Is there a gauge you can buy/hire? I have a table showing the desired flow for each vent but not sure how to achieve it, and have guesstimated it so far. Also it sounds a bit rumbly maybe this will improve with a balanced system? Is this a case where I should use a professional? All advice/experience welcome.
thanks
RobinB

Thanks John, Are there special ones to measure very low rates of airflow? I couldn't even tell if the kitchen extract was working except by wafting a bit of thin plastic nearby and watching it get sucked over the vent.

RobinB,

do you have calc's for expected/design flow rates for each leg/outlet for your system. Your kitchen extract ought to be one of the higher flow rates in your system.

For my system (which I designed myself, and am just about to commission), I have the following;

1. system schematic, with design flow rates at each intake/outlet (terminal)
2. anemometer (from Maplin for about £50)
3. a cone (made from cardboard) which (big end) fits over the terminal, and (small end with known cross sect. area) for the anmometer.

A little bit of maths gets you flow rate though the terminal, converted into a velocity read by the anmometer. I would do the calc for each terminal, and write that on the schematic. Then you hold up the cone, check the air velocity, and adjust the terminal valve to get the required velocity.

It is very important you do this, so our system is balanced, and the right amount of air is going the right way. It is not complicated but you need tobe robustin your approach.

Hope that helps?

GreenPaddy

The bin bag method is just a refined version of your 'wafting a bit of thin plastic nearby and watching it get sucked over the vent' test.

I haven't tried it, but I think it may actually be quite good.
If the flow from a single vent on an MHRV system is 10m^3 per hour, and a binbag is maybe 0.125m^3, it will take 1/80th of an hour to fill the bag, or 45 seconds. This would be quite easy to measure accurately, starting with a squashed flat binbag and waiting for it to be fully inflated.

At least good enough to make a relative comparison between different vents. I'm going to try it :)

for a Heath-Robinson route how about a child's beech type windmill?

hold it under the vent and count how many times it goes round in say 20 seconds. do this to all the vents at the same distance away and do some comparative maths with room volumes etc

I dont like the bag ideas much

Heath-Robbo could be improved by mounting the windmill on the cut out base of a waste paper bin then it would do flow and return and different size ducts too.

Bag or Windmill, which is best? There's only one way to find out!

I said that I didn't like the bag method much, it is difficult to define when the bag if full and as it nears full there is back pressure so slower flows would register slightly differently

I can see it working though and it would be cheap!

Posted By: SteamyTeapacket of Marlboro and matches

A joss stick will work quite well for those of a deep green going on purple persuasion. I can also imagine that using a piece of wet fish and substituting a cat for one of the people might be entertaining.

Posted By: evanYou don't want to be breathing in particulates y'know

I do sometimes

wookey, we are facing the same decision too, coincidentally, between the same options...
I had thought about using a mass air flow sensor from a car for inlet and exhaust. It could balance itself then. Any thoughts? Shouldn't be too much trouble with a pic.

Here's the instructions from our unit:

5.0 AIR FLOW BALANCING

WHAT YOU NEED TO BALANCE THE UNIT
• A magnehelic gauge capable of measuring 0 to
0.5 inches in water (0 to 125 Pa) and 2 plastic tubes.
• The balancing chart of the unit.

PRELIMINARY STAGES TO BALANCING THE UNIT

• Seal all the unit ductwork with tape. Close all windows
and doors.
• Turn off all exhaust devices such as range hoods, dryers and
bathroom fans.
• Make sure the balancing dampers are fully open.
• Make sure all filters are clean (if it is not the first time you balance the unit).

BALANCING PROCEDURE
1. Set the unit to high speed:
Make sure that the furnace blower is ON if the installation is in any way connected
to the ductwork of the cold air return. If not leave furnace blower OFF. If the outside
temperature is below 0°C / 32°F, make sure the unit is not running in defrost while
balancing. (By waiting 10 minutes after plugging the unit in, you are assured that
the unit is not in a defrost cycle.)

