Author: Bryan Orr

Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade

We have discussed DTD (Design Temperature Difference) quite a bit for air conditioning applications, but what about refrigeration? Let’s start by defining our terms again

Suction Saturation Temperature

Saturation temperature is the temperature the refrigerant will be at a given pressure if it is in the process of changing state. This change of state would be from liquid to vapor (boiling) in the case of the low side (evaporator / suction line). When we look at saturation temperatures instead of pressures we can use similar rules and we will see similar saturation temperatures across all refrigerants when the application is the same. Experienced HVAC and refrigeration techs pay far closer attention to the saturation temperatures than they do pressures.

Evaporator TD and DTD

Evaporator TD (temperature difference) is the measured difference between the suction saturation temperature (evaporator boiling temperature) and the box temperature. DTD (design temperature difference) is the designed or expected TD.

Delta T

Many A/C techs will confuse TD with Delta T. Delta T is the difference between the evaporator AIR temperature entering the coil to the air temperature leaving the coil. The Delta T will vary based on the humidity in the box where TD will not.

Target Box Temperature 

The temperature the refrigeration box should maintain when the system is operating properly


The increase in temperature between the suction saturation temperature and the suction line temperature leaving the evaporator. Superheat is the temperature (sensible heat) gained between the point that all of the liquid boiled off in the evaporator coil and the suction line at the outlet of the coil. in refrigeration, like HVAC 10°F(5.5°K) of superheat  is average with a range from 3°F to 12°F(1.65°K – 6.6°K) depending on the equipment type (10°F(5.5°K) for med temp, 5°F(2.75°K) for low temp, 3°F(1.65°K) for ice machines ).

Hot Pull Down

Refrigeration equipment is unlike HVAC equipment in that the evaporator will spend most of its life running in a very stable environment with minimal fluctuation in the box temperature.

On occasion a refrigeration system will see a huge change in load in cases where it was off and needs to “pull-down” the temperature, or when doors are left open or when a large quantity of warm product is placed in the box. When a piece of refrigeration equipment is in hot pull down it cannot be expected to abide by the typical DTD or superheat rules and must be allowed to get near the design box temperature before fine adjustments are made to the charge, TXV superheat settings or to the EPR (Evaporator Pressure Regulator) if there is one.

Design Temperature Difference (DTD)

In air conditioning applications a 35°F DTD is a good guideline for systems that run 400 CFM(679.6 m3/h) of air per ton of cooling (12,000 btu/hr). In refrigeration the DTD is much lower than in air conditioning.

There are several reasons for this but one big reason is the desire to maintain relatively high relative humidity levels in refrigeration to keep from drying out and damaging product. Keep in mind that NOTHING is a substitute from manufacturer’s data but here are some good DTD guidelines for traditional / older refrigeration equipment. Keep in min dthat the trend is toward lower evaporator TD on newer equipment.

Walk-ins  10°F DTD +/- 3°F
Reach-ins  20°F DTD +/- 5°F
A/C 35°F DTD +/- 5°F

You then subtract the DTD from your box temperature/return temperature to calculate your target suction saturation. You can then use this target saturation / DTD and compare it to your actual measured saturation and DT once the box is within 5°F – 10°F(2.75°K – 5.5°K) of it’s target temperature to help you set your charge, TXV and EPR as well as diagnose potential airflow issues when compared with suction superheat and subcooling / clear site glass.

For Example –

If you have a medium temp walk-in cooler with a 35°F(1.66°C) box temperature you would expect to see a suction saturation of  25°F +/- 3°F

When doing a quick inspection of a piece of refrigeration equipment without gauges you can use this data to do the following calculation –

35°F – 10°F DT + 10°F superheat = 35°F suction line temperature +/- 3°F 

In this particular case logic tells us that the suction line could be no WARMER than 35°F(1.66°C) because that is the temperature of the air the refrigerant is transferring its heat to. However by the time you factor in the accuracy of your box thermometer and line thermometer and the assumed saturation temperature you would still expect a 35°F(1.66°C) suction line temperature +/- 3°F(1.65°K)

For a -10°(-23.33°C) box, low temp reach-in you would calculate it this way

-10°F- 20°F DT + 5°F superheat = -25°F suction line temperature +/- 5°F 

Clearly, this is NOT the way to commission a new piece of equipment or to benchmark a system you haven’t worked on before, but it can give you a quick glimpse at the operation of a piece of refrigeration equipment without attaching gauges, especially on critically charged or sealed systems.

