Kitchen extraction - long run needed for ducting

[Chris Melluish]

My house, originally a three bed semi, has an extension round the back and side of the house, one storey behind the kitchen which is across the back of the original house, and two storeys elsewhere.

My wife has been designing her new kitchen, and it necessitates moving the cooker to a position a long way from any external wall.

There is no cooker extraction at the moment, just a large fan mounted in he wall of a cupboard in the kitchen. The ducting (8" diameter?) goes up into the ceiling and about 5 metres horizontally to the exterior between floors in the two-storey bit. While the fan worked it was very good, but has now packed up. The cupboard is coming down anyway in the new design.

The plan at the moment is to fit an extraction device over the cooker, route the ducting round the ceiling for about four metres, including one right-angled bend, and somehow join it to the existing ducting. This will give about four metres of 4" or 5" ducting connected to five metres of 8". I am also toying with the idea of a suspended ceiling which would hide the ducting (and the uneven bits and the Artex).

Will this work? Do I need a very high power fan? If so, would it be extremely noisy?

I have been looking round at integrated extractor fans, and the highest rating I have found so far is a Wickes one at 460 m3/hr. Non-integrated types can be more powerful but are more expensive.

An alternative would be to buy a 200mm replacement for the failed fan and mount it direct into the ceiling, or above the ceiling if it would fit. It would be about 2.5 metres from the cooker.

-- Chris Melluish

[anthony james]

I dont like answering without a good level of knowledge but since no-one else has after two days and I've looked into this a little...

That's a long run with a lot of bends. The flow rate drops significantly with each metre of duct and each bend. Smooth sided rigid ducting has less impact than flexible. I've seen figures for how much impact it has but cant find them quickly - google should be your friend here.

The big advantage of a purpose built extractor over a simple fan will be washable grease filters - your aim is to get the grease out of the extracted air to minimise the amount that condenses on the cool walls of the ducting.

There's a lot of good info on

Elica appear to make most of the branded cooker hoods available as well as under their own name - a lot of Bosch, AEG, Neff etc are rebranded Elica. Most of the Elica branded hoods seem to have the highest spec motor offered by the other brands - 750m3hr is common and many are available with an 'inline' (ie remote) motor 1200 m³/h. Our 750m3 model is remarkably quiet. Inline are meant to be even quieter.

i missed that you mentioned integrated - they do a 'built in' model with a 1000m3hr remote motor which might be of use.

[Dorothy Bradbury]

Fan Airflow is quoted as a Free-Air Maximum or or at a particular Resistance o Airflow is related to Resistance by a Pressure (P) v Airflow (Q) Characteristic Curve

---- Rising Airflow = Rising Static Resistance o Different fan technologies offer different P-Q characteristics & noise characteristics

---- Fans designed for ducts may not necessarily be noisy

Ducting present airflow resistance from 3 key areas: o Distance -- you have 9 metres, ~30ft which is somewhat long o Area -- larger area reduces airflow velocity and so reduces noise o Corners -- minimised with the run as straight as possible o Construction -- round-metal offers least resistance, flex-ducting most resistance

Ducting Resistance also presents a Pressure (P) v Airflow (Q) Characteristic Curve.

1) Identify the various ducting options:

---- Ideal ducting is Round + Large as possible + Straight as possible

2) You need to consider condensate -- re duct routing & drainage

---- an inline duct fan will also require servicing & cleaning

3) You need to identify the effective System Resistance

---- this will typically be from HVAC tables/charts re length/size/airflow

---- a ducting fan supplier should have this ready for over-the-phone

4) You then need to size a fan to suit that System Resistance

---- a fan that provides the desired house-outlet airflow for that resistance

If a supplier can't provide a valid P-Q characteristic for a fan, then it is likely the fan may not perform adequately and worse could overheat. Remember the fan motor itself must be cooled by the air passing over it, for a fan consuming

100W that in itself requires around 20cfm to be realised (NOT free air spec :-)

Most conventional cooker hoods I suspect will not cope: o Practical design limit is perhaps 10-15ft / 3-5m

---- you require usable exhaust airflow after some 30ft *and* some bends o Most cooker hoods look low profile, may be axial or diagonal flow fan

---- axial flow fan -- high airflow + low static pressure (useless)

---- diagonal flow fan -- high airflow + high static resistance (usable)

The difference is in the blades, and obvious from the P-Q characteristic curve. Radial flow fans, often called blowers, produce more noise than other types of fan - but produce very high static pressures to overcome ducting for less airflow.

