Why is the humidity so high
Why is The Supply Relative Humidity so High?
[Music]
hi i'm brian with hvac school hvac
school podcast hvac our school comm
that's a mouthful
making this video in conjunction with
true tech tools comm because they have
really great tools like the tesco pixel
by smart probe thermo hygrometers that
I'm demonstrating here
this video is going to be about
something that I noticed with the 605
eye and it just kind of struck me as
something that probably a lot of you are
going to notice and think this is really
weird and actually initially when I
first started kind of wrestling with
this I sort of understood what was going
on but not really and I reached out to
Jim Bergman and we had a conversation
and he gave me this description of how
he explained it to his students and I
thought it was a really good description
so I thought I'd Rob it and make a video
of it of course it's you know what's
what's a little robbery between friends
right when you're using the test fit of
605 eyes with the smart probes app like
I happened to be right now
verifying delivered capacity what you're
going to notice is that this supplier
relative humidity is really high and you
can see the return air relative immunity
is actually pretty low strangely enough
it's actually kind of dry here in
Florida probably because the state's on
fire right now but you can see this is
where my probe is here and my supplying
and then I've got my other probe hanging
down here below the return
[Music]
and so the question is why is my
relative humidity so high and the supply
doesn't that seem like a problem in fact
in previous testing that I've done I've
actually seen it significantly higher
than that almost at 100% relative
humidity so what gives so this right now
is reading 55 degree drive volt supply
air with 88 percent relative humidity
and the question is why is that because
the typical thinking would be well we've
got this air mass clinging over the
evaporator coil it's reaching dew point
and it's dropping moisture so the
moisture is draining out the drink right
now we're draining water outside so if
we're draining water outside why is the
relative humidity going up it's coming
in at 40 percent relative humidity and
going out at 88 percent relative
humidity so why is that so we're going
to do a demonstration that Jim Bergman
suggested that easy to do in his class
but he hasn't done a video of it yet and
I'm going to we're going to say this
sponge represents air it doesn't really
look like air but and we'll say that the
size of the sponge or how much it's
compressed represents the temperature of
it and then the water represents the
relative humidity or the moisture of the
air okay so the water is the moisture
the sponges the air how big it is is the
temperature so right now this sponge is
at 81 degrees we're going to say that
essentially this is a dry sponge it's
not dry but it's mostly dry doesn't have
much moisture relative to the overall
capacity that sponge can hold ok so
relative humidity 100% relative humidity
for this sized sponge would be it
completely saturated so this would be
100% relative humidity
totally saturated sponge to sponge is at
a hundred percent relative humidity
based on its size so I'm going to
squeeze out 60% of its water
now we're going to say that's forty
percent relative humidity
so it's forty percent of the water that
this air can hold and it's coming in
full-size it's coming in at 80 degrees
seventy nine point seven degrees dry
bulb right now some limbs of the coil
hits the coil and as I hit that coil the
air gets colder and that temperature
drop is represented by making the sponge
smaller so by squeezing it so now the
air now the sponge which is the air in
hold less moisture I dropped a bunch of
moisture but now it's at about a hundred
percent relative humidity it had to be
at 100% relative humidity in order to
drop moisture the Odom's an and had to
be at 100% relative humidity in order
for it to drop moisture because if it
wasn't that it wouldn't have dropped
moisture so when it's touching that coil
it's at a hundred percent relative
humidity
now the trick is that not a hundred
percent of that air actually made good
contact with the coil so it didn't go to
one hundred percent relative humidity
it only went to eighty eight point eight
and so instead of squeezing it all the
way I'm going to let off it a little bit
right now and so this represents eighty
eight point seven percent relative
humidity
because it's eighty eight point seven
percent saturated it goes out into the
air stream and as it comes out and as
soon as it into the dots as soon as it
goes out the duct into the rest of the
space okay back over here as soon as it
goes out the duct and back into the rest
of space then it's allowed to warm up
again and now it's lower than it was
before if you remember I started with it
at forty percent relative humidity I
went and I squeezed some out and then it
was it remained at eighty eight percent
relative moving in the duct and then
when it got AlphaDog then it expanded
and the question that my cameraman asked
me when I first proposed this was well
why doesn't wiesen the inside of a duct
mold great well the reason is is because
the inside of a dock when you're in
cooling mode is going if anything is
going to pick up heat not lose heat so
