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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