I have a Nest Learning Thermostat 3rd generation (installed March 2020) with a system boiler, 200 litre hot water cylinder & conventional radiator central heating (CH), serving 2 adults with normal usage. Domestic water heating (DWH) is scheduled for each day for 1 hr 30 min in the morning and 30 min early evening. Since about May 2022 we've noticed the boiler firing up outside the scheduled DWH times, but not to activate CH. The thermostat screen remains black; I must admit I've never noticed whether the water drop icon in a circle appears, which might be the telltale confirmation of what's happening. We couldn't understand why it was firing like this, seemingly arbitrarily, UNTIL I discovered the forum posts about Bacteria Prevention (BP) mode and our criteria appear to match those that would cause it to kick in. I've temporarily disabled the BP mode to see if that is what's been causing the firing up and will monitor it over the next week or two. I now strongly suspect the BP cutting in was the reason.
The Support page "How the Nest thermostat helps prevent Legionella bacteria"
is here: https://support.google.com/googlenest/answer/9256656?hl=en-GB
Please can anyone answer the following? (In these scenarios, apart from Q6, it is assumed that CH is not causing the boiler to fire up.)
1. When it says in the Support page "Bacteria Prevention mode will activate if your domestic water hasn’t been heated for at least two consecutive hours in the last 48 hours" is that literally the required condition: (a) the boiler actually fired up and heating continuously for at least 2 hours, or (b) is it simply that it's scheduled to heat for at least 2 hours?
2. If (a) is correct presumably it makes no difference whether the scheduled time is 2 hours or more as even with, say, a 3 hour schedule it's probably unlikely the boiler will stay fired up for a whole period of at least 2 hours within that scheduled period? The boiler might be going on and off during that period. Obviously if the schedule is less than 2 hours (as mine has been) BP will always potentially be activated as the boiler can otherwise never be operating for at least 2 consecutive hours for DWH.
3. If however (b) is correct would setting the schedule for 2 hours (or would it need to be 2 hrs 30 min?) resolve it, so the BP mode can stay enabled but it wouldn't kick in, because the "at least 2 consecutive hours" (schedule) criterion has been met? This would mean BP could (as it should) remain permanently enabled so that it provides protection for when we are away from home for lengthy periods, when we would turn the scheduled DWH off.
4. Is the "gap of more than 48 hours" in scheduled times a requirement before the 2 consecutive hours of heating criterion applies? Or does the "at least 2 consecutive hours" criterion kick in even if there is no such 48 hour gap between scheduled times? The wording of the Support page ("can happen" as against "will happen") suggests to me it is the latter state that applies.
5. Does the actual water temperature in the cylinder have any impact on whether BP kicks in or is it purely the timing criteria that does so? The water in our cylinder is almost always adequately hot so it's not as if high usage is causing the water temperature to drop significantly. Nothing in the Support page suggests water temperature is a factor.
6. If the CH was operating and the boiler was firing for more than 2 consecutive hours for CH (outside scheduled DWH periods) would this have any effect upon the "at least 2 hours" criterion for BP kicking in? I'm assuming it would not and the boiler firing for CH is irrelevant to BP activation. I don't imagine the BP activator knows whether the boiler is firing for DWH or CH (or does it?).
7. The screen examples shown in the Support page (showing the difference between the water drops for BP or Hot Water Boost) both show the screen as orange, presumably because the situation presented also has the CH operating. So is it the case that, without CH also firing up the boiler, if BP kicks in, the screen would remain black, but with the circled water drop icon? When I've observed the boiler firing when it seems it shouldn't be, it's obviously always been when the CH hasn't been operating (otherwise I wouldn't have thought it odd for the boiler to be firing).
And finally, 8. Just to complicate things, we've recently had solar panels installed with a diverter fitted to the DHW immersion heater so that any excess electricity generated (after load consumption does not require it and the battery is full) is diverted to operate the immersion heater. Is it correct to think that this would not have any impact on whether BP kicked in, if the criteria are not temperature related, but purely down to the timed periods of boiler firing/schedule)? (The boiler firing up issue goes back way before the solar was installed, in fact we've had a new Heat Link (July 2022 after a power surge bricked the original) and a new boiler (August 2022) since we first noticed the firing up issue in May 2022, neither of which made any difference to that issue.)
Apologies for the length of this enquiry but I guess it just shows how much potential there could be for uncertainty (especially if there were other factors such as incorrect wiring to the cylinder thermostat to muddy the hot water). Thank you to anyone who can come up with some reasonably definitive answers, some of which are probably perfectly obvious, dependent upon how literally one interprets the Support page information, and some could probably be confirmed through observation, but that's not always practical 🙄
Thanks again,
Richard
