Worcester Bosch Greenstar 8000 System Boiler Issues

[John Carroll]

46 minutes ago, EinTopaz said:

I’ll give that a try. But I know already from testing that the 90% range rated test I tried yesterday took 5 mins longer

to get To target. Still overshot but only by 3.6c this time. But then weirdly never did make its way down to 60. It settled at 62.5 with a modulated output of 54%
 

I assume the higher modulated % was because it was a colder day and the pipes are in the subfloor with the ventilated air bricks etc. so assume it was just working a little harder to keep them at temp. Or above! 
 

EDT ^ this was all on pump setting 5, which was essentially locking it at 87% pump speed 

If you are happy enough with that overshoot then why not leave well enough alone, you probably may not have even noticed it if you hadn't had the other big problem.

 

But no way  IMO should the there be a offset of 2.5C between the target and flow temperature once steady state conditions are achieved, these controllers are/should be PID, where the "I", (integral) will achieve zero deviation between the target and flow temperatures especially under these benign conditions, you might see + or - 0.1/0.2C deviation now and then, the colder day/pipes etc has nothing to do with the control, the controller is just looking at the process variable which is the flow temperature in this case.

 

Edited Thursday at 15:20 by John Carroll

[EinTopaz]

9 hours ago, John Carroll said:

If you are happy enough with that overshoot then why not leave well enough alone, you probably may not have even noticed it if you hadn't had the other big problem.

 

But no way  IMO should the there be a offset of 2.5C between the target and flow temperature once steady state conditions are achieved, these controllers are/should be PID, where the "I", (integral) will achieve zero deviation between the target and flow temperatures especially under these benign conditions, you might see + or - 0.1/0.2C deviation now and then, the colder day/pipes etc has nothing to do with the control, the controller is just looking at the process variable which is the flow temperature in this case.

It’s a fair point, it’s not the most egregious thing now everything is getting warm enough. To be honest the overshooting in and of itself isn’t bugging me, I’m fine with everything getting a littler hotter then I’ve set… though what is bugging me is more the fact it’s staying at 100% of its rated output long after it’s reached target. It’s working way harder than I’ve asked it to and the bottom line is that’s costing me unnecessarily everytime I run it.

 

For every hour it’s running at 35kw it costs around £2. So that will add up a lot through the year if it’s not modulating down when it could/should.

 

 

as a slight aside. Do you know the best insulation method for pipework that runs in a ventilated, unheated subfloor. The pipes are already lagged but wondering what else I can do to keep them as optimally warm as possible. 

[John Carroll]

8 hours ago, EinTopaz said:

It’s a fair point, it’s not the most egregious thing now everything is getting warm enough. To be honest the overshooting in and of itself isn’t bugging me, I’m fine with everything getting a littler hotter then I’ve set… though what is bugging me is more the fact it’s staying at 100% of its rated output long after it’s reached target. It’s working way harder than I’ve asked it to and the bottom line is that’s costing me unnecessarily everytime I run it.

 

For every hour it’s running at 35kw it costs around £2. So that will add up a lot through the year if it’s not modulating down when it could/should.

 

 

as a slight aside. Do you know the best insulation method for pipework that runs in a ventilated, unheated subfloor. The pipes are already lagged but wondering what else I can do to keep them as optimally warm as possible. 

 

Yes, this flat out firing is very hard to explain thermodynamically.

For example, with a flowrate of your now 87%, 18.71LPM,  then the rads will output 16.33kW(45% boiler output)  under steady state conditions with flow/return/dT temperatures of 60C/47.57C/12.43C, assuming 20C room temperature, if the boiler firing is then increased to 36kW then the dT at that flowrate of 18.71LPM has to be, 36*860/60/18.71, 27.58C, so the flowtemp should/must then increase almost instantly to, 47.57+27.58, 75.15C, there will obviously be a delay in the sensors reading this correctly as they are surface mounted but surely 4 or 5 minutes should be well more than adequate time for this? also the boiler fires at 100% output for well beyond this period so a mystery as to why you are not seeing anything remotely approaching 75C., the sky's the limit then up to at least 100C, see below.

The rad output theoretically/thermodynamically can output 36 kW but would require flow/return temps of ~ 100C/73C, (even assuming a 25C room temp), but I just can't see this happening.

 

Don't know anything really re insulation, I just use the cheap as chips lengths.

