Tag: humidity

This is the first of a three-part series of articles, which will dive deep into Advanced Psychrometrics. The source material for each of these articles may be found in ACCA Manual P Sections 3, 4, and 5. This article is based on information found in Section 3. 

Psychrometrics is the study of the physical and thermodynamic properties of gas-vapor mixtures. In HVAC/R, we are specifically interested in air-moisture mixtures, and how varying properties affect human comfort and equipment performance. The Psychrometric Chart is a tool used to describe all the possible combinations of gas-vapor mixtures, and can be used to calculate the sensible and latent loads associated with HVAC/R equipment.

Using a Psychrometric Chart can be a bit confusing at first, but with practice and familiarity of the formulas, a Psych Chart can be easily used for a wide variety of purposes. Basic Psychrometric education can be found in the Refrigeration and Air Conditioning Technologies Manual (RACT) and in the first two sections of ACCA Manual P. In this article, however, I’m going to show you how you can apply psychrometrics to calculating Design Room CFM and illustrate how psychrometry can be used to help a technician understand supply air properties. All of the information discussed here can be found in Section 3 of ACCA Manual P.

When selecting equipment for a home or building, it is recommended a Room-to-Room Heat Load Calculation be done as opposed to a Block Load Calculation (Wrightsoft is an excellent software for load calculations, just saying). Room-to-Room calculations result in a more accurate representation of the heat gains and losses per zone (room), and can greatly improve the accuracy and performance of system sizing and design. Assuming a Room-to-Room Load Calculation has been done on a building, the next step in utilizing the Psychrometric Chart would be to plot out the Room Sensible Heat Ratio Lines for each zone. Room Sensible Heat Ratio (RSHR) is the ratio of sensible heat to total heat (including latent) for a room (or zone). If, for example, a room had a total heat load of 2,500 BTUh and 1,800 BTUh sensible heat, the RSHR would be 0.72.

RSHR = Room Sensible Load ÷  Room Total Load

RSHR = 1,800 BTUh ÷ 2,500 BTUh

RSHR = 0.72

Now that we know the RSHR, it’s time to plot the RSHR Line on the Psych Chart. To do this, we need to find a “reference dot”.

80℉ db at 50% RH is considered the standard reference dot. Locate and mark the reference dot and then run a line through the reference dot using a straight edge that is lined up with the RSHR (0.72), which can be found on the far right-hand side of the chart. 

Now, locate the design conditions for the zone in question. Let’s say the design conditions (on a design day of 90℉) is 75℉ db at 50% RH. Plot that dot on the chart. Now, run a line straight through that dot heading to the left of the chart, making sure it is parallel to the reference line. This line is your RSHR Line. This line may now be used to select a supply air condition that will maintain the design room condition on a design day. However, the supply air condition must fall somewhere between the design room condition and dew point (which in this example is about 51.5℉). Theoretically, the lowest possible supply air condition would involve the evaporator coil in cool mode to be 51.5℉ (dew point), and the supply air leaving the register to be the same. However, this theory is in no way practical when you consider duct gains, air leakage, and bypass factors (let alone the fact no one wants a sweaty supply register). Practically, a supply condition falling somewhere between 80%-95% RH will result in good dehumidification, lower airflow, and low fan power consumption. 

Select a supply temperature condition. For this example, let’s choose 55℉ at 90% RH. The next step is to calculate the Design Room CFM. The equation for CFM is as follows:

CFM = Room Sensible Load ÷ (1.08 x ΔT)

Remember, the Sensible Load for this zone is 1,800 BTUh. The difference between the Room Condition and the Supply Air Condition is 20℉. 

CFM = 1,800 BTUh ÷ (1.08 x 20℉)

CFM = 83

The required volume of air given an hour of the runtime is 83 CFM for this room to maintain the design room air condition under design load.

 

But what if my ΔT is lower?

The required volume of air increases. The new supply air condition is 63℉ at 72% RH, giving us a ΔT of 12℉.

CFM = 1,800BTUh ÷ (1.08 x 12℉)

CFM = 139

Both of the different supply air selections will maintain the design room condition on a design day, because they each fall on the RSHR Line. But as the temperature difference between return and supply air decreases, the required CFM increases

 

What is 1.08 supposed to be?

