This tip is written by 19 year service tech Frank Mashione. Thanks Frank.

Here is a tech tip from the field that had stumped me. Lucky I knew the right person to call to get back on track.

This business has complained about heat calls for quit a while. I have heard other techs mention it in passing. I ran into our installation crew there about a month ago on a different job. Yesterday was my turn to take a crack at it.

They had called a different company day before and they said it was working fine. When I arrived checked thermostat set at 73 and 65 in store, so I checked another thermostat set at 73 and 70 in the store.

I Got up on the roof found both units locked out on high limit, which is 4 blinks for these Trane gas packs. I found it odd that the units were locked out on high temperature but the indoor fan was not running or inducer. I cycled power and the unit restarted. Next I checked gas pressure, it was on the high end of manufacturers specs so I adjusted it back just a bit.

In the back of my mind I thought I had it. I checked static pressure found it at 0.6″ wc which isn’t too bad especially for the area I work in which I regularly see static off the charts.

The temperature rise was 55° which was acceptable but the return temperature was reading high. I put my wireless air probe in the location of the high limit. After some run time the temperature in the high limit was approaching the trip point of the limit. This made it clear that the limit was doing its job by tripping.

The unit would run about ten minutes then trip on limit. After four trips it would lock out for an hour.

This got me looking again at the return temperature and I realized that it was much higher than the indoor temp.

The cause of the problem was the supply was installed too close to the return making return temperature high tripping out high limit. After nineteen years in the business still finding new things to learn.

Frank Mashione service technician


In HVAC/R we are in the business of moving BTUs of heat and we move these BTUs on the back of pounds of refrigerant. The more pounds we move the more BTUs we move.

In a single stage HVAC/R compressor, the compression chamber maintains the same volume no matter the compression ratio. What changes is the # of pounds of refrigerant being moved with every stroke(reciprocating), oscillation (scroll), or rotation (screw, rotary) of the compressor. If the compressor is functioning properly the higher the compression ratio the fewer pounds of refrigerant is being moved and the lower the compression ratio the more pounds are moved.

In A/C and refrigeration the compression ratio is simply the absolute discharge pressure leaving the compressor divided by the absolute suction pressure entering the compressor.

Absolute pressure is just gauge pressure + atmospheric pressure. In general, we would just add the atmospheric pressure at sea level (14.7 psi) to both the suction and discharge pressure and then divide the discharge pressure by the suction. For example, a common compression ratio on an R22 system might look like-

240 PSIG Discharge + 14.7 PSIA = 254.7
75 PSIG Suction + 14.7 = 89.7 PSIA
254.7 PSIA Discharge ÷ 89.7 PSIA Suction = 2.84:1 Compression Ratio

The compression ratio will change as the evaporator load and the condensing temperature change but in general, under near design conditions, you will see the following compression ratios on properly functioning equipment depending on the efficiency and conditions of the exact system.

In air conditioning applications compression ratios of 2.3:1 to 3.5:1 are common with ratios below 3:1 and above 2:1 as the standard for modern high-efficiency Air conditioning equipment.

In a 404a medium temp refrigeration (cooler) 3.0:1 – 5.5:1  is a common ratio range

In a typical 404a 0°F to -10°F freezer application 6.0:1 – 13.0:1 is a common ratio range

As equipment gets more and more efficient, manufacturers are designing systems to have lower and lower compression ratios by using larger coils and smaller compressors.

Why does the compression ratio number matter? 

When the compressor itself is functioning properly the lower the compression ratio the more efficient and cool the compressor will operate, so the goal of the manufacturer’s engineer, system designer, service technician and installer should be to maintain the lowest possible compression ratio while still moving the necessary pounds of refrigerant to accomplish the delivered BTU capacity required.

The compression ratio can also be used as a diagnostic tool to analyze whether or not the compressor is providing the proper compression. Very low compression ratios coupled with low amperage and low capacity are often an indication of mechanical compressor issues.