Edited 22 hours ago by John Carroll

[John Carroll]

Had a look at your data and did some calcs so things adding up much better there, you can see the measured flowtemps does lag the calculated ones but not hugely so you shouldn't be burning huge quantities of extra gas even if the flowtemp does exceed the target temp by 2.5c or so under steady state conditions, the boiler output will be outputting  ~ 1.0kW extra so say 15kWh over a 15 hour heating day or pro rata, at 6p/kWh?? = 90p/day, not huge?.

 

On for 5minutes
Flow 30.6
return 15.6   dT 15.00C
Burner 62% 22.32kW
Pump 93%   20.00LPM dT 16.00C Flowtemp 31.6C

On 15 minutes
Flow 50.9
return 29.0    dT 21.90C
Burner 100% 36.00kW
Pump   100%   21.50LPM dT 24.00C Flowtemp 53.00C

 

On 25 minutes (target temp reached)
Flow 60.1
return 39.0    dT 21.10C
Burner 100% 36.00kW
Pump   100%   21.50LPM dT 24.00C Flowtemp 63.00C

 

On 30 minutes
Flow 63.2
return 42.8    dT 20.4C
Burner 95%  34.2.00kW
Pump   100%   21.50LPM dT 22.80C Flowtemp 65.6C

 

On 35 minutes
Flow 64.8
return 46.4    dT 18.4C
Burner 85%  23.40kW
Pump   100%   21.50LPM dT 20.4C Flowtemp 66.8C

 

On 45 minutes
Flow 64.8
return 50.2    dT 14.6C
Burner 65%  23.40kW
Pump   100%   21.50LPM dT 15.60C Flowtemp 65.8C

 

 

On 55 minutes
Flow 61.9
return 50.7    dT 11.2C
Burner 49%  17.64kW
Pump   100%   21.50LPM dT 11.76C Flowtemp 62.46C

 

big time gap now as I stopped measuring till it settled to actual target temp of 60'c

On 1h35mins
Flow 59.9
return 47.8    dT 12.1C
Burner 49%  17.64kW
Pump   100%   21.50LPM dT 11.76C Flowtemp 59.56C

 

[marshian]

16 hours ago, EinTopaz said:

It’s a fair point, it’s not the most egregious thing now everything is getting warm enough. To be honest the overshooting in and of itself isn’t bugging me, I’m fine with everything getting a littler hotter then I’ve set… though what is bugging me is more the fact it’s staying at 100% of its rated output long after it’s reached target. It’s working way harder than I’ve asked it to and the bottom line is that’s costing me unnecessarily everytime I run it.

 

For every hour it’s running at 35kw it costs around £2. So that will add up a lot through the year if it’s not modulating down when it could/should.

 

From my non technical viewpoint and I'm going to phase it as such

 

For the burner to be running at 100% then the water flowing thro the boiler must be taking that heat input - which means the return temp water is cool enough to enable the boiler to "have at it" 

 

Only when the heat capacity of the return flow is less than the full power of the boiler will the boiler modulate down (OK it may rise above target set point before it does that but that will be in the programming of the boiler MBU)

 

Basically you have a circuit of water (Pipes and rads) and it's taking a fair old time before the return water starts to warm up enough to cause the boiler to modulate down.

[JohnMo]

4 hours ago, marshian said:

From my non technical viewpoint and I'm going to phase it

 

20 hours ago, EinTopaz said:

though what is bugging me is more the fact it’s staying at 100% of its rated output long after it’s reached target. It’s working way harder than I’ve asked it to and the bottom line is that’s costing me unnecessarily everytime I run it.

You have what you have, you seem to want run an on off system and have loads of modulation immediately. It isn't going to happen, the boiler has plenty of work to do playing catch up. Then you switch it off again.

 

20 hours ago, EinTopaz said:

Do you know the best insulation method for pipework that runs in a ventilated, unheated subfloor.

Thick stuff - 25mm wall thickness, cover in aluminium tape to stop mice eating it.

[marshian]

2 hours ago, JohnMo said:

You have what you have, you seem to want run an on off system and have loads of modulation immediately. It isn't going to happen, the boiler has plenty of work to do playing catch up. Then you switch it off again.

Perfect summary

[John Carroll]

In non technical terms I would say its behaviour is bordering on bizarre.

It takes 85 minutes to fall from 64.8c to its targettemp of 60C, despite a very steady ~ 17kW demand.                                                                                                                                               

 

As per Tests, all temps are Flow, (target temp is 60C)

zero hour 60.1C (after 25 minute warmup from 15C)

+5mins    63.2C

+10mins   64.8C

+20mins   64.8C

+30mins   61.9C

+95mins   60.0C

 

 

Edited 6 hours ago by John Carroll