That is the product of the following equation:

Runtime (minutes) x Isobaric Air Density x Isobaric Specific Heat of Air

60 x 0.075 x 0.24 = 1.08

Some caveats must be addressed regarding this formula, and I credit Alex Meaney with Wrightsoft and Genry Garcia with Comfort Dynamics, Inc. for helping me understand these complexities. Both gentlemen are brilliant-minded experts in their fields, and have contributed (and continue to contribute) to HVAC School.

 First, the runtime is specified in minutes, because we are solving for cubic feet per minute (CFM), but also using British Thermal Units per hour. Converting the hour of runtime to minutes gives us 60 minutes, and makes sure our units of measurement are compatible.

Second, you may notice the term isobaric. This refers to any property at a constant pressure. At sea level, atmospheric pressure is around 14.7 psia. At this presumed fixed pressure, the density of dry air is 0.075 lb/ft3, and the specific heat of dry air is 0.24 BTU/lb/℉.

In reality, atmospheric pressure is not fixed, and outdoor air is not always dry. While you may be able to correct for actual pressure and humidity, it may not always be practical. On the other hand, with the ability to use MeasureQuick (which corrects for air density and pressure in its calculations), the processes discussed in these articles may become more practical. It is important to note that manufacturers use isobaric air density and specific heat in their capacity ratings and airflow calculations. Therefore, the argument could also be made that even with this caveat, the end result will (on average) still land you nominally close to the actual air condition requirements. (Please note the wording used here) 😉

So how does this all circle back to practical application? It must be understood that a coil can operate in only one sensible heat ratio at a time, and it may not equate to any of the RSHRs calculated for any particular zone. In the case of a home with multiple zones, you may choose one of the following options when selecting a cooling coil to match the load conditions:

  1. If humidity control is critical to a specific zone, use the RSHR for that room to select a coil. All other rooms will vary in humidity, but the critical zone will be maintained.
  2. Average all the RSHRs together for a mean RSHR that can be used to select a coil. Each room will vary slightly from its individual RSHR, but it will be minimal and likely unnoticeable. 

And that, in a nutshell, is how you may use a Psychrometric Chart and data from a Load Calculation to determine Room Design CFM. This exercise, however, merely scratches the surface of the many factors that must be considered in an HVAC system. This exercise works only for a system that does not suffer from duct leakage, bypass factor, and has no ventilation whatsoever for the home/building. This exercise would fall short of providing any real-world insight into psychrometric properties involving an HVAC system. However, the skills learned here translate into the next phase of advanced psychrometrics! In the next two articles, I will detail how these variables can be accounted for (even solved for). In the end, I hope you will understand a little more about Psychrometrics in general, and how to add that knowledge to your ability to efficiently diagnose a system as a whole (including the envelope and people).

I’ll end this article with a quote from Alex Meaney, and I think it is important to keep this idea in mind throughout the rest of this series of articles:

“I’m of the opinion that local humidity is usually a[n] infiltration/ventilation/return problem, not a supply problem.”

–Alex Meaney

For access to the Testo Psych Chart I used for this article, click here.

 

— Kaleb

At the time of the publication of this article, COVID-19 (coronavirus) is spreading across the world at an alarming rate, and many people have self-quarantined to help slow and/or stop the spread of the virus. These precautionary measures are prudent and responsible. However, with the increased amount of time people will now spend inside their homes, there is a hidden factor to be aware of, which many people won’t think about. The prolonged occupancy of homes with increased cooking, bathing, and cleaning time will significantly impact the indoor air/environmental quality of these homes. An issue like this may not be measurable, but it is inevitable. In a time when many technicians, companies, and manufacturers will use this health crisis as a way to promote the sale of IAQ products in ways that can only be judged as unethical, it is imperative to the honest and curious technician to understand how to do her part in educating customers, and keeping everyone healthy.

This article will stay away from talking about specific types of boxed devices out there that “purify” the air, because that’s a topic for another day. The focus here is on the three main processes available to technicians and homeowners to improve indoor environmental conditions. Taking these one by one, technicians should have a thorough crash-course understanding of each and its importance to indoor air quality (IAQ). Ventilation, Filtration, and Humidity Control.

The first step in understanding a healthy indoor environment is to recognize the villains one must fight against in order to keep an environment healthy. Particulate Matter (PM), Volatile Organic Compounds (VOCs), Humidity (high or low), Carbon Monoxide (CO), Carbon Dioxide (CO2), Ozone (O3), etc. are just a few. These are the elements that tend to concentrate themselves in tight indoor environments. Each of the “Holy Trinity of IAQ” is designed to deal with these undesirables in their own dedicated way.

