Worcester Bosch Greenstar 8000 System Boiler Issues

[EinTopaz]

7 hours ago, SimonD said:

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?

 

Hey @SimonD thanks for the reply. It does short cycle when I run it in chimney sweep mode. When it exceeds 88'c it clips out. Even when not in sweep mode, it does overshoot the target temp by 6'c sometimes, causing it to turn the burner off for 2mins. As a safety thing I think. 

 

Also in my last post above, i added a video of a new symptom it's doing. After the WB visit, it's now getting to target temp of 80 (as suggested we do that by WB guy) then constantly changing between 20/80% burner output. You can hear it loading up and loading down as many as three times per minute. Its not cycling the burner off during that time, but definitely does not look or feel correct or normal in any way. 

 

7 hours ago, SimonD said:

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.

 

There is no ABV on the system. The boiler doesn't have one internally.... and externally the fitter back then (2yrs ago now or so) said as we have a towel rad with no TRV, it's fine to use that with no ABV. 

 

 

2 hours 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?

 

 

Good question. The flow pipe at 53/54 the return tops out at around 42. Generally speaking the delta stays at 20 ish or so for a while, then after the first hour once the flow stops climbing, the return closes the delta down to 12ish, worst case sometimes as low as 9ish if I leave the boiler on for ages. 

 

Edited 8 hours ago by EinTopaz

[Nickfromwales]

I watched the video.
 

That’s basically kettling, in a more modern sense of it. Shooting up, nowhere to go, shooting back down again. 

[EinTopaz]

3 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.

 

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. 

 

Hey @Nickfromwales thanks for the reply. I put a video on Vimeo of the valves under the boiler, and the mag filter etc, those are the only valves I can see physically. 

 

 

 

2 hours ago, Nickfromwales said:

Ah, missed that bit!

 

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

 

It is overshooting yes. In normal operation, it sometimes goes as high as 6'c over target and flicks the burner off for a minute or so. The temperature rapidly drops during that minute. Then when it comes back on its a very low% burner output and creeps back up to target temp then stays there pretty stabley. But still with the 20'c differential between LCD display and flow pipe. 

 

 

2 hours ago, Nickfromwales said:

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

 

I hope you're right. Let me know if anything looks untoward in the video link above.

 

1 hour ago, Nickfromwales said:

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. 

 

Sorry if i've not given a clear picture on the situation. it is turning into a bit of a saga and apologies for that.  I will say see my other post above this one with the update from the WB guy, it is now showing a new symtpom too after he did a software update and told us to set the target tempt to 80 rather than 70. The problem still does persist with big differential between boiler and flow pipe. But its now also showing a new symptom where the boiler is getting to 80 target, then going into this weird mode for an hour or so where its constantly alternating between 20/80% burner output while the target temp stays around 80. Flow pipe is showing around 60 during this time. But this is now a new problem after the WB guy updated the software, replaced the pump and sensors. 

 

1 hour 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? 
 

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

 

As above. before it was doing this new symtpom, yes it'd get to 70 within 10-15mins and often overshoot it by upto 6c 

 

 

[SimonD]

So next question, which you may have already said, but I can't recall, is if your radiators get warm at all? And does the towel rad serving as bypass get warm too?

[EinTopaz]

2 minutes ago, SimonD said:

So next question, which you may have already said, but I can't recall, is if your radiators get warm at all? And does the towel rad serving as bypass get warm too?

They do, eventually. The type 22's get warm within maybe 20mins, the triple column designer rads get warm after maybe 40mins.   When I say warm, their temperature is consistent with the flow pipe reading, all around low 50's.... whereas the boiler ofcourse is showing much higher.

Towel rad does get hot yes. I've tried it on and off, didn't seen to make a diff. 

[JohnMo]

Are all your radiator lock shields closed too far, so little or no water flows through system?

 

Are you certain you don't have a valve closed somewhere?

[EinTopaz]

25 minutes ago, JohnMo said:

Are all your radiator lock shields closed too far, so little or no water flows through system?

 

Are you certain you don't have a valve closed somewhere?

 

I've  thought about that too. I don't know the lockshield positions off by heart but i know atleast 4 of the 15 are all the way open as they've struggled to get hot in the past.  Also when i've tried different configs of which trvs are on and off the boiler has always done this, so a blockage or a closed valve close to the source seems more likely.  That said there are no zones on this system or anything like that. Just 1 CH only circuit directly piped to the boiler. Probably a silly question but how does one make sure the valves under the boiler are open, should they be all the way open or is it potentially dangerous for a nontechnical person like me to fiddling with that stuff? 

