Boiler flues (Viessmann)

[Tim S]

Hi,

Theoretical question (ie I'm not about to do it, well, not yet anyway[1])

Been reading the Viessmann Compact 100 24kW system boiler installation manual.

The flue is a typical round concentric pipe affair. The manual shows the flue sections being plugged together and into the boiler, where there are sealing rings shown.

Would one normally use a jointing/sealing compound *or* an assembly aid (in the way that silicone oil may be used on push fit sewer pipes) on these types of joints?

Manual doesn't make any mention but it may be assumed knowledge.

Other than that, it seems perfectly straightforward. The most difficult bit will be gauging the ventilation requirements as it will be sharing a common airspace with a gas cooker and a log stove.

Cheers

Tim

[1] As mentioned previously, I'm expecting to seek a CORGI chap to be a consultant if I DIY this - and to this end I've already had an encouraging talk with a fellow who prefers repair work to installation work, which obviously suits us both.

[Tim Downie]

Um, surely if it's a room sealed boiler with concentric flue etc. the other appliances are irrelevant?

Tim2

[Dave Plowman (News)]

They push together easily. They're a mixture of plastic pipe, painted steel pipe and 'rubber' seal - so I'm not quite sure which lubricant would be totally safe on all of these.

Eh? It doesn't draw any air from the room. Nor does it need any special ventilation for cooling the exterior surfaces - just observe the clearance given in the instructions if boxing it in.

[Tim S]

Dave Plowman (News) coughed up some electrons that declared:

Brilliant - thanks Dave.

I know that - it's only I thought I'd seen some ventilation requirements mentioned somewhere (perhaps the Viper handbook), even for a room sealed unit. I was only skim reading, but I have a thought in my head that says it's for backup in case something leaks.

But I'm probably talking bollocks. I haven't made a concerted effort to calculate the ventilation requirements of the various things yet so this will come out in the wash when I do the research properly :)

Cheers

Tim

[Tim S]

Tim Downie coughed up some electrons that declared:

Yep - Agree with you and Dave, now I've had a look at Part J (was actually looking at acceptable flue positions, but noticed a bit on ventilation).

Cheers

Tim

[Ed Sirett]

Modern boilers not only don't need air for the combustion (as it comes from outside) but also they don't need any for appliance cooling.

[Ed Sirett]

The (flueless) gas cooker only requires that there is some sort of opening direct to outside air. (e.g. openable window). The log burner does not come under BS 5440 but it would be a good idea to make sure that it has enough air.

I would suggest (unless some normative document states otherwise) that a permanent vent to outside air of at least 50cm² should do the trick.

[Tim S]

Ed Sirett coughed up some electrons that declared:

Excellent - thanks Ed.

I have a vent in the room already (put there for the current gas fire/back boiler).

I'll clean it and measure it, but it looks superficially like it's >=50cm2.

I'll have to be a bit careful because there'll be a cooker hood with exhaust to outside going in too and I'm aware this may load the vent (reduce its effective capacity) - I'll be sure to check on that too.

Cheers

Tim

[Tim S]

Ed Sirett coughed up some electrons that declared:

And to think that the old Potterton cast iron lump my dad installed in the mid 70's had a rated output of about one radiator from its case!

Cheers

Tim

[Tim S]

Ed Sirett coughed up some electrons that declared:

If you're still around...

I've come across some conflicting information on flues (this case, balanced, gas).

The Viessmann 100 Compact installation manual shows a flue lower than 2.1m above ground and *next to a "walkway"* as being unacceptable.

However, Part J makes no mention of any distances from "walkways" and neither do any other Viessmann documents.

I'm aware a flue must be guarded if people are likely to make contact with it.

Is this a new thing, or have Viessmann made an error?

The horizontal flue will be coming out of a wall on the back of a little annex on the back of the house. The roof is low and flat and the eves are about 2.1m above ground. Flue needs to go lower than 2.1m to both meet the

200mm clearance below eaves rule and Viessmann's requirement that there be 300mm clearance above the boiler casing.

Technically, there's a path (private, in the garden) going by there, although this could be tweaked.

Many thanks

Tim

[Dave Plowman (News)]

Indeed - my Kingfisher situated in a large bathroom kept it toastie warm even with the rad off - the Viessmann wouldn't stop ice forming on the window. ;-)

[Ed Sirett]

Forget Part J, if you follow the gas appliance makers instructions (which may or may not supercede BS 5440) then you will be Part J compliant. Part J are more general regulations on heat producing appliances - including ones that really could set the house on fire!

IIRC BS5440-1 permits GAS flues between 300 and 2000mm above the ground level of a walkway if there is a terminal guard. If Veissman wish to up the rule to not allowing any flue below 2.1m above a walkway they have every right to do so. However this may be a drawback commercially as it will be a discouragement to choose this make in certain locations.

Sounds like you need to make the area unpaved?

[Ed Sirett]

Although the wood burning stove does not come under BS5440 you would do well to treat it as if it did, unless there are regs or instructions that state things more accurately.

In BS5440-1 (open) flues must still work even when extraction devices in the room and adjacent rooms are set to their maximum settings. You may find that you then need a bigger vent to get things working OK. You would discover this when the warmed flue fails a smoke test but passes when you open the window.

[Tim S]

Ed Sirett coughed up some electrons that declared:

Thanks Ed. It is weird as they appear to have only applied it to this one model (not that I've read all the manuals but I did look at some others).

Think I'll give them a ring and see why they've stated that. They might have meant a public walkway rather than a garden path - it's not at all clear on their diagram.

That is of course the fallback. I can route the path further away from the house at that point so it's not really a problem - well, not enough of a problem to not use an otherwise ideal siting for the boiler.

Cheers

Tim

[Tim S]

Ed Sirett coughed up some electrons that declared:

Thanks again Ed.

[Tim S]

Tim S coughed up some electrons that declared:

I did and they said, for what it's worth, that the diagram is referring to a flue discharging onto public walkway at less than 2.1m in height.

Makes perfect sense.

Cheers

Tim