Worcester Bosch Greenstar 8000 System Boiler Issues

[John Carroll]

2 minutes ago, John Carroll said:

Double Post

 

Edited 6 hours ago by John Carroll

[MrPotts]

With the radiators getting to 52c isn’t that hot enough? My radiators normally get up to around 50c (with an 65c flow temp set but never reached) and are very hot to touch so I wouldn’t want them any hotter.

 

 

[JohnMo]

So, boiler will have a target flow temperature with overshoot hysterisis. But, it also has a control set point for dT. The circulation pump and boiler modulation will adjust to maintain dT then add more heat as dT closes down. I suspect the heating system return temperature is holding everything back, but this must be at or close to target otherwise the boiler output would not modulate down.

 

So boiler is doing what it should it has a decent run time, it's modulating down, it possibly massive for the duty. Other than a desire to have radiators you burn yourself on, I don't see a real issue.

[John Carroll]

1 hour ago, JohnMo said:

So, boiler will have a target flow temperature with overshoot hysterisis. But, it also has a control set point for dT. The circulation pump and boiler modulation will adjust to maintain dT then add more heat as dT closes down. I suspect the heating system return temperature is holding everything back, but this must be at or close to target otherwise the boiler output would not modulate down.

 

So boiler is doing what it should it has a decent run time, it's modulating down, it possibly massive for the duty. Other than a desire to have radiators you burn yourself on, I don't see a real issue.

The way I interpret how any boiler controls its output is that it bases it on the target flow temperature ONLY, the flow temperature is obviously influenced by the return temperature which in turn is determined (for a steady heating demand) by the flowrate and the room temperature.

You can see below the effect that the flowrate has, the design flowrate of a 36kW boiler is 25.8LPM/1547LPH which I doubt very much that this system is running at. We do know that the steady state output is 14.4kW (40% boiler output) so the numbers below show exactly the flowrates required at different flow temperatures to achieve this 14.4kw (40% output)

54C/46C/dT8C. 25.8LPM/1547LPH

57C/43C/dT14C. 14.67LPM/889LPH

60C/40C dT20C. 10.29LPM/617LPH

65C/35C/dT35C.  6.87LPM/412LPH

Its very unlikely IMO that the flowrate is > than 25.8LPM so 54C would seem to be the very minimum flowtemperature required to achieve 40% boiler output, its alos very unlikely that the flowate is as low as 6.9LPM so a 65C flowtemperature is unlikely at this end.

I would "guess" that the flowrate is ~ 14.67LPM/889LPH so the steady state flowtemperature is ~ 57C, return,43C, dT14C.

 

Edited 5 hours ago by John Carroll

[JohnMo]

34 minutes ago, John Carroll said:

The way I interpret how any boiler controls its output is that it bases it on the target flow temperature ONLY

But it has a modulating circulation pump which will change output based on dT, so not sure that can be fully true.

 

36 minutes ago, John Carroll said:

40% boiler output) so the numbers below show exactly the flowrates required at different flow temperatures to achieve this 14.4kw

If 14.4kW isn't enough the return temp will always be low. So if the boiler has been range rated to 40% it will constantly chase it's tail.

 

So @EinTopaz what is the boiler range rated too?

[SimonD]

2 hours ago, EinTopaz said:

The flow pipe is 20'c cooler than the boiler is saying.

 

1 hour ago, John Carroll said:

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?.

 

Any engineer worth their salt would open up the boiler and measure the actual temperature of the heat exchanger particularly where the flow exits the chamber and even better next to the sensors the boiler is using, not just the flow and returns outside the boiler. I don't think they measured a huge DT? but suggested increasing the set temp to increase the flow temp. Now they're by accounts coming back with a new pcb, which seems sensible.

 

There a just too many unknowns right now.

 

What controls is the boiler wired up to? Are they WB modulating ones or third party?

 

 

 

 

[John Carroll]

3 hours ago, SimonD said:

 

 

Any engineer worth their salt would open up the boiler and measure the actual temperature of the heat exchanger particularly where the flow exits the chamber and even better next to the sensors the boiler is using, not just the flow and returns outside the boiler. I don't think they measured a huge DT? but suggested increasing the set temp to increase the flow temp. Now they're by accounts coming back with a new pcb, which seems sensible.

 

There a just too many unknowns right now.

 

What controls is the boiler wired up to? Are they WB modulating ones or third party?

 

 

 

 

WB certainly confirmed that the external flow sensor was reading the same as their one and presumably the return, but not sure about this one. 

 

I might have missed it but what did or do  the external sensors read after a extended boiler run?. if both known, then very easy to calculate the flowrate as the constant boiler output will also be known, 40%??.

[John Carroll]

4 hours ago, JohnMo said:

But it has a modulating circulation pump which will change output based on dT, so not sure that can be fully true.

This boiler doesn't measure the dT because it doesn't have a return temperature sensor, also the pump is (customer) clamped to virtually constant speed.