Combi flue terminal position

[petek]

British Gas service engineer has just slapped a "Do Not Use" notice on my son's combi boiler because the flue terminal is, in his opinion, in a dangerous position.

The house is a stone built cottage in an 1850's terrace, separated from the adjoining cottage on one side by a covered (at first floor level) passageway. The flue terminates into this passageway, the ends of which are approx 1.2m wide by 2m high and are open. The "ceiling" of the passageway is about 3m high and at the same height as the kitchen ceiling (there are steps outside the back door down to surrounding floor level). The flue goes horizontally from the top of the boiler thro' the stone wall so exits into the passageway above the level of the open ends of the passageway IYSWIM. The BG engineer said that as this was a "dead" air space, carbon monoxide could build up and seep up into the room above the passage for example, hence it was dangerous.

The original boiler manual (Worcester Bosch - about 12 years old) shows various siting dimensions for the flue terminal bit I cannot relate any of these to this passageway.

Can anybody offer any advice, or tell me where I can get hold of relevant dimensions?

Cheers Pete

[Jim Alexander]

As usual the diagrams don't cover every possibility so the BG man may be (at least) using his judgement. Without answering the question here's how I see it. Make plans to relocate the flue because WHEN you go condensing the plume will fill the passageway. Some diagrams show a dimension of 200mm below the lowest part of a car port roof. IMHO the best you could claim would be below passageway opening header level, not above the lowest level as you have. I also think its a bit selfish polluting a space which is partly the neighbours.

Some interesting stuff about CO diffusion here. The BG may be right without understanding the process.

A

[Doctor Drivel]

The BG engineer was right to slap a notice on it. Best to extend the flue up and out of the passageway roof into clear air, or horizontal and out of the passage. Use a flue elbow and extension to do any of these, and have it well clipped and a mesh guard on it if low enough.

[dom]

Relevant guidance in the Building Regulations Approved Documents here:

Jim says, it simply can't cover every possible configuration.

But it does sound mad to discharge into that "dead air space" in an enclosed alleyway between houses.

[cynic]

here:

> As Jim says, it simply can't cover every possible configuration. >

ISTR something in a corgi mag last year about flues discharging into covered passageways. The usual blah factor, corgi know best etc but there was also something about the likelihood factor being taken into account. How many years has the flue been in the same position, is the floor above the passageway underdrawn, how far from the end of the pasageway and what is the natural air movement like in the vicinity. It may be possible to extend the flue if it a fan discharge. If it is a balanced flue natural convection type you may be stymied.

[Pete C]

Hi,

Putting a CO alarm on the ceiling of the passageway should warn of any dangerous CO buildup, there are ones with a digital display too for monitoring lesser levels of CO.

I'd have thought as the flue gases cool they will mix with the air lower down in the passageway.

Maybe a local company could give a second opinion.

cheers, Pete.

[petek]

corgi mag last year about flues discharging into

Many thanks for all these well-reasoned replies. On balance it looks like the BG man was probably correct. It's just unfortunate that the boiler/flue seems to have been in place for something like 12 years and it's only just now that somebody has questioned the terminal position. My son & d-in-law have only just purchased the cottage and moved in a few weeks ago so it looks like there is going to be some extra expense they hadn't bargained for. Surprised their surveyor didn't pick it up.

Now a further practical question (or two). Is it possible and/or acceptable to fit a horizontal right angle bend to the flue and exit the passageway above the opening, or would it be better to turn the flue thro' 90deg at the boiler and exit thro' the kitchen wall directly to the outside?

Also how difficult is it to get a flue (approx 125mm) thro' a 400mm approx stone wall? Diamond core drill? Either method above would seem to require this.

[Lurch]

On Mon, 06 Aug 2007 14:12:08 -0700, petek mused:

corgi mag last year about flues discharging into

All depends on the boilers instructions and what it currently has in terms of bends and length.

Diamond core would probably be a bit hard going. I'd probably stitch drill and lump hammer and chisel it out.

[Jim Alexander]

Is there another suitable location for the boiler? Bear in mind it's 12 years old and a replacement (condensing) boiler would likely have a different flue system. Flue parts are not best value. If there is suitable location maybe time for a replacement boiler, or move the existing boiler.

Jim A

[petek]

Unfortunately, the only logical position for the boiler is where it is now, which is in an alcove cupboard next to the chimney breast in the kitchen. I suppose it could be turned thro' 90deg and fixed on the outer wall, still in the cupboard, rather than on the passageway wall, but it may be easier to just turn the flue thro 90deg.

One of the options my son is looking at is having a new condensing boiler fitted but this is beyond my DIY skills and I think will be financially prohibitive (i.e. cost an arm and a leg!)

What did you mean by "flue parts are not best value"? Are they expensive? Do I have to get them from Worcetser Bosch or are there other "will fit" makes available on the market?

Pete

[Jim Alexander]

Boilers are easy to find discounted online, flue parts less so. Typical merchant prices for a 1m extn might be £40, £30 for a 90 deg bend, all plus VAT less whatever discount you can negotiate. Could only advise using the correct WB flue.

You asked earlier whether best to add a bend outside or rotate 90deg at the boiler and exit through the kitchen. You mentioned 125mm but its possible it's 100mm. The manual should specify but the max flue length could be as little as 4m for 100mm flue, less 1m per bend, so there is a difference between the methods.

May be worth getting a Corgi quote for the work as a reality check.

Jim A

[petek]

Thanks Jim The sizes quoted are based on my memory from my last visit to son's house. I'll check in the manual. He's getting quotes from a couple of Corgi plumbers, as you suggest, for a new boiler as well as separate quote just for the flue. Am I allowed to re-postion the flue myself or is this a Corgi job?

Pete

[Jim Alexander]

Well that question has been done to death loads of times here but is impossible to answer. If not working for gain you need to be competent and competent isn't defined. Assuming you identify and procure the proper parts and can follow the installation instructions its not rocket science for a competent diyer. It's one of the tasks that we properly use hacksaws for.....

Jim A