Tag: air flow

Imagine a glass of ice sitting on a table.

Now imagine you place a lid on the glass so all the water and ice is contained in the glass.

If the ice and water are well mixed the water and ice will both be at 32°F because the ice is slowly changing state from ice to water which we call melting. Becasue this is happening at atmospheric pressure we can know what temperature this will occur at and the heat being transferred is going toward melting the ice rather than changing the water temperature which we call latent heat.

Let’s say the temperature in the room is 75°F. In this scenario, heat leaves the air molecules as they contact the exterior of the glass and heat moves through the glass into the water and ice. Becasue glass is a pretty good insulator this happens pretty slow but this heat still moves from hotter to colder.

This movement of heat from the air to the exterior of the glass transfers THROUGH the glass via conduction.

What happens if we blow air toward the glass? what changes? 

If we move more air over the side of the glass we deliver more air molecules to the glass via convection but it doesn’t change the fact that the heat makes it through the walls and into the glass via conduction.

By delivering more air to the glass we warm the outside of the glass more which causes the water melt inside the glass faster, in other words more air over the glass means more heat transfer even though we didn’t change the temperature of the water or the air.

This same basic thing happens inside an evaporator and condenser coil, when we increase the flow of air we also increase the transfer of heat through the walls of the copper tubing in the coils. In the condenser more airflow increases the heat rejection out of the refrigerant and in the evaporator more heat is gained.

Because the refrigerant circuit is dynamic (refrigerant moving) and under pressure more or less heat entering or leaving the system impacts the process and changes the pressures of the refrigerant inside.

If we move less air over the condenser the pressure on the high side increases, if we reduce the air over the evaporator coil less heat enters the circuit, and pressures drop.

This is a basic picture for you to consider next time you see high or low system pressures and how coil airflow impacts heat transfer.

A more advanced but similar thought experiment is what would happen if the evaporator coil had no fins.

— Bryan

I ran a service call today with another tech where the previous tech had diagnosed an intermittent piston restriction. I read the history beforehand and for the past several years there were a lot of assorted comfort complaints and lot of little charge adjustments in both the Summer and winter, it is worth noting here the system, like many in our market this system a heat pump.

There were mentions of freeze-up during the Summer and high pressure cut out during the Winter which had me thinking airflow even before we arrived.

As we pulled up I noticed it was a townhome community with four homes per building.

There was no tenant home so we accessed the home via lockbox and as we walked up the stairs I was noticing the home was quite small… two bedrooms and two baths and when we opened the air handler closet it was….. 3.5 tons

The place had 12″x12″ tiles on the floor and it was a simple rectangle so we counted up length x width and the entire home was just under 1200 sqft.

Now sqft per ton is no way to do a load calculation… I admit it

But this townhome had occupied spaces on both sides meaning the only exposures and windows were on two sides with big trees shading the back, the building was built in 2007 and this system was installed in 2016.

What’s the next move you would make?

Common Sense

The great thing about having 4 other, almost identical units on the building is that we can easily see what tonnage they had installed, and they were 2-ton units…. makes more sense.

So in 2016 some fly-by-night company hacked in a 3.5-ton unit rather than using a 2-ton that may have been a little oversized to begin with given the low loads on this home.

The result is a system that is running REALLY low airflow resulting in low evap temperature and low superheat in the summer and high head pressure in the winter. Techs had been trying to “fix” the problem each year with little charge adjustments rather than finding and fixing the underlying issue.

After walking around the home we found some vents closed… likely because they were blowing somebody’s wig off with the high air velocity.

We went in the attic and found some ducts unsealed, some insulation pushed out of the way and two bath fans venting freely in the attic.

None of this required fancy tools or advanced diagnostic techniques to diagnose… just some common sense… some looking around and a little comparison to figure out the story.

Elementary my dear Watson… Elementary

— Bryan

When you start talking airflow, it can get pretty in-depth pretty quick. There is a big gap between what is useful for the average tech to apply every day and the whole story so let’s start with the simplest part to understand, Static Pressure.

Static pressure is simply the force exerted in all directions within any contained fluid, or in this case air. This means it’s not the directional force of air moving or blowing (that is called velocity pressure), it is simply to force pushing out on the positive side of the air system and pulling in on the negative side.

In other words, it’s energy exerted or inward in all directions instead of in one direction like velocity.