The best practice is to know the equipment you are working on, read up on it and properly log benchmark data the first time you work on a piece of equipment or during commissioning.

It should also be noted as Jeremy Smith pointed out, in recent years TD’s have been decreasing as manufacturers seek higher efficiency through higher suction and lower compression ratios.

This means that TD’s as low as 5 can be designed into some units but keep in mind… the suction line can still be no warmer than the box so as DTD drops so does superheat and the critical nature of expansion valve operation.

— Bryan

When I first started in the trade we used to run into shielded control wires on the Carrier Comfort Zone 1 zoning systems and also on a Carrier VVT system I used to maintain at a bank. I knew it has something to do with electrical “noise” and that communicating systems often called for it but I never looked any further into it.

Over the last decade there has been a lot of different residential communicating systems that have come out. Some require shielded cable, some recommend it and others don’t mention it all.

The fact is that whenever controls work on a low voltage “signal” rather than a simple “on/off” control they are more susceptible to induced charges from other nearby conductors, electronics and even transients from electrical storms.

A shielded cable has a  metallic jacket that surrounds the individual conductors and routes the induced charges to ground, keeping it away from the conductors inside.

As an example of this, I installed a Carrier Infinity system at my own house WITHOUT using shielded cable and almost every time there are lightning strikes nearby the unit will throw a communications fault, since I’m in florida that happens quite often.

If you do have the wisdom to run shielded cable you need to remember to bond (ground) one side of the shield securely to a good equipment ground on one end and ONE END ONLY. If you ground both ends you risk the sheild becoming a path in the case of a ground fault which could cause some bigger issues. If you ground both ends you can also create a “ground loop” that can cause the very noise you set out to eliminate.

In some cases, you can perform a similar function by grounding leftover/unused conductors on one end if you failed to run a shielded cable. There is no guarantee it will solve the issue depending on the severity because the other conductors don’t fully surround the conductors being utilized.

The lesson being, when working with communicating “signal” controls run shielded cable whenever possible. I was looking around and found this spec sheet from Southwire on their shielded 8 wire.

— Bryan

Most controls and thermostats will have some sort of cycle rate per hour setting that kicks in to prevent over cycling once setpoint is reached.

These cycle settings don’t kick in until the system starts achieving setpoint, so don’t worry that it will shut off if it’s set to 70° in heat mode and it’s 60° in the house. Once it gets to the setpoint the cycle per hour programming will prevent the system from running MORE cycles per hour than the setting.

When the cycle rate per hour is lower the run times will be longer and the off times will be longer.

When the cycle rate is higher the on times and off times will be shorter. 

As a general rule, a lower cycle rate is better for efficiency and a higher cycle rate is better for comfort. This is because most equipment is inefficient for a portion of its start-up time so more starting and stopping impacts efficiency due to this “ramp up” time.

Older homes that were poorly sealed and insulated often required more cycles because they would get cold fast when the equipment went off. Nowadays, it is great if we can use 3 cycles per hour with furnaces and 1 cycle per hour with heat pump systems to maximize efficiency.

If you get comfort complaints due to wider temperature swings between cycles then you would need to increase the number of cycles but this is unlikely to be an issue with modern construction.

Also, keep in mind that colder climates may require higher cycle rates due to more extreme differentials between indoor and outdoor temperatures.

You will notice that the thermostat manufacturers show that electric systems can have higher cycle rates, this is because there are no ramp up losses in electric systems so there is no real downside to shorter cycles.

