify the radiator for the heat loss and means flow temp through the radiator. You add up all the rooms and that gives you the boiler size.

 

But none of the above really matters, as either the boiler isn't pushing the heat outwards to the heating system, or the heating system cannot accept it due to something physically stopping it.

 

Looking at the datasheet the output of the boiler is 3.5 to 35kW. So pretty good modulation. Flow required through system is well below my heat pump and your piping is well suitable to my flow rates.

 

So unless there is a real issue with the boiler itself, you have the following possible issues

 

Filter clogged

A strainer in system is clogged (shouldn't be there if you have a filter)

Gate valve added and boiler control is just confused as it doesn't make a difference when pump speed is modulated. Or an isolator valve not fully open.

Or piping doesn't match your layout, plumber messed up with pipe routes.

 

[EinTopaz]

Yep, though I can definitely rule some of them out already.

Mag Filter - was checked by a plumber last week. Said it was clear. 
Strainer -  dont think we have any of these on the system
Gate valve - haven't seen one of these and assume if we had one it'd be above floor level for easy access etc? 
Piping - plumber confirmed last week no short cycles, i've been down myself and drew the layout myself.

That said, re: blockage, 28mm  flow/return pipes that runs from the boiler... I havent seen it myself but the plumber said those 28mm pipes each has a Y pipe that splits each into the 2 parallel runs of 22mm.... Do those Y pipes have a valve on them? Or is possible that the 28mm copper pipes could be blocked before the the Y pipe? Like maybe when they were solder welded or something? 
 

[John Carroll]

Can you recall what the boiler modulation was with your 41.1C/24.7C flow/return temperatures and was it and those two temperatures reasonably steady? If not, have you any numbers (modulation + flow/return temperatures) when it was?.

Edited Wednesday at 15:34 by John Carroll

[marshian]

2 hours ago, JohnMo said:

Looking at the datasheet the output of the boiler is 3.5 to 35kW. So pretty good modulation. Flow required through system is well below my heat pump and your piping is well suitable to my flow rates.

 

Small point not that it matters in this case but WB 35 kW boiler is 5.1 kW min according to the install manual

 

Was very surprised to see the 30 kW had a 3.0 kW min (Bet that it's impossible to achieve in reality 

[John Carroll]

Interesting, if the measured flow&return temperatures are a steady 41.1C/24.7C then the boiler is just about running at its minimum output, by calculation its running at 4.86kW, 13.5% modulation but at a utterly abysmal flowrate of 4.4LPM, on the other hand, if, by some miracle its running with a flowtemp of 75C but with the same return of 24.9C then it's output is 14.4kW, 40% modulation,  but at a huge dT of over 50C which most boilers would not continue running with, (max dT normally tolerated is ~ 30C), the WB does not monitor the return temperature.

 

 

 

Edited Wednesday at 16:45 by John Carroll

[marshian]

3 hours ago, John Carroll said:

Interesting, if the measured flow&return temperatures are a steady 41.1C/24.7C then the boiler is just about running at its minimum output, by calculation its running at 4.86kW, 13.5% modulation but at a utterly abysmal flowrate of 4.4LPM, on the other hand, if, by some miracle its running with a flowtemp of 75C but with the same return of 24.9C then it's output is 14.4kW, 40% modulation,  but at a huge dT of over 50C which most boilers would not continue running with, (max dT normally tolerated is ~ 30C), the WB does not monitor the return temperature.

 

 

 

 

Bloody love your maths based approach to everything - really makes me glad I joined up to this place

[marshian]

3 hours ago, John Carroll said:

the WB does not monitor the return temperature.

 

Neither does my Viessmann which I found really bloody odd - instead it just seems to modulate based on flow temp rise

 

If below target ramp up until target achieved

If at target ramp down till target maintained

If above target stay ramped down but keep running until a max of 4 Deg overshoot point then shut down

 

I thought it was weird not to monitor return temp but the 4 deg overshoot is a nice trick to reduce cycling and extend the run times

[John Carroll]

A modulating boiler cannot have the cut out at its target value, otherwise the burner will shut down every time the target value is reached, a non modulating boiler like my oil fired one, does cut out at its target value and has a ~ 8C hysteresis so will cut back in at  target value - 8C.

Also a modulating boiler will fire flat out (or whatever its range rated to) until the target value is reached, it will then ramp down once the target value is reached or slightly higher so its vital to have 4/5C leeway to avoid the burner tripping needlessely, I often see my daughters Vokera Vision 20S firing up and it exceeds the target value by 2/3C before it ramps down sufficiently to match the load, it will then maintain the target value within + or - a degree or two, as you say also useful when antcycling. I think modulating boilers also don't enable refiring until the measured value is at targer value - 5C or so, obviously, since most gas boilers fire up at ~ 65% output then the anti cycle period is timed so that the measured value is well less than this to avoid excessive overshoot on refiring.

[EinTopaz]

On 17/12/2025 at 15:33, John Carroll said:

Can you recall what the boiler modulation was with your 41.1C/24.7C flow/return temperatures and was it and those two temperatures reasonably steady? If not, have you any numbers (modulation + flow/return temperatures) when it was?.

I don't recall. But Generally speaking with all the rads on the boiler goes something like this (until the very recent kettling atleast). Below is when I had the target flow set to 68'c

- CH Demand
- Boiler starts at maybe 20% burner output for first few mins and gradually works its way up to 70% ish. 
- As it reaches target temp after maybe 10-15mins it starts to modulate down to around 40% after an hour or so. Then stays there. 