Measuring static pressure helps a tech know whether or not the system has excessive resistance to air flow overall or at a particular point.

Static pressure is measured in inches of water column (“WC) and is the amount of pressure needed to displace one inch of water in a water manometer.


A Magnehelic is a brand name for a high-quality Dwyer analog pressure gauge that comes in many different scales. Many techs will already have a high-quality digital differential manometer (like the  Fieldpiece SDMN5) for reading gas pressure, which makes getting a separate Magnehelic largely unnecessary.

When using a manometer or a Magnehelic, you will first zero it out to room pressure (for a Magnehelic make sure it is level). Next place the negative side probe in the return side of the unit after the filter but before the blower and place the positive probe in the supply duct. Keep the negative side probe away from the side of the blower and insert the probes in as straight and square as possible. It is advised to use a static pressure tip like the one shown below to prevent air velocity pressure or air currents from interfering with the static pressure reading.

With a static pressure tip point the tip against the direction of airflow (points opposite the airflow) in both the return and supply.

DO NOT confuse a static pressure tip with a pitot tube. A pitot tube is designed to measure velocity pressure or total pressure (velocity + static = total)  NOT static pressure, and it will have an open end and two connection points.

Total external static pressure is return plus supply, positive plus negative and in general, you would like to see it be 0.5″ or less…

If you see 0.8″ or higher that is when you start to see trouble on most newer residential systems, but as always, each piece of equipment is different depending mostly on motor design. Whenever possible design your equipment / duct system so the result is 0.4″ – 0.6″ of total static (Once again talking general residential / light commercial here).

If you do find it to be high, then read the return and supply separately to see which is higher which is just a matter of removing the hoses to your manometer or Magnehelic alternately. Whichever reads higher is the greater cause of the issue.

I could keep going on this, but instead, I will just link to some more in-depth articles if you want to do more reading.

— Bryan

Epic airflow write up from Dwyer 

Measuring Airflow from TruTech

Troubleshooting Ductwork by ACHR News


Airflow, Airflow, Airflow…. when we setup and commission comfort cooling and heating systems we need to pay more attention to airflow before we worry about the fancy controls or the refrigerant circuit.

So as a thought exercise let’s consider a typical 2-ton, straight cool, TXV, residential system and think through what happens when we alter airflow and what impacts that has on the system.

Rather than talk in terms of advanced psychrometric math we will keep the math to a minimum and focus on “If this than that” relationships between airflow and system function

Mass vs. Volume

First let’s establish that it is the molecules or “stuff” that makes up air that contains and can move heat energy. While we often talk in terms of CFM (Cubic Feet Per Minute) that is a measurement of volume rather than mass. The air conditioner cares about the mass flow of air over the coil not the volume flow which is why more airflow in CFM is required in high altitudes where air density is lower.

In other words…

Mass flow is what matters and when air get’s less dense we need more air volume to move the same amount of heat

So when we speak in terms of CFM/ton (Cubic feet of air per ton of cooling) that is referring to typical air at sea level and needs to be adjusted as air density changes.

400 CFM/Ton 

The 400 CFM/ton design has been used for years and it is an adequate baseline airflow for many types of equipment and in many moderate climate zones. There are several issues with the 400 CFM/ton rule where it needs to be adjusted.

  • Higher altitudes where air is less dense and therefore more air is required to maintain the same mass flow rate over the coil
  • The nominal or listed tonnage on a piece of equipment is often NOT what the equipment produces at current load conditions. A 2-ton system that is designed for AHRI conditions (95° outdoor and 80° indoor return temperature) could easily produce under 20K btu/hr at 73° indoor and 97° outdoor temperatures, so 800 CFM would be well over 400 CFM/ton in that scenario.
  • Areas with higher latent (humidity) load will run lower than 400 CFM/ton on purpose to remove more moisture from the air and areas with arid (dry) climates will often run higher than 400 CFM/ton to remove less or no moisture from the air.

How The Evaporator “Absorbs” Heat

In my refrigeration circuit basics training I call the evaporator coil the “heat absorber” because its end goal is to take heat from where you don’t want it and move it somewhere else.

The heat gained in the evaporator in this scenario comes from the indoor air being moved over the evaporator coil. The air is warmer than the refrigerant so heat leaves the air as it impacts the tubing and fins of the coil because “hot goes to cold”.