2. Place the magnehelic gauge on a level surface and adjust it to zero.

3. Connect tubing from gauge to EXHAUST air flow pressure
taps (see diagram). Be sure to connect the tubes to their
appropriate high low fitting. If the gauge drops below zero,
reverse the tubing connections.
NOTE: It is suggested to start with the exhaust air flow reading
because the exhaust has typically more restriction
than the fresh air, especially in cases of fully ducted and exhaust ducted installations.
Place the magnahelic gauge upright and level. Record equivalent AIR FLOW of the
reading according to the balancing chart.
4. Move tubing to FRESH air flow pressure taps (see diagram). Adjust the fresh air balancing
damper until the FRESH air flow is approximately the same as the EXHAUST air flow. If
Fresh air flow is less than EXHAUST air flow, then go back and adjust the exhaust
balancing damper to equal the FRESH air flow.

5. Secure both dampers in place with tape or with a fastening screw.

6. Write the required air flow information on a label and stick it
near the unit for future reference (date, maximum speed
air flows, your name, phone number and business address).

NOTE
• Use conversion chart provided with the unit to convert magnehelic gauge readings to
equivalent cfm values.
• The unit is considered balanced even if there is a difference of +/- 10 cfm or +/- 5 l/s
or +/-17 m3/h (+/- 0.015 inch in water) between the 2 air flows.

It would only be linear well into the laminar regime - so probably not.

sounds interesting though as I could probably borrow a pressure gauge for free

You could make a differential pressure guage, with a loop of clear plastic tube & some water. One end of pipe goes inside, the other side outside, and the height difference of the water is proportional to the pressure difference between the two ends of the pipe. 1mm difference indicates 10Pa I think. http://en.wikipedia.org/wiki/Pressure_measurement

jms452,

yes, that's the Maplin anemometer I've ordered. My calc's tell me that with an air flow for a room of 10 litre/s, and making a cone (to place over the terminals) with a final straight section of 100mm diam pipe (to get some sort of stable flow), the velocity would be about 2.5m/s - I think that anemometer should be adequate to measure that. Calc the required flows rates for each room, apply the cross sectional area of the cone inlet pipe, and that gives the target velocity. That's the method I have be involved with using, having balanced much more complex commercial facilities.


RobinB/wookey, I have attached a schematic which might help you.

As I understand it... there are 2 flow rate standards that must be met - one is a minimum flow rate versus each room needing an extract (wet rooms). This then gives you a total for the house. The second is a minimum flow rate based on the fresh air into the house and should be the greater of: (a) number of bedrooms, or (b) footprint of the house in squ metres. Again gives you a total for the house.

Taking these result for air from wet rooms, and fresh air to the house, you should then choose the higher of these as the target for your house total flow rate. This is the starting point. Total air in and out will be set at this higher value.

For each room flow rate, start with the wet rooms (extract air). The sum of these rooms flow rates, must equal the total flow for the house (which was calc'd above). You know the min. rate for each type of room (used it in the calc above), so each room must be set at more than that value. Then just use the room volumes to set the actual for each room, in proportion to each of the other extract rooms.

eg. 1 kitchen (100m3) and 1 bathroom (50m3); total flow rate for house is 30 litre/s.

Kitchen will be 20 litres/s (regulation min. 13l/s), and bathroom 10 litres/s (regulation min. 8 l/s).


For the supply rooms, spread the total flow rate (as above) across each room, again as a proportion of their volumes. You might want to play a little with this, and have a little more air to your lounge than say a child's bedroom.


To commission (I'm scraping the barrel of my memory banks), but as I recall, you open all the terminal valves fully, and then start with the "index" leg (ie. the terminal at the end of the duct with the greatest pressure drop - usually the longest run from the fan). Put the hood (cone and pipe) over the terminal valve, and start taking readings with the anemometer, and adjusting the valve to get the target value. It will be itterative, since when you set the valve on the next leg, it will alter the ones already set - but the impact gets less each time you get closer to the set values.

My system has 3 manually selectable set points for fan speeds, which I have decided will be;

"normal" - set at minimum to meet the regulations (calc'd at the start above)
"set-back" - for when the house is empty
"boost" - for when every shower, bath, cooker etc is in use (hopefully never)

Hope the above either convinced you it is reasonably simple and have a go, or that that you shouldn't even go there and just employ a technician to do it for you. Either way, make sure you have a schematic similar to the one I've (hopefully) attached, to either work against, or to have a record of what the technician has done.

GreenPaddy.

Green paddy thanks for helpful method. I'll post again when I finally get the job done. Part one - the flow requirements is done, part 2 actually setting these is to do.