Boiler randomly firing up

Hi all, sorry but im after a bit of advice.

I have a problem where my boiler randomly fires up 2/3/4 times a day when there is no call for heat or hot water.

My setup:
Unvented system
Ideal HE 18 boiler ( installed around 2013)
Tribune HE tank
2 zones (upstairs & downstairs)
Plus hot water
3x Danfoss HPV22
2x Nest Thermostats

The ideal HE18 is set to E - Efficiency/Economy which i think is 68.c
Hot water thermostat on the Tribune HE is set to 60.c
The hot water is set to come on at 6am and 4pm for 30 mins each time.
Heating is set to 19.5c for various times during the day.

When the boiler randomly fires up i check the nest app and according to it both CH & HW are off or in eco mode and not calling for heat.

Now by process of elimination i think its the HW calling for heat.
Turning the thermostat down to below 40.c on the tank forces it into action and the HPA2 whirs a bit then the boiler goes off.
Unplugging the HW Danfoss HPA2/HPV22 has the same result, both can take a few minutes to force the boiler to go off.

The upstairs Danfoss CH HPA2 failed a couple of years ago and was replaced, my first thought was another had failed.
So I swapped the CH & downstairs HPA2's around to see if there was any difference, but the same result the boiler randomly fires up.
I could move the valves with my fingers although they were a little stiff.

The tundish thing seems to be going more often than not.

Any thoughts what the problem could be?

thanks

Mark.

If the tundish is going, and you have Nest’s I believe these have a legionella program, so perhaps it’s this? Eg the cylinder is cooling down and the hot water is dropping in temp

Undertrained said:

If the tundish is going, and you have Nest’s I believe these have a legionella program, so perhaps it’s this? Eg the cylinder is cooling down and the hot water is dropping in temp

Click to expand...

Thanks that definitely seems to be the culprit, ive switched the legionella option off in the menu and it is now acting normally.
I know there is a small risk but i will alter my schedule to accomodate.

thanks