Bypass valve

[Johntheo5]

That boiler has a internal ABV (automatic bypass valve), If it took an hour for the boiler outle to feel like 70C then something definitel wrong, its possible, if unlikely, that th ABV is faulty. The pump head at its max constant (pressure) of 400mbar, 4.0M, should give adequate circulation except the boiler HEX is partially blocked.
Assuming the boiler/rad returns at a "lukewarm" 30C, then the rads are T30 with 51.4% output of their T50 rated output.

A reality check on the actual boiler output is, when the boiler is running steadily after say a hour, take a snapshot of the actual gas meter, then exactly 3 minutes (to the second) take another meter snapshot, the gas consumption/hour is the readings difference X 20, X this by 11.0 to get the boiler input in kWh, X this by say 90% (boiler efficiency) to get the boiler output in kW.

Have or can you you range rated the boiler to a lower output?.

 

[GeddyMortgage]

I wondered the same as you regarding some sort of failure with an internal ABV @Johntheo5 as in simple terms it does appear that the boiler simply can’t get the heat out of it… but I couldn’t see any evidence of one mentioned in the manual. Where are you reading that this boiler has an internal ABV? It’s a Greenstar Life 8000 system boiler. GR8300iw 35 S

Internal diagram and annotations attached

Also yes, it can be modulated down by setting the max % output to anything as low as 53% and you can see it’s % live output any time in the engineer menu.

 

[Johntheo5]

Not sure where I thought I saw this but your model certainly doesn't seem to have one so better from a trouble shooting point of view.

What is your % output showing "normally" and where are you feeling the boiler outlet, is it where it emerges from the boiler or further along?.

 

[GeddyMortgage]

Not sure where I thought I saw this but your model certainly doesn't seem to have one so better from a trouble shooting point of view.

What is your % output showing "normally" and where are you feeling the boiler outlet, is it where it emerges from the boiler or further along?.

Generally in the region of 50-75% output id say.

when I fire it up it starts low % and gradually creeps up but then as it hits the target flow it stays around 50% ish.

that said i have noticed many times it'll overshoot the target, turn off the burner for 1-2mins then resume, when it resumes it'll start at a very low output % again, around 20% and creep up until back to target, then stay at target at around 45-50% output.


Feeling the boiler outlet directly beneath the boiler, photo attached. .. Left read out was flow, right read out was return. But thermometers aside even to the touch there was a massive difference between boiler lcd temp and outlet temp.

Comparatively (and I know this is silly to say) but we have a 40kw conventional boiler upstairs and with that, you turn it on, and the temperature of the flow pipe of that one is always pretty consistent with the LCD read out. Leading me further to believe something is wrong with this downstair one that we're discussing.

 

[Johntheo5]

So, its running continuously after a few cycles?, if so the outputting ~ 18kW.
The two thermocouples show a rad output of ~ 17.2% of T50s so I think you said ~ 28kW of installed T50 rad output?, if so rad output only 28*17.2%, 4.8kW, if boiler still firing at 50% then the flow temperature should rise very rapidly so nothing adds up there, can you run the gas meter test?.

 

[GeddyMortgage]

I’ll need an engineer for that. Not sure how to run the gas test myself.

Also I’m ordering some better thermometers today I don’t trust the ones I’ve used there.

But yeah as above; even without the thermometers. Certainly that left flow pipe to the touch is no where near as hot as the LCD display is suggesting

 

[ericmark]

I noted my late mother's Bosch boiler had both internal and external by-pass valves, and with the lock shields set, so the current never exceeded target on the TRV head's computer display, and each room seemed to stay at the temperature set within 0.5°C.

But heat raises, so what is the point of these ? I see darker colours will radiate heat better, but the idea is to get the air circulating, and I have with both late mothers and my house found a problem with the hall, in that the heat goes upstairs, and does not circulate. In mother's house we used curtains across the stairs, in this house we have just given up controlling the hall temperature, and fitted thermostats linked to the boiler in other rooms. But in this house the radiator return temperature is not important, as not using a modulating boiler.

I also realise who ever designed the system, to put it blunt, did not do a very good job, in both homes. In the main, only the living room is important, the other rooms in the house can be cooler, we don't want them cold, but they do not need to be at 20°C to 22°C they can be 16°C to 18°C in the main. So we have a boiler cycling all the time, as the living room radiators simply can't sink the boiler's output.

This house does not really matter, as not a condensing boiler, but mother's house it did, and why anyone thought it needed a 35 kW boiler for a three-bedroom house I don't know? In the main, the house needed around 4 kW to maintain the temperature, and the boiler would not modulate down that low. OK the shower needs around 18 kW so that is around the minium size, but the boiler was twice the size required. The bath was super small, and not used anyway. Seems the builder had miscalculated when building the houses (1954) so made the house 1 foot narrower as to on the plans, to accommodate an ally between the houses, so had found a bath also shorter to make it all fit.