— Bryan


When a system has abnormally high head pressure (high condensing temperature over ambient) and compression ratio, one of the easiest things to look for is a dirty condenser coil and more often than not, that will be the cause.


There is another category of issues that can cause high condensing temperature (high head pressure) that result from improper practices rather than dirt and grime.

When a tech comes across a failed condensing fan motor or a damaged blade they will often go to their van and see what they have as a “universal” replacement part. I don’t have an issue with using aftermarket repair parts in some cases but you need to make sure that the part you are using will operate properly without sacrificing capacity, efficiency and longevity.

Often when using aftermarket parts a tech may be sacrificing one or more of these things and that can lead to issues.

When replacing a fan blade you need to ensure –

  1. The pitch is a match
  2. The number of blades is a match
  3. The Diameter is a match

If you change the pitch you will also need to change the # of blade and vice versa to end up with the same CFM airflow output which can be very tough to determine in the field.

The diameter really cannot change or you won’t have the proper gap between the blade edge and the shroud (Usually 1/2″ – 1″) which can greatly impact air movement.

When replacing a motor you need to ensure –

  1. The RPM (# of poles)  matches
  2. Voltage and phrasing matches
  3. The HP is the same or greater
  4. The physical size will allow proper installation

In some cases, the technical specs may work but the motor body may be deeper. When this happens you need to make sure that the fan blade can still sit high enough in the fan shroud for proper movement of air. In many cases the blade/shroud are designed so that the middle/center of the blade matches up with the bottom of the fan shroud (cowling) and if it isn’t it can decrease airflow.

This issue comes into play often in cases where a factory motor fails on smaller tonnage residential units with a less than 1/4 HP motor. In these cases when you replace the factory motor with a universal motor the larger physical depth of the motor and sometimes the width can result in less than designed airflow. Make sure when replacing the motor that you are still able to place the motor blade in the same position in relation to the blade to ensure proper air flow/condensing temperature.

Sometimes you will come across systems that are running higher head pressure than they should be. In these cases you will want to check and make sure the motor HP and RPM are correct and that the blade is properly sized and positioned in the shroud.

As always, being attentive is key to finding issues, even issues caused by others

— Bryan


I just noticed this portion of the Carrier air handler sticker for the first time the other day. I’m like most techs, it’s easy for us to ignore the great info posted right in front of you on the data tag because so many of the notices contain info you are used to seeing.

I like this list because it is very practical

  1. Verify airflow is correct – Easier said than done but this includes a visual inspection of the air filter, evaporator coil, and blower wheel, checking all of the air handler/furnace /control settings and verifying you are getting the correct calls/signals and then checking static pressure. Some purists suggest actually “measuring” total system airflow but this can be very tricky unless you own a TruFlow grid or are very experienced with a hot wire anemometer.
  2. Check Subcooling at the outdoor unit and verify charge – You need to have a solid line of liquid delivered to the TXV for it to do it’s job. Get the charge set first by subcooling before overanalyzing valve operation.
  3. Confirm TXV bulb is properly attached and insulated – This should be done with a factory brass or copper strap or with a stainless steel strap. In all cases, it should be snug with the entire bulb making good suction line contact. If a bulb is loose or uninsulated it will generally run lower than design superheat.
  4. Verify the system is free of contaminants and moisture – This is less something you can “verify” and more something you prevent by purging nitrogen, flowing nitrogen while brazing and evacuating to below 500 microns with a standing decay test. It is important as part of your diagnosis leading up to “bad TXV” diagnosis that you check for temperature drop across any filter driers or screens first.
  5. Be sure the evaporator and condenser coils are clean – This is just good general advice and something we should be checking along with air filters and blower wheels anyway.

— Bryan

Proper sizing and orientation of grilles, registers, and diffusers may seem like such a simple thing, but it’s an area where confusion and mistakes are commonly made.

First, let’s define some terms.