Compression ratio higher than designed = Compressor overheating, oil breakdown, high power consumption, low capacity 

Compression ratio lower than designed = Can be an indication of mechanical failure and poor compression

Understanding compression is critical to understanding the refrigeration process. Don’t be tempted to skip past this because it is a really important concept.

Look at the pressure enthalpy diagram above. Top to bottom (vertical) is the refrigerant pressure scale, high pressure is higher on the chart. Horizontal (left to right) is the heat content scale, the further right the more heat contained in the refrigerant (heat, not necessarily temperature).

Start at point #2 on the chart at the bottom right. This is where the suction gas enters the compressor. As it is compressed it goes to point #3 which is up because it is being compressed (increased in pressure) and toward the right because of the heat of compression (heat energy added in the compression process itself) as well as the heat added when the refrigerant cooled the compressor motor windings.

Once the refrigerant enters the discharge line at point #3 it travels into the condenser and is desuperheated (sensible heat removed). This discharge superheat is equal to the suction superheat + the heat of compression + the heat removed from the motor windings. Once all of the discharge superheat (sensible heat) is removed in the first part of the condenser coil it hits point #4 and begins to condense.

Point #4 is a critical part of the compression ratio equation because the compressor is forced to produce a pressure high enough that the condensing temperature will be above the temperature of the air the condenser is rejecting its heat to. In other words, in a typical straight cool, air cooled air conditioning system the condensing temperature must be higher than the outdoor temperature for the heat to move out of the refrigerant and into the air going over the condenser.

If the outdoor air temperature is high or if the condenser coils are dirty, blades are improperly set or the condenser coils are undersized point #2 (condensing temperature) will be higher on the chart and therefore will put more heat strain on the compressor and will result in lower compressor efficiency and capacity.

As the refrigerant is changed from a liquid vapor mix to fully liquid in the condenser it travels from right back left between points #4 and #5 as heat is removed from the refrigerant into the outside air (on an air cooled system). Once it gets to #5 is is fully liquid and at point #6 it is subcooled below saturation but ABOVE outdoor ambient air temperature. The metering device then creates a pressure drop that is displayed between points #6 and #7. The further the drop, the colder the evaporator coil will be. The design coil temperature is dictated by the requirements of the space being cooled as well as the load on the coil but the LOWER the pressure and temperature of the evaporator the less dense the vapor will be at point #2 when it re-enters the compressor and the higher the compression ratio will need to be to pump it back up to point #3 and #4,

This shows us that the greater the vertical distance between points #2 and #4 the higher the compression ratio, which means that both low suction pressure and/or high head pressure result in higher compression ratios, poor compressor cooling, lower efficiency and lower capacity.

In some cases, there isn’t much that can be done about high compression ratios. When a customer sets their A/C down to 69°F(20.55°C) on a 100°(37.77°C) day they will simply have high compression ratios. When a low temp freezer is functioning on on a very hot day it will run high compression ratios.

But in many cases, you can reduce compression ratios by –

  • Keeping set temperatures at or above design temperatures for the equipment. Don’t be tempted to set that -10°F freezer to -20°F or use that cooler as a freezer
  • Keep condenser coils clean and unrestricted
  • Maintain proper evaporator airflow
  • Install condensers in shaded and well-ventilated areas

Keep an eye on your compression ratios and you may be able to save a compressor from an untimely death.

— 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 make 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.


We’ve all been new at one time or another so there is no need to get all judgy about some of the mistakes new techs make just because they are inexperienced.


These are some very preventable mistakes that occur due to simple oversight and carelessness that need to happen 0% of the time.

Caps and Seals

Leaving caps off is never OK. While it’s true that Schrader valves and back seating service valves “should” seal completely and shouldn’t be left leaking it is always possible that a little leakage can happen. Besides, keeping bugs and dirt out of the ports is reason enough to keep the caps on.

Bill Johnson (co-author of RACT) made a really good point on a recent podcast. When a system is apparently low (which you can verify through non-invasive temperature tests) you shouldn’t just pull off the caps and attach the gauges. First, look for oil at the ports and leak check them to eliminate port leaks as a possible cause. Once you remove the caps and attach your manifold you won’t be able to know if the ports were a leak point or not.