Everyone should know what a bath fan is. If you don’t have a bath fan, you probably live in a house not updated since the 1970s, and you likely have other decor issues to deal with as well. Bath fans are the most common mechanical ventilation in homes today. They are a form of negative pressure ventilation. As the fan pulls air from the room and expels it (hopefully not in your attic), this creates a negative pressure on the building envelope, and air from outside is pulled in through the cracks and crevices around your windows, door frames, attics, and through Jerry’s mouse hole…which everyone has…right? This type of ventilation is by far one of the least desirable, because you exact zero control over the quality of air you are bringing into the home. The air could be high in humidity and temperature, or it could be passing through layers of blown-in insulation inside your attic; neither of which are ideal. Air from these places isn’t really fresh.

The general consensus is that positive or balanced pressure ventilation is best. Examples of positive pressure ventilation include Make-up air units (MAU), Dedicated Outdoor Air Systems (DOAS), and the use of a scuttle (a small duct run from outdoor air into the return ductwork for HVAC systems). Balanced pressure ventilation is accomplished through mechanical equipment like Energy Recovery Ventilators (ERV), Heat Recovery Ventilators (HRV), and Conditioning Energy Recovery Ventilators (CERV). Each of these technologies has their advantages and ideal applications. The reason positive/balanced ventilation is desirable is for its ability to control the fresh air. If you can control the air you breathe, you can keep it “fresh”. For all of these options, there are applications for which they can be used that actually improve upon the quality of the air entering the space. But why do we care about ventilation? What’s so important about it?

Houses used to be built loosely. This isn’t to say they were built poorly, but houses used to be loose enough to allow for tons of natural ventilation. The codes and standards have evolved, and we now construct assemblies more airtight than in the past. This is why the EPA has published that indoor environments are often 2-5 times higher concentrations of air pollutants than outdoor levels, and can reach upwards of 100 times worse! This is because as people bathe, clean, and cook, VOC concentrations, Particulate matter, and humidity levels increase dramatically. People thought bath fans were for bathroom odors, but really that’s just a nice side-effect. They are for removing water vapor during and after showers/baths. Ventilation is utilized to dilute VOCs, CO, CO2, and other chemicals in order to maintain a comfortable indoor environment. I know of people who grew up watching their mother open all the windows of the house for a couple hours a week in order to “flush” the house. Mechanical ventilation is just like that, except more controllable and technologically advanced.

Particulate Matter is another indoor environment characteristic, which can cause a variety of health concerns. Particulate matter is categorized by its size in diameter, which is measured in micrometers (or microns). A lot of buzz is generated around PM 2.5, which is particulate matter with a diameter of 2 and a half microns; that is due to PM 2.5’s ability to do major damage to the human respiratory system. To give you an idea of the size of PM 2.5, the EPA has published that PM 10 is considered inhalable. PM 2.5 is 75% smaller than that! This means PM 2.5 tends to stay in the air stream longer than larger, denser particles. However, PM 2.5 is not the smallest particulate matter that can potentially do harm. PM 1 and 0.5 are also in the air, and they can easily make their way to our lungs and bloodstream. In order to combat against these airborne particles, it is important to filter the air with a high-quality air filter. There are filters designed to trap PM 2.5 and smaller (MERV 11 up to HEPA), and they are a critical component to any air distribution system. The third edition (2018) of the EPA Technical Summary of Residential Air Cleaners states that a MERV 13 is recommended for every HVAC system, or as high a MERV rating as the system will allow. 

EPA Technical Summary: Residential Air Cleaners (2018)

It is important to note that Particulate Matter does not refer to just dust. Particulate matter can be made up of pollen, viruses, bacteria, fibers, fungal spores, vehicle exhaust, etc. This fact makes it clear that filtration is not only important for the HVAC system, but also for the incoming air to any mechanical ventilation system. Humans are constantly submerged in this fluid called air. We must give more thought to the quality of the air we breathe. 

The final head of our three-headed IAQ dragon is Humidity Control. This can refer to either high or low humidity levels. Either extreme is unhealthy and can create an environment prime for health risks. On one hand, high humidity can cause respiratory issues, encourage dust mite life, allow viruses and bacteria to increase, allow VOCs to become airborne, allow increased chemical reactions, and allow microbiological growth to take place. On the other hand, low humidity levels can also cause respiratory issues, irritate mucous membranes, allow viruses and bacteria to increase, and allow for the production of ozone. The happy medium is the generally accepted ideal humidity index, which falls between 35%-60% relative humidity.

