Are Cooker Hoods Over-Specced?

[MortarThePoint]

Are the hood manufacturers' numbers absurd, just designed to sell more expensive units, and the Building Regulations numbers more sensible?

@Radian you may have some interesting thoughts after your recent beef burning.

 

Power Use due to Extractor Fan:

Building Regs sets out a requirement of 30 L/s if "cooker hood extracting to the outside" or 60 L/s if "no cooker hood or cooker hood does not extract to the outside". Cooker hood manufacturers on the other hand take account of the room size and base it on 10 air changes per hour [1][2]. Our kitchen is 6.4m x 4.7m x 2.6m, so about 80m3. That works out as 800m3/h or 220 L/s.

 

In winter that is sucking out warm air to be replaced by outside air. The thermal heat flow is then:

Q = flow(m3/s) * dT(K) * 1kJ/K.m3 = 0.22m3/s * 10K * 1kJ/K.m3

Q = 2.2kW  (e.g. with dT=10K)

If your cooker hood is part of you MVHR and it has an efficiency of nu_1 and your heat generation (e.g. ASHP) has an efficiency of nu_2 then the electrical power consumption is:

P = (P_fan) + (Q * (1 - nu_1) / nu_2)

In my case nu_1=0 (i.e. no MVHR which is a whole different discussion). If dT=10C (e.g. 20C in, 10C out) and the ASHP nu_2 is at 300% then that suggests a power consumption of:

P = (240W) + (2.2kW * 1 / 3) = 1.0kW

And 75% of the electricity consumption is related to the heat flow rather than the fan's direct electricity consumption.

Based on cooking for 30 minutes per day, that works out as about 20p/day or £73/yr based on current (high, but perhaps to stay) prices.

Running the Building Regs numbers (based on 30 L/s and scaling P_fan):

P = (33W) + (0.3kW * 1 / 3) = 0.13kW

==> about £10/yr. A big difference.

 

Hood Fan efficiency:

The numbers in the table below are the maxima, not an actual operating point. But if it was an operating point, the efficiency of the fan itself would be:

nu_fan = P_flow / P_electricity

nu_fan = (Flow(m/s) * Pressure(Pa)) / P_electricity(W) = ((600/3600) * 520) / 250 = 35%

That's not too bad but isn't an actual operating point so the efficiency is probably 20-25% in INTENSIVE mode. In mode 1, unadjusted nu_fan = 18%, mode 2, unadjusted nu_fan = 31%, and mode 3, unadjusted nu_fan = 35%.

You're probably looking at an efficiency of around 20% best case. Remember, fans are really inefficient. but in extraction it doesn't really matter so much.

 

https://www.faberhoods.co.uk/hoods/victory-2-0-integrated-cooker-hood/

Edited February 4, 2023 by MortarThePoint