Every time I remove caps I look inside them to make sure they are in place unless it is a flare hex cap that doesn’t require a seal.

It’s a good practice to keep all caps and screws together and in the same place on every call. This helps to ensure they don’t get accidentally knocked into the dirt, lost or forgotten.  Put those caps back on, finger tight for caps with seals and snugged up with a wrench for hex flare caps (Trane residential units for example).

Leaving Disconnects Out / Off

Obviously, nobody TRIES to forget the disconnect but it still happens all the time and it’s almost always because the tech gets in a hurry or distracted and usually both, and it can be eliminated easily by some best practices.

Most often the disconnect is left off or out during maintenance or during very simple repairs. This is because the tech will often run test the equipment, then perform the maintenance or minor repair and leave without run testing again. This order of test first then clean / repair isn’t my favorite for several reasons will silly mistakes being one of them.

I advocate for performing the comprehensive run test at the very end of a repair or maintenance meaning you are observing the system running right before you leave with the last action being resetting the thermostat or controls back to the desired setpoint. When you run test last you don’t forget silly things that prevent the system from running.

Always do a final walk of the job before leaving and check disconnects, setpoints, cleanup and check for tools.

Making Poor Electrical Connections 

I see it all the time. Capacitors tested and the spade connections left loose, contactor lugs not properly torqued, stranded wires with some of the strands cut off to make the wire fit, crimp connections on solid wire…. the list goes on and on. Here are the top mistakes to avoid.

  • When forcing on a female spade (on a capacitor for example) it should be very snug. If it is loose at all, pull it off and pinch down the spade sides a bit to ensure it’s a snug fit
  • When making a crimp connection only do so on a stranded wire and use an appropriately sized connector. Position the jaws so that the indent crimp is made on the side of the connector OPPOSITE the split in the barrel. Even better is to use a crimper specifically designed for insulated terminals that compresses the entire barrel.
  • Never cut strands of wire to make a conductor fit under a lug. Use the proper connection (termination) type for the conductor.
  • Never leave exposed wire, strip back insulation only to the length required to make the connection and no more.
  • Don’t leave connections under tension. Use straps and zip ties to keep tension away from connections so that they aren’t left under a pulling/disconnecting force.
  • Make appropriate connections for the job, never leave connections open to the environment unless they are rated for it.

When making any electrical connection always pull the connection to make sure it is a snug fit before walking away.

Failing to See the Obvious 

So much is made of good workmanship (how things look) and diagnosis (figuring out what’s wrong) and rightfully so. However, for a new tech, nobody expects you to do the best looking work out there or to diagnosis the super difficult situation. You are expected to use common sense and spot things that are out of the ordinary or that can lead to issues. Here is a quick list of things to look out for that you can see with little to no experience.

  • Look for refrigerant oil stains, often oil stains or residue can lead you straight a refrigerant leak.
  • Use a mirror and a flashlight and look for dirty evaporator coils and blower wheels. You may make a diagnosis but if you leave the system with a dirty coil or a blower wheel you still look silly.
  • Check the air filter and let the customer know you checked it. A home or business owner may not know much about HVAC but they know what an air filter is and reporting the condition back helps give them confidence.
  • Watch for rub outs on copper lines, feeder tubes, external equalizers and sensing bulbs and wires. You can often find or prevent a problem just by looking for areas of contact between tubes and/or wires.
  • Inspect control wiring for cuts or UV damage outside. If the weedwhacker doesn’t get the wire often the sun will.
  • Look for past workmanship that may be done incorrectly. Just because that fan motor or capacitor is new doesn’t mean it is the right size and wired properly. Always double check your own work as well as work done by others.
  • Before making a repair double check the previous diagnosis and check that the part you have is actually the correct part. There is NOTHING worse than removing a compressor t find out the one you have isn’t the correct one. ALWAYS double check the diagnosis and the part.

There are many other things that could be added to the list, but for a new tech if you do the following you will be on the road to success even if you are green.

