Measuring airflow for residential forced-air systems for HVAC professionals

As a heating and cooling
professional, you work hard at what you do and take pride
in getting the job done right. That's why we're here to talk
today about measuring air flow for residential air
conditioners and heat pumps. We're not selling anything and we don't represent any
one's products or services. But as independent energy
efficiency researchers, we've had the opportunity to
make measurements on hundreds of residential systems. And we'd like to share
that experience with you. >> In this video, we'll talk about why proper air
flow is important, how it affects system efficiency
here, in the upper Midwest, and we'll go through
the pros and cons of different ways
to measure air flow. >> All right. Let's get started. >> Great. >> Okay. Here we are looking at a nice new high efficiency
gas furnace that also serves at the air handler for a two-and-a-half-ton
central air conditioner.

The question is, in the summer
is this furnace producing the right amount of air flow
for the cooling system? We'll find out in a minute,
when we actually test it. But first, let's talk about how
much air flow we're looking for and the consequences of not
having the right amount. So, you probably already know that manufacturers are
generally looking for somewhere in the range of 300 to 400
cubic feet per minute per ton of cooling capacity, in
terms of cooling air flow.

So, here, we'd be
looking for something in the 800 to 1,000 CFM range. And I'm sure you already
know the consequences of having air flow
that's too low. You're looking at very cold
evaporator coil temperatures, and potentially a frozen
coil and all the problems that go along with that. But when we test systems
here, in the upper Midwest, what we actually find is
that we're much more likely to find air flow
that's too high. And that's probably because
here, where it's cold, we tend to install big
furnaces with big blower motors. And we pair them up with
small air conditioners. And that's bad, too, because
high air flow creates its own set of problems. Number one, it hurts the
ability of the system to dehumidify the air. And it also means a lot more
blower electricity going through the system. And it just makes for a
noisier overall experience and less comfortable overall
experience for the homeowner. So, we typically find that about
half of systems that we test in our research in this part of the country have
air flow that's higher than 400 CFM per ton.

But only about one in ten have
air flow that's below 300 CFM per ton. When we correct those
air flow problems, we typically see an improvement
in efficiency performance of about five percent. But we've seen increases
as much as 30% when air flow was
really a long ways off. All right. So, we're going to take a look and see how well this system
is doing, in terms of air flow, by measuring it with a
number of different methods. There are lots of different ways
to go about measuring air flow. Here, we're going to
concentrate on just a few. We'll talk about using
a calibrated flow plate. We'll look at air flow by
measuring velocity in the ducts. We will look at static
pressure and how we can use that with manufacturer's
literature, to estimate air flow. We'll talk about adding up
air flow by measuring flow from individual registers. And finally, we'll look at
the temperature split method for measuring air flow.

Let's get started. >> Let's start with this device,
which is a calibrated flow plate that takes the place
of the filter. Basically, air flow is
very predictably correlated with the pressure drop
across this plate. So, just by measuring
the pressure drop, we can get a good measurement of the total air flow
going through the system. We really like this device. And we use it often
in our research. It's accurate and
relatively easy to use. In the kit, you get basically,
two flow plates, different sizes with a bunch of spacers
that allow you to adapt the particular plate
to any filter slot size. The main downside is
the up-front cost. Which is about $800, not including a good
digital manometer. Which would be a
digital, or another way to say it, is a pressure gauge. The one we have is made by the same manufacturer
as the flow plate.

It is right here,
that we'll be using. So, it can do the pressure to
air flow translation for us and give us a direct
reading of air flow. We combine the flow plate
with a pressure gauge. We can read CFM, cubic
feet per minute, directly. So, let's take a
look at it in action. Okay, I'm going to
drill the hole for the supply pressure
reference tap for the TrueFlow plate. This allows me to understand the
supply pressure at this point with the filter in place, and with the TrueFlow
plate in place. Which. [ Drilling Sound ] By measuring that pressure,
I can see the difference in resistance between the
filter and the pressure plate.

Helps me calibrate
the air flow and CFM. I'll stick the static
pressure probe in, point it towards the air stream. So, now that the
pressure tap is in place in my supply trunk line. I've turned the air handler on. We've turned it in the cooling
mode to get cooling speed. My digital manometer is
reading static pressure in the supply trunk with
the filter in place. I'm going to hit enter so that
my digital manometer records that pressure. Next, I'm going to
remove the filter and insert the TrueFlow
pressure plate. So, now that I have the
TrueFlow plate installed, then I've covered up the
filter slot opening just to eliminate any leakage
that might occur there. Now, I hook the TrueFlow plate
up to my digital manometer. My digital manometer is set to
read the particular flow plate.

