Using existing chimney/flue with condensing boiler

I'm considering having our existing boiler replaced with a Keston Celsius 25 and want to continue to use the existing fluing arrangements so that flue gas discharge is at a high level. I can't find enough information on the Keston website to be sure if what I want if feasible.
Current boiler flue:
4" pipe from the top of the boiler enters nearby chimney where it connects with a flue liner that takes it up through the first floor and through the roof space to a weatherproof terminal. The only bends are 45 deg above the boiler and 45 deg into the flue liner at the base of the chimney.
Suggested arrangement:
50mm pipe from the boiler to immediately bend 90 deg into the base of the chimney then another 90 deg bend to connect with the existing flue liner.
Any thoughts on the feasibility of this approach? Thanks, John Barnes
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AIUI, you need to stick the 50mm pipe right up the existing flue liner out the top of the chimney. If it is a straight run, this should not be difficult, although you'll need access to the chimney pot to fix up a decent flue arrangement.
You'll need to get the air inlet somewhere else, like through a nearby wall.
However, chat to Keston to see if the liner itself is suitable. My suspicion is not, but it is no more than a suspicion.
Christian.
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In article < snipped-for-privacy@h48g2000cwc.googlegroups.com

Although I have done almost exactly the same, I cannot recommend it to you, here's why: 1. The Celsius has a room sealed fan assisted balanced flue where the inlet and exhaust ports are intended to be close to each other. In the arrangement you describe the exhaust will be at roof level and the inlet will, I presume, be elsewhere. This breaks the fitting instructions for the boiler so it is unlikely to be signed off by a Corgi. 2. If you decide to use low level air you really shouldn't take inlet air from inside the living space but you could take it from outside or underfloor. If outside then in high winds there is a risk of significant pressure differences between the low level inlet and the high level outlet which will effect combustion and possibly result in blowouts. If under a well ventilated floor then you may view wind pressure to be balanced by the ventilation avoiding the risk of an outside inlet but Keston state explicitly that the warranty for the Celsius is void if you use underfloor air for the inlet.
Eco Hometec have a similar boiler to the Celsius, actually it is hugely better but is about twice the price, and they are far more flexible on their flueing options, they let you use a liner in a chimney and allow inlet air to be drawn down the chimney too.
I took their flexible approach to flueing and applied it to the Celsius by adapting the 50mm Celsius exhaust to a 100mm (new) stainless liner and took the inlet from underfloor using a filter the size of a small planet to give it clean air.
Be aware that I only did this because I didn't have any other choice, if you have an alternative, go for it.
By all means have a word with Keston but I don't think they'll like it.
--
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fred wrote:

can your boiler be sited near an outside wall? is it imperitive that it is located where the old boiler is? if you read the MI located at http://www.keston.co.uk/downloads/manuals/cel25-m.pdf it appears that the intake and flue don't have to be near each other but should be located "within areas of eaqual pressure" whatever that may mean in MI jargon!!lol.
how big is the chimney without the liner in it? if you can squeeze the the two pipes(intake and flue) into the unlined chimney then all you nee to do is return the flue and intake at the termination to stop ingress of rain, the bends appear only to allow 2 45 deg's and 90's at terminal so check that it will work. the length should be ok for a normal height property but do consider it(20 m for flue)
hope this helps you
Gav
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Thanks for all your comments, they have given me something to think about.
I suspect that connecting with the existing flue liner is not OK, because of the need for condensate to be able to drain back to the bolier, but I have contacted Keston to get their view on that.
The idea of running a new flue pipe up the existing chimney (which is perfectly straight and lined with 9" diameter ceramic) is attractive because it vents the flue at a high level.
The installation instructions have:
"2.8.3 Distance between Flue Outlet and Air Inlet There is no maximum - the terminations must not be on opposite sides of the dwelling but can be in areas of unequal pressure. A minimum clearance of 200mm must be left between the terminations."
The above (I think) allows me to run the flue up the chimney and take the air intake through the wall near the bolier. This is not quite as easy as I first thought because the boiler flue outlet is on the right hand side while the chimney is on the left and the boiler air inlet is on the left hand side while the route to an outside wall lies to the right, so some ugly cossing-over of the pipes would be required.
Running both the flue and inlet pipes up the chimney clearly gives a more balanced arrangment. The flue pipe could terminate at least 200mm above the inlet pipe to meet the minimum clearance constraint. I could make both pipes bend through 180deg after exiting the chimney so that the ends face downwards to keep out the weather. But some sort of chimney termination would be needed to provide a weatherproof seal and support the pipes - I can't find anything to do that?
Further comments please, John
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There's no need for the condensate to drain back to the boiler, you can have a separate drain for the vertical part of the flue. Mine was constructed from readily available muPVC drainage parts and securely (and gas tightly) connected to the vertical stainless flue. Flue gasses are blocked from escaping to the drain by a simple U trap which I think is described in the Keston manual. You're welcome to a copy of my design if you go this route. I should say that this was for a flexible stainless flue liner and may not apply if yours is different.

Yep, fine if it is _really_ straight although jointing lengths of 50mm muPVC on the roof could be :-) interesting.

Yes, I thought high level outlet & low wall inlet would be a no-no due to pressure difference but it isn't 'opposite sides' so I think you may have a winner.

200mm really isn't that much so you could just bend them 90deg in opposite directions and oppose by 100mm each. Support the pipe with builders' band, stuff the chimney pot with rockwool and finish with mortar.
Howzatt?
--
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On Fri, 13 Oct 2006 17:22:41 +0000, fred wrote:

Sorry... but I have a Celsius in my own home, have fitted several, commissioned several installed by others and know that the manual permits this. The only restriction is the terminal being on "opposite sides of the building".

OK I didn't know you could do that. Opens up possibilities of putting Kestons C25s in builders' openings.
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Ed Sirett - Property maintainer and registered gas fitter.
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Yep, you're quite right, I was going from memory rather than RTFM and had made the requirement stricter then specified.

Actually you can't, I broke the rules :-/. My only excuse is that I had no choice so I looked at the rules and guidelines from Eco Hometec and applied them to the Keston flue with a bit of conservative engineering nouse. I didn't run it by Keston.
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On Sat, 14 Oct 2006 20:11:17 +0000, fred wrote:

... um... who almost certainly would have refused!
My biggest concern is what would happen if the flue liner split, you'd then have flue gases forced out at positive pressure with the only escape being down the chimney into the living room. Of course the 55mm MUPVC pipe could fial but it's a magnitude tougher than flue liner.
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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That's why I didn't bother asking them ;-)

Yes, I'm bad but not quite that bad, the chimney is sealed to the room at the base with bricks & mortar but is ventilated into a well ventilated underfloor. I hear what you say about this being a pressurised exhaust system but I only considered options that Eco Hometec had ok'd for their similarly configured boilers.
My options were seriously limited due to the building layout and I considered this better using the only other possible alternative of old and inefficient CF flueing.
<slightly shamefaced>
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fred
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On Fri, 13 Oct 2006 06:03:00 -0700, john.barnes wrote:

I know it's not what you're asking but I heard that Halstead do a flue arrangement to use a conventional flue.
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