  • Read product manuals and never stop learning
  • Listen carefully to senior techs and ask lots of questions
  • Help other techs when they are in a pinch
  • Smile and treat customers with respect
  • Compete with yourself to do each job better than the last
  • Walk  every job before you leave to make sure everything is buttoned up (Screws, caps, disconnects)
  • Ask every customer is you have done everything to their satisfaction and if there is anything you can improve.
  • Do all the little things with exceptional detail. Cleaning drains, washing condensers etc… always do it with a level of detail that exceeds your peers and you will build a reputation for excellence.

If you do these things your co-workers, customers, and managers will generally overlook the mistakes you make just because you are green.

— Bryan


Michael Housh from Housh Home Energy in Ohio wrote this tip to help techs determine the air side charge on a pressure tank. Thanks Michael!

Determining the air-side charge of an expansion tank in a hydronic heating system is a relatively easy task.  A properly sized and charged tank is designed to keep the system pressure about 5.0 psi lower than the pressure relief while the system is at maximum operating temperature.


The proper air-side charge is equal to the static pressure of the fluid at the inlet of the tank plus an additional 5.0 psi allowance for the pressure in the top of the system.  The air-side of the tank should be checked and adjusted before adding water to the system, if the tank is already installed and the system has pressure in it, the pressure should be drained at the tank to 0 psi before testing the pressure on the tank.


The formula for calculating the air-side pressure is relatively easy and directly related to the highest point in the system from the inlet of the expansion tank.


Pa = H * (Dc / 144) + 5


Pa = air-side pressure in the expansion tank (psi)

H = height from inlet of the tank to highest point in system (ft)

Dc = density of water at its coldest state / typically filling (lb/ft3)


The above graph shows us the relationship between density of water and temperature between 50°F – 250°F.


A lot of the “rule of thumb” equations for hydronic systems are based on the density of water @ 60°F is 62.37, so we could simplify the above equation into a rule of thumb equation by first solving for the density (Dc).


Dc = 62.37 / 144 =0.433


Substituting ‘Dc’ into the original equation would give us a slightly less complicated equation that can be used as a rule of thumb to solve for the air-side pressure.


Pa = H * 0.433 + 5


Below is a graph that shows us this rule of thumb equation and the required air-side pressure based on the height of the system piping.

— Michel H.


This tip was a COMMENT on the sensible heat ratio tip left by Jim Bergmann. As usual Jim makes a great point, once you get the “sensible” capacity for a piece of equipment at a set of conditions you can easily calculate a true target Delta T.

Another interesting thing you can do with this information is to determine the approximate target temperature split under any load condition. There are some additional footnotes on that chart likely saying the return air conditions are at 80 degrees at each of the respective wet-bulb temperatures.

To do so, find the sensible capacity at any set of conditions, for example at 95 degrees outdoor air and 1400 CFM, the sensible capacity is:

At 72 wb 25,010 BTUH

At 67 wb 31,730 BTUH

At 63 wb 37,360 BTUH

At 57 wb 37,930 BTUH

Using the sensible heat formula, BTUH = 1.08 x CFM x Delta T

Delta T = BTUH /(1.08 x CFM)


Delta T = 25, 010/(1.08 x 1400)

or 16.6°

Delta T = 31,370/(1.08 x 1400)

or 20.74°

Delta T = 37,360/(1.08 x 1400)

or 24.70°

Delta T = 37,930/(1.08 x 1400)

or 25.08°

So you can see also that the target temperature split has a lot also to do with the return air and outdoor air conditions and it has a lot of variation

— Jim Bergmann w/ MeasureQuick


Every piece of air conditioning equipment is capable of moving a certain amount of heat BTUs (British Thermal Units) at set conditions. In most cases during cooling mode, a portion of those BTUs will go toward changing the temperature of the air and a part will go towards changing vapor water in the air into water that collects on the evaporator and then drains out.