A return draws air into a return duct system with negative pressure compared to the space usually via a fixed “grille” but also often called a “return vent” or a “return intake vent” or for some of you old school folks from up north the “cold air return”.


The supply vents, registers or diffusers blow air into the conditioned area with positive pressure and are responsible for distributing and mixing the air.


A vent is a generic word for a designed opening or cover that air passes in or out of. When in doubt, just say vent.


A grille is a fixed vent type that contains no damper or adjustable louvers. Grilles can be used for supply but are most commonly used in return applications. The grille shown above is a steel stamped return grille.


A register is a vent that contains an internal adjustment damper and often externally adjustable louvers. Registers have the same inlet neck and outlet face size. Air will move straight through registers and grilles.  Registers are the most common type of supply vent.  The register shown above is a common aluminum, adjustable, curved blade, one-way 10×6 ceiling register.


A diffuser is a vent that has a smaller inlet and a larger face resulting in a lower face velocity than that of the inlet duct. Diffusers often “turn” the air at a steep angle as it exits the face. Diffusers may or may not have adjustable dampers or louvers. The diffuser shown above is a typical tiered, acoustical ceiling 2’x2′ lay-in supply diffuser.

Sidewall Straight Blade vs. Curved Blade Ceiling 

Sidewall registers and grilles have straight louvers to force the air straight into the space with no turning at all at the face. Curved blades direct the flow at an angle and are generally used for ceiling applications.


When sizing grille or a register you will measure the OPENING that the grille or register is designed to cover or recess into, not the total external frame size of the grille or register.  The register shown below is a 10×6, sidewall supply register.


Look at the image at the top of the article. This is generally how we describe return grille orientation because return grilles are an instance where grille orientation/louver direction makes a big difference.

For return grilles, we state the dimension parallel (running the same direction as) with the louvers first and then the perpendicular dimension second.

For supply diffusers, they are almost oriented with the external louvers parallel with the long dimension on ceiling registers and with the louvers parallel with the short dimension on wall and floor registers like the shown below.

For floor registers, they follow the same rules as return grilles where you state the dimension parallel to the louvers first. This means that floor registers will often be smaller number first like 4×10 or 4×12.

Ceiling and some sidewall registers will usually just be described as long side first such as a 10×6 or 12×8 but that can vary from brand to brand.

Yes, it is pretty simple, but also essential for clear communication.

— Bryan

P.S. – This episode of the podcast with Jack Rise covers common duct and vent mistakes that you may want to know.

Does setting a thermostat too low cause an air conditioning system set in cool mode to freeze?

The answer is, no, at least not directly.

However, low evaporator load (low return temperature or low airflow) and low outdoor ambient temperature can both lead to evaporator coil freezing. Low indoor set-point can lead to low return air temperature which is a form of low load condition that can lead to coil frost accumulation on air conditioning systems… but it is actually pretty rare.

It isn’t the act of setting the thermostat too low that causes the freezing, it is only when the return temperature drops below the acceptable limit that the freezing can occur.

There is an important distinction we need to make before you go any further. Just because low indoor temperature or outdoor temperature CAN cause freezing, that doesn’t mean that it is the actual or only cause of freezing.

Many units are misdiagnosed as freezing because of these two causes when the actual cause (low airflow, metering device issues, drier restriction etc..) are left undiagnosed.

When a system is found frozen, it must be fully defrosted and tested for other issues before a conclusion is made that low set-point or low outdoor ambient were the root causes.

Most air conditioning systems are set up for around 400 CFM per ton of Cooling (but not always).

This will generally result in a 32° to 38° DTD (design temperature difference) on the evaporator coil with 35° being the typical standard. This means that the evaporator coil will be 38° to 32° colder than the return air that passes over it and ice will begin to form at 32°.

Based on this, return temperatures of below 70° begin to enter a zone where freezing becomes possible (on systems set up for 350 CFM per ton for example). At a return temperature of 64° frost formation on the evaporator coil becomes likely.