Edit: I forgot to mention, generally speaking I have about half the radiators TRV's  set to /4 and the other half all the way open. So they're certainly not closed down. 

Edited 7 hours ago by EinTopaz

[SimonD]

32 minutes ago, EinTopaz said:

They do, eventually. The type 22's get warm within maybe 20mins, the triple column designer rads get warm after maybe 40mins.   When I say warm, their temperature is consistent with the flow pipe reading, all around low 50's.... whereas the boiler ofcourse is showing much higher.

Towel rad does get hot yes. I've tried it on and off, didn't seen to make a diff. 

 

So it's a bit slow to heat everything up. That's not necessarily something to be concerned about as it really depends on system volume etc. 

 

But this is a curious issue. I'm sure the WB engineer would have checked the flow rate volumes and was satisfied the boiler was getting the right flow rate? Your radiators are getting warm to the same temperature as the flow out of the boiler, I presume throughout the whole house. And you're seeing approximately the correct delta T between flow and return. If the flow was very restricted your radiators would unlikely get to this temperature at all - from cold 20mins is not unusual, nor is 40 for 3 column radiators, especially if they aren't baffled. When you adjust the target temperature on the boiler the resultant measure flow temperature increases by exactly the same amount? E.g. 70C give flow of 51ish, 80C gives 60Cish flow. If you reduce the target temperature, does the same thing happen?

 

One of the things I would check is to log the pattern of flow and return temperatures - i.e. do they fluctuate or remain stable over time despite the boiler ramping output up and down? And while this is happening do your radiators maintain temperature?

 

I would probably also go into the installers menu and rate the max output of the boiler - IIRC the 8000s can still do this? Maybe drop the max output down to 22kW or even less and see what happens?

 

Other than check your mag filter isn't clogged up, I'm inclined to suggest this is an issue with the boiler and perhaps in his suggestion, the WB engineer knows something we don't. The reason is that somewhere in the back of my mind from a couple of years ago was an experience I had with a WB8000 where the onboard system control did very funny things that are reminiscent of this problem here, it's just my memory is too vague, but it's something that didn't get satisfactorily resolved even after being passed to WB technical and warranty (not my original installation mind you, just a problem I was asked to look at).

[marshian]

A few years ago when I was trying to make a glow worm boiler with 24kW rating and 10 kW min heat a house with a 4 kWh heat loss I had an issue with the boiler hanging or faulting out on warm up - the boiler did not like a DT at the boiler greater than 20 Deg C.

 

If it faulted out it required a manual reset when I got up which also meant a cold house - I wasn't popular when this happened.

 

The situation was always worse the colder the outside temps got - it wasn't unusual for the rad/room temps overnight to drop below 16 deg C when we were on scheduled heating slots (one of the reasons I wanted to go 24/7 with weather comp)

 

Boiler flow temp was set to 55 deg C so on start up the flow would quickly get to 55 but with 130 litres of water in the rad circuit the initial return would be 16 Deg C (55 -16 = 39 Deg C delta so you can see why the boiler might not be happy)

 

If it was just over 20 deg C it would modulate down to 10kW min until the DT at the boiler dropped below 20 Deg C at which point it would ramp up to max driving the DT to greater than 20 Deg C and at this point it would hang again dropping down to min 10 kW and then wait while the DT dropped below 20 again where it would rinse and repeat until all the rad were feeding back water lest than 20 deg below the boiler set point.

 

By this point the boiler would then short cycle as the circuit couldn't cope with 10 kWh being fired into it to replace a loss of 4 kWh

 

I did a number of things to limit the impact

 

Range rated the boiler to 10 kW (unfortunately the range rating is ignored on start up so once the house was warm it frequently short cycled)

 

Set a towel rail rad to be fully open on the lockshield - it was in a toilet (so small room and had a TRV so it provided a useful warming of the overall return until the TRV shut it down after 30 mins) by which point the overall circuit was at a temp level that meant the DT at the boiler was less than 20 Deg C

 

Reason for the Glow worm story is I wonder if this differential is driving the modulation behaviour 

 

I'd be inclined to actually go the other way - drop the target flow temp and run the heating period for longer and if you can range rate the boiler down I'd do that for sure especially if you know your house heat loss??

 

PS how is the HW (Hot Water) heated if not by gas boiler?

 

 

[Nickfromwales]

21 minutes ago, marshian said:

I'd be inclined to actually go the other way - drop the target flow temp and run the heating period for longer and if you can range rate the boiler down

Yea, same here, given the new information about rads slowly getting warm. 
 

Still stinks of poor circulation.

