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Dyson HP04 Pure hot+cool purifier/fan/heater
4.5Overall Score
Name: Dyson HP04 Pure hot+cool purifier fan heater
Price (RRP): $899
Manufacturer: Dyson

The new Dyson HP04 Pure Hot+Cool purifier/fan/heater is another addition to the Dyson stable improving on its highly successful, and highly intelligent, Pure Cool released last year.

The Dyson HP04 Pure Hot+Cool purifier/fan/heater is three devices wrapped into one. Its claim to fame is the purification mechanism that not only traps 99.95% allergens like dust and pollen but volatile organic compounds (VOC), petrol/diesel fumes (NO2) and those that emanate from flatpack furniture and cleaning products.

We put the Dyson HP04 Pure Hot+Cool purifier/fan/heater through its paces and see how it justifies the premium $899 price tag – not to mention the $100 each year or so to replace the filters. Dyson is, as always, an investment in a premium product, and it sells heaps.

First – as Dyson would love – the 101 Dummies Guide to Air movement and purification

A typical fan pulls air from behind it into the blades and expels it at the velocity (speed setting) you select. It is a one-way airflow, e.g. X litres of air-per-minute in and then the same X litres out.

Dyson uses an ‘air multiplier’ effect that can increase the output (not input) efficiency by up to 10 times.

Dyson HP04 Pure hot+cool air multiplier

Dyson scientists and engineers have very effectively harnessed the Coandă effect named after  Romanian aerodynamics pioneer Henri Coanda. Simply put Dyson ‘bladeless’ fans blow the air out over a curved surface, and it takes the surrounding lower pressure air with it increasing air output. For example one litre sucked in can pick up to nine extra litres of surrounding air to push out.


Air intake versus Air output or Clean Air Delivery Rate (CADR)

Dyson HP04 Pure hot+cool advertise 290 litres/second airflow. We know that is the air output, including the Coanda effect (Air Multiplier) at a maximum fan speed of 10. We can safely assume on fan speeds of 1-5 the efficiency is 1-5X etc.

What Dyson do not state is the actual air intake – volume sucked into the purifier in cubic feet or metres per minute – or CADR. This is useful to compare it with other purifiers that state CADR and helps to select the right size purifier for the room size.

Our guesstimate was about 29 litres-per-second (lps) using Dyson’s stated 10X multiplier effect. But the previous model (similar design specs) states a CADR of 60 cubic feet per minute or 1.688 cubic metres per minute. And guess what – that is 28.32lps or in CADR terms about 100m3 per hour!

More science

A cubic metre (1x1x1m) contains 1000 litres of air. So, if you have a small bedroom room 3 x 3 x 2.4m that is 21.6m3 or 21,600 litres of air. A larger room – say a lounge 5 x 5 x 2.4m is 60m3 or 60,000 litres of air.

Now many Aussie home have high or vaulted ceilings and large open space living areas. These have no confined area, so purification, heating or cooling is far more problematic as air can flow in from elsewhere including open windows, doors etc.

If you place the Dyson in that small confined bedroom (21,600 [email protected]) air would turn over every 12.5 minutes. But you will not run the fan on 10 because it is a little noisy in a small space so using linear assumptions:

  • Fan speed five = 25 minutes to turn over the air (approx. two times an hour)
  • Fan speed one (night mode is 1-4) = 125 minutes or less – (approx. once every two hours)

How many times do you need to turn over the air to purify it?

Air Change Rate (ACR) does not specifically relate to purification but to air coming in from outside to inside keep the home well oxygenated. That air can contain pollutants like pollen and noxious gasses.

Dyson’s role is to help purify the air once it is inside the ‘designated area’ and also to remove impurities emanating from inside – cooking fumes, furniture polish etc.

Recommended ACR for a bedroom (21.6m3) is 5-6 times an hour and a lounge (60m3) is 6-8 times per hour.