Fire doors letterplates and BS476

[kurnal]

Whilst I was browsing I came across the following good advice on the Chilternfire website:

17. Can I fit a letter plate or eye viewer to my fire rated door?

Fitting a letter plate or eye viewer will breach a fire rated doorset and has the potential to allow a direct passage for hot gases and flames. It is therefore important that:

There is test/assessment data for the fire rated doorset to tolerate the hardware
The hardware being fitted has been previously tested in a doorset of comparable construction
The maximum height at which the letter plate can be fitted is adhered to (see below).
 

The test conditions of BS 476: Part 22: 1987, are such that the neutral pressure axis is at 1000mm from the threshold of the door (500mm in BS EN 1634 – 1 2000); below the axis the pressure is negative and above the axis the pressure is positive. This means that above the neutral pressure axis hot gases will be  forced against the doorset and without supporting test data a letter plate fitted at this location may compromise the integrity of the doorset. The tested height of a letter plate relative to the neutral pressure axis is therefore an important constraint in limiting its location.

I always see the fitting of letterplates in a door as a bad move. Am I alone in thinking that the position of the neutral plane may be really only an important factor if you want to pass the BS476 test and the actual position of the neutral plane in a fire may be completely variable depending on the circumstances at the time? These may include the rate of growth the  fuel involved, ventilation conditions etc? Or are the BS476 parameters realistic and based on research for a wide range of real fire situations?

[Davo]

Prof

I seem to remember Envirograph doing a fire rated letterbox




davo

[wee brian]

K

Yes the neutral plain moves as the fire develops so it is a bit accademic. However this approach tends to keep the letter plate in the bottom third of the door which is probably the best place to have it.

I agree it's better not to have one and have pidgeon holes somewhere (but not by the final exit?? maybe??)

[K Lard]

See BS 8214 1990 Fire door assemblies with non-metallic leaves 14.4 Letterplates
Letterplates can allow the through-flow of air or gases. Under the simulated conditions of the fire test performed in accordance with BS 476-8 the upper part of a doorset will be subject to positive pressure while the lower part of the doorset is subject to negative pressure. A letterplate above the neutral pressure axis, more than 900 mm up from the foot of the doorset, will generally be unable to prevent flames and gases egressing from the furnace under positive pressure without the assistance of a second (internal) flap. A letterplate installed towards the lower part of the doorset will be exposed to negative pressure and air will be drawn into the furnace through the letterplate aperture. Whilst this is less likely to lead to a conventional integrity failure, the ingress of oxygen rich air is likely to cause a more rapid erosion of the adjacent door core and a large aperture may develop which could cause ignition of the unexposed face.
Therefore a letterplate should be installed in the neutral pressure zone, i.e. between 800 mm and 1 000 mm above the threshold. Letterplates with a maximum aperture size of 250 mm × 38 mm, complying with BS 2911, having well fitting sprung or gravity internal and external flaps manufactured from materials such as steel, stainless steel and some brasses, have proven successful for periods of
up to 30 min fire resistance when evaluated in accordance with BS 476-8 and only these should be fitted. Letterplates with larger apertures should not be used.
Now compare with latest version:
BS 8214 2008 11.3 Letter plates
Where a letter plate is fitted into a door it should be fitted together with an intumescent liner, as the use of intumescent liners significantly inhibits the spread of fire through the letter plate aperture of the door leaf. Only letter plates that have achieved the appropriate fire resistance period when tested in situ in a fire door should be used.
NOTE A third-party accreditation scheme exists for these products.
Since it is not possible to predetermine in which position a letter plate will be fitted, all letter plates for use in fire doors are usually tested in both the top and bottom of a door. Letter plates should be fitted in accordance with the manufacturer’s information provided with the door, identifying the acceptable locations for fitting. Letter plates with larger apertures should not be used unless they have been tested and have achieved the appropriate classification with respect to fire resistance and smoke leakage.

It appears to me that intumescent letter plates should be used.

[lingmoor]

Hi

I've been asked about fitting a letterplate to a 60 minute fire door (see clicky thing)

has anyone had experience of fitting them to fire doors?

Cheers

http://www.lorientuk.com/products/specialised-fire-resistant-door-hardware/item/high-performance-fire-smoke-security-letterplate

[William 29]

K.....you need to get out more. 

[nearlythere]

Whilst I was browsing I came across the following good advice on the Chilternfire website:

17. Can I fit a letter plate or eye viewer to my fire rated door?

Fitting a letter plate or eye viewer will breach a fire rated doorset and has the potential to allow a direct passage for hot gases and flames. It is therefore important that:

There is test/assessment data for the fire rated doorset to tolerate the hardware
The hardware being fitted has been previously tested in a doorset of comparable construction
The maximum height at which the letter plate can be fitted is adhered to (see below).
 

