Combi boiler - temperature falls and stuck at 38C when running hot water, also slight bad smell from water?

[igytk]

Hi, apologies for the slightly convoluted thread title! I have a fairly old Vokera Linea 24/28 that's been playing up. Previously, it was cutting out intermittently, with an error code 07 (thermistor circuit open) - I could turn it off and on again, and the hot water and heating would start to work again. I had a couple of boiler engineers come out to take a look - the first said it could be a combo of one or all of the PCB, fan and diverter valve, and recommended a replacement. Someone else came the following day and couldn't work out the problem (though it could have been the pressure release valve) but did a full service.

However, since then the hot water has only been lukewarm at best, although with no error codes. When the heating is on, the temperature on the boiler reads ~60C, whereas when the hot water is called it immediately drops to 38C and stays there, no matter how long the hot water is on for, and the water coming out of the taps and shower is lukewarm. After the service, the pressure dropped to 0, and I had to top it up to ~1bar, and when the hot water is on, the pressure falls to ~0.5-0.7 or so.

We've also noticed a bit of a mouldy / musty smell from the taps when the hot water is running, particularly the first second or two after turning them on. Having searched various things online, I'm wondering if there's a build up of sludge / limescale (we are in a heavy water area) which has damaged the heat exchanger. Originally, this was causing the error code, but since the service perhaps something has dislodged and now the hot water can never get above 38C? Is it possible the damaged heat exchanger is somehow causing the smell in the hot water? The boiler is upstairs in the back bedroom and all the taps are downstairs, so it would have to be via the hot water pipes. There's no obvious leak in the boiler cupboard either.

At this stage I'm pretty much resigned to getting a new boiler anyway, but would welcome any views on what the issue might be, and if the hot water smell is potentially related, or if that's something else that needs to be looked into? Thanks in advance!

 

[Johntheo5]

Could be the DHW thermistor, presume you have the DHW control set to max?, can you turn it down to say 45C and see does it have any effect, also turn the flowrate down to say 3 or 4LPM and watch for any changes. Theoreticall, a 28kw combi should flow ~ 12LPM at 45C from mains at say 12C.

 

[igytk]

Thanks for your reply! Yes, the DHW control was on max, I tried turning it down but it didn't help, the overall temperature was just stuck at around 36C instead. Not sure how to change the flow rate.

 

[Johntheo5]

You change the flowrate by throttling the hot tap to give you the required flowrate.

 

[igytk]

Ah I see! That helps make the water slightly warmer, although not enough for a comfortable shower for example, and the boiler temperature doesn't change.

 

[ianmcd]

you need to swap out the primary thermistor, for all they cost buy a pair they are both exactly the same, and change the DHW thermistor at the same time , your existing ones might be blue or black in colour , but these are better quality and as said very cheap so order two

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[Johntheo5]

The primary thermistor is reading OK (60C) on CH?

 

[ianmcd]

The primary thermistor is reading OK (60C) on CH?

So please tell me how you would test the said thermistor

 

[Johntheo5]

No idea, the CH reading was 60C but only reads 37C on DHW with lukewarm HW but maybe the water bypasses this thermistor when supplying DHW?.

 

[DP]

it was cutting out intermittently, with an error code 07 (thermistor circuit open)

Thermistor tolerances are on the cusp. Sometimes the electronics see the resistance as unacceptable so locks the boiler out.

When the heating is on, the temperature on the boiler reads ~60C, whereas when the hot water is called it immediately drops to 38C and stays there, no matter how long the hot water is on for, and the water coming out of the taps and shower is lukewarm. After the service, the pressure dropped to 0, and I had to top it up to ~1bar, and when the hot water is on, the pressure falls to ~0.5-0.7 or so.

Temperature shown will correspond to heating control set. With heating on and running for a while, the control will dictate what temperature needs to be reached and thermistor returns the temperature it sees to the control. If the thermistor is out of range, what you get and what you see will not be the samE. The control system is a closed loop- similarity being a scale. In one pan you put item to be weighed, in the other you put the weight to achieve balance. if the weight you use are no longer as marked, system does not work.
If the pressure drops, service you had done on the boiler might need done properly. Pressure will differ slightly when pump runs and return to original reading ( unless expansion has taken place) to indicate the expansion vessel has cushion of air in it.

 

[Johntheo5]

If the boiler is running normally, does or is the displayed boiler temperature show the actual temperature of the primary water entering the PHE?, IE, if the DHW temperature is say 50C would you expect to see maybe 65/70C if the boiler is firing at its full output?, one other query, any idea of the PHE heating surface of say a 30kw combi?.

 

[DP]

If the boiler is running normally, does or is the displayed boiler temperature show the actual temperature of the primary water entering the PHE?, IE, if the DHW temperature is say 50C would you expect to see maybe 65/70C if the boiler is firing at its full output?, one other query, any idea of the PHE heating surface of say a 30kw combi?.

On the Linea in heating mode, display shows primary temperature.
In HW mode, display changes to display showing hot water temperature and a dot
Issues highlighted by poster in first instance suggests further look at the boiler by perhaps someone who does repairs. While the Linea range has some minor flaws, repair often is not costly but results from a full service are satisfying

 

[Johntheo5]

So the 38C was the OP's DHW temp in HW mode.

 

[DP]

So the 38C was the OP's DHW temp in HW mode.

Yes, unless the thermistor sees that as 38 degrees but actual temperature is different
Could also be broken hot water knob that is causing additional issue.
I would run that boiler ( or any combi, for hot water, with control at max) then stop the hot tap- primary temperature should be around 70 degrees