Vaillant ecoFIT pure 825 thumping noise

[Intoxicat]

Thank you for the replies. I wasn't sure if anybody would still be interested in my little problem, so I would like to present it as a logic problem to solve.
If I run the hot tap only slightly open, after a few seconds the water comes out hot and there are no complaints from the boiler.
If I then open the tap further, after a couple of seconds there is the normal start-up 'whoosh', which then continues as in my video.
I don't know what it is that makes the sound that makes me think of the boiler switching to 'power mode' (all phrases of my own invention), but that sound is totally normal, and I can only guess is similar to what you hear when turning on burners on a gas room heater.
So what is making that thumping sound?
To my ears it doesn't sound like what I'd expect from gas ignition misfiring, but being only a layman I can't rule that out. In any case, after three misfires, wouldn't the boiler cut out as a safety measure?
So what would happen if, when the hot tap was turned on further, after the water still stored in the 'heater tank' (I assume there must be something like in which to heat the water) was exhausted, a partial blockage prevented the water from refilling it to the operating level, so that the smaller volume of water available was over-heating? Would that cause the noise on the video?
I'm rather hoping it is, because cleaning out the ECO-Mag or the canoe filter sounds a lot less drastic than all the other possible actions.

I'd love to offer a prize for the correct solution, but it's cost me enough money already.

 

[dilalio]

Firstly, your boiler is a combi so it won't do hot water and heating at the same time.

There's no "tank" for storing water.
Mains cold water enters the plate heat exchanger and is heated by water circulating from the main heat exchanger. This is also done in the plate. The waters don't mix.

Measure the litres per minute that you get from a hot tap only and compare this with the litres per minute that the boiler can heat up by 35°-45°.

This is in the manual and you may have more litres per minute than the boiler can handle. This often mean not fully opening hot taps or throttling the cold supply into the boiler.

Next check - when you are running a hot tap, see if the CH flow pipe from the boiler is getting hot. This should not happen.

A divertor valve is used within the boiler which directs heated system water to either the plate or the radiators.

If this is not fully moving over and is letting by, it "may" be the source of your noise.

Sound on the video doesn't say much and this is the best I can offer without being in front of it.

 

[Intoxicat]

Sorry dilalio - edited to say that I posted this before seeing your reply.
New day, new video. This morning I was out of bed when the boiler was firing up the heating for the first time for the day, and although it was noisy it didn't make me think it was going to blow up. I tried to take some vidoes with my expensive SLR, all of which turned out to be useless and on their sides, so I had to resort to my phone. What I've tried to show is wht happens when I first, turn the hot tap on gently, then increase it to full flow, then turn it down again. There are also occasional noises to suggest there are a few air bubbles in the system somewhere, although only a small amount that shouldn't cause problems. The pressure is 1.5 bar.
p.s. Confused by what you mean when you say it won't do heating and hot water at the same time. I can certainly get hot water (have a shower) while the heating is on, unless you mean it temporarily closes the heating down.

 

[dilalio]

unless you mean it temporarily closes the heating down.

Yes

 

[Intoxicat]

Firstly, your boiler is a combi so it won't do hot water and heating at the same time.

There's no "tank" for storing water.
Mains cold water enters the plate heat exchanger and is heated by water circulating from the main heat exchanger. This is also done in the plate. The waters don't mix.

I have to confess to not knowing how a heat exchanger works. The only way I could visualise the water being heated up for the hot tap is that it heated up inside a small storage tank of its own. I still can't visualise how cold water can possibly be heated up so quickly without the old hot water tank system, although I suppose an electric shower manages it.

In my old house I had a vintage Baxi back-boiler which sat behind a gas fire. One plumber advised me to hang onto it as long as I could, because although less efficient they are virtually bullet proof. The bills in my admittedly small terraced house were satisfactorily low anyway, but in any case, the cost of maintaining my condensing boiler would have made up for about a decade of extra fuel costs for my Baxi.