Month: June 2019


There are three reasons why I don’t like infrared thermometers for many HVAC tasks.

#1 – The Laser is Misleading
The laser dot is just a point of reference, not an exact point where it is reading. Often the thermometer will read lower, higher or over a MUCH wider area. Unless you are right up on what you are measuring you can’t be sure the result you are getting is correct.

#2 – They Only Read Surfaces
An infrared reads surface temp only, not air temp. This is necessarily a problem, but “shooting a vent” is not the same as measuring the air temperature coming out of it.

#3 – They Can be VERY Inaccurate
Basic infrared thermometers are only accurate on a surface that has high “emissivity” of near 1.0. These are usually darker, less reflective, generally non-metallic surfaces. Metals have a low emissivity (much less than 1 generally) which means that if you are reading a pipe an infrared could read much lower than the correct temperature.

Infrared thermometers can be useful to do comparisons where reading the correct temperature is less important than comparing one spot to another, such as looking for hot spots in a panel, or checking a zone to see if a damper is open.

So long as you use the right tool for the job you should be fine, but in general….

I don’t like techs using infrared thermometers for most tasks.

— Bryan

P.S. – While I don’t generally like infrared, I REALLY like thermal imaging. Check out these nice products from Trutech tools 


We’ve all heard about glide, but what is it really and how does it affect our system?

Glide, or temperature glide, is the difference between the bubble point and the dew point of the zeotropic refrigerant mixture.

Well that wasn’t very helpful, was it? All we did was introduce new terms without defining them and further confused the issue.

So, let’s start with zeotrope or zeotropic mixture. A zeotropic mixture is a chemical mixture that never has the same vapor phase and liquid phase composition at the vapor-liquid equilibrium state. Still unhelpful? I thought so, too, so let’s look at what it means to us rather than what the books say.

A zeotrope, is a refrigerant mixture or blend that boils across a range of temperatures at any given pressure. So, unlike water that boils at a constant temperature of 212°F at atmospheric pressure, a zeotropic mixture will boil between across a range of temperatures at that same single pressure. Using r407a as an example, at atmospheric pressure, the liquid would begin to boil at -49°F and will continue to boil until the last droplet boils away at -37.5°F. I know that it’s kind of weird to think of the process of boiling like that, but that’s what is happening with a zeotrope. Boiling takes place over a range of temperatures.
That temperature range is called the glide.


Now that we’ve got a basic concept that we can work from, we can start to understand glide and ultimately get to how it affects a refrigeration system. Let’s start with bubble point. Since we should have a solid understanding of states of matter and the transition between liquid and vapor, let’s assume we have r407a refrigerant in a 100% liquid state at 140 psig. If we start at 66°F, we’ll be just slightly subcooled which is a perfect starting point for this example.. If we start to add heat and raise the temperature of our refrigerant while holding our pressure constant, a single bubble will appear in the refrigerant as it begins to boil. That point is called the bubble point. For our purposes, we can define the bubble point of a zeotropic refrigerant blend is the point at which the first bubble appears.
Still making sense? I hope so.


Continuing with our r407a at 140 psig example, we’re going to continue to add heat to the refrigerant with the same constant pressure. The refrigerant continues to boil, but as the mixture of refrigerant changes, the boiling point changes, slowly rising as the liquid boils away. Eventually, we will have added enough heat to reach a point where one last droplet exists, that point is called the dew point. Like we did with bubble point, let’s state an operating definition for bubble point. The dew point is the point at which the last droplet of liquid evaporates. For our example, that temperature is 75.5°F or very near that. Since it’s boiling over a range of temperatures, it is also true that the refrigerant condenses over the same range of temperatures as we remove heat from it. That will happen in reverse of the process I just described.


What does this mean for the service guy?


Obviously, these different values affect our superheat and subcooling readings. Since the dew point is the point where the last droplet of liquid boils off, we need to know that value to measure and calculate superheat. Similarly, with the bubble point, we need that to calculate subcooling. These are the values found on the PT charts and that are programmed into your digital manifold gauges.

In refrigeration work, evaporator coil temperature can be used for a number of things. Most commonly, we will use it to control fixture temperature and to terminate defrost. It used to be simple to know what our evaporator temperature is. We looked at the gauge and transferred that number to a PT chart. We can no longer look at our evaporator pressure and know what our corresponding evaporator temperature is quite the same way.


Let’s look at numbers… say the manufacturer says that you need an 18°F coil temperature. With R22, you simply look at your trusty PT chart, find 40.9# and work from there. Easy enough, right?
Now, let’s look at the same coil with r407a. We have 2 points that are 18°F. The dew point (40#) and the bubble point (52.5#), so which one do we choose?
The correct answer winds up being neither one. Between manufacturer’s recommendations and field experience, I’ve found it best to use something closer to the average of dew and bubble point to find the actual, functional temperature of the evaporator.


52.5+40 = 92.5. 92.5/2=46.25


Looking at a PT chart, this shows us 13°bubble point and just over a 23° dew point. If you look, 18° will land right about in the middle. This isn’t always a perfect setting, but it’s as good a place to start as you can find. Set the control valve there and fine-tune it as needed to get the performance that you need. If we need to use a pressure reading to terminate defrost, we will need to reference bubble point because it is the colder of the two temperatures and will ensure a complete defrost. If we used dew point, the inlet of the evaporator would be several degrees colder than the outlet and frost may still very present.

