All fuel-burning appliances require oxygen to burn and sufficient oxygen to burn clean and safe, without soot and CO (Carbon Monoxide).

I live and work in Florida where most of our fuel-burning appliances are 80% efficient with open combustion that utilizes air and oxygen from the space for combustion.

With these low-efficiency appliances whether the appliance is forced vented or natural draft that combustion air is leaving the space, and exiting the flue.

This causes negative pressure that must be allowed to equalize as well as consumes oxygen from the space. It is because of this that these open combustion appliances must either be in a sufficiently large space or communicate with (be open to) a larger space or outdoors.

When you consider that other gas appliances also need to use oxygen and need to vent to outside you can see that without sufficient communication to outdoors that negative drafts can occur on natural draft appliances like water heaters.

This is why all open combustion appliances that utilize combustion air from inside the space must be in an “unconfined space” or connected to an unconfined space or the outdoors using an approved method.

I see many furnaces jammed into tight closets and mechanical rooms with little thought or planning regarding combustion air.

According to NFPA 31, 54 & 58 an unconfined space is a space that has at least 50 cubic feet of open area for every 1,000 Btu of input. This means that a 100,000 Btu furnace must be in a 5,000 cubic ft space to be considered unconfined.

If the appliance is not unconfined then additional combustion air must be made available to the space with one opening at the ceiling level and one near the floor.

If the air is coming from another unconfined space then the openings should be at least 1 square inch per 1,000 BTU and 1 square inch per 5,000 BTU if it is connnected to the outdoors.

While these openings and are needed in many cases to allow for proper combustion and venting it helps illustrate why modern sealed combustion “direct vent” appliances that take all of their combustion air from outdoors make so much sense.

Not only are direct vent appliances more efficient on the fuel utilization side, they also prevent the negative home pressures and/or thermal losses associated with having vents in walls and ceilings.

So either make sure you have an unconfined space, you are bringing air in from an unconfined space or outdoors or you have a direct vented appliance.

— Bryan


One of the most common parts to fail on a single phase HVAC system is a run capacitor, so much so that we sometimes refer to junior techs as “capacitor changers”. While capacitors may be easy to diagnose and replace, here are some things many techs may not know.

Capacitors Don’t “Boost” the Voltage 

A capacitor is a device that stores a differential charge on opposing metal plates. While capacitors can be used in circuits that boost voltage they don’t actually increase voltage themselves. We often see higher voltage across a capacitor than the line voltage, but this is due to the Back EMF (Counter electromotive force) generated by the motor itself, not the capacitor.

Current Doesn’t Flow Through The Capacitor, Just in and Out of It 

Techs notice that the one side of power is connected to the C terminal or the side opposite the run winding. Many techs imagine that this power “feeds” into the terminal, get’s boosted or shifted and then enters the compressor or motor through the other side. While that may make sense it isn’t actually how a capacitor works at all.

A typical HVAC run capacitor is just two long sheets of really thin metal, insulated with an insulation barrier of very thin plastic and immersed in an oil to help dissipate heat. Just like the primary and secondary of a transformer the two sheets of metal never actually touch, but electrons do gather and discharge with every cycle of the alternating current.  For example, the electrons that gather on the “C” side of the capacitor never go “through” the plastic insulation barrier over to the “HERM” or “FAN” side. The two forces simply attract and release in and out of the capacitor on the same side they entered.

The Higher the Capacitance, the Higher the Current on the Start Winding 

On a properly wired PSC (Permanent Split Capacitor) motor, the only way the start winding can have any current move through is if the capacitor stores and discharges. The higher the MFD of the capacitor, the greater the stored energy and the greater the start winding amperage. If the capacitor is completely failed with 0 capacitance it is the same as having an open start winding. Next time you find a failed run capacitor (with no start capacitor) read the amperage on the start winding with a clamp to see what I mean.

This is why oversizing a capacitor can quickly cause damage to a compressor. By increasing the current on the start winding the compressor start winding will be much more prone to early failure.

The Voltage Rating is What it Can Handle, Not What it Will Produce

Many techs think they must replace a 370v capacitor with a 370v capacitor. The voltage rating displays the “not to exceed” rating, which means you can replace a 370v with a 440v but you cannot replace a 440v with a 370v. This misconception is so common that many capacitor manufactures began stamping 440v capacitors with 370/440 just to eliminate confusion.