 

@EinTopaz, has this ever run correctly, or is this something recent / new? 
 

Have you stripped and cleaned the Maga-clean?

 

 

edit: the under-boiler valves are correct and fully open.

[John Carroll]

IMHO, there is only one logical explaination for the huge dT between the target/flow displayed temperature and the measured flow pipe temperature (and confirmed by the attending WB engineer) and that is, the displayed temperature is not tbeing taken from the flow pipe sensor but probably from the boiler HEX. It doesn't matter what kind of throttling or flowrates, as long as the boiler is firing continuously then because there is probably well less than 0.5L of water between where the boiler flow sensor is attached and the manual measuring point which means that even with a flowrate as low as 5LPM should only "take" this vol of water 5 or 6 minutes to reach the manual measuring point once it does then there should be little or no difference since the flowtemperature is then very constant for relatively long periods.

[SimonD]

2 hours ago, marshian said:

 

@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

 

 

21 minutes ago, Nickfromwales said:

Still stinks of poor circulation.

 

In my thinking, the flow is there. The flow right outside the boiler would equalise with the hex/internal temp shown at 80C, if it was indeed at that temp. Like @marshian says, this would easily happen within 10min of running. So the question to my mind is why don't we seen that? The laws of thermodynamics suggests that it isn't actually there. The WB engineer would have plugged the WB diagnostics in which would have confirmed sensor readings etc. including water temp., flow rate, surely. 

 

 

[SimonD]

2 minutes ago, John Carroll said:

IMHO, there is only one logical explaination for the huge dT between the target/flow displayed temperature and the measured flow pipe temperature (and confirmed by the attending WB engineer) and that is, the displayed temperature is not tbeing taken from the flow pipe sensor but probably from the boiler HEX. It doesn't matter what kind of throttling or flowrates, as long as the boiler is firing continuously then because there is probably well less than 0.5L of water between where the boiler flow sensor is attached and the manual measuring point which means that even with a flowrate as low as 5LPM should only "take" this vol of water 5 or 6 minutes to reach the manual measuring point once it does then there should be little or no difference since the flowtemperature is then very constant for relatively long periods.

 

You just beat me to it...

[marshian]

1 hour ago, SimonD said:

 

 

In my thinking, the flow is there. The flow right outside the boiler would equalise with the hex/internal temp shown at 80C, if it was indeed at that temp. Like @marshian says, this would easily happen within 10min of running. So the question to my mind is why don't we seen that? The laws of thermodynamics suggests that it isn't actually there. The WB engineer would have plugged the WB diagnostics in which would have confirmed sensor readings etc. including water temp., flow rate, surely. 

 

 

 

My favourite pastime is reading "Boiler Installation" manuals

 

Item 3 - Flow temp sensor at the heating block..............

 

 

Seems the boiler can be range rated although I'm not sure how low it can be rated - looks like it can't be range rated to min of 5.1 kW but it can be throttled back to around 15 kWh as a max

 

Settings under "heating" menu it seems

 

Also seems like the pump curves can be adjusted too but I think I'd start with capping it's enthusiasm to ramp up to 90%

 

 

 

[EinTopaz]

2 hours ago, SimonD said:

And you're seeing approximately the correct delta T between flow and return

For a while that's true, though if I leave the boiler on for maybe a few hrs, the delta drops to 11'c sometimes even single figures. But either way this made me think its unlikely that the system is too restricted. Otherwise that delta would be enormous, right? 

 

2 hours ago, SimonD said:

When you adjust the target temperature on the boiler the resultant measure flow temperature increases by exactly the same amount? E.g. 70C give flow of 51ish, 80C gives 60Cish flow. If you reduce the target temperature, does the same thing happen?

That's a good question, haven't tried that. But you are correct that the differentials so far have been fairly relative. I'll need to try it when im back home im away for a couple days now. argh.
 

 

2 hours ago, SimonD said:

would probably also go into the installers menu and rate the max output of the boiler - IIRC the 8000s can still do this? Maybe drop the max output down to 22kW or even less and see what happens?

The max can be reduced down yeah. To as low as 50%....though when It's firing up at the moment it is going as high as 90% output. So won't this just make everything take ages to warm up?  



 

[EinTopaz]

2 hours ago, marshian said:

PS how is the HW (Hot Water) heated if not by gas boiler?

 

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. 

[EinTopaz]

2 hours ago, Nickfromwales said:

Yea, same here, given the new information about rads slowly getting warm. 
 

Still stinks of poor circulation.

 

@EinTopaz, has this ever run correctly, or is this something recent / new? 
 

Have you stripped and cleaned the Maga-clean?

