Short cycling until flow temperature reached - error 2967

[Ccw71]

Hi all, I'm at a loss now and could really do with some suggestions please.

My boiler was working fine previously (should have left it alone!). I recently ran X400 around the system with a Magnacleanse attached to pick up the crud. I have drained down, flushed through, cleaned the F&E tank and refilled, adding x100.

Since then I noticed my boiler runs up for about 2 and a half minutes increasing the flow temperature by 5 to 10 degrees, then cuts off for about 2 and a half mins, before restarting, heating up a bit more and so on. (The cycling time makes sense as it apparently has a 5 minute anti cycling limit)

Once the flow reaches above 50 (close to the set flow temp), the cycling stops and it runs steadily. I presume this is due to the boiler modulating down?

I like to run it with a low set flow temperature of 55 (unless it gets really cold outside) gently heating the house up to 18 or 19.

When it cuts out, the status is 2967 "Temperature differential between flow temperature sensor and heat exchanger temperature limiter is too great"

I'm assuming I can eliminate it being a circulation issue in the central heating as I've the same happening when heating the water cylinder. I've bled all rads and made sure there's no air in the valves and the pump by cracking the nuts on the fittings slightly.

In an attempt to slow the heat increase down, I've set the boilers maximum output to 50%, but it makes no difference.

I've exhausted my limited knowledge and could really do with some ideas. Thanks in advance.


Worcester Bosch Life 8000 40KW regular Boiler -2 years old (not in warranty as supplied on the ECO scheme!)
Grundfoss Alpha 1 running in CP2 mode (though changing speeds makes no difference)
Open vented S Plan system
15 radiators, balanced with all but 2 having TRVs fitted. (same issue if wide open)
55 degree flow temp - 19 degree room temp

 

[Madrab]

I'm assuming I can eliminate it being a circulation issue

Not at all, if the differential between HEX and Flow temp is too great then the flow through the HEX can't be high enough and it will then short cycle but the restriction could be anywhere, even in the main HEX. Is the boiler protected by a normal system filter close to the boiler? It could be that the X400 has dislodged some crap that's now found it's way to a key area causing a restriction. Could also be an air lock somewhere, if the system is vented. Is everything heating up evenly when it gets up to temp?

 

[Ccw71]

Hi, thanks for the reply.
I assumed that as I have the same issue with heating the water cylinder, that may eliminate circulation issues in the central heating side of the system? The problem is affecting both CH and DHW.
All the radiators do heat up gradually and evenly.
The installer fitted a Magnaclean Atom on the heating and water combined return, but I also fitted a Magnaclean Pro 2 on the CH return.

 

[Johntheo5]

Looking at it from a few different ways, you are averaging a temp rise of 7.5C per 2.5 minutes, assuming cold temp at 20C and required flow temp is close to 50C (when boiler runs OK) then if the boiler fired continuously from cold, it would take it 10 minutes to rise from 20C to 50C, does this seem like a normal heat up time??.
Another view is if the burner is cutting out because of a dT of > 30C then, by calculation, at 50% output or 20kw, the flowrate is, 20x860/60/30, 9.6LPM.
Can you measure/feel the boiler flow and return pipes when boiler is running normally?. and estimate the difference, you know the flow will be 50C so should get a fair idea of the return?.

 

[Ccw71]

I really couldn't accurately say about the normal temperature rise when it was running before. I've got a sensor taped to the return but it got flat batteries, so I'm just waiting for batteries to arrive latter today. Is the differential fault as listed by Worcester, referring to the difference between flow and return, or the flow and the internal temperature of the heat exchanger? I'll get back with some real measurements as soon as my batteries arrive. Thanks.

 

[Johntheo5]

I really couldn't accurately say about the normal temperature rise when it was running before. I've got a sensor taped to the return but it got flat batteries, so I'm just waiting for batteries to arrive latter today. Is the differential fault as listed by Worcester, referring to the difference between flow and return, or the flow and the internal temperature of the heat exchanger? I'll get back with some real measurements as soon as my batteries arrive. Thanks.

Looks like that chemical clean may have blocked up your heat exchanger (hopefully not). Also running that pump in PP mode is no good if partial blockage, c/o to constant curve mode 3, and run the boiler if possible, with both CH&DHW on together. If this fails, you may have to drain down the system and connect a mains hose at the boiler return with another hose or a big bucket attached at the flow end and try back flushing it.
Before doing any of this ensure both filters are cleaned out.

 

[Ccw71]

Thanks for the reply. The most convenient point, avoiding a full drain down, to do the back flush would be via the Magnaclean connector on the CH return. I could then drain from the boilers drain point. Would this be pushing clean water through the correct path in the HEX?

 

[Johntheo5]

Depends on where the drain point is in relation to the Hx, but can't do any harm anyway, shut the boiler flow valve before flushing.

 

[Ccw71]

There is the main boiler drain at the bottom which conveniently drains out the condensate pipe. There's also the air bleed valve at the top, which has a hose connector. Which would you recommended?

 

[Johntheo5]

A schematic of your boiler might be of help, You could back flush through both but under no circumstances would I flush through the drain unless it's disconnected from the condensate drain, danger, however little, of getting water inside the combustion chamber or where ever.

 

[Ccw71]

Thanks, it shouldn't be to dificult to divert the drain point from the condensate drain.

 

[Johntheo5]

Flushing through the the air vent is a good idea as the the vent is/should be on the HX flow side so should flush through the whole Hx.

Probably something like this Vokera Vision 20S

 

[Ccw71]

Great, Thanks for all the help. I'll give it a go. I may leave it until after the weekend in case I make things worse.

The actual temps on the flow and return are Flow @ 54.8 Return @ 38 Room temp still climbing, but was around 17.5 at the time I recorded the boiler temps.

 

[Johntheo5]

Well, if its still range rated to 20kw and if its actually running at 20kw then a 16.8C dT gives a flow rate of 17.1 LPM, both quite normal.

You can get a very accurate feel for the actual flowrate if you keep range rating downwards in say 3 to5% increments until the flow temp just starts falling, flowrate is then 40 X %output X 860/60/dT.

What mode/setting is the pump on now?.

 

[Ccw71]

I ran it up to a steady flow temp of 64.5 where it's been sat for about an hour. All TRVS open with thr rads balanced to heat up about equally. Room temp is around 19-20. The boiler has modulated down to what sounds like minimum and the return temp is sitting around 48.8. The pump is running on CP3 as you recomended, 28 (watts?) on the screen.