How do I balance air outlet/inlet valves without an airflow meter?

As my local hire shops (UK) don’t stock airflow meters, I am trying to find another way to balance our newly installed MVHR (mechanical ventilation heat recovery) system.

Firstly I need to set the fun speeds, so I need to be able to measure the total ventilation rate.

I then need to set the air valves in each room, so as to split the ventilation correctly between the rooms.

(I need to be able to measure values up to about 60 litres per second and as low as 1 litre per second.)

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Lowes has a cheap meter for ~\$40.00, not sure if they ship to the UK though. – Tester101 Oct 14 '11 at 16:55
When mine was installed (Canada), the technician told me the house will always have a "negative" air pressure when the furnace is off because the MVHR fan runs continuously. She demonstrated this by holding a tissue next to a window opened a crack. When the furnace is on, she explained, it will draw in the fresh air it needs and the MVHR fan will slow down or shut off as required. – belwood Nov 12 '11 at 16:10
@belwood, I think the system are diffenet in the USA and Cananda as we don't have hot air ducted heating. – Walker Nov 13 '11 at 19:58
@Tester The meter from Lowes measures air velocity. To get airflow (volume) you will need to also need to have the duct/vent area. Multiply the average air velocity for the duct/vent by the area. Then you may need to convert units. 1 liter per second is equal to 2.12 cubic feet per minute. – RSMoser Nov 14 '11 at 20:53

I tried this approach on my senior project.
It is a little tricky, but you find the largest garbage bag you can buy, or any bag, and either a stop watch or better a video camera.

If you know the size of the bag 50 gallons in my case. And if you know the time it takes to fill the bag.

you can calculate the airflow through a particular outlet.

The bag can fill up pretty quickly, so I recommend making the calculations using a video camera. It helped to see when the airflow initially started.

I had a known fan that produced a airflow speed of 150 CFM, and I was able to measure out about 133 CFM with this technique.

The video camera is used to see more accurately when the blower starts and when the bag is full. Time( full bag) - time( blower starts) it can be hard to keep up with a stop watch, so I use the camera playback to see when each event takes place.

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How are you using the camera to do the timing? – Walker May 14 '12 at 21:14
I added a further description to my answer – Ashitakalax May 15 '12 at 15:14
Thanks, I did not release you were using a video camera not a normal camera. – Walker May 15 '12 at 15:37

Well, you could use the same concept used in a 15 knot aviation windsock...

Basically the aviation windsock is designed so that when the wind reaches 15kts or more, the sock is fully extended. This allows pilots in flight to gauge roughly the speed of the wind so that they can compensate for it on landing and takeoff.

In YOUR case, you can take a piece of fabric tied to a stick, and make a rough "meter" by drawing evenly spaced lines on the fabric and with each vent, seeing how much lift comes out of the vent by looking at how many lines are fully lifted before the cloth starts to dip.

But if you want accurate measurements of volume then you need speed x dimensions and, well, you need an accurate speed meter to get an accurate speed measure. Maybe you could order an airflow meter? Or call in a specialist?

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All the air vents are in the ceiling, so this will not work for me. In the UK there are not many specialists at present, as most homes don't have controled ventilation. – Walker Oct 14 '11 at 13:23