I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?




--
AnthonyL

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?


Has it always done this? As a mere user of the Baxi 105HE, not an
engineer thereof, I'm wondering where the CH & Hot Water temperature
knobs are set? Mine are set so the temperature never quite goes up to 70
(I was told to do this to reduce limescale deposits though I don't know
if that really works.) I never have any thermal cutout problems.

Hope this helps,

Nick

On Fri, 8 Mar 2019 13:06:28 +0000, Nick Odell
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?


Has it always done this? As a mere user of the Baxi 105HE, not an
engineer thereof, I'm wondering where the CH & Hot Water temperature
knobs are set? Mine are set so the temperature never quite goes up to 70
(I was told to do this to reduce limescale deposits though I don't know
if that really works.) I never have any thermal cutout problems.


I had some problem a few months ago that led me to deduce that having
the knob set too low appeared to be causing cut out. I was set to
about 2 'o' clock, I'm now on about 4 'o' clock. 6 'o' clock ie
vertically down seems to be the max. I'll wind it back a notch or two
and see what happens but I'm definitely hitting max temp at the
current setting.

--
AnthonyL

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

When my oil fired combi went wrong I did think that with four multiple
terminal relays it is to complicated for its own good. What I would
quite like is a series of LEDs on the front panel that show the status
of the relays, thermostats and other sensors. I was surprised to
discover that it does not appear to care about the pressure in the
central heating (and internal hot water tank) system.

--
Michael Chare

On Friday, 8 March 2019 12:24:34 UTC, AnthonyL wrote:
Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

It depends on combination of the boiler and the type of shower mixer. If the shower throttles back the flow of hot the temp rises and eventually the boiler will cut out.

Some thermostatic mixers and some combis play nicely together, others fight.

Owain

Wrote in message:
On Friday, 8 March 2019 12:24:34 UTC, AnthonyL wrote:
Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

It depends on combination of the boiler and the type of shower mixer. If the shower throttles back the flow of hot the temp rises and eventually the boiler will cut out.

+1 Sounds extremely likely.
Turn hw temp setting down, try experiment again?

Perhaps watching wife showering unless they are both boilers? :-)

--
Jim K


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

On Fri, 8 Mar 2019 05:19:03 -0800 (PST),
wrote:

On Friday, 8 March 2019 12:24:34 UTC, AnthonyL wrote:
Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

It depends on combination of the boiler and the type of shower mixer. If the shower throttles back the flow of hot the temp rises and eventually the boiler will cut out.

Some thermostatic mixers and some combis play nicely together, others fight.


I don't think it is a thermostatic mixer, simply turn and changes the
ratio of hot/cold.

--
AnthonyL

On 09/03/2019 13:38, AnthonyL wrote:
On Fri, 8 Mar 2019 05:19:03 -0800 (PST),
wrote:

On Friday, 8 March 2019 12:24:34 UTC, AnthonyL wrote:
Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

It depends on combination of the boiler and the type of shower mixer. If the shower throttles back the flow of hot the temp rises and eventually the boiler will cut out.

Some thermostatic mixers and some combis play nicely together, others fight.


I don't think it is a thermostatic mixer, simply turn and changes the
ratio of hot/cold.

That would be true of a thermostatic mixer as well though...

Do you know what make/model of shower it is?


--
Cheers,

John.

/================================================== ===============\
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|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?




I had that sort of problem from new with both a Vaillant and a Halstead,
admittedly in an older and rambling house with longish pipe runs. Given
up and gone to a proper system boiler.

Some friends have a small holiday cottage that we use occasionally, this
is a new compact house with very direct pipe runs and the shower is
flawless, even with other demand on the small combi.

On Fri, 8 Mar 2019 16:22:03 +0000, newshound
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?




I had that sort of problem from new with both a Vaillant and a Halstead,
admittedly in an older and rambling house with longish pipe runs. Given
up and gone to a proper system boiler.


I thought the Vaillant was a proper system boiler?

--
AnthonyL

On 09/03/2019 13:39, AnthonyL wrote:
On Fri, 8 Mar 2019 16:22:03 +0000, newshound
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?




I had that sort of problem from new with both a Vaillant and a Halstead,
admittedly in an older and rambling house with longish pipe runs. Given
up and gone to a proper system boiler.


I thought the Vaillant was a proper system boiler?

Vaillant make Heating only boilers (vented and unvented), System
boilers, and Combi boilers. Typically the 400, 600, and 800 series
respectively. So a HE 624 is a 24kW System boiler and a 838 a 38 kW Combi.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Sat, 9 Mar 2019 15:19:02 +0000, John Rumm
wrote:

On 09/03/2019 13:39, AnthonyL wrote:
On Fri, 8 Mar 2019 16:22:03 +0000, newshound
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?




