Why is Indoor Relative Humidity Low in Winter? | Explained for HVAC Techs

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This video is brought to you by ewc Controls the best controls for forced Air zoning on the market find out more At ewccontrols.com I think that everybody who has a gas Furnace out there will realize that Their house gets very dry in the Wintertime now the question I want to Answer today what I want to take a look At is does the gas furnace actually Remove moisture out of a house how does It get so dry inside what I think we Need to do is take a look at the Psychometric chart everybody just goes You know it's going to be okay this is a Simplified version of the psychometric Chart and it's going to go a long way For explaining why the air becomes so Dry or has a low relative humidity while You're running your gas furnace so Here's our psychometric chart now we're Missing a lot of stuff here we're trying To keep it really really simple for the Purpose of explaining this particular Theory So if we have a dry bulb temperature you See down the bottom of the screen we Have dry bulb temperatures ranging from 40 to 120 on the right side we have Humidity ratio a little curved lines Throughout the chart you see 20 40 60 80 That's our relative humidity then you Have our wet bulb on the left curve of The chart

So we're going to take a normal Condition let's say 70 degrees at 50 Humidity we don't have 50 so we're going To go down to 40. so we're going to be Right here on the chart right on this Line right here so what we're going to Do is we're going to put that air that's At 40 percent humidity relative humidity And 70 degrees dry bulb to a gas furnace And let's say that that air comes out With a 50 degree rise which means it's Going to be 120. so on the far right of Our chart we've gone from 70 all the way To 120 and we're going to go in a Straight line the reason why we're going In a straight line is because we're Neither taking moisture out of the air And we're not putting moisture back into The air unless we have a humidifier but For the purpose of this discussion we're Ignoring the humidifier so we're warming The air but keeping the same moisture Content so it's going in 70 degrees 40 Percent relative humidity now if we go Straight across here in the most Unscientific way we reach this point About right here so we're right on the 120 line but as you see because we went Straight across We're no longer on the 40 curve we're Not even on the 20 curve We are somewhere between zero and twenty Percent if we're sort of going to Extrapolate where we're at based on just

A rough idea if zero is the corner there And twenty percent is that curve then We're probably somewhere around 12 13 14 Maybe so our relative humidity has Dropped from 40 percent at 70 degrees Dry bulb and because the furnace is Neither Acquiring more moisture or removing Moisture the content is the same so what Is different the specific volume of the Air is different as air gets warmer and Warmer it expands it can actually hold More moisture But because our moisture content didn't Change The percentage of moisture that is in The air compared to its capability has Dropped because this expanded warm air Can hold more water but the same amount Of water is there To put it very simply So we've gone from a relative humidity Of 40 down to below 20 percent Down to our nosebleed section if you Guys are like me in the winter time You'll wake up in the morning with Nosebleeds because here we have heat Pumps we don't have gas furnaces we Would have the same problem right here It may not be as pronounced because it Doesn't get as cold here so we don't run The heating system as much it tends to Be a little bit more humid where I'm at Because we're close to the ocean so we

Have that going for us but in the winter Time it still gets very dry because we Have the same effect so let's take a Look one more time on the chart and see What a heat pump might put out and see How that differs from a gas furnace as Far as that Supply Air So we're going to start right there on 70 again which is 40 humidity that's Where we're at If you need to measure something Different if it's 60 of course you can Start there if it's 20 you can start There But we're going to start right here on The same spot 70 degrees dry bulb 40 Relative humidity let's say we only go To 90. We only rise 20 degrees now heat pump Typically does a little bit better than That unless it's very very cold outside So we'll put right in the middle there Let's say let's say it's 90 degrees it Might be a little bit lower Might be a little bit higher depending On what the outdoor ambient is So if we go straight across in My Unscientific Method at 90 degrees we're Still down to 20 relative humidity so You can see the big difference even if We start at 60 relative humidity at 70 Right here and we go directly across to 90. we are still roughly at the 30

Percent humidity mark So it's going to still seem like it's Gotten drier and drier when in all Actuality it's just the air expanding it Can now hold more moisture than it could Before It's the same Theory That we use to explain why Supply Air And air conditioning has a higher Relative humidity even after it's gone Through the cooling cycle and lost some Water as it condensed on the evaporator We'll talk about that at a different Time but for this time we're going to Kind of talk about Why is it that our relative humidity Drops if we add moisture and we do need To add moisture in the form of a Humidifier especially when we have a gas Furnace operating in very cold ambient Conditions outside you're going to need That extra moisture because it becomes Unbearable to live in such a dry climate Your skin hates it your lungs hate it And if you have certain medical Conditions it can be quite a misery So I just want to explain that to Everybody out there maybe we'll talk About humidifiers at a different time But just want to focus on the Supply Air And its mannerisms as it goes to a Fossil fuel furnace especially but any Sort of heating appliance that doesn't Add water to the air is going to lower

That relative humidity on the other side

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