You need a capture hood to get any sort of accuracy at all, but there are three main types:

• The basic is the Testo or Airflow anemometers, Testo better built. What you have to recognize with these hoods is that they are seriously affected by turbulence, so if you put them on a supply terminal designed to give a jet your readings will be seriously out – BSRIA have done a report on this. Extract ok. The other main issue with them is that they cannot measure much above 60 m3/h (~20 l/sec), which means they cannot be used for measuring total flows on outside terminals, which in turn means you cannot check for duct leakage (sum of internal flows should equal the external flow). This limit is simply due to the small size of the vane: 100mm diameter, which at higher flows puts an extra resistance on the mvhr system and so gives daft readings. Cost a few hundred £ depending on how many hoods. Very light ( 1kg?)

• The next step up is the large area hot wire anemometers with a much larger throat (200mm sq) so can measure larger volumes and doesn’t seem to have the turbulence issues above. £2k? Swemma main brand. Fairly light ( 2 kg?)
• And finally we have the balometers, which have a powered fan which runs to balance the pressure inside the measuring chamber to outside, so there is little change on the whole system balance. Most accurate but a little heavier (3kg?) and cost (£3k?).

Which you get depends on what you are doing. The anemometers are fine but only if the system has no leaks (tricky to know) and you can’t really do a Passivhaus without a separate duct leakage test.

The hot wire are fairly light and accurate, and for all day use are the best choice.

If you get mostly called out for fault finding, as we are, then the balometer is best for accuracy, so we put up with the extra weight.

Remember that you will have to calibrate the unit once a year, which costs a couple of hundred quid. It worth also thinking about hiring: BSRIA has reasonable rates, and each unit is calibrated before it is sent out. Its also a good way to find out what each unit is like to use.

ps: It has been pointed out that the CIBSE commissioning code says the only absolute measurement recognized is a pitot tube, with measurements taken at intervals across the duct. This is fine for commercial pipe ducts, but hopeless for residential, where the small ducts and lack of straight runs before and after measurement points means that it is wholly impractical. Don’t try it!