–Jeremy Smith


Every year when outdoor temperatures rise there is a rash of news stories and articles about air conditioning. We had an early heat wave this year and lot of people have come out and referred to the idea of a rule of thumb of what temperature you can achieve indoors based on the outdoor temperature, most commonly used is the “20° rule”. Here is a link to an article like this.

There is no such thing as a universal 20° rule, it is simply the difference between the indoor and outdoor design conditions and it varies based on location and design

There are times where 20° is the design difference between indoor and outdoor temperatures, specifically when the design outdoor temperature is 95° and indoor is 75° for cooling. Before we go any further let’s specify EXACTLY what we are talking about

This is all about designing an air conditioning system and has nothing to do with DIAGNOSING it. When a tech goes to a home their job is to diagnose and test the HVAC system not to quote rules of thumb.


When a contractor designs an air conditioning system they have to size it for the space being cooled (I’m just going to focus on cooling here). The size of the unit needs to be based on a design indoor and outdoor temperature and humidity.

The indoor temperature design for homes is fixed by ACCA at 75° and the designer can choose 45%, 50% 0r 55% indoor design humidity.

The outdoor design temperature comes from temperature data specific to the location and is based on a temperature that will only be (statistically) exceeded 1% or 0.4% of the time in that location.

We do not design air conditioning systems for the hottest possible day with the lowest possible indoor temperature the customer may want because that would result in over-sizing for 99.6% of the year and over-sizing isn’t a good thing for many reasons including –

  • Higher initial cost
  • Larger Ducts Required
  • Poor humidity control
  • Short Cycling resulting in less efficiency and shorter system life
  • Generally lower rated efficiency on larger capacity systems

Take a look at the chart above, you can see from a glance that Florida has outdoor design conditions of 90° to 95° depending on the city and which column you use for design. Because the indoor design conditions stay fixed at 75° regardless the design difference on a peak design day vary from 20° down to as low as 15°.

Nevada (for example) is completely different –

You can see Reno is much like Florida in terms of dry bub temperature but Las Vegas is 106° – 108° and still a 75° indoor temperature design so a 31° to 33° difference must be designed for.

ACCA manual J does allow some oversizing to find a proper system match, from 15% greater than the load for straight cooling and 25% greater on heat pump systems (where the heating load is greater than the cooling load) but that isn’t a lot of wiggle room.

It’s also important to remember that system performance also changes based on outdoor and indoor temperatures and we must select out equipment capacity based on the specific design conditions rather than AHRI conditions which are 80° indoor and 95° outdoor temperatures.

So here are some facts to get straight-

  • There is no universal design rule of thumb 
  • This 20° rule has no relationship to the old 20° delta T rule (which is also a bad rule of thumb) 
  • Oversizing isn’t a good idea ESPECIALLY in humid climates
  • The fact that you can get your home to certain temperature in one part of the county has nothing to do with another location
  • How cold you can get your house on a hot day is just a representation of the capacity of your system in comparison to the load, it’s not something to be proud of.
  • Did I mention that over-sizing isn’t a good thing? Just because you can get your house to 66° on a 140° degree day in Nome, Alaska doesn’t mean you have a better air conditioner. It means someone put in a unit that is much too large. 

So when a contractor emails their clients before a heat wave (Like I did recently) or when a LOCAL news channel runs a story and quotes something like, “You can expect your home to maintain only 20° lower inside than the outdoor temperature” withhold judgement for a minute and consider. That 20° rule may be a good guideline for their market and they may be telling it to consumers to reduce nuisance service calls on a rare 100° day in a place like Savannah where the design temperature is 93°.

As HVAC professionals we understand that some designs will do better than others and with modern multi-stage / variable speed systems we can get away with a little more over-sizing than we used to. We also (should) know that we size systems based on heat gain and loss and not based on square footage and that oversizing a system because the customer wants it like a “meat locker” has unintended consequences.

Now there will always be the techs who use silly rules of thumb rather than proper diagnosis procedures. If a customer calls you to look at their A/C don’t just walk up to the thermostat and say something like “It’s 80° in here and 100° outside so it’s doing good”. You need to properly test the equipment and I would suggest doing actual capacity calculations using in duct psychrometers and MeasureQuick in addition to everything else if it seems like the system just isn’t keeping up.

Check the charge, check ducts and insulation, do a good job of making sure everything is as it should be for the customer…. but sometimes, on unusually hot days, a properly designed and installed system may bot maintain 75° inside.

So these are my takeaways –

  • Size systems properly not based on rules of thumb
  • Still use proper system diagnosis and commissioning. Don’t use a rule of thumbs as an excuse not to do a proper diagnosis.  
  • It’s OK to use a rule of thumb specific to your market to communicate with customers so long as it’s based on real design conditions 
  • Don’t oversize systems especially in humid climates
  • Don’t be a jerk to people on the internet 

So in Orlando… you can expect your A/C to maintain 75° on a 95° day in most cases and if the temperature rises above that it may not keep up.

— Bryan

If you want to know more about the ACCA design process take a look at this quick sheet with design instructions


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