No, I think DB Mandrake is right in saying that everything is working as designed. My cylinder is a Megaflo 210i for which the online adverts boast
"Patented cold water inlet diffuser minimises mixing of cold and hot water" . (See:
https://www.plumbnation.co.uk/site/h...ater-cylinder/ on the "Description" tab.) I think the probe is measuring the temperature close to the inlet and so, while hot water is being drawn off, it will report a temperature close to the cold supply temperature. Once the flow stops, there is indeed some mixing and I do see the temperature rise a little.
Here is a picture of the cylinder.
Megaflo cylinder.jpg The temperature probe is on the end of the thin black wire and is in the purpose-built pocket in the cylinder. The mechanical thermostat is under the grey cover to the right of the probe and is wired in series with the BDR91. Its performance implies to me that the temperature reporting of the CS92 is about right, because when the CS92 overshoots and then catches up (i.e. when the mechanical thermostat has limited the temperature rise before the CS92 reports a temperature rise), the Evohome final reported temperature matches the mechanical setpoint to within a couple of degrees.
The frost protection has come on again this morning and I attach a couple of photos showing the Evotouch showing 6
oC and the blue HW "Off" icon but the System Summary screen showing "Stored Hot Water" demand at 100%. (I guess it had dipped below 5
oC briefly to trigger the DHW heating.)
20171229_100339_DHW frost protection.jpg 20171229_100353_System Summary.jpg
I do have unusually cold mains water. In Scotland all our mains water comes from surface sources (rivers and lochs) whereas in England a significant proportion comes from underground sources (aquifers and boreholes) so I think our water is more susceptible to changes in the ambient air temperature, otherwise we would have warm mists over our lochs in the mornings. My house is a large old property and is connected to a large diameter main which, I guess, gives only a short residency time for the water between the reservoir and my house. I know that some housing developments will have a long reticulated distribution network which can mean that water takes as much as 24 hours to come out of a household tap after treatment. In those cases, with smaller diameter pipes, I guess the water will gain temperature from the ground and from the internals of the property before being used which could explain why some on this forum report winter temperatures not falling below 10
oC.
Anyway, my point in my recent posting was not to suggest anything was not working as intended. I think I am in a minority of one in wanting to disable the frost protection so I will stop advocating that. I was just keen to share the information that the "Off" setting for the Evohome hot water is actually just a change in the setpoint to 5
oC + the differential. And that, if you have a cold water supply below 5
oC, then the Evohome hot water will act like a combi boiler and always reheat when hot water is drawn off, whether the hot water is set to "On" or "Off".