You Can Test a Capacitor While the Unit is Running

You simply measure the current (amps) of the motor start winding coming off of the capacitor and multiply it times 2652 (on 60hz power 3183 on 50hz power) and then divide that number by the voltage you measure across the capacitor. For a full write up on the process, you can look here





The term “short” has become a meaningless phrase in common culture to mean “anything that is wrong with an electrical device”.

A short circuit is a particular fault that can mean one of two things in technical lingo.

Any two circuits that are connecting in an undesigned manner. This would be the case if a control wire had two conductors connected together due to abrasion. Like a Y and G circuit “shorted” in a thermostat wire between the furnace and the thermostat. This would result in the condenser running whenever the blower is energized.

A short can also be described as a no load path between two points of differing charges. This would be a traditional “short to ground” low voltage hot to common connection or a connection between legs of power without first going through a load of appropriate resistance.

Both of these conditions will result in something occurring that should not be occurring. Either something being energized when it shouldn’t be or fuses and breakers tripping or blowing or damaged components.

This is different than an Open circuit which is no path at all. So if a load has power applied and NOTHING is happening it is an open. If power is applied and breakers or fuses trip or blow or something comes on at the wrong time or order, that is a short

— Bryan


Back in the “good old days” controls were all analog and mechanical which simply means that they acted in a directly connected and variable manner based on a change in force. Both pneumatic (air pressure) or hydraulic (fluid pressure) systems are examples of mechanical, analog controls. When pressure increased or decreased on a particular device it signaled a change in in action on another device like a pump / valve etc…

In the HVAC/R industry, we still see these types of controls with a TXV being a common example. The TXV is controlled by pressures in the suction line, bulb and spring to set the outlet superheat. These forces are all mechanical without electrical inputs or specific “data points”. The feedback from these forces are in constant tension to output the proper amount of refrigerant to properly feed the evaporator coil.

Digital vs. Analog

As controls have changed from mechanical to electrical we now have systems that are controlled by analog electrical signals and digital electrical systems. Analog is simply a varying electrical signal (either voltage or amperage) that signifies changes in a system or device. A digital signal means data encoded into “digits” which can be communicated using many different computer languages, rules or protocols (these all mean essentially the same thing). In digital controls the “signal” can include a combination of voltage, amperage and On/Off changes to communicate between devices.

So what about 4-20ma?

When industry started to change over from mechanical to electrical they created a protocol (set of rules) that controls could use that would still function in a similar way to the old pneumatic controls. They decided that the range would be  4ma(milliamps) as the bottom reference of any sensor and 20ma would be the top reference. If you were setting up a sensor to indicate the fluid level in a tank you would set the bottom output to 4ma (meaning empty) and the top output to 20ma (meaning full).

In the case of a pressure transducer, you set the top range to the max rating of the sensor to 20ma and the bottom pressure reading to 4ma… you get the point.

ma controls are great because of their simplicity and ruggedness. You supply power to a “sensor” (Actually a sensor / transmitter to be exact) and based on the measurement the sensor reports to the transmitter that produces a milliamp signal. This signal is connected to an input on the control that measures the amperage and converts that to a reading.

Because amperage is the same at all points in the circuit the 4-20ma circuit is not impacted by voltage drop or interference like a voltage sensing circuit. Because the “bottom” of the scale is 4ma the control can also sense the difference between an “out of range” condition below 4ma and an open circuit.

The downside of a 4-20ma control is that each device requires it’s own conductor. In digital controls, many devices can be controlled by a single conductor set or “trunk” making it much easier to route, configure and manage complex controls.

Testing 4-20ma Circuits 

There are two different ways to measure milliamps. One is to use a special milliamp clamp called a “process clamp meter” that allows reading the amperage without disconnecting wires. These meters are expensive and it is unlikely that a typical HVAC/R tech will have one.

The more common way it to use alligator clips on a quality meter set to the milliamp scale and connect in series with the circuit. this means you will need to disconnect a wire or terminal and put your meter in the path. This is the same way we test microamps on a gas furnace flame sensing rod only using the milliamp scale.

— Bryan



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(5.5°K) DTD +/- 3°F(1.65°K)
Reach-ins  20°F(11°K) DTD +/- 5°F(2.75°K)
A/C 35°F(13.75°K)) DTD +/- 5°F(2.75°K)

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 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 commision 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




I get questions all the time about performing “load calculations” and “rules of thumb” as well as how to do it properly. This article isn’t about load calculation but the only good answer is to find a quality ACCA approved Manual J software and get used to using it.

