Greenstar 24i System Boiler Low Output: Advice appreciated

[teledu]

I bought a 6-month old Greenstar 24i System boiler that had been taken out by a flat dweller who needed to ditch their cylinder and move to a Combi for space reasons.
It has sat around for a few months, but was installed recently by my friendly neighbourhood CORGI heating engineer who has no experience of this particular boiler (the firm he has worked for tends to use Ariston).
All seemed well until the recent cold spell triggered some CH demand.
The boiler continued to operate as it has for HW demand only: ie at full output for only around 10-15 seconds before modulating down to a low level. A thermocouple on an insulated section of pipe on the boiler output registers around 45°C on a cool CH system, climbing over the course of about an hour to 55° as the rads and HW tank are heated. If the CH is eventually satisfied, the output will rise to 59° on completion of a 10 second firing, and fire again when cooled to around 38°. All this with the temp control at MAX which should give us around 80°+. In Service Mode with CH only on, the boiler should run continuously at max output, but does not (it still winds down after 10-15 seconds).
There are no fault code indications, which suggests that the primary sensor and overheat stats are all ok. The previous 20 year-old Glowworm boiler ran the same system without any flow issues, so something must be amiss with this boiler? The pump is set to max output (III) and isn't making any distressing noises suggesting partial seizure, but is low pump performance a likely (or even the only) cause for these symptoms?
Thanks.....

ps: the boiler is still notionally under guarantee... anyone know if Worcester might honour their guarantee after a reinstall and owner change?[/i]

 

[bengasman]

IF the boiler was properly installed, and IF you have the benchmark sheet filled out, and IF system was properly commissioned, your installer should be able to sort the problem out.

I doubt it is due to a warranty issue. I doubt even more that Worcester would repair it for free if it was a boiler problem.

Silly question maybe, did your friendly installer have his own CORGI number, or was the company he used to work for registered? You can find the details on the commissioning papers.

 

[teledu]

Thanks bengasman:
to answer your questions... The benchmark sheet has not been filled in (other than by the first installer). It isn't a very comprehensive sheet is it; but if I asked him to, there's no question that the flow and return figures (88 and 69 in its original location) would not be met by a mile. The installer is my son's best friend (and we've known him since he was 2)... he worked for a heating outfit for around 8 years but has now started his own business. He's got his own CORGI registration. I wasn't here when he re-checked the installation, but says everything is as it should be apart from the temps. NO doubt he can sort it out, but as we're dealing with a used boiler it will be at my expense, so I'm just trying to make sure we don't swap a £100+ pump for example only to find that it wasn't the problem. As a 2-man band normally working with new gear, he hasn't got a stock of Worcester bits (or probably the time to work on this 'favour' job for me when he's stacked out with 'proper' jobs).
I fitted a flowmeter inline when I did my own solar hot water: is there a simple way that you professionals use to temporarily fit a flowmeter in the CH or DHW circuit to check pump performance?

 

[Gasguru]

Pumps are tested by removing the pump head and feeling the torque on the impeller, however, unless the impeller is very clogged it is very unusual to have a pump problem.

Since the boiler has stood around have you considered the boiler bypass may have jammed fully open?

Have you considered the system may have a partial blockage? With the old boiler (and separate pump perhaps without bypass) the full pump pressure head could have been applied to the system.....now with the integral pump (and bypass) there may be less pressure head available to overcome the blockage.

 

[teledu]

Good thoughts...
I don't suppose the integral bypass provides any indication of it's status, so presumably that's a drain-strip-check? (no, not by me). If the boiler can be run without its casing (that seems to be necessary for commissioning...) it may be possible to monitor the temperature of the bypass pipework to get an idea of whether the valve is open?
The old system had a 15mm + gate valve non-auto bypass on the DHW, which is still there, but almost closed. No bypass on the CH (one rad without TRV). In case anyone asks, yes the old pump has been removed.
Hadn't considered a system blockage, but will 'bounce off' my man.
In case it adds anything to the understanding I just did some detailed measurements with CH off and my tank already at 53° (so not ideal). Turned the tank stat up to max to restore demand:-
TANK FLOW RETURN BURNER(sec) TIME(Min)
53........ 52...... 42.......19HI+5LO......0
53........ 57...... 50...........NO.............2
53.........57...... 52...........NO.............3
53.........57...... 53...........NO.............4
53.........56...... 53...........NO.............5
53.........55...... 53......15HI+6LO.......6
53.1......60.......53...........NO.............7
53.1......60.......55...........NO.............8
53.1......60.......55...........NO.............9
53.1......60.......55...........NO............10
53.1......60.......55...........NO............11
53.2......58.......54...........NO............12
53.2......57.......54.......12HI+8LO.....13
53.3......60.......54...........NO............14
53.6......61.......55...........NO............15

Probably not very instructive with such small differentials:
The firing duration would be little or no different though with a cold tank or cold CH.
For ref, the tank is a recently installed Ariston Primo 210 litre twin coil. The tank monitor is in the top immersion heater stat sleeve, so not very quick to see changes.

