Ventilation – Chimney or Flue?

Since RdSAP10, a number of questions are now present in relation to ventilation. 

Let's have a closer look at how we record whether to record an item as a chimney or a flue.

Please see Convention 9.01 for details regarding Open chimney/fireplace
count (for ventilation)

As per Convention 9.01 an open chimney is "defined as a vertical duct for combustion gases of diameter 200mm or more, or a
rectangular duct of equivalent area (125,000 mm2)."

Further to the above, the Convention states:
"Include all open chimneys/fireplaces in the count
(both downstairs and upstairs) only when they are unrestricted and suitable for use".

So how do we treat specific cases as open chimneys? 

Convention 9.01 states that the following should be included as open chimneys:

• chimney attached to a decorative gas fire,
• chimney fitted with a damper
• temporarily blocked using cardboard, newspaper bungs, chimney balloons and similar, and,
• decommissioned chimney that is provided with a ventilation area greater than 30,000mm2, otherwise it is treated as a blocked chimney

Therefore assessors should note the above, and take photographic evidence to confirm if the presence of an "open chimney".

Number of blocked chimneys

Count the number of permanently "blocked" chimneys that have a small vent on the inside of the house, measuring up to 10cm x 30cm (or equivalent).

Note: If the vent exceeds 10cm x 30cm (30,000 mm2) or equivalent, then it is entered in the open chimney count instead.

So what happens if a vertical duct (such as a chimney) is less that 200mm?

Convention 9.01 says that it should be counted as a flue.

Convention 9.01 states "A vertical duct with a diameter less than 200mm should be counted as a flue. This includes the case
where a flexible gas flue liner is sealed into the chimney as this reduces the diameter to less than 200mm."

Specific examples can be found below:

• Open flue gas fire with flue products outlet sealed to the chimney including radiant gas fires and ILFE (Insert Live Fuel Effect or Insert Living Flame Effect) 
• Chimney or open flue attached to wood or solid fuel boiler
• Chimney or open flue attached to wood or solid fuel stove/appliance with controlled air supply.

Note that this relates only to the number of open fireplaces (it affects the ventilation rate assumed for
the calculation). Other rules apply when considering the choice of main or secondary heating system. See also Convention 5.02. (for heating).