I had that sort of problem from new with both a Vaillant and a Halstead,
admittedly in an older and rambling house with longish pipe runs. Given
up and gone to a proper system boiler.


I thought the Vaillant was a proper system boiler?

Vaillant make Heating only boilers (vented and unvented), System
boilers, and Combi boilers. Typically the 400, 600, and 800 series
respectively. So a HE 624 is a 24kW System boiler and a 838 a 38 kW Combi.



Goes to show what I know about boilers. Too many years with coal
fired heating.

I'm guessing our bungalow used to have a system boiler in the loft
with supplementary immersion heating. In fact that's pretty well what
I had in my previous coal fired property and to be honest that worked
well as the immersion heater wouldn't need to do anything when there
was a fire and kept a stock of hot water using E7 when heating was
needed.

--
AnthonyL

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

I've seen this when the secondary plate exchanger was scaled up.
Boiler would throttle the burner back, but when the plate exchanger gets
so bad it can't conduct the minimum burner power through the lime scale,
then the burner ends up cycling on and off.

You might get a short improvement by turning the hot water temperature
to max, as a higher temperature differential will enable more power to
be passed through and it might be able to handle the full min burner
power without cycling, but this will also scale up the plate exchanger
faster, so it's only a very short term fix.

I took the plate exchanger out and descaled it. It required an enormous
amount of descaler - fortunately I had a 2kg tub of Furnox DS-3, and it
took quite a while. A plumber would probably just replace the plate
exchanger.

--
Andrew

On Fri, 8 Mar 2019 16:30:09 +0000, Andrew Gabriel
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

I've seen this when the secondary plate exchanger was scaled up.
Boiler would throttle the burner back, but when the plate exchanger gets
so bad it can't conduct the minimum burner power through the lime scale,
then the burner ends up cycling on and off.

You might get a short improvement by turning the hot water temperature
to max, as a higher temperature differential will enable more power to
be passed through and it might be able to handle the full min burner
power without cycling, but this will also scale up the plate exchanger
faster, so it's only a very short term fix.

I took the plate exchanger out and descaled it. It required an enormous
amount of descaler - fortunately I had a 2kg tub of Furnox DS-3, and it
took quite a while. A plumber would probably just replace the plate
exchanger.


Makes sense. Just a bit of a limit on what I should spend on a 2006
built boiler perhaps. Needed a new diverter valve 18 months ago.

--
AnthonyL

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

If we assume for the moment that the boiler is working as it should then
there is a likely cause that could result from the boiler DHW
temperature going over the set point.

IIUC, your Baxi does not have any form of flow regulation. So all it can
do in response to varying DHW flow rates is modulate its burner output.

So normally you set a temperature, say 60 deg, and the boiler will
attempt to deliver water at that temp.

If you are drawing DHW at a high rate, then its unlikely to have the
power to reach that temp - and so the temp will fall below the max.

If however draw at a low rate, then it will have to modulate its burner
output power down to prevent the temperature exceeding the limit. If it
can't keep under the limit even at minimum burner power, then it will
start to cycle the burner.

You can test this theory, but running the shower as normal, but by
adding some extra demand - say running the hot tap in the basin to
increase the DHW flow rate a bit.

If that stops the burner cycling, then you may be able to fix the
problem simply by increasing the max DHW temp on the boiler (the TMV on
the shower should keep the person in there safe). If that still does not
do it, then you may need to change the shower head for a more "thirsty"
one, or use the basin tap trick for every shower.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Friday, 8 March 2019 18:19:00 UTC, John Rumm wrote:
On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

If we assume for the moment that the boiler is working as it should then
there is a likely cause that could result from the boiler DHW
temperature going over the set point.

IIUC, your Baxi does not have any form of flow regulation. So all it can
do in response to varying DHW flow rates is modulate its burner output.

So normally you set a temperature, say 60 deg, and the boiler will
attempt to deliver water at that temp.

If you are drawing DHW at a high rate, then its unlikely to have the
power to reach that temp - and so the temp will fall below the max.

If however draw at a low rate, then it will have to modulate its burner
output power down to prevent the temperature exceeding the limit. If it
can't keep under the limit even at minimum burner power, then it will
start to cycle the burner.

You can test this theory, but running the shower as normal, but by
adding some extra demand - say running the hot tap in the basin to
increase the DHW flow rate a bit.