In order to control humidity indoors, a technician must be aware of her climate zone, and whether she must work to increase or decrease humidity levels indoors in relation to outside levels. For arid climates, humidification is necessary, and options such as higher airflows and in-duct steam humidifiers are great solutions. For humid climates, running lower airflows and adding mechanical supplemental dehumidification is ideal. Some dehumidification systems allow for ventilation as an option, and they include a high MERV filter to cover all the bases. This option is an ideal solution for certain applications. Humidity must be controlled in an occupied space for that space to be comfortable. People are much more sensitive to humidity than temperature. 

Looking at these three paths to creating and maintaining healthy air inside a home, it is important to realize these are Indoor Air Quality solutions. To create and maintain a fully comfortable indoor environment, air leaks, insulation, and load matching are other issues that would need to be addressed. However, in addressing the current issues with air quality in homes, this “Holy Trinity” is all any technician needs to exert energy into in order to help keep occupant air clean. There is a mindset that humans are never more intimate with their surroundings than when they inhale the air into their bodies. Technicians must take action to educate consumers and recommend the most effective solutions for IAQ improvement. 

There are many companies and manufacturers using this health crisis to promote the sales of popular air “purifiers”, which use chemistry to “clean” the air in lieu of ventilation, humidity control, and filtration. The technology of these products will be discussed in a later article, but the most important take-away at this juncture is how important it is to maintain control over the humidity, the outdoor air coming into the space, and the concentration of particulate matter in the air stream. The methods of dealing with the issues mentioned in this article are the only methods that have been time and volume tested over decades, and they have standards in place to help ensure their effectiveness on IAQ. 

So what do technicians do right now? Many homeowners may not want to spend the money on advanced in-duct filtration, mechanical ventilation, and humidity control during this time of uncertainty. Joe Medosch from HaywardScore.com has shared a very ingenuitive and affordable solution for many people to effectively filter indoor air. 

This DIY method is a great way to help encourage homeowners to remain healthy as they spend more time inside their homes. This “box fan filter” may also make it more viable for sensitive people to open their windows and doors for longer periods of time during pollen season, as this enhances the circulation of air inside, and adds filtration throughout the home. Another recommendation for homeowners is to utilize the bath fans and kitchen exhausts as a way of ventilation. ALWAYS run a bath fan during bathing activity, and continue to run it 10-15 minutes afterward in order to prevent as much water vapor as possible from remaining inside the home. Portable dehumidifiers and humidifiers are also available. 

Another recommendation for every technician, business, and the homeowner is the use of IAQ monitors throughout the home. Real-time monitoring and translation of data over time allows people to see the effects of their activities on IAQ. For technicians and businesses, it is a great way to track the effectiveness of your work over time. Without measurements and testing, you can only guess!

As we work together to combat the spread of viruses in our communities and around the world, the HVAC/R industry has a large opportunity to help educate customers on how to create and maintain a healthy indoor environment. We must take care to avoid fear-mongering and sales tactics geared toward the exploitation of people’s vulnerability and miseducation. Practice integrity, do your research, and implement industry best practices always

 

– Kaleb

Sometimes I beat around the bush too much in these tech tips, so let’s get right to the nitty-gritty! (as Nacho Libre would say)

Humidity inside a home should be maintained between 30% and 60% relative humidity.

I like to shoot for 50% in humid climates when possible (and by possible I mean financially feasible for the customer because anything is possible).

Causes of High Indoor Relative Humidity 

  • Low Heat Load / Short Equipment Run Time / System Oversizing
  • High External Humidity Drivers / Humidity Entering the Home
  • High Internal Humidity Drivers / Humidity Being Generated Inside the Home
  • Poor or No Spot Ventilation in Kitchen’s and Bathrooms (or it Isn’t Being Run)
  • High Evaporator Coil Temperature / High System SHR / High Evaporator Dew Point Temperature
  • Insufficient Total Dehumidification Capacity 
  • Low Space Temperature
  • Relying on the A/C alone to Dehumidify

This is the list of everything that causes high relative humidity in a home or building. Total humidity drops when you pull out more water than you put in and it increases when more moisture enters the space than you pull out.

Before we cover what to look for and how to fix it let’s first address some common fallacies that often crop up.

