I suspect your boiler is too powerful for your intended use and there is nothing wrong with it.
Boilers use cheap thermistors that have a high thermal mass...y
We are assuming that the system has actually operated satisfactorily while the low flow taps and flexes have been fitted?
Have there been any other modifications to the plumbing side?
All those bits has no one checked or changed the main sensor
Totally agree with that
I would say the culprit is the thermal mass of the metal mass in the heat exchanger. The thermistor and associated circuity reports over temperature, the flames are shut down but is then several seconds before the water temperature starts to drop.
From the video in the first post.
The indicated temperature increases up to 56° and then at 53 seconds into the video a click can be heard and then after a few more seconds the temperature starts to fall.
The temperature continues to fall until 1 minute 16 seconds when it reaches 42° and then a double click can be heard and then after a few seconds temperature starts to rise again.
As the OP suggested in the very first post....the boiler is cycling ON and OFF
EDIT also take into account the thermal mass of the water circulating between primary and secondary heat exchangers
The temperature of the mains supply water may now in this hot summer be high enough that the heat required is less than the minimum the boiler can modulate down to.
I can say the boiler did once work correctly with the low flow fittings as when talking a shower it is most obvious due to the constantly fluctuating temperature which we never had before.
This is without rereading previous replies, do you get consistent hot water at other taps?
If yes, then I would be looking at shower mixer being the culprit
Why are you specifically saying the shower? Did it work correctly with a single tap on?
Does the shower on it's own now present the problem?
I have previously had to replace the shower mixer for problem you are having. Before rushing out to do that, I would put a clamp thermometer on 15mm pipe right of flow pipe or left of gas pipe under the boiler. If nothing else, run a hot tap full bore and feel above pipe for consistent heat. If I remember correctly, boiler will display primary temperature. It should read around 70 degrees
Now run the shower, do above test.
Then run shower as well as a cold tap in the bath, see if temperature is consistent
Sometimes high water mains can cause problem in the shower mixer
Just to clarify you want me to monitor the temperature of the DHW flow pipe under the boiler whilst running a hot water tap a full bore.
Then with the hot tap still running switch on the shower & the cold tap on the bath.
See if the temperature of the flow pipe is constant?
I know if I set the DHW thermostat to 50 degrees the boiler will let the primary water be heated to 60 degrees & then turn the burner off.
In that case as Gas112 says, control issue. With primary temp at 60 ( are you sure it is primary temperature, some boilers show secondary temperature during hw demand). I think a service and check boiler operation is on the cards.
I would focus initially on a hot tap run full bore. If boiler switches off at 60 for primary temperature, something is telling the controls things are getting too hot and burner goes out. Does the fan keep going full tilt to 60 degrees or does the fan starts to slow down till 60 degrees stop takes place
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