The heat is transferred from the air though the walls of the copper tubing and into the refrigerant via conduction while the heat is transferred through the air and refrigerant itself via convection because they are both dynamic (moving) fluids.

The air temperature is decreased because heat is removed from it into the refrigerant. The refrigerant in the evaporator coil is at saturation (boiling) so the coil temperature doesn’t change directly as heat is added to the refrigerant but it does begin to increase indirectly because as the total heat energy in the evaporator increases so does the coil pressure and vice versa. This is similar to the pressure cooker effect where as the water boils in the pressure cooker the pressure increases and so does the boiling temperature of the water.

When the temperature of the coil is below the dew-point of the air moving over it there is also a transfer of latent energy from the air as some of the water vapor in the air condenses to liquid water (condensate) on the evaporator coil. This latent heat transfer does not result in colder air but rather lower moisture content in the air, this heat does impact the evaporator in the same way as sensible heat as it is added to total heat picked up in the evaporator.

Evaporator Coil TD

We use the term “coil TD” a bit differently in different parts of the industry but in air conditioning it is the difference between the air temperature of the return air entering the evaporator coil and the saturated suction temperature often called the “coil temperature”. In typical 400 CFM/ton applications this difference will be around 35° with a higher number meaning a colder coil and a lower number meaning a warmer coil. There are several things that can impact coil TD including refrigerant mass flow rate (how much refrigerant the compressor is moving), metering device performance, return air dew point (moisture content) and most commonly…. airflow.

What Happens When Airflow is Decreased?

In this theoretical system when the airflow is decreased and all else stays the same the following things will occur –

  • Mass airflow will decrease, meaning there are fewer molecules moving across the coil
  • Air velocity will decrease, meaning the air is moving over the fins and tubing more slowly
  • Bypass factor decreases, this means more of the air molecules will be touching the metal as a ratio
  • Air temperature decreases (to a point) due to the air moving more slowly across the coil with less bypass factor
  • Coil temperature decreases because less overall heat is being picked from the air
  • Coil drops further below dewpoint, causing more moisture to be removed from the air increasing dehumidification
  • Suction pressure decreases because less heat energy being picked up means less pressure and as the superheat falls the TXV also futher throttles the flow of refrigerant through the coil
  • Compression ratio increases as the suction pressure drops meaning the compressor moves less refrigerant as the refrigerant density entering the compressor falls
  • Coil TD increases as indicated by the colder coil in relationship to the return air

We all know that if you have far too little airflow a system can freeze up when the coil temperature drops below 32°F. The other consequence of dropping airflow is lower overall sensible capacity and therefore a drop in EER and SEER rating. On the positive side in humid climates, a system with lower airflow will remove more water from the air which can be desirable.

The lesson is, sometimes you need more airflow and sometimes you need less but no matter what, changing airflow changes a lot about how the system operates and should be done carefully and thoughtfully.

— Bryan



One of our techs called me the other day and gave me a story of woe.

He had been working on a system and he had the following readings

  • Low superheat
  • Low suction pressure
  • Low head pressure

He reassured me that the system airflow was correct and wondered what could have been wrong.

I asked him how he could be sure his airflow was correct and he told me that he had “checked everything”. By that he meant he has looked at the coil, blower wheel, filter and inspected the ducts, NOT that he had measured the airflow.

This isn’t a tip on how to measure airflow but there are many ways it can be done with varying levels of accuracy in the field. From a hot wire anemometer in duct to an air flow hood measuring airflow can be done and is certainly better than just guessing, especially when you get stuck on a diagnosis. My favorite way to measure airflow is to use factory fan tables and static pressure but that method just doesn’t work when anything in the system has been altered from factory test conditions (dirty blower wheel, wheel or motor replaced etc…)

While there is validity to visual inspection and to airflow measurement there are some issues that can be tough to notice that can lead to the symptoms the tech was observing.

Low Load

While we often think of the combo of low suction, superheat and head pressure as being caused by low airflow it actually falls under a larger heading of low evaporator load. This simply means that the quantity of heat being picked up in the evaporator is lower than the refrigerant mass flow rate requires for desired operation.

This can be caused by low air temperature passing over the coil, low air flow, or an undersized coil.

Here are some things to look out for that can cause these symptoms that are more uncommon.

