Odd behaviour with Greenstar Ri boiler

15 Feb 2007
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United Kingdom
My Greenstar Ri boiler works well generally but it behaves oddly in two ways.

When it is just heating the DHW it goes into over-temperature shut-down when the demand stops. This is okay because the boiler cannot modulate low enough so the water is almost bound to go over-temperature. However the manual says that the boiler should be able to come on after a three minute delay. But it doesn't - it comes on much later when the water inside the boiler has had time to cool down I suspect.

The other odd behaviour occurs when the demand for DWH stops whilst the demand for CH continues. The boiler turns off for a few seconds and then starts up again.

Has anyone else experienced these odd behaviours?
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This odd behaviour as you call it is because the boiler has not been correctly wired so that the boiler controls the pump to provide an overrun after any demand.

Rewire the boiler so that it correctly so that it can do its job properly.

There is an alternative that if you dont have a mid position valve but have two two port valves then the problem is that no auto bypass has been fitted.

Both these senarios are as a result of installer error as we politely call it !

I dissagree weith Agile and say its working perfectly.

If you have a mid pos system then the boiler will go off for a second when the valve motors between mid position and heating only.

The reason it may not be coming on after its three minute timer delay is that it hasnt cooled down enough. It can take a while to cool down its its purley heating the DHW.

I say its working perfectly.
Thanks for your replies so far.

The pump is wired from the boiler properly to give pump over-run. So the pump over-runs every time the demand stops, except when the over-temperature shut-down occurs. This is correct according to the manual.

I have a mid-position valve. The idea that this causes a momentary interruption in the demand when it moves from one position to another is convincing.
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It still does not explain his """except when the over-temperature shut-down occurs""".

Even if the valve motor is moving whilst there is no call for heat the pump over run should still be operating if the boiler is still hot.

Well you have convinced me that my boiler behaves properly according to its design. Thanks for that as it has saved me being tempted to buy a new PCB!

But could the design have been better? After all, in certain circumstances my boiler takes about 15 minutes to cool down after a DHW cycle when the house really needs heat!

At first I agreed with Tony and thought the pump should over-run after an over-temperature shut-down. It could even be designed to run after the 3 minute delay. Then the boiler would know that the rest of the system was cold.

But then I realised that this might endanger the boiler. It might result in cold water being drawn into a very hot boiler. So maybe designing boiers is more subtle than I thought!
If your boiler is not allowing heating does not kick in for fifteen minutes after hot water demand there is something up somewhere providing the heat is being called for
O-B you're saying it goes to "over temperature shut down". That implies an overheated condition - which it shouldnt reach. Or do you mean it turns off because it's simply up to temperature?

Are you saying that when the HW stat is satisfied the burner goes out then the boiler goes into overheat (fault) mode? If so then yes, the pump should be running to stop it doing that.

Unless the rads were really hot I'd expect it to start up within a couple of minutes , for the CH.
I agree with you ChrisR (in an ideal world) boilers should not go above the set temperature and if they do it should count as a fault. However the designers of the Greenstar Ri seem to think otherwise.

The flowchart in the manual shows nothing about the boiler shutting down if the set temperature is reached mid-cycle. Instead it shows the boiler only making a decision at the end of the cycle.

Then the boiler either turns off normally with a 3 minute pump over-run or it goes into what they refer to as "over-temperature shut-down if water temperature is 5 deg C above set value". In the second case there is supposed to be a 3 minute wait without pump over-run. I think this is their idea of an anti-cycling routine.

As far as I can see the over-temperature condition is likely to occur when the boiler has been heating the DHW alone. The DHW simply cannot absorb the 6 KW that the boiler is putting out at its lowest modulation.
If after DHW usage when CH is selected the boiler turns off the pump than I am of the opinion that its as a result of a fault.

I would also expect the fault may be a scaled up primary heat exchanger although a blocked plate heat exchanger might cause this effect too.

Was the system power flushed and which which chemical? Which inhibitor was used?

Yes, I originally thought that but his continued use of the term DHW and talking as if it never powered both HW and CH at the same time made me suspect it might actually be a combi in spite of being headed Ri.

Assuming that its really a heat only boiler he should check that it has a permanent live supply going to it ALL the time regardless of being OFF/CH/HW/CH+HW. Is this the case ?

If so then it might still be a scaled up heat exchanger.

BUT first can you confirm that you have a mid position 3- port valve ??

If so can you confirm you have one rad without a TRV or an auto bypass valve?

If NOT and you have TWO 2-port valves then do you have an auto bypass valve?

We might be able to work out the problem if only he can answer each of these questions correctly !

Was with Agile on this by description of fault it sounded like a combi fault, If the Ri is like the old CDi the wiring is not straight forward and permanent lives for valves etc all come from the boiler . There used to be a separate wiring diagram as an amendment sheet .

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