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Tim Cooke
Tim Cooke,
Category: Appliance
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Experience:  Owner at Self-employed
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Worcester Greenstar Highflow 440 Cdi ErP. Set to level 6.

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Worcester Greenstar Highflow 440 Cdi ErP. Set to level 6. Not on ECO setting. Runs hot for a few minutes, boiler temperature reads 59-62, then drops to luke-warm (47), and does not recover. It has done this before, and reverted back to normal operation, not sure why.
JA: Are you getting any error codes or blinking lights on your Erp boiler? How old is it?
Customer: No error message whilst running, nor after. It is 2 years old
JA: How long has this been going on with your Erp boiler? What have you tried so far?
Customer: This first started about 1year ago, but resolved. My plumber thought it was probably due to faulty Mira shower valves mixing cold water into the system.
JA: Anything else we should know to help you best?
Customer: We have a fairly high water pressure, can reduce this with an installed reducing valve if necessary.

Hello, I can help you with this.

I will just look out the manual for your boiler, and the shower, and will post again shortly.

Please could you tell me the model of your Mira shower?

Customer: replied 6 months ago.
Mira Excel
Customer: replied 6 months ago.
Mira Excel no other information

If you turn your Mira temperature control to the maximum hot setting, and run your test again, does that make any difference?

Customer: replied 6 months ago.
I will try, but later. can we pause this session?

Of course. This isn't an interactive chat session, and you can continue posting for hours, days, or weeks - as long as you need to :-)

If you post again and I'm not immediately available, please be patient - I will get a notification and will respond as soon as I'm able.

Customer: replied 6 months ago.
I've tried with the Mira valves turned to max hot, makes no difference. I see how cold water mixing from the valves into the hot water side might cool the water at the tap, but that can't affect boiler temperature, which is what the problem seems to be.
With the central heating turned off to prevent confusion, the sequence is as follows.
Without HW running, boiler not firing, boiler temperature 47.
Turn on HW, boiler fires immediately, temperature rises to variable level, sometimes 64, sometimes 72. After a minute or so the temperature starts dropping, then falls steadily to about 47 and stays there; no error message (EM).
Turn off HW, boiler continues to fire. Temp drops to 35. EM back to back "t"s. Temp rises to 60, EM single "t". Temp fluctuates slightly, then the boiler stops firing and the EM stops.
Temp falls back very slowly.
I have repeated this a number of times.
My bathroom is far from the boiler and there are clearly heat losses on the way, so that the water temperature needs to be 60 at the boiler to deliver a decent hot bath, which it has until recently done. Do not know why the change. The setting at the boiler is No. 5, not 6 as I said earlier. Can you help?

Your approach is logical - kudos.

The shower-to-max-hot experiment is a valid one that often reveals odd faults. Although I don't like eliminating anything before the real fault is found, it tend to agree with you that it seems more likely that it's a boiler fault.

The back-to-back 't' symbol on the boiler display (alternating with 't') is supposed to mean that the boiler is filling the internal DHW tank. This tank is intended to reduce delivery time when an outlet is opened, so I wouldn't expect to see it while a demand is being satisfied. However, if the flow sensor stops detecting flow, rightly or wrongly, the boiler should then revert to heating the internal tank. Otherwise the DHW demand should override all other demands.

So, it's possible that the flow sensor is faulty, but I'm still thinking.

Meanwhile, what's the hardness value of your local cold water supply?

Customer: replied 6 months ago.
Thanks. The local water is rated "soft to slightly hard" by Northumberland Water.
Customer: replied 6 months ago.
Their values are Soft = 0-20, Slightly Hard = 40-60.

OK. So it's unlikely that the DHW heat exchanger has scaled up.

It seems like it's either a flow sensor fault, or the control board. Lets hope it's the sensor. Do you have the means to test its resistance?

Customer: replied 6 months ago.
I do. The question is where?

I'll look it up. Bear with me.
It's an NTC thermister and I'll have to look up the likely value too...

Customer: replied 6 months ago.
Waiting. Just hoping this isn't going to get too technical!

Resistance at room temperature : 14.7K Ohms
Resistance at 80C : 1.7K Ohms

I'll attach a couple of diagrams to help you locate it. You'll need to power off and remove the outer casing.

Customer: replied 6 months ago.
I take it that the temperature referred to is flow temperature, which means that the 80C must be with the boiler running. Or is it possible to get the temperature up, then power off, case off and take the measurement before the temperature falls?
Customer: replied 6 months ago.
Nothing attached so far.

I was just giving you the range - you don't have to make the boiler hot.

However, I've found a different range, so am ambivalent.
That other range is (33.5k Ω- 500 Ω). Let's see what yours measures and go from there.

The diagram took me a while to extract to a suitable file size.
It should be attached now.

Customer: replied 6 months ago.
Got it. I may be gone some time.

I hope you return sooner than Captain Oates!

I'll look out for your reply later on....

I've been pondering, and another possibility to add to the list is low burner pressure.

This needs to be tested and set by a Gas Safe registered engineer, as it's possible that your gas meter is at fault instead of the boiler.

The problem wouldn't show itself in CH mode as the temperature differential is smaller, so the boiler tends to module down and you wouldn't notice anything.

Tim Cooke and other Appliance Specialists are ready to help you
Customer: replied 5 months ago.
Tim, thank you very much for your help and advice. On reflection, since the boiler is still under guarantee I have decided to get the Worcester Bosch engineer out. It has been most helpful to have an understanding of what might be the problem; had I had this before it might have saved me the cost of 2 new Mira shower units. At least I will be able to have a (fairly) intelligent conversation with the engineer. Once more, many thanks. Michael.

Hello Michael, I humbly apologise - I've just noticed that you replied!

I hope you had a productive visit from the Worcester engineer.