If that stops the burner cycling, then you may be able to fix the
problem simply by increasing the max DHW temp on the boiler (the TMV on
the shower should keep the person in there safe). If that still does not
do it, then you may need to change the shower head for a more "thirsty"
one, or use the basin tap trick for every shower.


the funny thing is these things are marketed as energy saving


NT

On Fri, 8 Mar 2019 18:18:50 +0000, John Rumm
wrote:

If that still does not
do it, then you may need to change the shower head for a more "thirsty"
one, or use the basin tap trick for every shower.


Or --depending on your water-- descale the shower head...


Thomas Prufer

On Fri, 8 Mar 2019 18:18:50 +0000, John Rumm
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

If we assume for the moment that the boiler is working as it should then
there is a likely cause that could result from the boiler DHW
temperature going over the set point.


It's working as well as I can tell for the CH.

IIUC, your Baxi does not have any form of flow regulation. So all it can
do in response to varying DHW flow rates is modulate its burner output.

So normally you set a temperature, say 60 deg, and the boiler will
attempt to deliver water at that temp.

If you are drawing DHW at a high rate, then its unlikely to have the
power to reach that temp - and so the temp will fall below the max.


That I'd be happy with as even though the water will not get very hot
it is simply a matter of turning the shower knob anticlockwise to
reduce the amount of cold. As stated the problem is when the burner
goes out.

If however draw at a low rate, then it will have to modulate its burner
output power down to prevent the temperature exceeding the limit. If it
can't keep under the limit even at minimum burner power, then it will
start to cycle the burner.


That's what seems to be happening.

You can test this theory, but running the shower as normal, but by
adding some extra demand - say running the hot tap in the basin to
increase the DHW flow rate a bit.


Good point. I'll give that a go.

If that stops the burner cycling, then you may be able to fix the
problem simply by increasing the max DHW temp on the boiler (the TMV on
the shower should keep the person in there safe).

No TMV afaia, simply turn anticlockwise to reduce the cold water.

If that still does not
do it, then you may need to change the shower head for a more "thirsty"
one, or use the basin tap trick for every shower.


Interesting, I'm wondering now when I did change the shower head
because my wife found the other one too gentle. But that is probably
the opposite to what you are proposing.


--
AnthonyL

On 09/03/2019 13:50, AnthonyL wrote:
On Fri, 8 Mar 2019 18:18:50 +0000, John Rumm
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

If we assume for the moment that the boiler is working as it should then
there is a likely cause that could result from the boiler DHW
temperature going over the set point.


It's working as well as I can tell for the CH.

IIUC, your Baxi does not have any form of flow regulation. So all it can
do in response to varying DHW flow rates is modulate its burner output.

So normally you set a temperature, say 60 deg, and the boiler will
attempt to deliver water at that temp.

If you are drawing DHW at a high rate, then its unlikely to have the
power to reach that temp - and so the temp will fall below the max.


That I'd be happy with as even though the water will not get very hot
it is simply a matter of turning the shower knob anticlockwise to
reduce the amount of cold. As stated the problem is when the burner
goes out.

Yup, just including the description for completeness.


If however draw at a low rate, then it will have to modulate its burner
output power down to prevent the temperature exceeding the limit. If it
can't keep under the limit even at minimum burner power, then it will
start to cycle the burner.


That's what seems to be happening.

You can test this theory, but running the shower as normal, but by
adding some extra demand - say running the hot tap in the basin to
increase the DHW flow rate a bit.


Good point. I'll give that a go.

Alternatively you could try running the shower with just the hose (i.e.
head removed)

But note also the comment from Andy and others. A scaled shower head
could cause a similar issue as could a scaled up secondary HX in the
boiler.


If that stops the burner cycling, then you may be able to fix the
problem simply by increasing the max DHW temp on the boiler (the TMV on
the shower should keep the person in there safe).

No TMV afaia, simply turn anticlockwise to reduce the cold water.

In which case you will need to be wary of setting the DHW top limit too
high since you don't want to have a scalding risk. If you set it at 60
max, then by the time you allow for a bit of loss in the pipework etc
that ought to be still cool enough to get out of the way should it start
getting too hot.

If that still does not
do it, then you may need to change the shower head for a more "thirsty"
one, or use the basin tap trick for every shower.


Interesting, I'm wondering now when I did change the shower head
because my wife found the other one too gentle. But that is probably
the opposite to what you are proposing.

Well a high flow rate head, with lots of wide jets may give a "softer"
more rain like flow, that a lower consumption one with a smaller number
of narrower more focussed jets. How hard the shower feels will be a
product of the speed of the water and the weight of it hitting an area
of the body.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Sat, 09 Mar 2019 13:50:19 GMT, lid (AnthonyL)
wrote:

On Fri, 8 Mar 2019 18:18:50 +0000, John Rumm
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

If we assume for the moment that the boiler is working as it should then
there is a likely cause that could result from the boiler DHW
temperature going over the set point.