Bad Advice = Just Get it Colder in The Space 

Truth = Lower Temperature Alone Means Higher Relative Humidity

The evaporator coil running below dew-point and water leaving the pan and going out the drain is what dehumidifies the space. This is called latent heat removal and it’s our friend when we are looking to drop the RH% in a space.

Sensible cooling is decreasing the space temperature and while this is a necessary part of comfort in most seasons, it is the enemy when it comes to dropping indoor RH%.

When air is cooled without being dehumidified the relative humidity in the space actually INCREASES because the lower the temperature the less water vapor the air can contain before turning into liquid water.

When we dehumidify with cooling equipment it is the water leaving the drain that matters (latent heat removal) not dropping the temperature of the space (sensible).

For dehumidification getting water out (latent heat removal) = good

dropping room temperature (sensible heat removal) = bad

Bad Advice = Add More Insulation to Drop The Humidity 

Truth = Adding Insulation Will Decrease The Heat Load and Generally Increase the Relative Humidity 

In order for an air conditioner to pull out humidity and drip it down the drain, it needs to run. In order for it to run it needs to be warm enough in the space for it to run.

When you add typical insulation in the ceiling, floors and walls you decrease the heat load without changing the humidity load. This will result in the RH% going up.

There are some insulation materials such as closed cell foam that will also act as an air & vapor barrier helping to block moisture from making it in. This can help reduce humidity but it is the air/vapor barrier portions that do it not the insulation.

Bad Advice = If Humidity is High It’s The Air Conditioner

Truth = Many Humidity Issues are Caused by Abnormally High Moisture Not the A/C

The air conditioner needs to be properly sized and selected with sensible and latent capacity that matches the building design. There are many cases where homes aren’t built or lived in exactly to design and cases where the weather doesn’t act like the models predict.

In Florida we have a lot of Hurricanes and tropical systems, In these cases we get tons of moisture, high winds that create big pressure differential across our homes and forces it in, low sensible temperatures so the A/C doesn’t run much and power outages that keep it from running for days in some cases.

For months afterward owners will complain of condensation, biological growth, high relative humidity etc… and everyone tries to “solve” the issue by messing with the air conditioning. These tropical weather events increase the amount of moisture in the home while at the same time impacting the ability of the equipment to remove the moisture.

My own house is another example of an extreme internal moisture condition. I have great insulation, good vapor and moisture barriers and excellent HVAC equipment (if I do say so myself).

However… I have 9 kids and we homeschool so they are home most of the day, we live in the country so we do tons of laundry (lot’s of dirt and mud) and we cook 3 meals a day at home …

Needless to say, our home has internal moisture loads that no model will be able to account for. This is why we added a whole-home dehumidifier to keep that humidity in check.

Final case study…

Many years ago I had a customer who always had high humidity in the main living area and the vents in the ceiling would sweat. I kept going back and messing with the equipment over and over and nothing I did seemed to help. I finally asked another tech and he laughed and said; “they have a pool don’t they?” I thought about it and sure enough, they did have a pool. “How did you know that?” I asked. He smiled and said “They are leaving the slider open when the kids play in the pool to keep an eye on things or they are in and out all time, that’s why the issue is always in that room”, I’ll be darned, he was right. You may be able to use a data logging humidity sensor to find these sorts of client caused intermittent issues.

What to Do About High Humidity

There are many approaches you can take on this depending on the types of tools you have at your disposal, as well as the severity and the budget and patience of your clients. I’m not going to give every possibility and test but here is what I would suggest for the average HVAC tech even if it makes my more hardcore building science friends cringe a bit.