It's been programmed with
the appropriate flow plate. We hook up the pressure
hoses on either side of the pressure plate to
the digital manometer. And the manometer is going to
read out cubic feet per minute, CFM, of air flow across
that pressure flow plate. Right now, we are reading
seven ninety, seven hundred and ninety-eight
hundred CFM, I have here. So, that's right on the low
end that we're asking for, for this two and a half ton
air conditioning system. That's it. It's very simple, easy to use. And as we stated
earlier, very accurate. >> Okay. So, our second way of measuring air flow
involves measuring the velocity of the air in the ductwork. If we know the velocity
of the air that's going through a section of duct,
in say, feet per minute. And we know its cross
sectional area in square feet, we can put those two pieces
of information together and calculate cubic
feet per minute. Now, there's a couple of
different ways that you can go about getting at this velocity.

One is to use a pitot
tube, like this one, where you have a short
tube with a hole in the end that you face into
the air stream. And the greater the velocity, the greater the pressure
on that hole. And by measuring that pressure, you can translate
pressure into velocity. But today, we're going to
use a different device. And that is this
hot wire anemometer. Now, this thing works by
having a very tiny little wire in its tip that's heated. And as the air goes across
that heated wire, it's cooled.

And by measuring
that cooling effect, we can translate
into air velocity. Now, it would be great
with either the pitot tube or this anemometer, if
we could just stick this into the ductwork
in any given spot, take one reading,
and be done with it. But the problem is that air
velocity varies on the inside of the duct from one side to the
other and from front to back. And sometimes, if varies a lot.

So, if we really care about
getting a good measurement of air flow, we're really
gonna need to make a number of measurements to get the
average velocity of the air through that section of duct. So, okay. So, here we are on
the backside of the furnace. And I've marked the location
of five holes that I'm going to drill in this nice long
straight section of return duct. And those holes are not
exactly in the same place. We won't go in today, why
they are where they are.

pexels photo 5877456

But you can look that up,
online for more detail. But we're just going to go
ahead and drill five holes. And we'll run our
hotwire anemometer through each one of them. [ Drilling Sound ] Okay. So, I've drilled
my five holes. And now, what I'm going to do is
take this probe, and I'm going to extend it all the way
into each one of the holes. And then, slowly withdraw
it while we're using our anemometer, here, to
average the velocity across each one of those holes. Alright, last hole. So, we've done our
five traverses through the five holes. And we've got our
overall average velocity of 540 feet per minute. And when you multiply that
by the sides of the ductwork, which is ten inches
by 23 inches, that works out to
just about 860 CFM.

But I can tell you
that we saw everything from 190 feet per minute,
with the velocity, there, to over 800 feet per minute. And one thing we can show you, is because this little
unit can data log and we recorded those traverses,
we'll show you a graph of what those five velocity
traverses look like. So, another way to
measure air flow is to take some static
pressure measurements at different places
in the system. And then, take that information
to the manufacturer literature where we can look up air flow as
a function of static pressure. And we've really got
two choice, here. One is this static pressure rise
across the air handler, itself. And the other one would be
the static pressure drop across the evaporator coil. In this case, we're
going to use the rise across this furnace unit, where we have the
literature available.

And you can see that we've put a
static pressure probe down here in the blower return cabinet. And then, we have
another one up here in between the furnace
outlet and the bottom of the evaporator coil. And we have the system running. And you can see that we're
measuring just a little under half an inch of water
column across this system. So, with the manufacturer
literature in hand, we can look up, based
on the speed tap, the model of the furnace, and the static pressure
rise that we're seeing. And we read off of the chart that we are seeing
about 975 CFM. Which is just a little bit
higher than we measured with the flow plate, but
in the same ballpark. Now, this method will work well if you have the manufacturer's
literature. It won't work at all, if you
don't have that available. And generally, it's probably
preferable if you have a choice, to make the measurement using
the total external static of the furnace, rather than the
pressure drop across the coil. Because with the coil,
you can have issues with, is the coil wet,
is the coil dry? And if that coil has been
in place for a while, it may have some
dirt buildup on it.