The theory is great, as each TRV closes, the pressure increases, until the by-pass opens, so hot water is returned to the boiler, and the boiler modulates (turn down output) to allow the return water to be cool enough to gain the latent heat from the flue gases. And if it can't modulate enough, then it uses a mark/space ratio to turn the boiler off/on and further reduce output, to do this it needs either TRV control or a modulating wall thermostat using the ebus in some way.

Then we mess it all up by fitting an on/off wall thermostat.

 

[Johntheo5]

I’ll need an engineer for that. Not sure how to run the gas test myself.

Also I’m ordering some better thermometers today I don’t trust the ones I’ve used there.

But yeah as above; even without the thermometers. Certainly that left flow pipe to the touch is no where near as hot as the LCD display is suggesting

No need for a engineer. Just your smart phone, as stated above just take a photo of your house gas meter readings exactly 3 minutes apart, ensure boiler running normally at 45/50% with no gas cooker/oven on etc and post the two readings on here.

 

[GeddyMortgage]

No need for a engineer. Just your smart phone, as stated above just take a photo of your house gas meter readings exactly 3 minutes apart, ensure boiler running normally at 45/50% with no gas cooker/oven on etc and post the two readings on here.

Current % read out from boiler, its been on about an hour and a half by this point

12:33pm

12:36pm

Obviously our meter is m3 not kw

 

[Madrab]

New(ish) boiler so pump should be fine. With 28mm pipes running down to a 22mm backbone and then into 15mm feeds then there should be minimal system resistance. If everything is taking ages to warm up and the boiler's reaching set temp before the heat reaches the rads then I'd suspect either a restriction in the system somewhere or when the system's been balanced the lockshileds have been wound down too much limiting flow.

With 15 rads all fed with 28mm>22mm>15mm then I'd be tempted to open all the rads up - noting where the lockshield's are currently set at - how many turns to close - and see how the system performs then

 

[Johntheo5]

Current % read out from boiler, its been on about an hour and a half by this point
View attachment 397863

12:33pm

View attachment 397864

12:36pm


View attachment 397866

Obviously our meter is m3 not kw

Gas consumption, 067023.71 minus 067023.635, 0.075m3= 0.075*11, 0.825kWh= 0.825*60/3, 16.5kWh, at 90% boiler efficiency = boiler output of 0.9*16.5, 14.85kw......~ 14.85/36, 41% output so there are there abouts the displayed 39% output, average of say 40% output/14.4kW.

If your rad(s) T50 output is 28kW then % rad output is 14.4/28, 51.5%.

 

[GeddyMortgage]

New(ish) boiler so pump should be fine. With 28mm pipes running down to a 22mm backbone and then into 15mm feeds then there should be minimal system resistance. If everything is taking ages to warm up and the boiler's reaching set temp before the heat reaches the rads then I'd suspect either a restriction in the system somewhere or when the system's been balanced the lockshileds have been wound down too much limiting flow.

With 15 rads all fed with 28mm>22mm>15mm then I'd be tempted to open all the rads up - noting where the lockshield's are currently set at - how many turns to close - and see how the system performs then

Good call, will give this a try, though I’m doubtful because perhaps 3 rads in the system are already all the way open. The ones furthest from it struggled to get hot otherwise.

 

[GeddyMortgage]

Gas consumption, 067023.71 minus 067023.635, 0.075m3= 0.075*11, 0.825kWh= 0.825*60/3, 16.5kWh, at 90% boiler efficiency = boiler output of 0.9*16.5, 14.85kw......~ 14.85/36, 41% output so there are there abouts the displayed 39% output, average of say 40% output/14.4kW.

If your rad(s) T50 output is 28kW then % rad output is 14.4/28, 51.5%.

Yep, so it’s correctly modulating down and using the appropriate amount of gas.


Though it’s still leaving me head scratching that the boiler is modulating down before that flow pipe gets hot or anywhere near target temp.

I have just found an option in the hydraulic menu that has confused me and I’ll need to investigate.

This one here in 2-A3… on my boiler this was set to 1… but in the manual it looks like 1 shouldn’t even be of the options available… could be a printing error but I’m also wondering if 1 is making the boiler think there is a separate pump somewhere and doing less work internally to compensate? Maybe that’s a really long shot.

 

[Johntheo5]

Circulation flow seems a problem, you can see below that if 75C is the flow temperature then the return must ba as low as 25C with a dismal flowrate of 4.13LPM & dT of 50C, can't see any boiler accepting these conditions.
If 40C is the return temperature then the flow temperature must be as low as 60C with a flowrate of 10.3LPM & dT of 20C,

 

[GeddyMortgage]

Circulation flow seems a problem, you can see below that if 75C is the flow temperature then the return must ba as low as 25C with a dismal flowrate of 4.13LPM & dT of 50C, can't see any boiler accepting these conditions.
If 40C is the return temperature then the flow temperature must be as low as 60C with a flowrate of 10.3LPM & dT of 20C,


View attachment 397897

I see, ive ordered some more accurate measuring tools for the flow / return so can say later / tomorrow what’s actually going on there… though if those figures are right …what would cause this? Too much resistance or even a blockage in the pipes somewhere? Or something else?