Flue lengths and angles

[PaulAH]

Morning all

Trying to design a flue to fit between an existing boiler and chimney stack.

Does anyone have a formula for calculating the offset achieved by two elbows of given angle separated by a pipe of given length? Also the height of the three items together.

There was a time when I could have worked it out but the O-level trigonometry books were printed on parchment.

Many thanks
Paul

 

[oilman]

Forumla? What's wrong with a big piece of cardboard, and draw it full size?

 

[chrishutt]

Since flue offsets are invariably 135 degree angles, you're dealing with an isosceles right triangle (a right angled triangle with 45 degree opposite angles). Pythagoras states that the square on the hypotenuse equals the sum of the squares on the opposite sides.

So if you decide what horizontal offset you want (measuring from centre lines of flue), you square it, double it, then square route it and that will give you the actual length of the offset (pipe and fittings).

For isosceles right triangles, this can be reduced to a figure (known as Pythagoras's constant) = 1.414. So for 135 degree offsets, just multiply the horizontal offset by 1.414 to get the actual offset length. To get the length of offset pipe, deduct the fitting lengths (measuring on centre lines).

 

[PaulAH]

Neat one, Chris. Thanks. But elbows come in various angles (mercifully - no way could I do this job using 135deg fittings without major surgery to standard-issue pipes).

So here, dusting off the parchment for the benefit of the thousands of readers who are gagging to know....

Where
L is the length of angled pipe
C is the angle of elbow (NB in "traditional" measurement: 135deg = 45 deg, 150 = 30 etc)
R = rise
O = offset

R = L * COS C
O = L * SIN C

In more practical terms, where
P = gross length of angled pipe
A = gross length of each arm of the elbow
S = depth of spigot, to allow for the overlap

we have

R = 2A+(P+2A-2S) * COS C
O= P+2A-2S * SIN C

Phew! Thank you Mr Tudsbury (maths teacher, 1960-63).

If you're using Excel, which thinks in Radians, you will need to express the angle as SIN(RADIANS(C)).

Oilman: Following your advice as always, I drew it not on a piece of cardboard but on the wall behind the boiler. What better?

The supplier www.hotline-chimneys.co.uk thoughtfully gives dimensions for various combinations of pipe and elbow. My wall drawing was out by at least an inch using the above formulae. So much for theory... not bad though, eh?

Paul

 

[oilman]

Not forgetting twice the spigot length for the pipe, it's too expensive to make a mistake when itmeans it's too short. (Unless you are doing it for a customer )

 

[PaulAH]

2S

 

[PaulAH]

Too expensive? I'm now going to have to scrap 6m of SS liner, Chinaman's hat, blanking plate... and pay out for this lot... all because you told me I had to fit a solid flue between boiler and stack. I want my money back.

What am I bid, along with a little-used Boulter Camray horizontal exit balanced flue?

 

[chrishutt]

So you were just testing us? You had the answer all the time.

 

[PaulAH]

Bleedin well wasn't. Spent all evening working it out. Sad git.

Mr Tudsbury (aged 112) was a help, through the good offices of Madam Petulengro.

 

[oilman]

Too expensive? I'm now going to have to scrap 6m of SS liner, Chinaman's hat, blanking plate... and pay out for this lot... all because you told me I had to fit a solid flue between boiler and stack. I want my money back.

What am I bid, along with a little-used Boulter Camray horizontal exit balanced flue?

Depends what you mean. I don't see why you have to throw everything away. In simple terms, the solid bit fits between the boiler spigot and the point where you can get into some solid bit of building structure. That might be what you meant, but I thought the stack was the bit above the roof, where there might be a lot of pots in one stack.

What about a Boulter Camray High Level Balanced Flue?

 

[chrishutt]

through the good offices of Madam Petulengro

Elementary, my dear Watson....

 

[PaulAH]

I thought the stack was the bit above the roof

Sorry, my terminology is that of an amateur. Let me explain. What we have is a chimney, 6 ins diameter, which had a previous life with the old boiler. It runs from the boiler cupboard downstairs to a chimney stack on the roof. The solid tubular lining seems to be some kind of fibrous compound (asbestos, possibly ?) apparently in good condition. Can't see all the way up but had it swept and there are no obvious faults.

Now that we've decided on a solid flue between boiler and base of chimney, instead of running the SS liner all the way from cowl into the boiler, there seems to be no need for a new liner at all. The flue can just be stuffed up the hole (suitably sealed, of course) letting the chimney function as before.

Does this sound reasonable?

The cowl is suspect and may have to be replaced. It's like a tin box with concave sides and the vertical edges sliced off to provide breathing slots. I'm sure it was partly responsible for downdraughts. It certainly lets rain in. Do they have a reputation?

The Chinaman's hat came with the (now redundant?) flue liner; it terminates with a 4ins spigot that fits into the liner. Presumably this would be no use as a replacement for the tin box, perched on top of the 6 ins chimney stack. Any suggestions? Are you familar with http://www.hotline-chimneys.co.uk/cowls_vedette_info.htm ?

What about a Boulter Camray High Level Balanced Flue?

Max length only suitable for single storey, according to the blurb.

 

[oilman]

You could do what you suggest, though the ss liner would be a good thing to keep J. Prescott Esq. happy. You will have a problem if the solid pipe from the boiler does not go OVER the existing liner, so you will probably neen an adaptor. (you may need one anyway, ask for a flex-to-vent connector).

I don't know of problems with your existing type of cowl, and I haven't seen any test results for the Vedette, but you probably wont have a problem.

Why not just fit the liner and chinamans hat? You've got them now, and the Vedette will cost more money.

 

[PaulAH]

You do make life very difficult for me sometimes, Oilman. There I was thinking we could just stuff the flue up the hole. Now I (rather, my mate Ivan) will have to climb up a ladder to the chimney top, which will certainly not make the Fat Controller happy under his Working Heights Directive.

A question: How in practice do you attach a liner, which is slightly short of the chimney height, to a solid flue when you can't get into the chimney to attach it? You could have someone on top lowering it a couple of feet with a piece of string but, having joined flue and liner, how can you haul it back into place without risking fracture of the joint?

You will have a problem if the solid pipe from the boiler does not go OVER the existing liner

Is this because it may trap condensate? I was planning to get a 4in to 5in conical adaptor, which will allow a little leeway in the 6in hole because of uncertainty over whether the flue will line up perfectly.

 

[oilman]

Difficult? I was hoping for impossible.

There's no problems with working at heights, ladders can be used if the job is quick, you just have to think about and plan the operation.

The vent to flex connector usually has some clamping screws to grip the flex. You can put it on then give it a shove, might work.

All condensate should be able to flow down to the boiler, so the upper sections must fit inside the lower sections. If you do it the wrong way up, the black gunge will dribble down the outside of the flue.