Calculating the effective duct length

Installation - venting

Calculating the effective duct length:

Friction against the inner walls of the vent duct will slow down the expelled air as it passes through the duct. An el- bow, bend, non-return flap or wall vent creates more friction than a straight duct. The friction created by each of these is expressed as an additional length factor. The additional length factors are calculated by comparing the friction created by, for example, an elbow to that created by 1 meter of straight plastic piping (Table I).

The additional duct length factor for each item needs to be added together to ascertain the effective duct length which will be needed for the venting system.

A larger internal duct diameter also helps reduce friction. If the effective duct length is more than 20 m the

duct diameter should be increased (Ta- ble II).

1.To calculate the effective duct length:

^First measure the distance which will require straight ducting. Multiply this amount by the corresponding addi- tional duct length factor in Table I.

^Establish the number of angles, bends etc. necessary and add their additional duct length factors to- gether with the help of Table I.

^Add all the additional duct length fac- tors together to give the effective duct length.

2. Internal duct diameter

^Refer to Table II for the appropriate duct diameter for the effective duct length.

Table I

	Additional duct length
Flexible ducting*
- 1 m laid straight	1.8 m
- 45° angle (Bend radius = 0.25 m)	1.5 m
- 90° angle (Bend radius = 0.25 m)	2.5 m
	2.5 m
Flexible or aluminium vent hose* / plastic piping
- 1 m laid straight / or 1 m straight pipe	1.0 m
- 45° angle / elbow (Bend radius = 0.25 m)	0.6 m
- 90° angle / elbow (Bend radius = 0.25 m)	0.8 m
Wall vent kit* or window vent kit*
- with louvred grille	3.8 m
- with non-return flap (hinged flap)	1.5 m
Non-return flap* to be integrated into the vent ducting	14.3 m
(see "d Main venting duct" for example)
Flat ducting (for washer dryer stack)*	11.0 m
Venting connection to the left/right	1.0 m

* These can be obtained from the Miele Spare Parts department or from builders’ merchants.