Hi there

My Vokera 24e Maxin combi developed this problem a couple of weeks
ago: when HW is continuously demanded, the burner switches itself on/
off, every 30 seconds or so. It means having to step in and out the
shower, waiting to catch the next stream of warm water, etc.

It's an old boiler that came with the house - we've been here 3
years. I haven't tested to see if the same problem occurs when
running the CH.

Any ideas?

thanks

Daljit

In article . com,
Daljit writes:
Hi there

My Vokera 24e Maxin combi developed this problem a couple of weeks
ago: when HW is continuously demanded, the burner switches itself on/
off, every 30 seconds or so. It means having to step in and out the
shower, waiting to catch the next stream of warm water, etc.

It's an old boiler that came with the house - we've been here 3
years. I haven't tested to see if the same problem occurs when
running the CH.

Can be caused by a furred up plate exchanger (in which case it
won't happen when running the CH). The scale in the plate exchanger
reduces the heat it can transfer to the HW, so the primary circuit
gets hotter than expected and the boiler cuts out.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Can be caused by a furred up plate exchanger (in which case it
won't happen when running the CH). The scale in the plate exchanger
reduces the heat it can transfer to the HW, so the primary circuit
gets hotter than expected and the boiler cuts out.


Hi

I've just tested if the burner cuts out when demand is from the CH,
and it does. Ignites for about a minute and then off for about 30
seconds.

thanks

Daljit

In article om,
Daljit writes:

Can be caused by a furred up plate exchanger (in which case it
won't happen when running the CH). The scale in the plate exchanger
reduces the heat it can transfer to the HW, so the primary circuit
gets hotter than expected and the boiler cuts out.

I've just tested if the burner cuts out when demand is from the CH,
and it does. Ignites for about a minute and then off for about 30
seconds.

Different problem then.
Possibly lack of flow through boiler. Maybe a blockage
or the pump is on it's way out. Guessing now though.
Is the flow pipe very hot just as it cuts out? (Be
careful not to burn yourself if it is.)

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Different problem then.
Possibly lack of flow through boiler. Maybe a blockage
or the pump is on it's way out. Guessing now though.
Is the flow pipe very hot just as it cuts out? (Be
careful not to burn yourself if it is.)

Hi

Apologies for taking a week to respond, but daytime job's been murder.

I've checked the pump by removing the vent(?) cap, and it does seem to
spin freely. However, i note that the flow pipe does heat up almost
too hot to touch, just before the burner cuts out - but i thought this
normal? It does then rapidly reduce in temperature, which again
suggests pump working. Question i guess is, is it working
correctly?

I found this page on pumps: www.pumptool.co.uk/Combi%20Boilers.html.
It provides instructions on how to overhaul a pump. Any thoughts on
this? Seems a lot of work.

Other info:
- the water in the system looks clean
- the pump's a Grundfos UPS 15-50
- boiler must be about 5 - 8 years old

Taking a different tack, would it be worthwhile looking at the
ignition box? Not quite sure what that is exactly, but a friend
mentioned it as a possibility.

thanks

Daljit

On Sun, 16 Sep 2007 04:10:45 -0700, Daljit wrote:


Different problem then.
Possibly lack of flow through boiler. Maybe a blockage
or the pump is on it's way out. Guessing now though.
Is the flow pipe very hot just as it cuts out? (Be
careful not to burn yourself if it is.)

Hi

Apologies for taking a week to respond, but daytime job's been murder.

I've checked the pump by removing the vent(?) cap, and it does seem to
spin freely. However, i note that the flow pipe does heat up almost
too hot to touch, just before the burner cuts out - but i thought this
normal? It does then rapidly reduce in temperature, which again
suggests pump working. Question i guess is, is it working
correctly?

I found this page on pumps: www.pumptool.co.uk/Combi%20Boilers.html.
It provides instructions on how to overhaul a pump. Any thoughts on
this? Seems a lot of work.

Other info:
- the water in the system looks clean
- the pump's a Grundfos UPS 15-50
- boiler must be about 5 - 8 years old

Taking a different tack, would it be worthwhile looking at the
ignition box? Not quite sure what that is exactly, but a friend
mentioned it as a possibility.

thanks

The next step would involve measuring the gas pressure to the burner to
see if it is being modulated. If the boiler is still cutting out on a
reasonable (5-10 litre/min) flow rate or the gas pressure is always at
maximum then other actions are needed.

I'd still go for the secondary heat exchanger being scaled up, as this is
a stock fault on all combis - but not in a soft water area.

Although I would have expected the burner to stay on longer for the
start of CH mode. This is where experience of just feeling around and
measuring a few things to get an accurate diagnosis is difficult to put
into text.

In a healthy system I would expect the initial burn in CH mode to be
around 5-10 minutes depending on the size of the system. At the end of
that period I would expect all the radiators to be hot. If the system has
more than 5 or more than 10 rads the longer times are expected needed.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

I'd still go for the secondary heat exchanger being scaled up, as this is
a stock fault on all combis - but not in a soft water area.

Although I would have expected the burner to stay on longer for the
start of CH mode.

This indeed is the case - the burner's lit longer when demand is for
CH - about 30 - 45 secs longer - altho doesn't stay on anywhere near
long enough for eight radiators to get warm. And yes, it's soft
(Scottish) water.


