Worcester Bosch Greenstar 8000 System Boiler Issues

[EinTopaz]

Hi all,

This one is turning into a bit of a saga... but, I've been having a hate / hate relationship with a boiler we had fitted a couple years back. Now it's getting colder again and we've started to use this boiler again, it still has issues. I'm hoping to get some help diagnosing what the problem/s are with it and perhaps some guidance on good next steps to take and focus my efforts.


Boiler / System Info

This is a 36KW system boiler and CH only. It powers 15 radiators, 11 x type 22 and 4 x triple column which sum total around 28kw total @ T50. It's 28mm pipes for the first couple meters, then splits to two parallel runs of 22mm (each around12Meters or so) then down to 15mm for the last meter or two to each rad (i've added a topdown image of the system design below). The pressure is 1.3Bar when not on and raises to 1.8Bar when on... Also worth noting; the boiler, pipework and all radiators were fitted at the same time around 2yrs ago, all new at the time.


Operational Issues
 

• The most notable issue is that is the boiler seems to be reaching (and actually slightly overshooting) target temp of 70'c within 10minutes of it firing up from cold. This is displayed via the boilers LCD display . Yet despite this, the flow outlet pipe and all radiators take longer to get hot to the touch, and the flow pipe and radiators never seem to get higher than around 53-54degrees, which ofcourse is way cooler than what the boiler LCD display is showing. So there seems to be a fairly large temperature differential between what the boiler thinks the flow temp is and the heat of the stuff outside the boiler. Almost as if the boiler can't get the heat away from itself...? I've added a photo of this too showing the LCD display and thermometers I put on the flow/return pipe.

 

Questions based on the above
 

• Would upping the 12 meter 22mm runs to 28mm help in my case?
• Could there be a blockage in my boiler, and would this show an error code if so? (it's clear of error codes)
• Could there be a blockage in my system? and if so what's the best way of finding it?
• If I open all the lockshields on the rads and the problem persists, would that rule out bad balancing?

System design image
(thick lines 28mm, medium lines 22mm, thin lines 15mm, black square outline is the 4 x triple column rads)

 

Lcd readout image (left thermometer is flow, right is return)

 

 

 

As well as all the above, it'd be great to know if anyone else has this boiler so we can compare notes.

 

Lastly, i've got a plumber and someone from WB coming out this week to try and rule things out. But it'd be good to arm them with as much knowledge as I can so any and all suggestions and help here are massively appreciated.

Thanks

Ged

 

 

Edited 23 hours ago by EinTopaz

[SimonD]

I'm guessing there is an auto bypass fitted somewhere on the system near the boiler? IIRC the minimum length of pipe for this according to the MIs is about 3m?

 

You're pretty much overpiped, so on a gas boiler upping the 22mm pipes to 28mm would make no difference.

 

As you've surmised, you have a flow problem, but it may not be low flow, but flow returning back to the boiler through a portion of your pipework acting as an uncontrolled bypass. Last time I had this was with 2 37kW Worcesters piped into a low loss header.

[SimonD]

I've just re-read your op where you say the the flow pipe out of the boiler doesn't rise in temperature. Again, you've surmised correctly. This can often happen when the heat load in the circuit is too low for the output of the boiler - the boiler fires up and before it gets to modulate down the water in the heat exchanger gets too hot. Are you experiencing short cycling? As it does the boiler get really hot, like 80C plus and then turn itself off for a while before re-starting?

[Nickfromwales]

@EinTopaz

 

Sounds like a restriction. Can you post some pics of the valves under the boiler, the ones that actually connect to it.

 

This stinks of one of the flow or return valves being only partially open.

 

Next check is to remove the head off the zone valve and move the spindle manually. Do this with the boiler at 70° and then see if the temp shoots up on the flow pipe stat. 

 

Pipework / sizing is A OK. 👍. No issues there. 

[Nickfromwales]

Does the hot water cylinder get the flow temp displayed going to the cylinder when calling for hot water? 

[marshian]

12 hours ago, EinTopaz said:

Boiler / System Info

This is a 36KW system boiler and CH only. It powers 15 radiators, 11 x type 22 and 4 x triple column which sum total around 28kw total @ T50. It's 28mm pipes for the first couple meters, then splits to two parallel runs of 22mm (each around12Meters or so) then down to 15mm for the last meter or two to each rad (i've added a topdown image of the system design below). The pressure is 1.3Bar when not on and raises to 1.8Bar when on... Also worth noting; the boiler, pipework and all radiators were fitted at the same time around 2yrs ago, all new at the time.

