Tag: wet bulb

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.


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.


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



Both wet bulb temperature and air enthalpy are extremely useful to understand when calculating actual system capacity as well as human comfort. Dry bulb temperature is a reading of the average molecular velocity of dry air, but it does not take into account the actual heat content of the air, or the evaporative cooling effect of the air.

Like we mentioned in the last tip, when air is at 100% relative humidity the dry bulb, wet bulb and dew point temperatures are all the same. This is because at 100% relative humidity the air is completely saturated with moisture and can have no evaporative effect.

When air is less than 100% RH it will provide an evaporative cool effect and wet bulb temperature is a measurement of that effect. In fact, wet bulb temperature is the temperature a damp thermometer bulb will read when exposed to a 900 FPM (Feet per minute) air stream. If you have ever seen someone using a sling psychrometer, that is exactly what is happening (Hopefully you have a wrist that is well calibrated to 900 FPM). The lower the wet bulb in comparison with the dry bulb (This differential is called wet bulb depression) the lower the relative humidity and the greater the evaporative cooling effect.

Enthalpy is the total heat content of the air and is represented in BTUs per lb of air. By converting lbs of air to cfm we can calculate the amount of heat in an air mass as well as the change in the enthalpy across a coil to calculate the heat moving capacity of a coil, BTU losses/gains over a length of duct and much more.

You will notice that wet bulb and enthalpy are slanted lines, descending from left to right and they are equivalent. This means that a particular wet bulb temperature is also equal to a particular enthalpy (At 14.7 PSIA at least). In the chart above you can see that a 62.8 degree wet bulb mass of air contains approximately 28.4 BTUs per lb. The tricky part is reading at this extreme level of resolution, because 28.4 vs. 28.6 can make a significant difference when it is multiplied out over a large air mass. This demonstrates why VERY accurate tools and careful calculations are required for enthalpy calculations in HVAC/R.

— Bryan

For a full WB Enthalpy calculator go HERE and look for the enthalpy chart

In this episode Bryan speaks with Jim Bergmann about his path to being a test instruments business owner. we talk about –

– The Challenges in the past with poor quality instruments

– Enthalpy calculations

– The Future of test instruments and readings

– Taking readings with a pitot tube

As always if you have an iPhone subscribe HERE and if you have an Android phone subscribe HERE

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