  1. Make sure you have a few good quality psychrometers/hygrometers. I use the Testo 605i as my go-to. Never trust a cheap tool with humidity measurement.
  2. Ask the customer about how often they cook and note if they have a range hood that vents outdoors.
  3. Ask the customer if they use bath vent fans when bathing and showering.
  4. Look for roof leaks, proper grading around the home, ponding water etc…
  5. Test the space humidity, temperature, and dewpoint at various locations around the home. Often you can find the source of an issue this way. keep in mind that the closer you get to the ceiling the dewpoint tends to increase due to that fact that water vapor is less dense than air.
  6. Check the HVAC equipment in detail. When humidity is a challenge setting up the equipment for 350 CFM per ton is generally a good practice. Make sure it all wired properly if it is multi-stage or has dehumidification features. Confirm the system airflow, for newer equipment using the total system static and fan chart method is usually the easiest for a tech. I use the Testo 510 and 440dp for this.
  7. Inspect the ductwork and seal any leaks. Leaking ducts cause pressure imbalance in the home and can either drive air in or out of the home.
  8. Make sure there are no dryer vents, bath fans or kitchen ventilation leaking or discharging into attics or crawlspaces. Make sure the dryer vent is well-connected to the dryer.
  9. Check and measure any incoming fresh through fresh air intakes, ERV’s or HRV’s. If it is too much it may be reduced but proper calculations and likely blower door testing will need to be done before reducing outdoor air.
  10. Look for can lights, gaps around boots into the space, holes in walls between the attic and crawl space and the living space etc.. Sealing these can greatly reduce the moisture drivers.
  11. Check seals, sweeps & weather stripping around doors and windows
  12. Make an assessment if the equipment may be significantly oversized. If so then do a Manual J calculation to determine.
  13. Discuss supplemental whole-home dehumidification with the customer, especially when the issue is a big priority for them.

The goals in inspecting the home and equipment is to make some of the following recommendations that can reduce indoor humidity when they are appropriate

  • Run or Install Point Ventilation in the Kitchen and Baths to Remove Excess Moisture at the Source When in Use
  • Alter Habits (like leaving doors open) That Lead to Moisture Issues
  • Install New Weatherstripping and Door Sweeps
  • Seal or Install Sealed Can Lights, Seal Around Boots and Seal Other Gaps Between Attic/Crawlspace and the Home or Walls
  • Make HVAC System Settings Changes to Run Longer with a Colder Evaporator Coil (Reheat is an extreme example of this)
  • Advise Properly Sizing Equipment or Installing Whole-Home Dehumidification Where Appropriate

Quick caution. It is possible to seal a building so tight that it can become unhealthy. Whenever sealing is in order it is best to do a before and after blower-door test on the space and decide if mechanical outdoor air needs to be brought in.

When this is the case I generally suggest a ventilating dehumidifier (and an ERC in some cases) in humid climates, otherwise, you can just make the situation worse.

Also keep in mind that when you run a colder coil the equipment, ducts and vents will be more likely to condensate as they will also be colder. While a colder coil will decrease the space humidity it may not be an option if it results in excessive equipment, duct and vent sweating. This is situation dependent and often dictated by where the equipment and ducts are installed… attics are the WORST for this.

When condensation occurs you can either drop the dew point (humidity) of the air around it or increase the temperature of the surface that is sweating. Sometimes decreasing the dewpoint of the air is very hard (like ducts in an attic) so we are left with increasing the temperature of the duct with either more insulation or warmer air going through it.

Another thing worth mentioning is that varible speed blowers and multi-stage compression paired with humidity controls can help a lot with the coil temperature and run time side of the equation. Even then, they aren’t a silver bullet to fix all issues and if you over promise you may end up with a dissatisfied customer.

Once more… For lower humidity in a home, you want..

  • Longer run times
  • Colder evaporator coil
  • Less moisture coming in from outside
  • Less moisture being generated from outside
  • Higher indoor temperatures
  • Extra moisture removal with dehumidification when required
  • Spot ventilate when cooking or bathing

— Bryan

 

 

 

Air conditioning was about humidity control from the very start. Willis Carrier’s very first air conditioning system was all about controlling humidity with the side effect that it also could reduce the sensible temperature. Theaters caught on that this new fangled contraption could lead to big Summer numbers when they installed it to keep patrons cool and the rest is history.

We often add water to the air (humidify) and remove water from the air (dehumidify) as part of our work but let’s take a different look at what happens when we do that, but first let’s all get on the same page with some terms. (Skip down past the terms if you already know them because that isn’t the point of the article)

Dry Bulb Air Temperature

When you measure with a typical thermometer you are measuring dry bulb temperature. It is a measurement of the average movement of the molecules in the air you are measuring with no consideration for the amount of water in it. We also call dry bulb temperature sensible temperature and changes in dry bulb, sensible heat change.

Latent Heat 

When we change the state of matter by boiling, evaporating or condensing we cannot track the changes or movement of heat strictly with a thermometer. This heat the moves during a change of state is called latent heat.

Relative Humidity 

The relative humidity is just a percentage that tells us how much water is evaporated in the air in relation to how much there could be at the same temperature. It’s like describing how full a glass is, it tells you how full or empty the glass is but by itself it doesn’t tell you how much energy or water is in the glass.

