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Viessmann 343-F gas/solar combi boiler fault - leaking for 3yrs poor design?

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I have just found this website while browsing, so I have signed up.


This, my first post, is both to seek advice and, perhaps, to alert others to the problem I am facing.


It does not concern a utility company as such,

but the manufacturer/supplier of energy equipment

- in this case Viessmann,

a well-respected company who make gas, solar and other systems for domestic hot water (DHW) and central heating (CH).


Having looked through the forums I am hoping that this is the right sub-forum in which to post:

it seems that there are quite a few plumbing and heating experts who contribute to the discussions here.


The essential details are as follows:


In October 2010 (ie just over three years ago) we had our old, traditional system renewed/replaced.

We had a Viessmann 343-F "tower unit" installed.


This is a gas-fired condensing boiler, integrated with cylinder, pumps and solar water-heating circuit and controls,

all packaged into one single box about the size of a domestic fridge.


So as well as the mains gas, we have a panel of solar tubes on the roof which helps to heat the water.


This tower unit stands on the floor of our utility room, in the corner, with all pipe-connections at the top.


The system works superbly, is very efficient and has cut down our gas bills (since especially in sunny weather, the solar circuit heats most of our DHW needs).


However...... several weeks ago we noticed a puddle (indeed a pool) of water on the floor around, beside and behind the boiler - or rather the tower unit.


As it is located against the walls in the corner of the utility room there is very little visibility and vitrually no access beside or behind the unit

- and of course it is immovable as it is plumbed-in and very heavy.


We mopped up the apparent leak: it wasn't a pouring flood.

But the puddle soon reappeared.


There was much more liquid when the CH was on. With DHW only, there was still a leak, but noticeably less.


There is no local Viessmann agent in our area, and the firm which installed our system had ceased trading and no longer exists.


To cut a long story short, telephone discussion with Viessmann UK customer services and tech helpline,

and eventually a call-out to a local plumber, suggested that our initial fear - that one of the CH pipes or joints behind the boiler unit leading to

and from the radiators had sprung a leak - was mistaken.


Rather the suspicion was that there was a condensate leak.


This tallied with the evidence that there was more of a puddle when the CH was on (boiler running long and often) than when DHW only (boiler running much less).


Now here is the problem.... and the warning.


Further checking with the help of a friendly local plumber revealed

(a) that there was no flow at all out of the condensate drain pipe,

which was normally plumbed in to the waste outflow pipe under the sink, and which we disconnected to check. Also


(b) so far as we could see by tracing that drain pipe back, it had not become disconnected from the boiler,

and the joints and connections (with jubilee clips) along its length inside the tower unit were all sound.


BUT.... more checking eventually turned up the cause of the problem.

The condensate drain pipe is a flexible ribbed plastic tube, like a thin vacuum cleaner hose or some washing machine outflows.


This tube had been wedged, or crushed, beween a internal copper pipe and the casing panel inside the tower unit,

just under the boiler components and above the HW cylinder.

When we removed it and looked closely, we discovered a small but significant split on the underside (invisible until the tube had been removed).


The good news is that - having found the source of the leak - the immediate solution is to replace the tube. Simples!


There is however some bad news. It is obvious that this split was not sudden or recent.

The tube had evidently been wedged and squashed under the other pipe when the unit was originally assembled.

The tube was actually held inplace by cable-ties. So the split had been there for many months, indeed possibly years.


The really bad news is that all the components inside this tower unit casing are very tightly packed-in, so inspection is difficult;

but we established that the condensate seeping from the split plastic tube had flowed down through a gap in the panels on to, and into,

the spongy insulation jacket which completely encases the HW cylinder.


This means two things. First, that the leak had been there for so long that the condensate had eventually saturated the whole of the insulation jacket,

and then soaked into the floor immediately below the unit, the skirting boards and the base of the walls below.

Only then, and when the flow was substantial, did it begin to "puddle" on the floor-tiles under and behind the unit.


Second, (and any plumber reading this will be ahead of me!) the fact is that condensate is highly corrosive.

So.... we have had a boiler/cylinder assembly inside this tower unit casing where for many months,

and possibly years ever since the new unit was first installed,

the cylinder and all its pipe-connections have been sitting (so to speak) immersed

and saturated completely in corrosive acidic condensate which was being constantly replenished.

It is of course impossible to inspect, as all this is inside the moulded insulation jacket.


Although condensate only flows when the boiler is running, I had assumed the discharge would only be a matter of a few cupfuls.

Since the immediate short term repair of fitting a new drain tube,

I have been staggered to observe that the boiler discharges around 5-6 litres per 24 hours.

And the boiler has been running for over three years!


I have reported all this (complete with photos) to Viessmann's service department,

and made clear that I am looking to them to come up with an appropriate response

- which to me means complete inspection, overhaul and replacement of all affected parts or (probably easier) replacement of the entire unit.


Notwithstanding the fact that the unit was installed just over 3 years ago

(the warranty is for 3 yrs) the evidence points clearly to a manufacturing/assembly fault or error,

possibly a design flaw, and certainly a problem and a leak which has been present for many months at least (ie it arose well within the warranty period).


I now await Viessmann's response.

So in a way I am jumping the gun by posting this now in a forum.


If Viessmann accepts responsibility and undertakes to replace the unit and make good the damage I shall be the first to applaud them.


If however they don't, we could be in for a serious fight over liability and compensation.


I, and I hope Viessmann, do not see that as in either of our interests. But on a contingency basis I am having to think about what that might involve.


So, for anyone else who has or works on similar Viessmann units

- check the condensate drain hose immediately and very carefully!


For anyone who is a plumber - any comments on the nature and consequences of this problem?

For anyone who has experience of claiming for a fault of this kind - any advice to offer if I have to go down that road?

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No-one seems to have picked up on this thread, which is now a year old! In fairness I ought to provide an update and to say that Viessmann UK have been outstandingly helpful in their response on this issue.


Even though the unit was strictly speaking beyond the warranty date, they accepted responsibility for the fault. They immediately provided a new condensate drain tube to replace the split one, which solved the immediate problem. But more than that, Viessmann UK agreed that there was the possibility of corrosion damage as a result of the leaking condensate soaking through the insulation and affecting the water tank, pipes and joints inside the "tower". So they agreed to replace the entire unit, free of charge.


Quite outstanding customer service, in my view. So this post is mainly to put on record firstly, that Viessmann products are still - in my view - extremely good (the split plastic pipe was I think one of those one-in-a-million pieces of bad luck). Secondly, Viessmann's unhesitating readiness to solve the problem and replace the unit was exceptional and very welcome. So hats off to Viessmann customer service: they deserve credit and recognition for doing the right thing.

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