Vaillant Ecotec 824 Plus with new Nest thermostat

1 Apr 2020
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United Kingdom
I’ve recently had a nest 3rd gen thermostat installed into this boiler, seemed nifty at the time however, this replaced no previous thermostat (we used to manually switch it on and off and there’s a panel at the front with a analog timer). So the nest 3rd gen replaced this.

It’s been a few days since the install and I’m hesitant to pay the call out fee, the thermostat says it’s on but the house is cold and the boiler has a flashing hourglass symbol on it... from excessive googling I found it’s called, Boiler anti-cycling time is active... I turned the dial on the boiler itself to off and on, but it didn’t really do anything except get rid of the hourglass, heaters are still cold, yes I know the nest thermostat should control this...

Any ideas that’ll save me a call out?
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What temperature is the boiler thermostat set to?
What temperature is the boiler thermostat set to?

The target flow rate is 70c... after googling around I found a hidden menu, ran electrical tests, successfully, decided to check the “External pump” on T.06 which makes the relay sound but doesn’t sound like it’s pumping anything so I assume the pump is knackered.

I might be wrong, but that’s as far as my knowledge will get me...

Also set the boiler to Comfort for d.18 and reduced the d.02 anti cycling mode minutes from 20 to 5 minutes.

There was also existing fault codes F.28 failure in start up ignition unsuccessful and F.75 pulp water shortage.

So hopefully the engineer will come tomorrow and swap out the faulty pump or do a magical expensive reboot of the boiler and charge me and arm and a leg anyway!

Is the analogue timer still in place and do you still have hot water ?
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Is the analogue timer still in place and do you still have hot water ?

Yep the timer is still in place at the front of the boiler, there is hot water just no heating.

It seems the boiler ignites and tries to heat water for circulation but cuts off with the timer when the flow temperature exceeds what’s set, think I saw it go 5 degrees higher than what it’s set at before it decides to theoretically “time out”
The turn dial for the heating is supposed to be left on unless we want to switch to manual mode from the nest and use the static timer on the front of the machine...

Pretty sure all signs point to the damn pump!


Appreciate all the responses!
Turn heating on, and press the i button, and report back.
Turn heating on, and press the i button, and report back.

Interesting Picasso, I only assumed this because the test menu had an internal/external/circulation pump.

Having turned on the heating and pressing the i button the boiler began making usual noises doing its thing, it heated up to 65c as I’ve recently set and continued to climb to 70c where it stopped and the hourglass egg timer has shown up.

Since the anti cycling time out was changed to 5 minutes and it took that long to write this... the boiler is heating up again, the temperature has stayed at 65 for about three minutes, then began to climb again but very slowly... and again the anti cycling mode has turned on with the hour glass sand timer flashing.

The sound it makes sounds like there is air in the system but the pressure looks fine to me and no visible leaks around any of the rads, I think the heating temperature was recently upped to 70c and before it was circa 50c due to SWMBO. Looking at the bottom of the boiler there’s a cup like thing which looks like a filter, looks like there’s quite a bit of sedimentation in it, dark at the bottom for about 15mm high and reddish throughout!

Hope this helps someone down the lane!
So after a bit more research I reckon the guy will tell me it’s the diverter valve. From my little understanding, and having done a bit of research plus your guidance;

The pump is ruled out from fault since it’s a combi and I have hot water. I am then assuming, it’s not a sensor; since the hot water flows at target rate, or the hall sensor as boiler only ignites when hot water is called for, burner/fan etc all good since there is adequate hot water without flaking...

So we have fire, fuel, spark just no throttle for CH, ergo either the diverter motor or valve.

Hopefully the engineer puts a new valve in anyway for preventative maintenance, looks like they leak often (overtime) anyway with Vaillant (if the motor/valve is the culprit)

Nice work guys, do correct me if I’m wrong, I’ll give an update in 12 hours.

Much appreciated
Thank you all for the time and help,

The engineer changed the divertor valve as the plastic on it had broken. They also relieved the excessively high pressure of 3.5 bar.

For reference the pressure currently reads 2.3 bar, the heaters were on for a bit though, they are now off for the last hour or two, target flow rate is 75c and the dhw temp is 46c, warm enough to mix with cold but not hot enough to scald or be uncomfortable.

Thanks again,
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