Wet Bulb Air Temperature

If you were to use an old school sling psychrometer you would simply wet a little sock around a bulb thermometer and sling it through the air. If the air humidity was less than 100% then the wet bulb temperature would be less than the dry bulb temperature. The difference between the wet bulb and dry bulb temperature is called “wet bulb depression” and is can be used to calculate relative humidity.

DewPoint 

The temperature at which air becomes 100% saturated. Wet bulb, dry bulb and 100% relative humidity lines all intersect at the dewpoint. Dewpoint is the glass 100% full.

Air Enthalpy

A psychrometric chart displays the total amount of heat energy in the air between the dry air AND the evaporated water vapor in the air in heat per mass, in the USA that is usually BTUs per LB. Wet bulb temperature and enthalpy of air run VERY CLOSE to right along with one another so 63° wet bulb air will have an enthalpy of 28.3 btu/lb at typical conditions and often we use wet bulb as a proxy for enthalpy.

Absolute Humidity / Moisture

We can calculate the total moisture in the air in pounds or grains of moisture per pound of dry air. This is the TOTAL quantity of evaporated water in a pound of dry air by weight and shouldn’t be confused with relative humidity.


Here is the part I want to get to –

When we remove water from the air with an air conditioner or a typical dehumidifier we are cooling the air sensibly until it hits dewpoint. The evaporated water in the air will then begin to condense on the surface of the evaporator coil and will give up latent heat to the coil because the coil temperature is below the dewpoint temperature (at least most of the time on most systems).

When we dehumidify by cooling in an air conditioner we are dropping the enthalpy, temperature and absolute moisture of the air all at the same time and all of that combined heat is entering the coil.

In a dedicated dehumidifier we do the same thing but then run that air back over the condenser to add back enthalpy via sensible heat so the end result is less total moisture with higher air enthalpy to prevent overcooling.

What happens if we humidify or dehumidify the air by increasing or decreasing the total moisture WITHOUT adding or removing heat? This does (mostly) happen with evaporative (swamp) coolers and dessicant dehumidifiers.

When the humidity of air changes WITHOUT a change in enthalpy (total heat content, sensible + latent) the temperature of the air also changes, as a result, this is called an adiabatic process. Adiabatic simply means a change in temperature without a change in total energy/heat content within a system.

When you simply add or remove grains or pounds of water vapor to the air you would obviously change the enthalpy of the air UNLESS the temperature changed to compensate. This may sound like crazy science but we experience it every day.

The inside of your body is about 98.7°F but the outside of your skin is cooler than that, often more like 93°F.  Let’s say it’s 100°F in Phoenix and 40% relative humidity. We know that hot goes to cold so the heat from the air is headed into your skin and your body reacts by beginning to sweat.

What temperature is the sweat as it leaves your body? Well, it would need to be somewhere between 93°F – 98.7°F right?

So how can 93°F sweat cool you?

It cools you because as it evaporates into the air the water in your sweat takes energy to make that change, maintaining enthalpy (total heat) in the air around your skin but dropping in temperature.

If you were to measure the wet bulb or enthalpy change entering and leaving a swamp cooler (evaporative cooler) you would notice that it stays (mostly) constant but the temperature of the leaving air is still lower temperature.

Take a look at the two sets of air conditions shown above. If you were to use a dessicant dehumidifier that could decrease the total moisture content of 75°F air from 64.66 grains to 33.88 grains (per lb of dry air) without any exchange of energy (constant enthalpy) the temperature of the air leaving that dehumidifier would increase by 20°F to 95°F. This can and does occur in dessicant dehumidifiers every day.

Now…

Rarely do processes in real life abide by idealistic conditions plotted on a psychrometric chart. If the water is a different temperature than the surrounding air in a swamp cooler than there will be an enthalpy change, if the dessicant wheel is a different temperature than the air passing over it then there will be an enthalpy change.

The cool thing here is gaining a deeper understanding of the relationship between dry bulb temperature, enthalpy and total moisture content of air by understanding some of the edge cases many of us don’t experience as often.

Evaporation by itself leads to lower sensible air temperature

Condensation by itself leads to higher sensible air temperature

— Bryan

 

 

Does heating the air cause the humidity in the air to decrease? Yes and no

Heating air causes the RELATIVE humidity percentage to decrease but it does not change the overall moisture content in grains of moisture per lb of air.

