Fluctuating Water Temperature

For years I have had issues with the domestic hot water not staying a constant temperature. There are no error messages that I can see and over several years I have changed both GT153 sensors, the main PCB and even the heat exchanger. However, it continues to be a problem.

We overcome the issue by either switching on the immersion heater whilst the non-thermostatic shower is in use, or by use of a thermostatic controlled shower in our en-suite. That at least negates the need for the immersion between spring and autumn.

The temperature change is less noticeable if I use one of our PCB's that have been repaired by that excellent chap, Murray Martin in Newark, so I can only assume he uses better tolerance components than Gledhill did. However, the temperature still fluctuates enough that our daughter needs to switch on the immersion when she uses the shower in the main bathroom.

Can anyone offer advice on where to go next?

Have you checked the DHW heat exchanger pump is set to speed III - when mine was at lower speeds, the DHW fluctuated

Not something I have considered. Is this something I can see by looking at the pump? I will take the front cover off later in the week and have a look. I will post the result here.