All in the numbers
The HETAS Technical team has been busy pulling together some of the important numbers that you use in your everyday work. From connecting flue pipe distances to hearth projections, this article has all the numbers you need.
Frequently Asked Distances
- 20% RULE
HETAS recommend that any diagonal distance between bends should be no more than 20% of the total length of the chimney for example, 5 metre chimney height then maximum distance between bends would be 1 metre.
Training Reference: HTU04 p51 and HTU06 p24
- TERMINATING BELOW CEILING BS EN 15287
Where a twin wall flue system passes through a ceiling for connection to an appliance it must project a minimum of 425mm below the ceiling. Fire stopping as per manufacturer’s instructions will also be necessary.
Training Reference: HTU04 p54/HTU06 p36
- XX DISTANCE TO COMBUSTIBLE MATERIALS – European Chimney Designation
The XX rating from the flue manufacturer must be maintained at all times. This does not include shielding to try to reduce the distance required.
ADJ Reference: p24 1.45a d13. Training Reference: HTU06 p30.
- HORIZONTAL LENGTH
Appliances with a rear outlet horizontal length should not exceed 150mm.
ADJ Reference: p26 1.48 d15. Training Reference: HTU04 p51.
- HORIZONTAL LENGTH EXCEEDING 150mm
Training Reference: HTU06 p25 and HETAS Technical Bulletin 1.
Maximum of 4 bends between appliance outlet and flue outlet (each up to 45 degrees). ADJ Reference: p26 1.49. Training Reference: HTU04 p51.
- HEIGHT OF FLUES
If a minimum chimney height of 4.5 metres cannot be achieved then as an alternative approach a flue calculation to BS EN 13384-1:2005 can be obtained from a flue manufacturer as the basis for deciding whether a chimney design will provide sufficient draught. Results of the calculation need to be left with the installation instructions with the client.
ADJ Reference: p30 2.8.
- TERMINATION TO EASILY IGNITED ROOF COVERINGS
- At least 1800mm vertically and at least 600mm above the ridge
- At least 1800mm vertically and at least 2300mm horizontally
Note; not less than 1000mm above Flat roof felt with lime stone chippings
Note; Plain felt, Rubberised or Sedum covering then 1800mm
ADJ Reference: p32 2.12 d18. Training Reference: HTU04 p48.
- CONNECTING FLUE PIPE DISTANCE TO COMBUSTIBLE MATERIALS
3 X Diameter of flue pipe to combustible material or 1.5 x diameter with shielding (note minimum 200mm) non-combustible shield must have 12mm air gap between shield and combustible material
ADJ Reference p33 2.15b d19. Training Reference: HTU04 p53.
- Maximum Single skin flue pipe
Maximum allowable length with pipe support 2,000mm. Training Refernece: HTU04 p54 and to BS 15287.
- COMBUSTIBLE MATERIAL FROM FIREPLACES AND MASONRY FLUES
A)200mm from the inside surface of a flue or fireplace recess; or
- B) At least XXmm from a flue product with designated separation distance (GXX); or
- C) at least a 40mm air gap from the outer surface of a masonry chimney or a fireplace recess unless it is a floorboard, skirting board, dado or picture rail, mantel-shelf or architrave. Metal fixings in contact with combustible materials should be at least 50mm from the inside surface of a flue.
ADJ Reference: P34 2.18 D21. Training Reference: HTU04 p52.
- SUPERIMPOSED HEARTH
Minimum thickness of 12mm as long as surface temperatures not exceeding 85 degrees.
ADJ Reference: p37 2.23a d24. Training Reference: HTU04 p23.
- CONSTRUCTIONAL HEARTH
- In a recess at least 150mm either side of jambs and a minimum of 500mm depth from jambs.
- Free standing at least 840mm square.
- Minimum thickness 125mm
ADJ Reference: p37 2.24 a&b d24&25. Training Reference: HTU04 p20.
- HEARTH PROJECTION
In front at least 225mm for a closed appliance or 300mm for an open appliance and a closed appliance that can be used with its doors open.
ADJ Reference: p38 d26. Training Reference: HTU p21.
- APPLIANCE MINIMUM EFFICIENCY
Room heater/stove 65%. Website Reference: www.hetas.co.uk/consumer/appliance-minimum-efficiencies.
- SMOKE TESTING
Pellets that produce 24m3 over 60 seconds. Training Reference: HTU05 p6 & 7.