And that could affect the
readings that you get. >> Okay. Another way to
measure air flow is at each of the registers using
our hot wire anemometer. The hot wire anemometer
measures feet per minute, if we know the register size and its net free area
in square inches. So, it'll require going to the manufacturer's
specification sheet to make sure we know the open
area of the face of this grille. The hotwire anemometer
measures feet per minute. And we have to do a
traverse across that register to get an average
feet per minute. Then, we multiply that by
the square feet opening of the register. And we then are able to
calculate cubic feet per minute. So, I'll start our traverse
across this register. And we'll take several passes. Now, measuring air
flow like this is great for diagnosing comfort problems
and distribution issues. But we don't generally
recommend it for getting at system air flow back
at the air handler.

It just takes too long to
measure all of the registers in the home, and there's too
much uncertainty from adding up all those individual
register measurements. Maybe you're wondering how
accurate these methods are. And we were, too. So, we made one extra
measurement on this system, using a calibrated fan to
match the system pressure and give us a very accurate
measurement of system flow. Compared to that gold standard,
the other measurements are in reasonable agreement. The flow plate measurement
was a bit lower. And the measurement based on static pressure
was a bit higher. But they were all
within ten percent. We didn't try to measure system
flow from register measurements, because if would
have taken too long. And the temperature split method
is just an indicator of flow, not a direct measurement. Plus, we couldn't do that test because the indoor
conditions weren't right. Generally, we favor
the flow plate test. Because it takes a quick and
reasonable accurate value. The hotwire anemometer
approach can be accurate, but it takes more time. The static pressure
approach is the quickest. But we have found that the
readings can vary some, depending on where you
measure the static pressure.

And you have to have
an air flow chart for that equipment
you're measuring. But our tests on this
particular system show that if you take your
time to do it right, you can get good air flow
measurements using a number of different techniques. One other note; if you're way
above sea level, you might need to make adjustments for
air density at altitude. >> One final method for assessing cooling air flow
is a really easy one to do, but you can only do it
under certain conditions.

And it's called the
temperature split method. And it's a lot like
measuring temperature rise when you're assessing
air flow for heating. In that we're looking
at the difference between the warm return
air that's coming back to the system, and the cool
supply air that's coming out the backside of
the evaporator coil. That temperature
difference it called the ''temperature split''.

So, if we have a system that's
running really high air flow, that air is going over that
evaporator coil very quickly and it's not getting
cooled down very much. So, this temperature difference,
or this temperature split, will be on the low side. On the other hand, if we have a
system with very low air flow, that air is dwelling on that
evaporator coil for a long time, it's getting cooled down a lot. So, we will see a
large difference between the return air and the
supply air that's coming out.

And by gauging the magnitude
of that temperature split, we can get a sense of
whether our air flow is in the right ball park. But there is a rub. Only part of the
energy that's going into this cooling coil is going to actually cool the
temperature of the air. The rest of it is going to
removing humidity from the air and it doesn't reduce
the temperature. So, depending on how humid
the air is, that's coming into the system, it will affect
the target temperature split that we're looking for. If we have very dry air that's
coming in, we'll be looking for a very high temperature
split for our target. On the other hand, if we have
really humid air that's coming back on the return
side, we'll be looking for a relatively low
temperature split.

So, to make this
measurement, all we really need to do is measure the
temperature on the supply side. Remember, measure
the temperature, the dry bulb temperature, and the wet bulb temperature
on the return side. And then, use our target
temperature split table to look up, based on
return dry bulb temperature and return wet bulb temperature,
what our target number is. And then, compare that to
what we actually measure. But there's one other thing
that we have to worry about. If we see a low temperature
split, it could be that
air flow is high, but it could also be
something else that's wrong with the system. For example, if refrigerant
charge is low, our system will not be putting
out its full amount of cooling, that in and of itself will lead
to a low temperature split. And if we're not careful,
we could misdiagnose that as an air flow problem, instead of what is really
a refrigerant problem. On the other hand, if we see
a high temperature split, then that's a pretty
good indication that we have low air
flow on our system.

Now, the final consideration is that our temperature split
target table only works for a certain range of dry bulb and wet bulb return
temperatures. And if we're outside of
that range, we're not going to be able to make
that measurement. And that's the cases,
here today. We're off the charts
as far as being able to get a target temperature
split. So, we won't be able
to measure it, today. Well, there you have it. Some of the ins and outs of various ways to
measure air flow. I hope this inspires you
to start measuring air flow on the systems that
you install or service. >> If you found this
video useful, please watch our other
videos related to details of our research findings
and tips for homeowners. Thanks, for watching..

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