Boiler cutting off and not modulating

[hoylez]

I have installed a Worcester Greenstar 8000 35kw system boiler
Instead of the boiler modulating it gets to 69 degrees and cuts off drops down to 50 degrees then kicks in again this cycle happens every 5 mins like clockwork
I thought it may be air but I put got water only on I have an unvented cylinder next to the boiler so it's a small circuit and boiler gets straight up to 72 degrees and cuts off again this cycle is every 5 mins surely if it's a modulating boiler the hot water even on a small circuit should just modulate at about 70 degrees and stay on until the cylinder stat is up to temperature? Anyone else experienced this or got any ideas what's causing it?

 

[John.g]

This is a re occuring theme with gas fired boilers on HW only. If mine, I would check that the balancing valve (if fitted) on the cylinder coil return is fully open, shut the cylinder zone valve, keep draining off hot water until it runs cold, refire the boiler on HW only, observe boiler temp/modulation and cylinder temperature, when cycling starts the coil isn't able to absorb the output from your fully modulated boiler and I'm afraid you can't do much about it except to then run the CH&HW together.

 

[hoylez]

This is a re occuring theme with gas fired boilers on HW only. If mine, I would check that the balancing valve (if fitted) on the cylinder coil return is fully open, shut the cylinder zone valve, keep draining off hot water until it runs cold, refire the boiler on HW only, observe boiler temp/modulation and cylinder temperature, when cycling starts the coil isn't able to absorb the output from your fully modulated boiler and I'm afraid you can't do much about it except to then run the CH&HW together.

I've had the balancing valve on fully open but it also does this with both H/W and Heating on together and Heating on it's own it just doesn't seem to stay lit for more than 5 mins at a time

 

[John.g]

The minimum output of your boiler is 5.5kw so if the boiler is in fact modulating properly, it means that your total heating demand is < 5.5kw, this would be normal enough with a fully heated house and only one zone on but if all zones on with a cold/cool house the heating load should be well in excess of this so no cycling.

 

[hoylez]

The minimum output of your boiler is 5.5kw so if the boiler is in fact modulating properly, it means that your total heating demand is < 5.5kw, this would be normal enough with a fully heated house and only one zone on but if all zones on with a cold/cool house the heating load should be well in excess of this so no cycling.

So what are you saying? The boiler surely shouldn't cut off every 5 mins it's doesn't stay on long enough to get the heat round the rads
I installed another of these boilers with a cylinder next to it and it's modulates lovely at a constant 70 degrees without ever cutting out

 

[Brambles]

Are you currently using this boiler only with a DHW circuit?

I think that your issue probably relates to the system installation. The boiler is firing up and cutting out after 5 minutes because the return temp is too high, driving the flow temperature to cut out.

Look at the bypass arrangement and settings.

Is this a new boiler on an existing installed system, or a complete new installation?

 

[John.g]

I think its the one on the rads that's cycling so return temps shouldn't be a problem if demand is > 5.5kw, cycling on/off every 5 minutes would suggest a heating load of ~ 3 kw, very unlikely.

 

[Brambles]

I think he/she is only running it on DHW - hence the reference to a small circuit - that is really a large thermal circuit, but small in length

 

[hoylez]

It's a new boiler

I think he/she is only running it on DHW - hence the reference to a small circuit - that is really a large thermal circuit, but small in length

Its a new boiler which it's on a large property mainly old pipework but main primary run is new 28mm pipe
The boilers being used for both hot water and Heating there was a few radiators not getting hot after lots of messing realised boiler was cutting out
Status code on boiler suggests it's cutting out because the flow is too hot
I also tried hot water only to see if that would run without cutting out and to see if it was just when the heating was on but it does the same for hit water also
Worcester have said even on hot water on it's own it shouldn't cut off that quick and boiler should modulate
I wanted to eliminate anything else other than faulty stats in boiler just in case it's a system fault and then getting charged for calling them out
[automerge]1607378649[/automerge]
It's a new boiler

I think he/she is only running it on DHW - hence the reference to a small circuit - that is really a large thermal circuit, but small in length

Its a new boiler which it's on a large property mainly old pipework but main primary run is new 28mm pipe
The boilers being used for both hot water and Heating there was a few radiators not getting hot after lots of messing realised boiler was cutting out
Status code on boiler suggests it's cutting out because the flow is too hot
I also tried hot water only to see if that would run without cutting out and to see if it was just when the heating was on but it does the same for hit water also
Worcester have said even on hot water on it's own it shouldn't cut off that quick and boiler should modulate
I wanted to eliminate anything else other than faulty stats in boiler just in case it's a system fault and then getting charged for calling them out

 

[John.g]

It depends on HW coil size, if a rapid recovery cylinder coil then should be OK even with cylinder up to 60C as long as boiler at 70C but to rule it out completely I would run off the hot water as even the most basic coil will absorb ~ 8kw and if the boiler still cycles then you have a starting point IMO. Presume magnetic filter installed and a clean Hx even though its a new boiler.

 

[Brambles]

Set the boiler only to drive the DHW circuit. Ensure that the flow and return are clear of any restrictions - if the problem persists, call out Worcester Bosch, but make sure that the system is completely cold when they arrive. They would need to be there at least an hour and a half to prove it is not the boiler.

My view is that the issue is within the system restricting the return - not the boiler. Which I guess you do, hence the reluctance to call out Worcester Bosch. You will have measured the boiler return temperature and the DHW cylinder return temperature - if they are not nearly identical - the issue is unlikely to be the boiler.

The Greenstar 8000 courses are well worth attending - the 90 minute online course is free, the one day attended is also free if recommended by your local rep or £25 if you apply direct.

The attended course works through the detail of the boiler logic sequence and more specifically the behaviour of the boiler on high or very low return temperatures

 

[Chuck]

What's happening to the return temperature measured at the boiler while the cycling is occuring? How long after the flow temperature starts to rise does it take for the return temperature to start rising?

 

[John.g]

Interpreting that status code may tell a lot.
If the boiler is modulating to 5.5 kw then even a flowrate of 4 LPM will give a deltaT of 20C so if the boiler is cutting out at 75C (SP+5) then the return would have to reach 55C, very unlikely, at least with a cold HW cylinder but quite possible on CH, if in fact the flowrate is that low. Boiler may not be modulating?.

 

[Gasmk1]

Is pump up to the job does sound like slow return and flow getting up to temp to quick what pump is in the system boiler

 

[John.g]

Could be low flowrate but as the boiler fires up at ~ 70% output then to avoid cut out before modulation the flowrate would have to be, theoretically, 18 LPM to maintain a 20C deltaT.
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This is probably the fault code displayed but possibly may be displayed every time the boiler cycles off as the flow temperature will be 5C higher to switch off the boiler.

"204 O Current prim. water temp. higher than set val. The actual boiler water temperature is higher than the target boiler water temperature. The boiler is switched off. "