You may have heard from others in the field that Manual J tends to “undersize” the equipment. If you are an engineer or designer you may be shocked at how “oversized” most equipment is when compared to Manual J. Like most things, the truth can be found somewhere in between and here are the reasons for this.

System selection is just as important as Manual J because if you don’t match the proper equipment to the BTU load or if you fail to consider the factors you can easily end up with a system that is undersized or that does not deal with the humidity load of the space.

Here are some common factors that contractors & designers fail to properly factor into their Manual J

Duct leakage rate 

We can guess, but the only way to really KNOW the rate of leakage is to do a duct leakage test like the one shown below by Corbett Lunsford.

Building Envelope Leakage Rate

The rate of leakage in and out of a structure is one of the most overlooked aspects of a load calculation. Once again, we can guess based on the age and construction type but the true leakage rate can vary wildly. The only true way to test leakage rate is by measuring it with a blower door.


Even insulation is often a guess in areas where you cannot access walls or portions of the attic. It takes a combination of experience and thermal imaging or other R-value measuring tools to truly calculate heat loss / gain through insulation.


This one is very challenging to calculate. You may have two homes with nearly identical construction, orientation, and layout, one with significant shade from trees and another without. This shade can represent a significant decrease in radiant heat transfer to the walls, windows and roof and will vary seasonally based on the season and various times of day. Shade is something you can factor in using common sense. While I wouldn’t suggest “under sizing” just because of shade, you can be sure that a well-shaded structure will have lower radiant gains which will have an impact in all seasons.


For every cubic foot of air  you move out of a building you are also moving one cubic foot of air into the building, either through a designed path, through cracks and gaps, or when a door or window opens. One way or another when you move air out you are also moving it in. It is always better to move that air into the building through a designed path where the air can be controlled, measured and likely treated (ERV, HRV, Dehumidification) instead of through cracks and gaps that can be in any number of undesirable places. In addition to this, there are new standards being enforced surrounding ASHRAE 62.1 & 62.2 that mandate mechanical fresh air be brought into all structures. This fresh air needs to be considered as to heat gains and losses both sensible and latent.

Because of all these factors and industry pressures I have found that design professionals have a tendency to underestimate heat gains and losses (on existing, untested structures) while contractors and field personnel tend to oversize equipment based on “experience” or “rules of thumb” and usually a combination of both. This happens because it is much more common for a contractor to get a complaint of a system “not keeping up” than humidity, or power consumption issues because the thermostat displays the temperature in big bold numbers while humidity and power are a bit more abstract.

Design professionals are under pressure not to oversize equipment by the industry (and rightfully so) but may not be fully aware of all of the “as built” conditions that exist.

But for sake of argument, lets say the heating season losses and cooling season gains have been perfectly calculated

We now bump up against some of the most common areas of misunderstanding by contractors and field staff which is properly matching the equipment to the load. here are some of the biggest mistakes.

  • Failing to cover both sensible and latent (humidity) loads in the cooling season
  • Using nominal tonnage of equipment to estimate capacity (no a 3-ton cannot be counted on to produce 36,000 BTU)
  • Looking at AHRI ratings to find capacity instead of at the manufacturer performance data
  • Oversizing / Undersizing either the heating or cooling side by considering one and not the other

In order to properly select equipment, it is recommended that you use ACCA manual S to ensure that you don’t miss any of the steps. ACCA has a great quick guide on system selection you can read HERE

Here are some tests you can apply to your residential design to help double check that your selection matches the design.

  • The sensible cooling capacity of the system you choose should not be more than 15% greater than the sensible heat gain of the space
  • The latent cooling capacity of the system must be equal to or greater than the latent load of the space
  • In heat pump applications with greater heating loads than cooling loads (common) the cooling system must not be more than 25% than the sensible cooling gain
  • The heat Pump + electric aux heat capacity should not exceed the heat loss by more than 15%
  • In the case of fuel-burning appliances choose the next size greater than the heat loss of the space

All of these need to take into account the manufactures specifications matching the load calculation conditions to the specifications of your equipment at the same conditions. While a furnace may produce the same heat output no matter the conditions, air conditioning and heat pump equipment output will change depending on indoor and outdoor conditions.

The final step is configuring the equipment to the proper air flow levels so that the sensible / latent capacity will match the design. If the system was designed for 400 CFM per ton then ensuring that the equipment is set to output that airflow is critical.

