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.

 

[dilalio]

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.

Have you checked the lpm and if the CH flow gets hot during DHW usage yet?

As I alluded to?

 

[Intoxicat]

Have you checked the lpm and if the CH flow gets hot during DHW usage yet?

As I alluded to?

I'm very reluctant to leave the boiler banging away for a full minute in order to check that. By DHW do you mean direct hot water? I'm not sure how to check the central heating flow while I'm running the hot tap. It sounds as if it needs two people and the plumber didn't come today. I'm starting to look and smell as if I've been to Glastonbury for a week. I'll see if I dare test it, perhaps by running it for 30 seconds and then doubling it.

Done that as described above, starting from the instant the hot tap was turned on and it turned out to be a very neat 8 litres per minute. I've looked through the entire manual and can find no mention of this anywhere.
The only pipe getting warm was the one arrowed on the left. The room thermostat hadn't triggered the central heating to come on for hours, so all other pipes were either cold or with only residual heat.

 

[Stuckinarut]

If your flow pipe is getting hot/warm, then it suggests let-by into the heating when hot water being used. Usually the diverter valve

 

[Intoxicat]

Fin you’re flow pipe is getting hot/warm, then it suggests let-by into the heating when hot water being used. Usually the diverter valve

I have no idea how that works or how you even recognise the pipe, but thank you.

 

[dilalio]

I'd be running the boiler for longer on hot water to get a good grasp of the noise and to aid diagnosis.

The boiler has safety devices which will shut it down if there are combustion issues.

The video is still dominated by the sound of the tap running so, it's hard to hear the boiler noise... Do another on your phone and run a tap in the bathroom so it's not interfering with the sound.

The pipe you have indicated is the flow pipe. It should not get hot when DHW is operating. But, feel the pipe further up as you may be getting some convected heat nearer to the boiler, which gives a false reading. Make sure Heating has been off for a while so that it's cool before doing the test.

You only need to pop a litre jug under the hot while it's running on full, and time how long it takes to fill a litre. Then divide 60 by this time to give you your lpm.

Other than this. Get a boiler repairer, who knows vaillants to come and assess it. Not just a "plumber" - I assume your guy is Gas Safe?

 

[Intoxicat]

I'd be running the boiler for longer on hot water to get a good grasp of the noise and to aid diagnosis.

The boiler has safety devices which will shut it down if there are combustion issues.

The video is still dominated by the sound of the tap running so, it's hard to hear the boiler noise... Do another on your phone and run a tap in the bathroom so it's not interfering with the sound.

The pipe you have indicated is the flow pipe. It should not get hot when DHW is operating. But, feel the pipe further up as you may be getting some convected heat nearer to the boiler, which gives a false reading. Make sure Heating has been off for a while so that it's cool before doing the test.

You only need to pop a litre jug under the hot while it's running on full, and time how long it takes to fill a litre. Then divide 60 by this time to give you your lpm.

Other than this. Get a boiler repairer, who knows vaillants to come and assess it. Not just a "plumber" - I assume your guy is Gas Safe?

Thank you for your continued attention. I should have referred to him as a 'plumbing and heating specialist'. He is Gas Safe registered but is a Worcester specialist, which I didn't know until he told me and phoned a local Vaillant specialist for advice.
The boiler was making a rumbling noise this morning, purely with the heating starting up from cold, which wasn't ideal, but not frightening. What I did notice, if this helps, was that when I turned the hot tap on this morning I actually heard the heating click off, heard the hot water heating kicking in, and that's when the much louder thumping began. I turned the hot tap off, the thumping stopped, I heard another click, a few seconds pause, and then the heating kicked in again. I have another very short video, taken this morning, of the rumbling sound when purely in heating mode. It seemed to be coming from the top half although it's difficult to be absolutely sure. I really must clean the top of that boiler. I will do what you say and add the results to this post in a few minutes.
I tried it, but not having a stopwatch and being unable to keep my eyes on the clock and the jug at the same time, the result could have been anywhere between 7.5 and 8.5, so I'd say my first attempt was correct. It would be easy to conclude that the sound on this video is rumbling pump
bearings, yet it is absolutely silent except when there is a sudden demand. Also, that noise isn't constant, which it would be if it was the pump alone (and it also wouldn't explain the thumping noise).