The BTUs that go towards changing the TEMPERATURE of the air are called SENSIBLE and the ones that go toward removing water from the air are called LATENT. The percentage of the capacity that goes toward sensible cooling at a given set of conditions for a given piece of equipment or space is called SENSIBLE HEAT RATIO (SHR). So a system that has an SHR 0f 0.70 and 30,000 Total BTUs of capacity at a set of conditions would produce 21,000 BTUs of sensible cooling and 9,000 BTUs of latent removal because 30,000 x 0.7 = 21,000 and the rest 30,000 x 0.3 = 9,000.

Higher SHR (closer to 1.0) = More change in temperature and less humidity removed

Lower SHR = less change in temperature and more humidity removed

In the HVAC industry, there is a set of standard conditions used to compare one piece of equipment to another. When a system has an SHR rating listed it would often be at AHRI conditions unless the specs state otherwise.

When doing a load calculation a good designer will calculate and consider the internal and external latent and sensible loads and match up with equipment accordingly based not only on one set of design conditions but on the range of seasonal and occupant conditions that the structure is likely to experience based on the use, design and climate. By following ACCA (Manual J & S) and ASHRAE (62.2 & 62.1 for example) standards a designer will have guidelines to follow and this includes matching the space SHR to a piece of equipment that will make a good match at similar conditions. It does often need some digging into manufactures specs to interpret this data for the equipment.

In the example above from a Lennox unit, you can see that the SHR is listed and highly variable based on outdoor temperature, air flow setting as well as indoor wet bulb and dry bulb temperatures. In this example, you would need to multiply the total capacity x SHR to calculate the actual sensible and latent capacity.

This example from Carrier has no SHR listed, instead, it lists the specific sensible and total capacities. You can easily calculate the SHR by dividing the sensible capacity by the total capacity and the latent is simply the sensible subtracted from the total.

The cool thing is that this understanding can help both designers and commissioning technicians to match equipment properly and even make further adjustments using airflow to get a near perfect match which leads to lower power consumption, less short cycling and better humidity control.

— Bryan







This tech tip was written by a friend of HVAC School, Brian Mahoney HVAC instructor at Western Suffolk BOCES/Wilson Tech. Thanks Brian!

The podcast on delta T for A/C the other day got me to thinking about the formula I learned in school about calculating the GPM of a hydronic system using a handy formula. We will be using the following values:

Td – temp difference of your supply vs return

Net boiler output(btu) use the boiler plate rating or get fancy and do an efficiency test and multiply your rated input multiplied by your efficiency rating. On an oil system, the unit could be down-fired.

It may be rated for 1 gallon per hour (140,000 BTU per hour input, but it may be firing with a .85 gallon per hour nozzle. So you have to do the math:
1 gallon of #2 fuel oil contains about 140,000 BTUs. Multiply that by .85 (your nozzle size) and you get 119,000 btu/hr input. Input would be 119,000 x .80 efficiency = 95,200.

500 – a constant which stands for a pound of water times 60 minutes – 8.33 x 60 = 499.8 (we fudge a bit.)

This is the weight of water at 60 degrees. You could look up the weight at the temp you are working with and multiply by sixty but it wouldn’t be far off.

To find a system’s gallon per hour:
BTU/ (500 x TD)
100,000/(500 x 20)
100,000 / 10,000= 10 GPH

Nice, but is there anything else you can do with this? How about a room that’s not warm enough. Is your baseboard supplying enough heat? You could look up the specs for that product, maybe. But what if it has dirty fins or mud in the pipe that is affecting temperature transfer. How would you know?

By using your Testo temp clamps on either end of the baseboard you find your temperature difference and using the data from the last calculation you solve for net BTU output of the baseboard

Btu = GPH x 500 x td
10 x 500 x 2 = 10,000 btu/hr

Now you know what you are getting. So you can check the specs of that baseboard and see if it’s giving you its rated output. If it is you don’t have enough baseboard or you have a problem with the room; thermal bypass for instance.

If it’s not performing as rated and the fins are clean you have an internal problem such as mud in the pipe insulating it.

Just something for the wet-heads.

— Brian M.

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