— Bryan


This article was written by Christopher Molnar, a licensed Florida mechanical contractor. While I’m not personally a practitioner of the “check static every time” doctrine, I certainly appreciate Chris and his passion for this topic. Thanks Chris!

Why Check Static Pressures on EVERY call 

You wouldn’t go to the doctor’s office and walk away without having your pulse and blood pressure checked. To do so would be malpractice. Your pulse and blood pressures are a sign of how your blood is flowing through your body and how well your heart is pushing that blood around your body. Measurements outside of the standards are cause for further investigation. 

 Why is it acceptable to walk away from any HVAC service call without checking the air pressures in the ductwork? I would suggest that not taking these pressures on each and every service call is malpractice and you are cheating your employer and customer by not taking these readings.


What is static pressure? The definition is the outward pressure of the air (without movement) in the ductwork. The number is either a positive or negative based on which side of the blower/fan you take the pressure on. The air on the supply side of the system is always positive pressure, in other words the pressure is greater than that of the air outside of the duct. Duct pressures are generally measured in Inches of Water Column in the US.

 The air on the return side of the system is always negative, or a pressure that is below the air pressure outside of the ductwork. Think about it, it makes sense. In order to blow air into a space the air in the supply duct must be above the pressure in the room we are moving it into. In order to draw air into the return duct, the air in the return duct must be under that of the room we are pulling from. Air will always move from an area of high pressure to an area of low pressure.

 What causes this static pressure? Ductwork is just a pipe, correct? So why does pressure build up in this pipe, shouldn’t it just “Flow”?

 The blower is designed to work against a certain amount of pressure in order to provide comfort to the customer and achieve peak heating/cooling efficiency. 

The less air that moves through the ducts and over the coil the greater the temperature of the air will drop and more humidity it will give up. However, we also must maintain a proper level of air flow so that we do not overheat a furnace, or freeze a coil. For air conditioning this is about 400CFM (cubic feet per minute) per ton in most regions. 

On the data plate or in the specs of every air handling device or funace you will find the “total external static pressure” that the device can handle. For example, on the nameplate above you will see that the total maximum static pressure is .5 inches of water column. So, what does this mean?

A technician will take two pressure readings using a static pressure tip. One will be in the supply duct above the coil in the case of a fan coil / airhandler or between the blower and coil on a furnace. The other will be in the return duct right before the blower compartment, or sometimes in the return plenum. 

The number in the supply duct will be positive. The number in the return duct will be negative. To find the total external pressures you will drop the sign (+ or -) and use the absolute value. So, a +.25 will become .25 and a -.25 will become .25 as well. Add the two numbers together as .25 + .25 equals .50 inches water column (.50 “w.c.).


To properly get these readings you will need a mimumum of one static pressure tip and a manometer, such as the Testo Smartprobe 510i that interfaces with a smart phone.

You want to see a measurement as close to, but not over the “maximum external static pressure” on the data plate. If the number is too low it most likely means that the blower is not operating at the correct speed, or that there is a leak in the ductwork that is preventing static pressure from building up.


Low supply static pressure: Look for duct leaks. Look for missing register covers. Look for proper blower speed. Check for proper cooling (if system has a cooling coil that should be wet). Finally, double check supply duct sizing.


Low return static pressure: Look for duct leaks. Look for missing filters. Look for proper blower speeds.
High return duct static pressure: Dirty filter? Blocked return grills? Insulation or something else stuck in the return duct. Collapsed return duct (if fiberboard or rusted steel ductwork).

 High supply duct pressure: undersized supply ducts. Closed registers and grills. Closed doors in buildings with central returns. Collapsed ductwork. Blower set at too high of a speed.

 Static pressure that is too low will affect latent capacity (moisture removal) on PSC motors in some cases. 

Static pressure that is too high will cause noise, excess wear on the blower motor, high amperages, cooling and heating inefficiency, and in many cases can pull the condensation off of the air conditioning coil causing leaks and damage. 

 We all learn in trade school to measure voltage, amperage, and the resistance of components. Most technicians understand what causes low superheat, high subcooling, and pressures. We understand what high amperages can do to a system. We know that motors will burn out if the amperages are too high. We know that a clogged condensate line can cause a flood. Taking the additional 5 minutes to take two static pressure measurements when the system is operational will allow you to do a quick air flow check and make sure that there is not something else happening that will impact system health, performance, and customer comfort. Not taking static pressure on each and every Heating or Air Conditioning service call should be malpractice.

— Chris

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