Typically one can use a large radial blower, or more usually assistive inline fans: o Serial inline fans are not strictly additive in their airflow, but in pressure o Serial use of fans boosts pressure nearer to the free-air specification of one fan

Likely solution -- you will need a cooker-hood AND/OR inline ducting fan. o In the USA it is not uncommon to have a 20-30ft exhaust duct o It does require proper sizing or you get little airflow & a very overheated fan

9 metres is a good length, so pressure capability will be a key factor.

[Dorothy Bradbury]

Some example figures...

Two ways of assessing kitchen extractor requirements:

1) Based on market offerings

---- most hoods offer 600-1200m3/hr free airflow

---- using a 50% derating re ducting that is 300-600m3/hr or 175-350 cfm desired

2) Based on air changes per hour

---- a kitchen is typically 15 air changes per hour

---- so 1) calculate the volume of the kitchen re cubic feet

---- then 2) work out what cfm you require for 15 changes per hour

That is the out-of-wall final cfm or m3/hr required.

Two ways of identifying your duct system resistance:

1) Based on HVAC tables/calculations for a figure in Pa

---- this is a kitchen, single duct, not a multi-duct/fan HVAC

2) Based on supplier tables & charts for effective duct length

---- adjust for duct construction

-------- 4" smooth = 1.00*duct-length, flexible = 1.25*duct-length

-------- 6" smooth = 1.00*duct-length, flexible = 1.50*duct-length

---- adjust *per* elbow

-------- 4" elbow = 12-15ft, 6" elbow = 17-20ft

---- adjust for outlet plate & wind resistance

-------- 4" typically 20-40ft, 6" typically 30-40ft

So 30ft 4" flexible, 3 elbows & 1 outlet = >110ft effective duct length.

Now you ask a duct fan supplier for a duct-fan & duct-length P-Q curve: o The curve consists of 2 plots overlaid on the same Pressure/Airflow scales

---- Fan curve - showing at Pressure (P) the resulting Airflow (Q)

---- Duct curve - showing at length-1-2-3 the Pressure (P) drop for Airflow (Q) o Choose the duct-curve corresponding to your effectively duct length

---- eg, 110ft o Identify where the duct-curve crosses the fan P-Q curve

---- where the 2 curves intersect is the fan's cfm in your duct system o If the fan CFM is above your requirements, the fan will do

Simpler than it sounds.

Note: o You can't just go on free-air cfm comparisons necessarily

---- the static pressure matters - you need the actual curves o Few cooker hoods will push sufficient airflow thro 30ft of ducting

---- both from an extraction perspective and re fan cooling

---- inline fans in particular require inline airflow for cooling

A duct fan supplier will simply need to know your application, and the length/elbows/outlet to match up duct & fan accordingly.

[Rick Hughes]

There are lots of maths around to calculate resistance & pressure drop, airlfow etc ... but for your purpose the limit for a typical 5" extract is

5m stright run, about a m less if a 90 degreee bend included.

Rick

[N. Thornton]

If you have 8m of duct and you buy fans rated to do 4m of duct, use 2 fans plumbed in series. Check the duct length ability figs, as they may be for very idealised ducts, and yours may be less than ideal, meaning the fan would work with less length.

NT

[Chris Melluish]

Many thanks for the usual sound advice from the group.

After some discussions with the manufacturer I have bought an Elica remote motor extractor hood, which as well as being more powerful than others I considered will hopefully be a lot quieter.