Again, this is due to the return air temperature, not just the thermostat setting.

Just because the thermostat is set to 67° doesn’t mean the return temperature will achieve 67°. In the case of a supply to return bypass damper as is common in some residential zoning systems, you may see a low return air temperature even when the thermostat is set normally.

Systems that have long run times and high humidity return air are the most likely to freeze the coil due to low return temperatures caused by low thermostat set point. For example in Florida, I have seen vacation homes on a rainy Spring day where a vacationer sets the thermostat to 50° in cool and leaves for the day… that makes for a nice frozen coil when they return because of the combination of low sensible load (outdoor temp), lots of moisture to freeze and all-day continuous run time.

A typical residential system with no special low ambient controls or freeze protection shouldn’t be set below 70° indoors or run for a significant amount of time when it’s below 65° outdoor.

This isn’t to say that they WILL freeze if you set it colder or run when it’s colder than that, but given enough runtime it is possible.

It isn’t a system “running too hard” that causes it. It isn’t the set-point itself, it is the heat load on the evaporator coil that can cause the suction saturation to drop below 32°.

Once again, as a technician, never blame set-point until you have exhausted all other possibilities.

— Bryan

Disconnects are on the edge of being HVAC or electrical, but most places the HVAC technician is allowed to repair and replace disconnects. Even if you work at a place where it isn’t allowed the ability to find problems is a diagnostic skill you will want to have. Always work safely with proper safety gear and on de-energized equipment whenever possible.

Check Pull Condition

On a pull-out disconnect, the pull should fit tightly with little to no carbon buildup or signs of arcing. In some cases, you can replace the pull alone but most of the time if it is damaged you will need to replace the entire disconnect.

Check for Proper Connections 

Poor connection and improper wire sizing are the two biggest causes of disconnect damage. Confirm the wire matches the MCA (Minimum Circuit Ampacity) of the connected unit and then check the connection lugs for a snug fit while unenergized. It’s best to follow the torque recommendations and use torque screwdriver when possible.

When in Doubt, Check for Voltage Drop

Whenever you suspect an issue with a disconnect you can simply check for voltage drop across the line and load sides of the disconnect on each leg. You should read little to no voltage between line and load as shown above. If you have a thermal imaging camera or an infrared thermometer you can also check for hot spots on the disconnect lugs.

Seal Behind Disconnects w/ Rear Penetrations 

In many cases, a disconnect will be fed with the wire coming from the wall behind. In these cases, it can be difficult to properly seal the wall and the penetration into the disconnect. This is why the top of the disconnect should be sealed to the wall to prevent rainwater from running down behind and into the wall or disconnect.

It is a good practice to replace any damaged connectors and make sure there are no signs of moisture intrusion or corrosion in the disconnect and inspect the whip for proper fittings and strapping at the same. Look at the ground connections in the disconnect and where they connect to the system and make sure they are properly connected under a dedicated grounding screw.

There have been many breakdowns and even fires over the years caused by issues with disconnects, make inspecting them a regular part of your maintenance and service call process and you can save some issues.

— Bryan



You may have noticed that in 5-ton and under equipment 3/8 liquid lines are generally the norm. We went to a job recently where the system had a 1/2″ liquid line and it got me thinking about the ramifications of going larger or smaller on the liquid line.

Liquid Line Basics

The liquid line should be full of liquid with additional subcooling to prevent flashing due to pressure drop from the length, rise, fittings and filter/drier. Because liquid refrigerant is much denser than vapor the liquid line contains a relatively large amount of refrigerant compared to the much larger vapor line.

Even small changes in liquid line size can have a big impact on refrigerant velocity in the liquid line as well as the amount of charge contained in it. This is why we see a big variation in suction line size but very little change in liquid line in residential applications.

Pressure drop in the liquid line is only a concern when it results in flash gas or when it results in an unacceptably low pressure drop across the metering device. Flashing occurs when the refrigerant pressure drops to the point that all of the design subcooling is “used up” and the refrigerant in the liquid line begins to boil off. Up until that point, the pressure drop will result in negligible temperature drop.

