Vaillant 418 unnecesary purge cycle

Without actually getting as far as firing, my recently installed Vaillant 418 seems to occasionally do a quick purge cycle - the fan runs for a few seconds, then stops. The display shows there is no call for heat. I have noticed it on two mornings. Now why would it do that?
Time clock is set to allow heating only to come on, but the stat is set to 15C, much lower than the house actually is. HW is set to come on at 16:30 in the afternoon.
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Harry Bloomfield wrote:

Probably runs on windows. This is a Linux newsgroup!
:)
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On 17/05/2018 12:01, Harry Bloomfield wrote:

It may well have an anti seize cycle to exercise things like the pump and fan during periods of low use.
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Cheers,

John.
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My Keston does that every 24 hours of non-use, according to the manual. In 16 years, I only heard it do it once, but you need to be in the bathroom at the time, and not having a shower or anything else noisy, in order to hear it.
I included the same feature in my heating controller for the mid position valve (which controls upstairs and downstairs heating zones, not hot water). If it's not operated in last 24 hours (which would normally be the case all summer), it winds it to the end and back. (One thing I didn't check but was intending to was that it actually got to the end and operated the microswitch.)
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Andrew Gabriel
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Andrew Gabriel presented the following explanation :

My boiler is in a kitchen cupboard, which makes it noticeable when it is quiet in the kitchen. I noticed it this morning again, at 09:45. It is set to allow CH, but the stat is set to 15C so it should not be called in this warmer weather at all. The last time it was called to provide any heat was yesterday, for HW - I had a bath, then Jean had a bath in the afternoon and it was timed to come on heat HW at 16:20. So probably there would be no call for the boiler to fire any later than 17:20 yesterday.
I have not read the manual thoroughly, there is a lot of it, but I didn't spot any mention of it doing an occasional purge, so I have emailed Vaillant to ask them about it.
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On 17/05/2018 12:01, Harry Bloomfield wrote:

Does this occur just as you run a cold tap close to the boiler?
--
Adam

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On 17/05/2018 17:17, ARW wrote:

Forget that, yours is not a combi.
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ARW was thinking very hard :

Not that I have noticed, but there could be no link between the two anyway - it is an open vented system.
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