 

[bengasman]

for starters, any external bypass needs to be completely shut or removed if you want to do it properly.
you will NEVER get your required 20 k drop with an open bypass.
dunce cap time for installer

 

[teledu]

I'm busy making up 2 dunces caps right now...
Just looked more carefully at the pipework: the bypass is in the middle of the 'T' branch to the 2-port valves; so it bypasses CH and HW.
Even with the gate valve hard closed the bypass pipe is getting fairly hot....tum-te-tum....watch this space (need sleep).

 

[bengasman]

Ha ha, my claim to fame is being half way in stripping my bike to find out why I had no power when my mate asked if I had checked the MAIN fuse as well. ahum. Had to hear that one for years

 

[teledu]

Now that looked pretty promising, and we have some serious progress.
I've not shared this with my installer yet, but I'm not sure if we're quite out of the woods. Ramming that bypass tight shut has certainly made a big difference. I'm only getting a maximum burn still for around 10-12 seconds before it modulates to low (almost silent operation) but with a cold CH loop it stays lit pretty much continuously with a 13° differential (currently 58-45).
Is this typical, or should I be getting nearer the 'design' figure of 20°?
I also still have the CH temp dial set to absolute max, and I don't think we're going to hit 80+ degrees.
The other reason I'm still concerned is that when the CH loop was cool, and with HW off, in Service Mode it was shutting down after ~15 seconds (reset required; slow flash, normally on, which means flue or heat exchanger overheat). This seems to take us back to some sort of restriction in the system, as the book says it should run for long enough to check gas rate...at least 10 mins. Maybe time to change all my 20 year-old Drayton TRV's?
It's now about 50 mins since firing up, and the rads are all very hot; Flow 59, return 47 and pretty much steady (heat in = heat radiated), and the burner has just gone off, so that's as high as we're going to get (60° in round figures).[burner back on at 52-47 and cycling just to maintain 60-ish].
I believe the best efficiency will be with flows in the high 50's, so I'm not bothered that we can't hit 70 or 80; just concerned that something is still not right if we can't.
Thanks for the insights!!

 

[bengasman]

Theoretically, a steamer works at full efficiency with the return at 55 C.
In reality, the lower the return, even below 55, the more condensate you find which must mean increased efficiency.

If the boiler itself does not get over 60 C, my guess would be there is something wrong with the settings. The older models CDI greenstar mark 2 and I think the mark 1 as well, can be limited and adjusted every which way electronically, overriding the user controls.

Could be of course a faulty pot meter or ntc.

Before I change a boiler, I powerflush the system with acid using the old boiler and after that carefully check all the valves, trv, wheelhead and ls alike.

It has been the subject of heated discussions here whether you should have your 20 degrees over the flow and return at the boiler or over every rad.
Technically only at the boiler is required, but at the same time if your rads only cool it 10 degrees, you won’t get a drop of 20 at the boiler.

Ideally, you should have the best part of 20 degree drop at every rad, which will ensure the same difference at the boiler. Having said that, it is neigh impossible to get a 20 degree drop on small rads.

One of the tricks I use to avoid problems is to stick the roomstat in the lounge and 2 lockshields on the lounge rad. This way you never get the system cycling endlessly because some numpty turned off the heat source working on the roomstat.
Also makes sure you never have the boiler on without somewhere for the heat to go.

If you have twoo small rads close together, stick them in series, makes it a lot easier to get your drop.

 

[teledu]

It'll all go quiet now while I check out individual rad drops, get some new valves where they are sticky, balance them; etc. but thanks again.
With hot rads the return temp at the boiler is generally around 35-40 when it fires up again, rising slowly to 50-ish when it decides to shut down (flow at ~60). With DHW only, the return soon gets close to the flow when the tank is up to 50°, but I guess that's to be expected as there's little transfer possible with a 10° differential.