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.

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