It's working as well as I can tell for the CH.

IIUC, your Baxi does not have any form of flow regulation. So all it can
do in response to varying DHW flow rates is modulate its burner output.

So normally you set a temperature, say 60 deg, and the boiler will
attempt to deliver water at that temp.

If you are drawing DHW at a high rate, then its unlikely to have the
power to reach that temp - and so the temp will fall below the max.


That I'd be happy with as even though the water will not get very hot
it is simply a matter of turning the shower knob anticlockwise to
reduce the amount of cold. As stated the problem is when the burner
goes out.

If however draw at a low rate, then it will have to modulate its burner
output power down to prevent the temperature exceeding the limit. If it
can't keep under the limit even at minimum burner power, then it will
start to cycle the burner.


That's what seems to be happening.

You can test this theory, but running the shower as normal, but by
adding some extra demand - say running the hot tap in the basin to
increase the DHW flow rate a bit.


Good point. I'll give that a go.

If that stops the burner cycling, then you may be able to fix the
problem simply by increasing the max DHW temp on the boiler (the TMV on
the shower should keep the person in there safe).


No it didn't do the job.

Also if I turned the temperature down the boiler started cycling a bit
erratically and with bit of what sounds like valves or circuit breaker
noises. If I turned the temperature up it was better but would still
cut out.

No TMV afaia, simply turn anticlockwise to reduce the cold water.


It is a Mira 415. I suspect put in at the same time as the boiler so
2006?


If that still does not
do it, then you may need to change the shower head for a more "thirsty"
one, or use the basin tap trick for every shower.

I'm fairly sure the problem is in the boiler somewhere. The shower
has worked fine in the 3 1/2 years we've been here. The only changes
have been:

1) Flow reducer valve put in a couple of years ago
2) Change shower head to one that doesn't have such a fine spray
3) Replacement diverter valve about 18 months ago

It's definitely cycling when it shouldn't and as I understand it now
the burner should simply adjust and not turn off/on though I presume
that it correct operation for the CH?


--
AnthonyL

On 10/03/2019 10:40, AnthonyL wrote:
On Sat, 09 Mar 2019 13:50:19 GMT, lid (AnthonyL)
wrote:

On Fri, 8 Mar 2019 18:18:50 +0000, John Rumm
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out? Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar? Is it easy to replace?

If we assume for the moment that the boiler is working as it should then
there is a likely cause that could result from the boiler DHW
temperature going over the set point.


It's working as well as I can tell for the CH.

IIUC, your Baxi does not have any form of flow regulation. So all it can
do in response to varying DHW flow rates is modulate its burner output.

So normally you set a temperature, say 60 deg, and the boiler will
attempt to deliver water at that temp.

If you are drawing DHW at a high rate, then its unlikely to have the
power to reach that temp - and so the temp will fall below the max.


That I'd be happy with as even though the water will not get very hot
it is simply a matter of turning the shower knob anticlockwise to
reduce the amount of cold. As stated the problem is when the burner
goes out.

If however draw at a low rate, then it will have to modulate its burner
output power down to prevent the temperature exceeding the limit. If it
can't keep under the limit even at minimum burner power, then it will
start to cycle the burner.


That's what seems to be happening.

You can test this theory, but running the shower as normal, but by
adding some extra demand - say running the hot tap in the basin to
increase the DHW flow rate a bit.


Good point. I'll give that a go.

If that stops the burner cycling, then you may be able to fix the
problem simply by increasing the max DHW temp on the boiler (the TMV on
the shower should keep the person in there safe).


No it didn't do the job.

Also if I turned the temperature down the boiler started cycling a bit
erratically and with bit of what sounds like valves or circuit breaker
noises. If I turned the temperature up it was better but would still
cut out.

No TMV afaia, simply turn anticlockwise to reduce the cold water.


It is a Mira 415. I suspect put in at the same time as the boiler so
2006?

Ah, ok the 415 is not a traditional TMV but a pressure balanced valve
instead. These are specially designed for use with combi's since a
traditional TMV can have unexpected results.

(i.e. if the temp falls, a normal TMV will compensate by increasing the
flow of hot. On a combi that might actually lower the temp of the hot
supply even more. A PB valve will keep the pressure on the inlets
balanced, this tends to better reflect the effect of demands being made
elsewhere in the house)

If that still does not
do it, then you may need to change the shower head for a more "thirsty"
one, or use the basin tap trick for every shower.

