Why is The Supply Relative Humidity so High?


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


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


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


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


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


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