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I have no cables into the airing cupboard so no tank thermostat, during the summer months the intention was to have 1/2 hour every other day with boiler on, which is enough to get hot water to wash hands, with both Sunday and Monday with 1/2 hour giving an extra boost on Monday.

The boiler actually runs for 19 minutes, then starts to cycle, and in this weather no central heating.

On the days programmed to come on it works fine, but on the days programmed not to come on, I find the boiler has fired up, although the thermostat, PC, and phone all say it's not running, in real terms it is.

If I switch off the schedule then it does not fire up, either on schedule or any other time, but with schedule active seems to have a mind of it's own, firing up today at 10 am.

Simple cure is turn off the schedule and just press boost when I want DHW, also fitting a cistern thermostat would cure it, but would not need that if it actually followed the schedule set.

Since it does not fire up is schedule is not enabled, seems clearly down to software fault, but is it some thing I have done? Or some learning process? or some thing else?

Any ideas please.
 
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davesparks

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If the hot water has not been on for a period of 2 hours at any time in a 48 hour period the nest will automatically run a bacteria prevention cycle which heats the hot water periodically. This is to prevent the build up of legionnella bacteria.
There will be a small yellow and black hot water symbol on the display when this is running.
This setting can be switched off from the nest settings if you wish, just look online for the instructions on how to do this.
 
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Thanks it seems that was exactly what was happening, not going to call it a problem, as it may be a good thing, anyway now disabled, the boiler only runs for 19 minutes before is starts to cycle due to return water temperature so may not need 2 hours running, but I will test temperature of water.
 

davesparks

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The temperature of the return water doesn't necessarily indicate the temperature of the stored water in the tank.

Presumably this is a gravity system if there is no cylinder stat?
 

UNG

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Thanks it seems that was exactly what was happening, not going to call it a problem, as it may be a good thing, anyway now disabled, the boiler only runs for 19 minutes before is starts to cycle due to return water temperature so may not need 2 hours running, but I will test temperature of water.
I think you need to check the temperature of the whole tank not just the water at the tap as you could be providing the ideal home for legionella growth
 
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I have tried to measure temperature, however the pipes are clearly not a good indication of tank temperature, set Monday for longer time in hope once a week getting it hot is enough, however set for 1/2 hour or hour makes very little difference, as second 1/2 hour boiler hardly runs, all it seems to do is keep pipes hot.

Winter is coming so once we need central heating then DHW will get really hot, but the pipe run is that long turning on kitchen tap you have a whole bowl full of cold water before hot arrives, so some local water heater may be a better option?
 
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