Missing Blower Cutoff Plate

The blower housing cutoff plate helps to direct the airflow from the wheel out of the housing. It’s there so the blower wheel can be removed but if it’s missing it can greatly reduce airflow.

Incorrect Blower Wheel

We’ve seen several occasions where a homeowner or handyman has replaced a blower wheel with a wheel off of another system where it is too small. This will generally be visually obvious but is certainly worth looking out for.

Incorrect Evaporator Coil

We had one instance where we were consistently seeing symptoms of low load and later found that someone had put in an Evaporator coil that was a smaller tonnage than the original.

Oversized Compressor

Sometimes a compressor will be replaced with a compressor a size or two larger than the original. This will show low suction and superheat but will show higher than usual head pressure rather than lower like a typical low load evaporator condition.

Incorrect Blower Motor

In the old days you would simply match HP, RPM and Voltage on a Motor and you would get a fairly consistent result. There are now off the shelf ECM/X13 Motor replacement kits that can produce very different results from the original factory motors depending on how they are programmed.

Concealed Duct Issues

Issues like a collapsed inner duct liner or an old filter pulled deep into a return can be tough to find visually. I will generally use a combination of measuring total system airflow and measuring static pressure at various points in the duct system to help find these concealed issues.

Air Bypassing or Recirculating

Open bypass dampers are a common source of issues but there can also be cases where there are gaps around the coil where air can pull around the coil without adding heat to the coil like usual.

Blower Spinning Backwards

This is an extreme case but I’ve had techs chasing their tails on many occasions just to find out the blower was running backwards. Some older ECM motors would fail and run backwards though I haven’t seen that issue occur recently.

Oil logged evaporator

Over time an Evaporator can become logged with oil that can impede the transfer of heat through the tubing walls. This can look like a low load condition and often accompanies low refrigerant velocity CAUSED by low load over time. This was more common in older mineral oil systems especially when the system has had a compressor changed or oil added over time. The only way to fix it is to flush the coil internally or use an additive designed to help with oil return.

The way to find these more uncommon causes is to

  • Measure total system airflow against design
  • Use static pressure to help isolate issues
  • Look for signs of past repairs or newer parts and confirm the replacements are correct and setup properly

— Bryan

This article is written by one of the smartest guys I know online, Neil Comparetto. Thanks Neil!

Recently I posted a question in the HVAC School Group on Facebook, “when designing a residential duct system what friction rate do you use?”. As of writing this, only one answer was correct according to ACCA’s Manual D.

I feel there is some confusion on what friction rate is and what friction rate to use with a duct calculator. Hopefully, after reading this tech tip you will have a better understanding.

So, what is friction rate?

Friction rate (FR) is the pressure drop between two points in a duct system that are separated by a specific distance. Duct calculators use 100′ as a reference distance. So, if you were to set the friction rate at .1″ on your duct calculator for a specific CFM the duct calculator will give you choices on what size of duct to use. Expect a pressure drop of .1″ w.c. over 100′ of straight duct at that CFM and duct size / type.

Determining the Friction Rate

First, you need to know what the external static pressure (ESP) rating for the selected air handling equipment is. ( external static pressure means external to that piece of equipment. For an air handler, everything that came in the box is accounted for, including the coil and typically the throwaway filter. For a furnace the indoor coil is external and counts against the available static pressure)

Next you have to subtract the pressure losses (CPL) of the air-side components (coil, filter, supply and return registers/grilles, balancing dampers, etc.). Now you will have the remaining available static pressure (ASP). ASP = (ESP – CPL)

Now it’s time to calculate the total effective length (TEL) of the duct system. In the Manual D each type of duct fitting has been assigned an equivalent length value in feet. This is done with an equation converting pressure drop across the fitting to length in feet (there is a reference velocity and a reference friction rate in the equation). Add up both the supply and return duct system in feet. It is important to note that this is not a sum of the whole distribution system. The most restrictive run, from the air handling apparatus to the boot is used. Supply TEL + Return TEL = TEL

The formula for calculating the friction rate is FR= (ASP x 100) / TEL
This formula will give you the friction rate to size the ducts for this specific duct system. If you test static pressure undersized duct systems are very common, almost expected. This is because a “rule of thumb” was used when designing the ducts.

This is just an introduction to the duct design process. I encourage you to familiarize yourself with ACCA’s Manual D and go build a great system!

— Neil Comparetto

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