Boiler drops from 1.2 to 0 in an hour!

Earlier this week one morning we got up to no heating and on checking the water pressure on the boiler it was at 0. We opened the taps to refill it up to approx 1.2 but within an hour it had all gone again. An engineer came out and said he had cleared some gunk out of the system which may have been causing the pressure relief valve to stick, but it did not fix the problem. A second engineer came out and bled all the radiators and said there was a lot of air in the system but still the problem has persisted. Literally every hour we are having to refill the system. The latest engineer has suggested that there must be a leak under the floors in the pipework somewhere. This has been going on for 5 days now so I am fairly sure that the leak cannot be upstairs otherwise we would have noticed it coming through the ceiling or something by now. Assuming therefore it is downstairs, the engineer did say he was surprised that, given the volume of water that must have leaked out by now, that we would have probably noticed a damp smell by now, but we haven't.
I have today lifted a few flooboards up in various rooms downstairs, no evidence so far of any leaks. There is a lot of brickwork under the floors partitioning the rooms off but there are holes in between these bricks so I am sure that water would have seeped around a bit once down there.

As far as the boiler is concerned there is no evidence of any water coming out of the pressure relief valve. However as a test somebody advised isolating the boiler at the return and flow valves and if it still dropped then it would be the boiler, else it would be the pipework. I did this and sure enough it did drop, however could somebody check the image attached to confirm that I shut off the correct valves - I turned the valves numbered 1 and 5 off - this is CH flow and CH return as labelled shown by the arrow.

The engineer also did this test a couple of days ago and said that when he did it the boiler did start to leak (not enough to have explained the amount it drops by), but he is going to order a new part to fix that particular leak. It did not leak when I did the test, although occasionally when I refill the system it does drip a little bit.

I am sure it must be a leak as opposed to the boiler leaking, but then the test I did seems to indicate the opposite?

Also we have refilled the system, turned the thermostat right down so it does not kick in and still it drops, which to me can't be the boiler as it's not even firing up to burn it off or anything!

Any help gratefully received - in the attached image I shut off 1 and 5 via the valves circled. The pressure then continued to drop. Is this a valid test? (Click on the image to make it bigger).

The boiler is a Vaillant Ecotec 831 Plus.

Thanks in advance for any advice.

Quite possible that the primary heat exchanger is leaking via the condensate discharge (assuming the expansion vessel and prv have both been checked and are ok.) If boiler still under warranty call Vaillant.

Many thanks for your reply dangermouse. If this was the case is it easily possible to see evidence of this - the condensate discharge currently outputs approx a tea-cup full of water each day which I am told is normal?

Even if it were a leak from the boiler I can't see where all the water could be going to - the fact that it also happens without the boiler firing up means it is not burning/evaporating it away?
Thanks again.

pump up system to 2 bar. isolate boiler at flow and return valve.

turn boiler off and do not use during test period.

let alone overnight and see in the morning; if pressure has dropped, it is the boiler.

remember to first open both valves after test

Thanks bengasman.

"Isolating the boiler and flow and return valve" - can you confirm from looking at my picture that this is shutting the valves I have circled, (numbers 1 and 5)?

Thanks again.

unfortunately, the picture comes up too small to see, best is to look at the manual.
another way of doing it is to feel (careful, hot) when the boiler is on heating, which pipes are hot.
when you do the test, shut the valves firmly, but with some sense.

Thanks. If you click on the picture it should make it bigger, big enough to see.

it should.
i should earn a lot more money
i hate that word: should

Yep thems the isolators

Thanks.

I just don't understand it then. We must have re-filled the system completely at least 10 times a day for the past 6 days - that's a heck of a lot of water - and I just don't understand where it is all going! It must be a leak under the floor I guess but as mentioned an initial look under a few boards has revealed nothing. But what is really confusing me now is the test I did by isolating the boiler and the pressure still dropped - which points more to the boiler - but if so where is the water going in that case - it does not discharge in the same volumes and the boiler is not kicking in as the thermostat is down so it doesn't get burned off - where else can it have gone!!??

tie sandwich bag over over prv outlet to eliminate that option.(outside)
next likely one is the white pipe on the right hand side under the boiler.

Thanks bengasman.

We have done the sandwich bag test - nothing.

Is the white pipe the consender discharge? If so then yes we do get this chucking out about a tea cup full a day outside (I think this is normal?). No evidence of any deposits under the boiler itself however. Although when the engineer did the isolation test this is when he said it did start to leak within the boiler so has ordered a replacement part for that, but he did not think the leak it was making was anywhere near enough to account for the volume that we have lost.

white pipe is indeed for condensate.
1 cup a day is quite little, indicating either very little use or less than optimum performance

to go from 1 to 0 pressure, i would expect at least a litre, if not 2

Just an update on this.

We continued to have to refill the boiler literally every hour. Then all of a sudden after about 10 days of doing this it would not need refilling for about 4 hours. This concerned me at first because I had visions of great lakes of water under the floor and the reason it was slowing down was because the water level under the floor was now on top of the pipe with the leak....

I started to take up various floorboards where easily accessible in various rooms. Not a sign anywhere of any leak. Even where we could not see totally under the whole of a floor I would have thought that looking under a part of the floor would have shown some evidence of a leak elsewhere in that room just purely judging by the volume of water that we lost.

Engineer number 3 came and we looked under a few more floorboards, including under some laminate flooring - not on my list of top 10 things to do just before xmas....anyway nothing found at all.

We got to the stage where we had to decide that we either took up literally every floorboard and floor, one room at a time, until we found it, or (my preferred option by now) just wait for it to eventually show itself - if it caused any damage then we would address that as and when.

The engineer as his last course of action put some sealant into the system.

For 4-5 days after this nothing changed, we topped up every 4-5 hours. Then all of a sudden it didn't need topping up, and we have now gone for about 2 weeks without topping it up at all, occasionally the level drops to 1.0 when idle but when firing up and 'in action' it gets up to 1.3-1.4. I can only assume that the sealant has managed to block the hole. So for now we are reasonably happy, although obviously aware that it could go again at any time.

One thing though that the engineer said that really confused me, was that he had recently fixed a similar leak in another house where the pressure was dropping so fast you could almost see the needle on the gauge drop before your eyes - but when he eventually found the leak, beneath the living room floor - it was just a steady drip, drip, drip - not gushing out as I would have imagined? Does this sound right?

No. You've a major problem or did have. I suspect the boiler HEX as DM has already pointed out.

I had problems with Vokera recouperators (sec heat exchangers) bursting internally,was at one where I could hear the water going down the condensate waste as I was topping up the pressure.

Easiest way to tell is to disconnect the condensate waste from the boiler and stick a bucket under it whilst your filling the system or leave the bucket there to see what happens.

Obviously you will have to wait until the problem re-occurs.