Measuring Air Flow With Garbage Bags...
 

I have often wondered how to get a fair measure of air volume, but I have been reluctant to buy the proper equipment for such a seldom required event.

Here's the basic idea:


Original Link HERE.

-AC

Simple and cheap. I like it.

Out of curiosity, what are you measuring the airflow of?

Hi,

Here is a paper on bags and other simple air measurement:
http://www.builditsolar.com/Referenc...lbnl-51550.pdf

and some more: http://www.builditsolar.com/Referenc...lbnl-51551.pdf

I tried a really big bag to measure solar air heating collector flow rate. It worked fairly well: Adventures in Measuring Airflow for Solar Collectors

Another way to do it is to get one of the little Kestrel wind meters and do a velocity survey across the duct, and average the results.

Gary

I don't know if you saw the photo, but...


After much searching I was able to get my hands on a Fine Wire heat exchanger that is meant for small house or single-room ventilation. The guy who sold it to me said he had tested it an it was only good to 82% efficiency. From where I'm standing, that's acceptable.

It is the same core for the HRVs built by THIS COMPANY. They have a CO2 sensing and humidity sensing controller that actuates the fans when the need arises. I guess this helps raise the efficiency even further. I tried to buy a controller from them, but they only make 220V 50Hz controllers.

I have the HX, some fans and some ideas on how to build the fan controller. Haven't quite got the box design figured out yet.

I'll want to measure airflow for that project.

-AC

The chart doesn't show for 1 second.......Is that a bad thing? Even the longest duct vent opening is at 1 second.

I'm not quite sure I understand what you mean by, " Even the longest duct vent opening is at 1 second".

But as far as the 1 second thing goes, it would be about double the CFM of the 2 second fill rate (on both charts)... because the 2 second CFM rate is about double the 4 second CFM rate.

Yes?

Best,

-AC

I was thinking that if the air is moving too fast, it wouldn't be as efficient as if it was flowing at the optimum speed. Its coming from a natural gas furnace if that makes any difference?

So, are you saying that you are trying to measure air flow rate using the garbage bag technique and your garbage bag is filling up in one second or less?

As far as air flow goes, if your air flow is too fast, you can have excessive noise from the air rushing through the ducts.

So, do you have reason to think that your air flow might be too fast to be efficient, or is it too fast to measure?

I'm still puzzled...

-AC

Yes, too fast to be efficient. My understanding of under-sized or restricted ductwork will reduce airflow over the furnace heat exchanger and could overheat or damage the furnace heat exchanger.

But what if it is blowing so fast as to not pick up the heat from the heat exchanger?

I will check the temperatures at the supply plenum and then at the vents to see what the loss of temperature. I read that conventional furnaces should be around 140 to 170 degrees and high efficiency gas furnaces lower at 120 to 140 degrees.

Good approach.

I had central heating and I started reducing the amount of the house I was heating, and so all of the output of the furnace was gushing into a smaller space. It was not very peasant, because all that air was just coming into one room.

I was saved by my mini split.

-AC