I'm fairly sure the problem is in the boiler somewhere. The shower
has worked fine in the 3 1/2 years we've been here. The only changes
have been:

1) Flow reducer valve put in a couple of years ago
2) Change shower head to one that doesn't have such a fine spray
3) Replacement diverter valve about 18 months ago

It's definitely cycling when it shouldn't and as I understand it now
the burner should simply adjust and not turn off/on though I presume
that it correct operation for the CH?

The main difference between CH operation and DHW is the addition of the
secondary plate heat exchanger for the DHW.

Since we have now eliminated a lack of adequate DHW demand as a cause,
it points the finger of suspicion at the Plate HX, and lack of adequate
rate of heat transfer. The MO would be the system fires, but the primary
side runs hotter since you are getting less cooling effect from the PHX.

Hard water area?


--
Cheers,

John.

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On 10/03/2019 13:57, John Rumm wrote:
On 10/03/2019 10:40, AnthonyL wrote:
On Sat, 09 Mar 2019 13:50:19 GMT, lid (AnthonyL)
wrote:

On Fri, 8 Mar 2019 18:18:50 +0000, John Rumm
wrote:

On 08/03/2019 12:24, AnthonyL wrote:
I've been trying to understand what happens with the hot water when my
wife screams that it has gone cold in the shower so I watched it the
other day.

Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum.Â* Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams.Â* Burner comes back
on - repeat and rinse.

Shouldn't the burner flame just adjust to maintain the temperature
rather than cut-out?Â* Obvious behaviour of the CH but not for DHW.

Otherwise is this indicative of a thermistor problem with hysterisis
or similar?Â* Is it easy to replace?

If we assume for the moment that the boiler is working as it should
then
there is a likely cause that could result from the boiler DHW
temperature going over the set point.


It's working as well as I can tell for the CH.

IIUC, your Baxi does not have any form of flow regulation. So all it
can
do in response to varying DHW flow rates is modulate its burner output.

So normally you set a temperature, say 60 deg, and the boiler will
attempt to deliver water at that temp.

If you are drawing DHW at a high rate, then its unlikely to have the
power to reach that temp - and so the temp will fall below the max.


That I'd be happy with as even though the water will not get very hot
it is simply a matter of turning the shower knob anticlockwise to
reduce the amount of cold.Â* As stated the problem is when the burner
goes out.

If however draw at a low rate, then it will have to modulate its burner
output power down to prevent the temperature exceeding the limit. If it
can't keep under the limit even at minimum burner power, then it will
start to cycle the burner.


That's what seems to be happening.

You can test this theory, but running the shower as normal, but by
adding some extra demand - say running the hot tap in the basin to
increase the DHW flow rate a bit.


Good point.Â* I'll give that a go.

If that stops the burner cycling, then you may be able to fix the
problem simply by increasing the max DHW temp on the boiler (the TMV on
the shower should keep the person in there safe).


No it didn't do the job.

Also if I turned the temperature down the boiler started cycling a bit
erratically and with bit of what sounds like valves or circuit breaker
noises.Â* If I turned the temperature up it was better but would still
cut out.

No TMV afaia, simply turn anticlockwise to reduce the cold water.


It is a Mira 415.Â* I suspect put in at the same time as the boiler so
2006?

Ah, ok the 415 is not a traditional TMV but a pressure balanced valve
instead. These are specially designed for use with combi's since a
traditional TMV can have unexpected results.

(i.e. if the temp falls, a normal TMV will compensate by increasing the
flow of hot. On a combi that might actually lower the temp of the hot
supply even more. A PB valve will keep the pressure on the inlets
balanced, this tends to better reflect the effect of demands being made
elsewhere in the house)

If that still does not
do it, then you may need to change the shower head for a more "thirsty"
one, or use the basin tap trick for every shower.

I'm fairly sure the problem is in the boiler somewhere.Â* The shower
has worked fine in the 3 1/2 years we've been here.Â* The only changes
have been:

1) Flow reducer valve put in a couple of years ago
2) Change shower head to one that doesn't have such a fine spray
3) Replacement diverter valve about 18 months ago

It's definitely cycling when it shouldn't and as I understand it now
the burner should simply adjust and not turn off/on though I presume
that it correct operation for the CH?

The main difference between CH operation and DHW is the addition of the
secondary plate heat exchanger for the DHW.

Since we have now eliminated a lack of adequate DHW demand as a cause,
it points the finger of suspicion at the Plate HX, and lack of adequate
rate of heat transfer. The MO would be the system fires, but the primary
side runs hotter since you are getting less cooling effect from the PHX.

Diverter valve.

DHW thermostat.

--
Max Demian