cms sq versus sq cms - CH boiler ventilation


I am making some changes to my kitchen and adjoining dining room (no door between them), and as my CH boiler is situated in the kitchen thought I would just check the ventilation requirements. I have the installation instructions for the boiler (Potterton Kingfisher, CFL70 - conventional flue) and this states that the free area should be 84 cm sq.
Now when I did math at school, 84 cm sq meant a square measuring 84cm x 84cm, whereas 84 sq cm is only a little over 9cm x 9cm, a big difference. I hope that the instructions actually mean the latter? If not I might just as well take the back door off its hinges and freeze in the winter!
Just as an aside, the boiler is currently ventilated via openings at the back to the underfloor space (between 5ft and 2ft all over the house) and from there via 7 air bricks around the walls. This would be well in excess of the 84 sq cm, but is there anything in the regs that disallow this? BGas/Transco have a couple of times shut the boiler off cos they didn't realise where the ventilation came from, and I wasn't home when they visited to explain things. David.
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My boiler is a conventionally flued Potterton C70 (early 1980s) so probably similar in size to yours. It's sited in its own under-stairs boiler room to which the house builders supplied ventilation air through a ~ 4" diam pipe, at ceiling level, nothing at low level. When the domestic extension to CORGI kicked in it was condemned until I'd put ventilation panels in its door at both low and high level. In addition, I had to have a permanently open vent in one of my lounge windows off which the boiler room is sited. (All vent sizing details are in the appropriate BS - viewable at your local library.)
So I now get cold draughts in the winter from the window vent, but I guess I'm not going to die from CO poisoning etc. Another downside is that we can now clearly hear the boiler firing whereas before its door was ventilated, the firing was normally inaudible. Prior to CORGI intervention I kept open the considerably larger chimney opening above the open fire space where my TV has always sat. That's now closed off with minor vent holes...
84 cm sq does seem rather large compared to anything I've got here which certainly satisfied the CORGI fitter.
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M Stewart
Milton Keynes, UK
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DavidM wrote:

In my book, 84 cm sq ( cm^2) is unequivocally 9cm x 9cm (and it was at school, too!)
David
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So Lobster, in your book, what does 84 sq cm mean ?
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snipped-for-privacy@hotmail.com says...

But but but... 9 x 9 = 81!
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Burn him! He's an alien, with 9.1 fingers.
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Lobster wrote:

Have you tried multiplying 9 by 9 recently? :)
9cm x 9cm = *81* cm^2, which is read as '81 square centimeters'.
The phrase '84 centimeters squared' is ambiguous to me. If I heard it I would probably presume the source is unreliable, or at least confused.
Rem
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wrote:

Possible, although we don't have "coffee shops" in the UK and Meneer van Rijn was not a native of these shores.
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On Sat, 06 Aug 2005 14:40:30 +0000, DavidM wrote:

They mean 84 square cm.
In the absence of instruction to the contrary.... Provided the air is direct from outside. Provided there are no other conventionally flued heating appliances (including a solid fuel grate).
The 70 kBTU/hr that 20.5kW (but boilers of this period tended to quote output rating so it's likely input rating is c. 25kW) The first 7kW is free provided it's in a room not a cupboard. This leave ventilation for 18kw which can be provided with 4.5 cm^2 per kW. I.e about 81cm^2
There are rules for ventilation from under floor spaces. Off the top of my head 1) Not for terraced or semis unless partition walls to ground level (usually the case) 2) Air bricks have tapered holes so take the minimum they can be much smaller than you might think. 3) They must be >300mm over ground level (to prevent snow and leaves trouble). 4) The connecting hole between below and the room for the boiler must have at least twice the area for the direct vents.
HTH
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Ed Sirett - Property maintainer and registered gas fitter.
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9.2cms x 9.2cms and work from there.
Thanks also to Ed for the comments on underfloor ventilation, I will check the air bricks and stick a big notice on the front of the boiler for the benefit of visiting gas engineers.
Now I just need tofind a Corgi plumber who will reconnect the gas and do all the saftey checks after I have reinstalled the boiler. Any offers (mid Herts)? David.
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I guess that there is a difference between 84 cm square ...and 84 sq cm.
84 cm square = 7056 cm sq 84 sq cm = 84 sq cm ........................or 9.1651513 cm sq
If there is still any confusion I would ring potterton and ask which one they are defining. for your boiler. Sometimes this sort of detail is written for the USA market and they do use different phrases to define their "math "
Barry
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Whatever definitions you have, I don't think you can be so inconsistent from one line to the next.
My definitions, for what it's worth:
84 sq cm = 84 cm sq
An 84 cm square = 7056 sq cm = 7056 cm sq
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You are of course quite right........its good to see that some people do read things carefully. At least we seem to agree on the principal.
Barry .----------------------------------
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>> I guess that there is a difference between 84 cm square ...and 84 sq cm.
>>
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