 

If this is CH Only then HW is covered by some other system?

 

I'm guessing you have a fairly large house with a high heat loss if you are running 70 deg flow temp with T22 rads and triple columns

 

I've never worked out my T50 rating for my rads as I sized them for a 40 deg flow temp (I'm actually running lower than that) but having done a quick check it seems my 13 rads work out to be 22 kW

 

Do you have a Automatic bypass anywhere on the system - if you do what pressure is it set too?

[Nickfromwales]

11 minutes ago, marshian said:

If this is CH Only then HW is covered by some other system

Ah, missed that bit!

 

12 minutes ago, marshian said:

I'm guessing you have a fairly large house with a high heat loss if you are running 70 deg flow temp with T22 rads and triple columns

I think more the point that it’s getting to 70° as it’s unable to dissipate heat and therefore is overshooting? 

[marshian]

15 minutes ago, Nickfromwales said:

I think more the point that it’s getting to 70° as it’s unable to dissipate heat and therefore is overshooting? 

 

I didn't read it that way either - so is the boiler short cycling?

 

@EinTopaz How has the system been balanced - have you screwed the lock-shields down to get a DT at the rads of 20 Deg C?

 

It's very easy to concentrate on the DT at the rads and forget that the boiler will have a min flow rate required to be happy!!

[SimonD]

13 minutes ago, Nickfromwales said:

I think more the point that it’s getting to 70° as it’s unable to dissipate heat and therefore is overshooting? 

 

Op says target temp of 70C. With the flow temp on pipes ony reaching 53-54, I wonder what the return temp is on the pipes as that would tell up a lot. @EinTopaz?

 

[Nickfromwales]

15 minutes ago, SimonD said:

 

Op says target temp of 70C. With the flow temp on pipes ony reaching 53-54, I wonder what the return temp is on the pipes as that would tell up a lot. @EinTopaz?

 

Zoom in and see the two Amazon stats on the pipes. 
 

[Nickfromwales]

I reckon flow is choked. Usually a partially open valve on the jig, or a partially open gate (pump) valve. 

[SimonD]

41 minutes ago, Nickfromwales said:

I reckon flow is choked. 

 

Yes, possibly, but it's intriguing that the Delta t across flow and return is about 16C.

[marshian]

53 minutes ago, Nickfromwales said:

Zoom in and see the two Amazon stats on the pipes. 
 

 

They aren't normally too bad for accuracy as long as they are tightly encased in some foam lagging - if just cable tied to pipes they are pretty poor temp indicators

[Nickfromwales]

1 minute ago, SimonD said:

 

Yes, possibly, but it's intriguing that the Delta t across flow and return is about 16C.

Or that there’s just heat being forced out of the boiler by expansion, meaning there’s no heat any further downstream.

 

A full survey required and loads of info missing. 

[marshian]

5 minutes ago, SimonD said:

 

Yes, possibly, but it's intriguing that the Delta t across flow and return is about 16C.

 

If the boiler is producing 70 deg heat (according to the display) then unless the display is miles out or it's indicating a set point rather than an actual temp out - I would expect a similar temp at the flow pipe - so I'd wonder where the heat is going!!!

[Nickfromwales]

3 minutes ago, SimonD said:

 

Yes, possibly, but it's intriguing that the Delta t across flow and return is about 16C.

The boiler would shut down the burner way in advance of reaching 70° though, if there’s any discernible flow out to a circuit getting rid of heat? 
 

This looks like it ‘kettles’ quite soon after the burner ignites and there’s an unavoidable overshoot. 

[Nickfromwales]

1 minute ago, marshian said:

 

If the boiler is producing 70 deg heat (according to the display) then unless the display is miles out or it's indicating a set point rather than an actual temp out - I would expect a similar temp at the flow pipe - so I'd wonder where the heat is going!!!

It’s not going anywhere. 
 

Light, overheat, shut off, cool down a bit, repeat. 

[Nickfromwales]

2 hours ago, Nickfromwales said:

 

@EinTopaz

 

Sounds like a restriction. Can you post some pics of the valves under the boiler, the ones that actually connect to it.

 

👆👆👆

[marshian]

12 minutes ago, Nickfromwales said:

It’s not going anywhere. 
 

Light, overheat, shut off, cool down a bit, repeat. 

 

10 mins to overheat that's not exactly a short cycle scenario IMO..............

 

My old glow worm 24kW with a min of 10kW could heat up, overshoot and shut down in less than 4 mins if the house was up to temp and so not a lot of heat transfer going on from the rads.

 

The water leaving the boiler would be same temp as the display said it was!!!