if the temperature outside of the duct
was colder than what was in the duct
then it could condensate inside the duct
but
doesn't happen because obviously we're
including those that's the point so
that's the reason why your supply air
temperatures will be near 100% relative
humidity in fact any differential
between 100% and what you actually have
displays how much what we call bypass
factor or bypass air is making it into
the supply air stream so if we had no
bypass factor if all of the air was
reaching 100% relative humidity that it
would still be a hundred percent
relative humidity when it hit the duct
the fact that it's not all 100% relative
humidity either means that the coils not
a dew point which we know that it is a
dew point so that means that we have
that percentage of air bypassing so it
doesn't mean that it's like bypassing
like it's going around the coil it just
means that that percentage of air isn't
making full contact with the coil isn't
reaching you point or 100% relative
humidity which those are the same thing
you point hundred percent relative
humidity or the exact same thing in fact
when you're dry bulb temperature wet
bulb temperature and dew point are all
the same is 100 percent relative
humidity
100 percent relative humidity a lot less
than 100 percent relative humidity see
what I did there see still one percent
relative humidity here when the air gets
colder so it's still totally saturated
as long as water is coming out 100%
relative humidity air but as soon as it
heats up and expands boom can hold a lot
more again now this is like 10 percent
relative humidity
just to be a little more detailed in the
description for those of you who want
that sort of thing I wanted to plot this
on the second metric charge and actually
show it in action so if the air coming
in is 81 degrees so the return air is 81
degrees dry bulb at 40 point five
percent relative humidity which is what
we were showing early on when we were
looking at the smart probes app so we
plot up to the 40% line from 81 degrees
and then go right to see the grains of
moisture and you can also if you go
further you can see the dew point but
the grains of moisture here are 64 and
the dew point is approximately 54 point
five degrees so this means that the air
touching the coil must get to at least
54 point five degrees Fahrenheit for any
condensation to a court curtain that
means demystification so if you don't
have condensation then you don't have
demystification and vice versa this also
means that the evaporator coil itself
has to be colder than 54 point five if
it's not colder than 54 point five then
the air can't get to that temperature so
we're talking about dew point we're
actually talking about the temperature
of the air achieving dew point now when
we go into the supply we're measuring a
55 degree drive all the supply air
temperature in the doct above the unit
and we're reading it about two three
feet above the unit and it's now reading
88% relative humidity and the point of
this whole video is kind of to
demonstrate that that 88 percent
relative humidity isn't a problem
because when you look at that a lot of
texts are going to thank oh my gosh
that's an issue but if we plot that over
to the right we can see that the air now
contains 56 grains of moisture per pound
instead of 64 like it did before so that
proves that dehumidification is in fact
occurring and the reason that the supply
is at 88 percent relative humidity and
not a hundred percent relative humidity
is due to oil bypass factors so not all
of the air is actually coming in in
solid contact with the coil duct gains
meaning that there is some dry bulb
temperature added even in that short
piece of duct and blower gains because
the blower actually adds heat so based
on what we know because we see a change
in the grains per pound of dry air we
know that it's dehumidifier we also know
that we have some bypass factor we have
some gains because our relative humidity
is 88% and not 100 percent which is what
you would expect
I said that really animatedly like so
what gives why do you think that is
let's say your tribal temperature is 80
degrees if your wet bulb temperature was
also 80 then that would mean that you're
at 100% relative humidity or if you're
at 100% visibility humidity that would
mean that your wet bulb and dry bulb
would be all the same isn't that
interesting what would it be if you were
at a hundred percent humility which is
what you said several times is that what
I said yeah they need to be like me oh I
think then you just feel like everything
I do is so bad I'm the worst one hundred
percent relative humidity which means
that whenever you go to a family reunion
all you do is talk about about yourself
but it's only a hundred percent if
everybody else talks really good about
themselves because it is relative right
if you're in a bunch of other to humble
team now you're doing reflection twice
so allative relative humidity humility
oh right yeah I think we just lost the
entire audience in addition to enjoying
squeezing sponges I would also suggest
that you subscribe to the YouTube
channel here up there no no no you can't
so don't you just move the camera is it
in shape
hey how this works is it stays in one
place on the screen so you subscribe
over there no no now you're not supposed
to move the camera that's not how the
sports