Many old timers will swear a blue blaze that oversizing a furnace will directly result in lower humidity, cracking furniture etc…

The problem with that theory is that no matter how much you heat the the air you don’t change the overall moisture content and when you blow that air into the space it quickly acclimates with the room.

But there still may be some truth in this oversizing dries stuff out theory

In the Winter during cold climates the moisture content is very low outside regardless of the relative humidity. When you use a larger furnace than you need you also tend to move more air than you need to.

When you move more air there is often greater negative or positive pressurization of the conditioned space due to zonal imbalance and duct leakage. This pressure imbalance will tend to drive more dry air into the space or more of the inside air out resulting in lower humidity.

Neil Comparetto also pointed out that when the appliance takes its combustion air from the space this can cause significant negative pressures which also draws dry air in from outside. The larger the BTU output the greater volume of air that must come in for combustion.

The other factor is the supply air temperature itself. If the hotter supply air is blowing directly on an object it will tend to dry it out more quickly due to the increased temperature of the object itself.

In conclusion –

Furnaces don’t reduce air moisture quantity directly no matter how big or small

There are other reasons why oversizing can cause issues so don’t do it.

— Bryan

I was listening to someone talk about air relative humidity the other day while looking at a psychrometric chart and he commented that the chart ends down at freezing (32°F) because “all the water freezes out of the air at that point”

I think I made this Jed Clampett face

The psychrometric chart is designed to deal with typical air conditions (especially indoors) and it is true that as air gets cooler the grains of moisture per pound of dry air do drop significantly.

Take a look at the difference between these two air conditions where I hold the relative humidity to 45% and just change the temperature.

First at 40°F

And then at 25°F

So while the moisture content in relationship to saturated air is 45% in both cases, the total moisture content in grains (1/7000 of a lb of water) goes down.

This is why colder air is dryer in an absolute sense because it often contains significantly less water vapor than warmer air.

Relative humidity is only in relation to the maximum saturation of the air.

Moral of the story… air can (and does) still contain water vapor even below freezing and the fact that it may be “off the chart” doesn’t mean there is no moisture in the air.

— Bryan

Let’s first state the obvious. Most techs are intimidated by Psychrometric charts and Mollier diagrams, we JUST ARE. While there are some pretty complicated formulas that back up these diagrams, using them isn’t a big of a deal once you understand the different elements and then focus on one at a time.

BUT WHY DO YOU CARE?

Dew point is one example of a very useful measurement to understand, design for and test for in an HVAC/R system. Take an evaporator coil, do you know how to calculate the exact temperature at which that evaporator coil will start to condense moisture? can you tell the exact temperature at which a surface inside of a space will start to condensate and possibly grow mold? These are both cases where a basic understanding of a psychrometric diagram can help a technician.

While some of the elements on the chart are represented by curved or slanted lines, dew point temperature and humidity ratio / absolute moisture content are just straight lines horizontally across the chart.

So if we focus on a 65°F(18.33°C) dew point on the right side of the chart you will notice it crosses  over 92 grains (there are 7000 grains of moisture per lb) of moisture line and then goes all the way across until it intersects with the curved 100% humidity line on the left side. This shows us that at a 65°F(18.33°C) dew point the air always contains 92 grains of moisture per lb.. ALWAYS.

This also shows us that when the air is at 100% relative humidity the dew point, wet bulb and dry bulb temperatures are ALL THE SAME.

If we have a dew point of 55°F(12.77°C) the air contains 64 grains of moisture per lb. If the dew point is 30°F(-1.11°C) the air contains 24 grains… you get the point.

So now if you find the dry bulb temperature and the relative humidity you can easily calculate the dew point at which that same air will reach saturation and begin to form condensation.

Let’s say we have 75°F(23.88°C) dry bulb air at 50% relative humidity. We would simply draw a line up from the bottom at 75°F23.88°C) until we hit the curved 50% line. Then go right (or left) until you bump into the the grains of moisture and then the dew point scale. Now you know at what temperature that same air mass will start to condense water.

So we can see that this if this 75°F(23.88°C) dry bulb 50% relative humidity mass of air comes in contact with a surface that is 55.5°F(13.05°C) or less, it will begin to condense water. We also know that this air stream contains 65 grains of moisture per lb of air.

Forgive me for saying so, but I think this is pretty cool.

— Bryan

P.S. – If you want a good quality Psychrometric chart you can use THIS ONE

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