 

Here it seems to be a big disconnect that doesn't make sense

[Nickfromwales]

1 minute ago, marshian said:

10 mins to overheat that's not exactly a short cycle scenario IMO..............

10 mins of doing what I suggested its doing, means it’s short cycling excessively, repeatedly, over the stated 10 mins that we’ve been shared info on. 

[marshian]

2 minutes ago, Nickfromwales said:

10 mins of doing what I suggested its doing, means it’s short cycling excessively, repeatedly, over the stated 10 mins that we’ve been shared info on. 

 

@EinTopaz probably needs to explain this better - right now it reads that the boiler is running for 10 mins and reaching it's target before overshooting

 

I fail to understand how a flow pipe leaving the boiler can be 30 deg lower than the water temp reached inside the boiler - it's likely to be 30-40 cm of pipe max so unless it's going via a blast freezer there is heat going somewhere

 

14 hours ago, EinTopaz said:

The most notable issue is that is the boiler seems to be reaching (and actually slightly overshooting) target temp of 70'c within 10minutes of it firing up from cold.

[Nickfromwales]

7 minutes ago, marshian said:

I fail to understand how a flow pipe leaving the boiler can be 30 deg lower than the water temp reached inside the boiler - it's likely to be 30-40 cm of pipe max so unless it's going via a blast freezer there is heat going somewhere

I say it’s creeping out, vs going somewhere. These are my assumptions from the info I’m seeing.

 

First place I’d look would be isolation valves.

 

Then I’d switch off every rad.

 

Then I’d fully open both valves of the rad nearest the boiler. 
 

Then do the observations over again to see what is actually going on here.

 

Another option is a duff pump. 
 

Another, which I’ve had with WB before, is the pump relay on the PCB giving sporadic intermittent power out, like a child flicking a light switch to annoy you, which took ages to figure out.
 

Kept the pump active just enough to have the boiler fooled, so it didn’t lock out. Temp was bobbing up and down like a yo-yo. 

[SimonD]

37 minutes ago, Nickfromwales said:

The boiler would shut down the burner way in advance of reaching 70° though, if there’s any discernible flow out to a circuit getting rid of heat? 

 Nah, I've had these 8000s go over 100C when there's insufficient flow and call for heat. The bangs are really quite scary!

 

4 minutes ago, Nickfromwales said:

I say it’s creeping out, vs going somewhere. These are my assumptions from the info I’m seeing.

 

Yes, and it may be that if the boiler is able to modulate down in time, that's why we haven't heard to op tell us it sounds like the boiler is blowing up and switching off.  

[marshian]

14 minutes ago, Nickfromwales said:

Another option is a duff pump. 

 

Good call on that as a potential reason but the system and boiler are relatively new I wouldn't expect it to be clogged up like mine was once

 

 

 

[EinTopaz]

Hey all, thanks for all the replies. I didn't expect it to be this popular. I'll start replying now in order. Massively appreciate the responses. I'll start working through them now.

 

 

Before I do though, quick update, I  had a WB engineer here today to swap the pump and sensors to rule those out. He has, and the problem has persisted. He also said something which I completely disagree with albeit me not being technical. He said there should be a different between LCD display target temp, and actual system temp. I said by as much as 20degrees?! he said the boiler target temp isnt you setting the temp for the system its just the internal temperature of the Hex. he advised we simply turn the target flow temp on the boiler up to 80. Which also sounds bizarre to me, but giving it a try. The same problem persists, 20odd degree differential between boiler temp and temp on flow pipe. (BTW i've measured the flow pipe now with 4 different instruments, including a rental expensive one, and the one Wb brought with them) they all say the same number. and that number is consistent with all the flow pipes on each of the rads. They're all within a few degrees of each other. But the boiler is the one that's 20 or so above everything else. 

Anyway, the stuff the WB guy did was, software update, factory settings,  replacement pump and sensors, checked the hex and said it was clear, and also set the target temp to 80. 

Same problem persists, and a new problem has arisen now too which i'll put a video below of. After half hour the boiler gets to target temp of 80, then from half an hour to an hour afterwards it then constantly alternates between 20/80% burner output. Gradually with that window getting smaller 30/70 - 35/65 etc, then eventually settles on 45% stabley. Im not sure if this is the massive flow temp now, or the factory reset, or the new parts. But it definitely is a new thing that wasn't happening before. Would this help suggest to any of you what the problem is? Its not cycling the burner off per sae, but almost as if it cant decide what to do with the heat etc.

video below. 
 

 

Edited 8 hours ago by EinTopaz