 

 

edit: the under-boiler valves are correct and fully open.

Mag filter was stripped and cleaned last week, when the boiler was serviced.

From memory it's never run correctly no. It's always seemed to cost a lot this boiler but all the rads on it's system never seemed to get as hot as what the LCD was displaying, until i recently starting actually measuring the flow temps... it confirmed what it 'felt' like living with it, if that makes sense?

The boiler only went in 2 years ago when we did the extension, all rads new, all pipework new, boiler new. It's a problem that only presents itself in the colder months as I don't use CH at all until maybe mid-october-ish! 

Edited 4 hours ago by EinTopaz

[EinTopaz]

22 minutes ago, marshian said:

Seems the boiler can be range rated although I'm not sure how low it can be rated - looks like it can't be range rated to min of 5.1 kW but it can be throttled back to around 15 kWh as a max

 

Settings under "heating" menu it seems

 

Also seems like the pump curves can be adjusted too but I think I'd start with capping it's enthusiasm to ramp up to 90%

Yes, on the pump side, it's set to setting 0 (the mode that unlocks the pump mapping) this also puts it relative to the burner rather than constant Mbar, i've tried all the settings with not much difference. Currently its on setting 0 relative to burner mode, but with the min % set to 80 and the max set to 100. WB Advised that was a good setup to ensure the pump is not being lazy. As their initial suspicion was the pump wasn't getting rid of the hot water fast enough hence the differential between boiler and flow pipe. But it hasn't fixed the issue. 

I will try loading it down when im back home and see if that makes any difference. 

[marshian]

3 minutes ago, EinTopaz said:

The max can be reduced down yeah. To as low as 50%....though when It's firing up at the moment it is going as high as 90% output. So won't this just make everything take ages to warm up?  

 

That's the thing about boilers they don't always respond in the way you think they will - range rating my old glow worm boiler to 10 kW consistently reduced the warm up time by about 20 mins - yet it was a 24kW boiler

 

Right now it's modulating up and down like a whores drawers - get it in a more steady state and it might actually improve the house warm up and reduce consumption

[SimonD]

2 hours ago, marshian said:

Seems the boiler can be range rated although I'm not sure how low it can be rated - looks like it can't be range rated to min of 5.1 kW but it can be throttled back to around 15 kWh as a max

 

4 hours ago, SimonD said:

would probably also go into the installers menu and rate the max output of the boiler - IIRC the 8000s can still do this?

😉

[SimonD]

1 hour ago, EinTopaz said:

The max can be reduced down yeah. To as low as 50%....though when It's firing up at the moment it is going as high as 90% output. So won't this just make everything take ages to warm up?  

 

Not necessarily. If the boiler puts a slug of heat into the system and it senses it gets too hot in the heat exchanger because heat isn't dissipated quickly enough into the system, then reducing output can resolve this and enhance the heat output. It's behaviour I've seen when boilers can't modulate down enough for the heat load of the property so it fires, tries to modulate, then shuts of and not enough heat gets delivered out to the system. But in these circumstances you often get poor distribution of heat to the radiators, so they fail to even reach set flow temp due to poor circulation.

 

I'm just suggesting this as a process of elimination.

 

Maybe there's a slight problem with flow rates, but then as you say you'd end up with either a very wide DT or a very narrow one. So my suspicion is more leaning more towards the flow temp not being the same as the temp displayed.

[marshian]

20 minutes ago, SimonD said:

 

😉

 

Fair point - but as I said I quite like reading boiler install manuals and it never hurts to check

[SimonD]

Just now, marshian said:

 

Fair point - but as I said I quite like reading boiler install manuals and it never hurts to check

 Lol, yes indeed. With you reading the boiler manuals and @JohnMo reading heat pump manuals, we're pretty much covered on the BH library of knowledge side. I only really read the manuals when I'm on training or on site! 

[marshian]

1 minute ago, SimonD said:

 Lol, yes indeed. With you reading the boiler manuals and @JohnMo reading heat pump manuals, we're pretty much covered on the BH library of knowledge side. I only really read the manuals when I'm on training or on site! 

 

It really is amazing how much detail is in the install manuals if you dig into the detail - first manual I ever read was for my glow worm - I only read it because it was driving me nuts and I thought I could dial out it's problems - I got to understand the valiant/glow worm anti cycle settings which was a complete revelation when trying to manage a boiler with a minimum output 2.5 times the heat loss of the house.

 

Despite the boiler being a cheap end of the market the manual at least laid out the critical basics like min flow rate required which is often absent in a lot of manuals I read 

[JohnMo]

21 hours ago, EinTopaz said:

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

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.