The next step would involve measuring the gas pressure to the burner to
see if it is being modulated. If the boiler is still cutting out on a
reasonable (5-10 litre/min) flow rate or the gas pressure is always at
maximum then other actions are needed.

I need to do a bit more background reading before being able to report
this (reckon it'll be next Sunday).

thanks for advice

Daljit

The next step would involve measuring the gas pressure to the burner to
see if it is being modulated. If the boiler is still cutting out on a
reasonable (5-10 litre/min) flow rate or the gas pressure is always at
maximum then other actions are needed.


Ok - checked burner pressures and they matched those given in service
manual - approx 10mbar and 1.5mbar for max/min DHW demand. I did need
to remove the modulator coil to establish output at the minimum
setting, but this isn't unusual (according to the manual). I noticed
that the boiler didn''t cut-out at the min setting (which of course
makes sense).

I also checked the soft light ignition pressure, and this too was
fine, cutting out after 10 secs like the manual says its should.

Any other suggestions?

thanks

Daljit

On Sat, 22 Sep 2007 06:22:45 -0700, Daljit wrote:

The next step would involve measuring the gas pressure to the burner to
see if it is being modulated. If the boiler is still cutting out on a
reasonable (5-10 litre/min) flow rate or the gas pressure is always at
maximum then other actions are needed.


Ok - checked burner pressures and they matched those given in service
manual - approx 10mbar and 1.5mbar for max/min DHW demand. I did need
to remove the modulator coil to establish output at the minimum
setting, but this isn't unusual (according to the manual). I noticed
that the boiler didn''t cut-out at the min setting (which of course
makes sense).

I also checked the soft light ignition pressure, and this too was
fine, cutting out after 10 secs like the manual says its should.

Any other suggestions?


What flow rate are you getting for the DHW?
Pump problems (OK I accept they are more likely now we have eliminated the
common stuff)
temperature sensors.
PCB problems.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

What flow rate are you getting for the DHW?
Pump problems (OK I accept they are more likely now we have eliminated the
common stuff)
temperature sensors.
PCB problems.

Flow rate is approx 7 l/min when DHW control turned to max. I haven't
noticed the flow in the

Is there anyway i can check pump is working optimally?

Temperature sensor - i've located one - the thermistor (part no 8484)
- but how do i check it's working properly?

PCB - I'm not that good with electrics, so would be lost if the
problem's here.

thanks for your perseverance!

Daljit

On Sun, 23 Sep 2007 03:24:06 -0700, Daljit wrote:


What flow rate are you getting for the DHW?
Pump problems (OK I accept they are more likely now we have eliminated the
common stuff)
temperature sensors.
PCB problems.

Flow rate is approx 7 l/min when DHW control turned to max. I haven't
noticed the flow in the

OK with that flow rate the boiler should modulate and give a steady stream
of HW at a steady temperature.


Is there anyway i can check pump is working optimally?

We have the surprising behaviour of the CH cutting out as well as the DHW.

Here are some of the possibilities in no particular order:
1) There is inadequate flow in the primary circuit; so the temperature
rise in the primary heat exchanger is too great and the boiler cuts out.

To diagnose you need to work out what is getting hot and what is not.
In CH mode make sure at least one or two TRVs are full on.
Correct operation: The flow pipe is a little warm and continues to warm up
steadily over 5 -10 minutes to untouchable. If the pipe is cold and then
very hot there is poor primary flow. Either pump is bad or the heat
exchanger is blocked or some other fault with the same effect.

2) The Sensor is over enthusiastic,T so the PCB thinks the boiler is
over heating and shuts the burner down.
raden/geoff can say better than me - but 10k Ohm cold and 1k Ohm hot
would be in the range.

3) The modulator is working right mechanically but is being told to do
the wrong thing?
The ignition rate looks plausible. The gas pressures during operation are
crucial here. Full for DHW until steady state has arrived in which case
for 7l/min something in the mid range should be OK.



PCB - I'm not that good with electrics, so would be lost if the
problem's here.

Alas, the game nowadays requires electrical, mechanical and IT skills.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

IT'S FIXED! Magic! You'll never believe what the problem was - am
still amazed myself.

This morning i phoned the Vokera tech support line - the one for
CORGIs - and confidently described my problem, the checks i had
undertaken, and the results. Told him i was completely stumped. He
asked if the fan was working, and i said it was. Then he asked if i
had checked the flue something-or-other. He said the name a couple of
times but i still didn't catch-it, so i asked him what he meant. He
said if i push the flue (at the top of the boiler) to one side and
peer in, i should be able to see a 10mm pipe sticking up. He told me
to scrape away anything that might be blocking this (think he said it
sucks in air). Anyway, that's exactly what i done and found there was
a tiny bit of white material on the lip (less than 4mm diameter).
Scraped it away, assembled the flue, and tried it out - and bingo -
worked first time!

Great stuff - have now just run a bath to celebrate. Wife thinks i'm
a god too.

Anyway, cheers for all your advice. I reckon if i got a man in, he
would have done all the same checks you suggested and then phoned
Vokera for the solution - it's happened to me before (with a
Vaillant). Anyway, i've saved some cash so name your charity - will
send em a chq for £25 - email your address to rev_funk@hotmail with a
co. and uk. and i'll send you the receipt.

cheers

Daljit