The test conditions of BS 476: Part 22: 1987, are such that the neutral pressure axis is at 1000mm from the threshold of the door (500mm in BS EN 1634 – 1 2000); below the axis the pressure is negative and above the axis the pressure is positive. This means that above the neutral pressure axis hot gases will be  forced against the doorset and without supporting test data a letter plate fitted at this location may compromise the integrity of the doorset. The tested height of a letter plate relative to the neutral pressure axis is therefore an important constraint in limiting its location.

I always see the fitting of letterplates in a door as a bad move. Am I alone in thinking that the position of the neutral plane may be really only an important factor if you want to pass the BS476 test and the actual position of the neutral plane in a fire may be completely variable depending on the circumstances at the time? These may include the rate of growth the  fuel involved, ventilation conditions etc? Or are the BS476 parameters realistic and based on research for a wide range of real fire situations?

I seem to remember from a time that, because of their size, eye viewers were acceptable in fire doors .The small area of glass was not significant enough that it would be subjected to differing areas of linear expansion.  
Maybe things have changed somewhat.
Of course the type of material used in their construction and the means of installation might be important.

[lingmoor]

I'm not sure if your aware that this thread is well over 2 years old...I just thought I'd tag on as it's the same subject

[Paul2886]

http://www.envirograf.com/category/carpenters_and_builders/6.intumescent_letter_flap_system.html

[Mike Buckley]

The company I work for does fit letter boxes into 1 hour fire rated doors. Provided the letterbox itself is fire rated to the standard of the door and it is properly fitted by a qualified contractor following an approved procedure then there is no problem. However going down to your local DIY store and buying a standard letterbox then hacking a hole in the door is not satisfaqctory.

[Tadees]

See attached photo.  Ignore the fact that the door is most probably not FD30S with or without the letterbox. 

1. Focusing solely on the letterbox in an existing door, would the location of this letterbox be suitable?
2. If so, is intumescent liner still necessary?
3. Is it possible to fit intumescent liner to existing letterboxes such as the one in the photo, where there is the standard metal plate on either side of the door or is it more economical/practical to replace the letterbox?

[Auntie LIn]

Follow published guidance.   If the door is fire rated (look for an identifying mark) and if its approval cover the use of a letterplate then the most sensible thing is to buy it complete with a correctly fitted letterplate assembly.   Looking at the style of door in the photo it appears to be moulded mdf faces so positioning of a letterplate only where the approval says it can be fitted is crucial - you can completely wreck even the best of fire doors by trying to put the letterplate where you fancy it, not where the appropriate support framing is located.

[kurnal]

The letter flap is around the neutral plane but I would also consider who would be at risk.... If for example the door opens directly into the base of a staircase  or a dead end corridor it would be on my action plan for an upgrade but if say it opened into a two flat lobby or corridor with two way travel I might accept it as a tolerable risk.

[Fishy]

Interesting question.

Is the BS 476 / BS EN 1364 test "realistic" ? no it isn't and it was never meant to be so.  How could you ever develop a test that would replicate any and all possible fire scenarios?  It would be completely impracticable.  It is, however, reasonably representative of a real fire & the evidence is that it adequately explores the key performance requirements of fire-resisting construction.  The whole world tends to use the same time/temperature curve (ISO 834 - though there are others for specialist applications), so it can't be that bad!

Our regulators know this, so they give us reasonably conservative requirements/recommendations (30 mins, 60 mins, 2 hours or whatever - the rating being based upon risk) knowing full well that the fire-resisting construction isn't likely to last for those times in a real fire; it might do more or it might do less, depending on the particular fire scenario.  Similarly, with a neutral pressure plane at about 1m height in the test you at least expose a typical 'person-height' element to both positive and negative pressure in the furnace - whereas in a fire it is unlikely to see exactly the same pressure conditions - it will vary principally depending upon fire size, ventilation and compartment height.

Coming back to the issue of letter plates in fire doors - the key issue for me is about the 'good practice' test.  If such an installation was reasonably modern & it would significantly compromise the ability of the door to pass a BS 476 / BS EN 1634 test then personally I'd be looking at remedial action.  Plenty of companies make fire-resisting letter plate assemblies that'll demonstrably give up to 60 mins F/R in a typical solid-core fire door, so it's probably pretty difficult to claim it isn't reasonably practicable to correct?  After all, if the fire performance of the doorsets came into question, then it's not beyond the bounds of possibility for one of them to be removed and tested (Trading Standards have done this a number of times in the past).

[Tadees]

If the flat door is an existing one, which provides 'notional' fire resistance (whatever that means in the guide) and the flats are 2 per floor, 3 floors in total, single staircase, would you ask for letterboxes to be replaced, door to be replaced or existing letterboxes to be provided with intumescent lining, provided the flaps are metal and on both sides of the door?