Those are the LCD boiler display temps ^ Not the ones im measuring on the flow pipe myself. That's always a good 20'c lower than the LCD display as above. 

[EinTopaz]

On 17/12/2025 at 16:44, John Carroll said:

Interesting, if the measured flow&return temperatures are a steady 41.1C/24.7C then the boiler is just about running at its minimum output, by calculation its running at 4.86kW, 13.5% modulation but at a utterly abysmal flowrate of 4.4LPM, on the other hand, if, by some miracle its running with a flowtemp of 75C but with the same return of 24.9C then it's output is 14.4kW, 40% modulation,  but at a huge dT of over 50C which most boilers would not continue running with, (max dT normally tolerated is ~ 30C), the WB does not monitor the return temperature.

 

 

 

I suspect it's a coincidence. I have measured at the gas meter when the boiler burner output says 40%, I did the 3minute measure at the meter and it is using the correct amount of gas at that modulation.

 

[EinTopaz]

On 17/12/2025 at 22:27, John Carroll said:

A modulating boiler cannot have the cut out at its target value, otherwise the burner will shut down every time the target value is reached, a non modulating boiler like my oil fired one, does cut out at its target value and has a ~ 8C hysteresis so will cut back in at  target value - 8C.

Also a modulating boiler will fire flat out (or whatever its range rated to) until the target value is reached, it will then ramp down once the target value is reached or slightly higher so its vital to have 4/5C leeway to avoid the burner tripping needlessely, I often see my daughters Vokera Vision 20S firing up and it exceeds the target value by 2/3C before it ramps down sufficiently to match the load, it will then maintain the target value within + or - a degree or two, as you say also useful when antcycling. I think modulating boilers also don't enable refiring until the measured value is at targer value - 5C or so, obviously, since most gas boilers fire up at ~ 65% output then the anti cycle period is timed so that the measured value is well less than this to avoid excessive overshoot on refiring.

Ours sort of does this. When we had the target temp set to 68, it never went up to 100%, max it went upto was around 70ish. But it also doesn't hit this output straight away, it starts at 20% when the CH is demanded then gradually works its way upto 70, then when hits target (on the boiler atleast!) starts modulating down. Ours does often overshoot, but it's set to only turn the burner off if it overshoots by <6'c... it does do that sometimes but less commonly. 

If I run it in sweep mode however, it exceeds 88'c and turns itself off every few mins. Which I guess is also indicative of a boiler that can't get rid of heat quick enough, or is reading its internal temperature incorrectly. 

[JohnMo]

4 minutes ago, EinTopaz said:

CH Demand
- Boiler starts at maybe 20% burner output for first few mins and gradually works its way up to 70% ish. 
- As it reaches target temp after maybe 10-15mins it starts to modulate down to around 40% after an hour or so. Then stays there. 

So really confused - what is the issue with that? Is your house warm enough ?

[EinTopaz]

On 17/12/2025 at 14:29, EinTopaz said:

That said, re: blockage, 28mm  flow/return pipes that runs from the boiler... I havent seen it myself but the plumber said those 28mm pipes each has a Y pipe that splits each into the 2 parallel runs of 22mm.... Do those Y pipes have a valve on them? Or is possible that the 28mm copper pipes could be blocked before the the Y pipe? Like maybe when they were solder welded or something?

Did anyone have thoughts about this? I haven't had time to check yet, could I be onto something here? 

[EinTopaz]

3 minutes ago, JohnMo said:

So really confused - what is the issue with that? Is your house warm enough ?

The issue is to get the radiators hotter than 52degrees, the WB technician has told me to set the boiler target temp to 80.  

When the boiler is set to target temp of 68, the flow pipe coming out of it is measuring 54ish, and all the subseqent radiators are measuring around 52ish. 

I've checked the gas meter and it is using the amount of gas that the boiler modulation would do. So my concern is two fold.

1 - the boiler is way hotter than the water coming out of it, this is costing me money to heat my house but my house isn't hot.

2 - to get my house hot i've got to set the boiler to a target temp that is now making it overheat and kettle when it gets to the new target temperature recommended by WB of 80'c... for a modulating boiler that sounds crackers to me.

Edited 18 hours ago by EinTopaz

[JohnMo]

Do your add on temp gauges have the sensor in insulation, if not you will read meaningless numbers. To get them to read close to correct, you need thermal paste, aluminium tape and covered in insulation.

[JohnMo]

1 minute ago, EinTopaz said:

house hot

Do you want a hot house? Is it getting to target room temperature?

[EinTopaz]

3 minutes ago, JohnMo said:

Do your add on temp gauges have the sensor in insulation, if not you will read meaningless numbers. To get them to read close to correct, you need thermal paste, aluminium tape and covered in insulation.

Done all that, WB brought their own too to verify, used 4 different measuring instruments now including clamps. K-type sensors, all within lagging and paste. Whole thing. The flow pipe is 20'c cooler than the boiler is saying.

 

2 minutes ago, JohnMo said:

house hot

I'd like the radiators to go to circa the value that im setting on the boiler. Like every other boiler ive ever owned does. Then I can easily control everything. 

[John Carroll]

42 minutes ago, EinTopaz said:

Done all that, WB brought their own too to verify, used 4 different measuring instruments now including clamps. K-type sensors, all within lagging and paste. Whole thing. The flow pipe is 20'c cooler than the boiler is saying.

 

I'd like the radiators to go to circa the value that im setting on the boiler. Like every other boiler ive ever owned does. Then I can easily control everything. 

 

What's your next move?. Since WB know that a huge dT exists as measured by their own instruments then they just can't wash their hands of the problem?.

Edited 17 hours ago by John Carroll