From the Lennox Design and Fabrication Guide

Liquid Line Sizing Factors

We need to size the liquid line with the following factors in mind

  • Keep the velocity low enough to prevent noise
  • Minimize pressure drop to prevent flashing
  • Do not oversized the liquid line to prevent excess refrigerant charge

At first, it may seem like bigger would be better on the liquid line but that isn’t the case. An oversized liquid line can lead to a lot more refrigerant charge which will result in a greater likelihood of off-cycle refrigerant migration and flooded starts in addition to the cost associated with more charge for no good reason.

Lennox Liquid Line Chart

Our goal should be to use the smallest liquid line size that will still reliably provide a full line of liquid to the metering device under all load conditions that the system will be reasonably operated under. Luckily for us, we don’t need to guess as the manufacturers provide us guidelines for liquid line sizing.

Carrier Liquid Line Sizing Chart

Vertical Pressure Drop / Gain 

In general, on an R410a system, we don’t want more than about a 35PSI pressure drop in the liquid line otherwise we run the risk of flashing. When the condenser is LOWER than the evaporator the liquid line pressure loss is about 0.5 PSI per foot of vertical rise which limits the rise to around 60′ for R410a systems by the time you consider the other pressure drops.

If the condenser is ABOVE the evaporator then the pressure actually increases the longer the vertical separation allowing the liquid line to be downsized in some cases.

Each manufacturer has their own piping guide or has the details in the install instructions or the product data. In most cases 3/8″ liquid line is a safe bet but just like the suction line there is some wiggle room depending on the system and the specific application.

Here are some great guides

Carrier Guide

Lennox Guide

Johnson Controls / York Guide

— Bryan


In residential most techs and installers size the suction (vapor) and liquid lines to the stubs on the equipment and in larger built-up systems it is rarely the responsibility of the technician to size the piping.

But what happens if we show up to a job and the lines cannot be (reasonably) replaced and the size is different? or what if the stub on the evaporator coil is a different size than the condenser?

The Factors in Suction Line Sizing

  1. Pressure Drop – Any pressure drop in the suction line due to size, length or fittings results in lower refrigerant density and higher compression ratio at the compressor without a corresponding decrease in evaporator temperature. In other words, you get all the bad stuff like compressor overheating and poor capacity without a lower evaporator operating temperature. This is why pressure drops in the suction line are generally recommended to be less than 3PSI on R22 and 5PSI on R410 but the lower the better.
  2. Oil Carry – When mineral oil was used oil carry was a bigger deal than it is now but manufacturers still design the systems with a minimum suction refrigerant velocity which means that bigger isn’t better. The York / Johnson controls guide says the following

Minimum (suction) velocity of 1000 fpm for vertical lines and 800
fpm for horizontal lines guarantee proper oil return

Equivalent Length

Before we look at an example of a suction line selection chart we must first square on this topic of “equivalent” length. You need to calculate the actual line length and then add-in the equivalent length of fittings (very similar to duct design). By just a quick glance at the chart above you can see the advantage of using long-radius 90’s.

Manufacturers Chart

Now it’s time to look at your manufacturer’s literature to see if a particular suction line size is acceptable. The one above applies to standard Carrier R-410a single-stage heat pumps and comes from the system product data.

Take a look at the 5-ton system at the bottom of the chart. You may have thought that a 3/4″ line set could NEVER be used on a 5-ton but this chart clearly shows that on line sets with equivalent lengths of under 50′ the % of capacity loss will only be 1%. While this isn’t ideal it may be the only economical option in some cases. On the other hand, a 5/8″ suction line on a 3.5 ton is “off the chart” meaning it isn’t acceptable under any conditions.

Either way, our goal is to keep suction line pressure drop to a minimum while maintaining the minimum suction velocity and this is the sweet spot manufacturers are keeping you in with these charts.



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