Month: October 2017


Air-to-water pool heat pumps are seeing more and more popularity in the climates that can support them over the more traditional gas and electric pool heaters we usually see.  While they definitely contain some familiar operating principles to an air-to-air heat pump, there are some striking differences in the way the heat is transferred as well as how the components are controlled and protected.


Heat Exchange in a Pool Heat Pump

Most residential and light commercial heat pumps are packaged systems, containing all but a few necessary components for it to perform its duty of heating the body of water it serves.  We have some strong similarities with an air-to-air system: a large air coil mounted on its perimeter, an outdoor fan to perform heat exchange over this coil, a compressor to pump refrigerant, and a metering device to control the capacity delivered to the evaporator.  On many, you’ll also find a liquid receiver and / or a suction accumulator to make up for the wide temperature loads they’re designed to handle.  The system gets interesting with the introduction of a water coil.

There are two different types of common water coils in use currently.  The first one we’ll go over is the coaxial coil.  This is a coil inside a coil, designed to rapidly exchange heat between the refrigerant and water we’re adding heat into.

The mainstay material for coaxial coils has been a unique alloy of copper (Cu), nickel (Ni), iron (Fe), and manganese (Mn), known as cupronickel (pictured above.)  It has all of the pressure characteristics of copper, but with the added benefit of being much more resilient to corrosion from a variety of sources.  Interestingly, all of the “silver” U.S. coinage is made with this same alloy.  Titanium (Ti), while less noble than copper, has also entered the market as an alternative to cupronickel with a lighter weight and greater corrosion resilience.  However, these coils are often larger in size given titanium exchanges heat less effectively than copper and is much more difficult to work.

Coaxial coils are fairly simple in their construction.  The outside of the coil carries refrigerant, while the inside handles the water.  These coils are designed to counterflow the two mediums in order to exchange heat rapidly.  The inside of the coil is also rifled, creating a swirling effect that helps this process along further (see the diagram below.)

An alternative heat exchanger that’s used is the tube in shell coil, which, just as its name implies, is a refrigerant coil inside of a water tank.  Counterflow makes an appearance here as well, passing the influent water across the effluent refrigerant.  Depicted below is a dome-style tube in shell titanium coil.  This particular coil is found in a water-to-water pool heat pump.  Note how the copper adjoins to the titanium using a ferrule just before entering the coil.


Safeties and Controls

Pool heaters are thermostatically controlled just as you would expect.  In most cases water temperature is measured with a thermistor inserted into a temperature well that contains a heat conductive paste to provide the most accurate measurement of the water flowing across it.  These are made of either a polymer composite, brass, or copper.  They also double as a safety measure on systems employing cupronickel as they will breach first should a chemistry imbalance present in the pool water, alerting service persons before a water flooded refrigeration system occurs.

Another unique safety we’ll discuss is the water flow switch.  This switch is akin to the pressure fall switches found inside induced draft furnaces.  They contain a pressure sensitive diaphragm connected to a normally open switch that closes when pressure is applied, thus informing the unit control when water flow is absent.  Just like air-over designs, a refrigerant coil with no water to transfer heat into can be severely detrimental to the compressor it serves.  High discharge pressures can result, leading to premature mechanical failures.  While a high-pressure refrigerant cutout may also be employed, this water flow safety acts as a first response to protect the system from catastrophic damage.  This will prevent the system from operating whether the pool pump is off due to scheduled downtime, a power interruption occurs, a pool filter becomes impacted, or any other condition that ceases water flow.

A majority of systems contain an internal water bypass that controls how many gallons per minute are allowed to flow through the condenser coil.  Too much supply water and the temperature rise will be too low; too little, and the temperature rise will be too high.  This is one of the most important contributing factors to efficiency in an air-to-water system.  These bypasses are usually preset by the factory without any adjustment required in the field.  Be aware that there will likely be an external bypass as well which can wreak havoc on the heater operation should it be tampered with.  Remember to check these first if the water flow switch is keeping the system off!

Similarly to air-to-air systems, air-to-water heat pumps will occasionally require defrost cycles during cold weather. This is achieved primarily by demand defrost with a temperature sensor affixed to the air coil.   Care needs to be taken with the placement of these sensors, as a quickly frosting circuit can produce short cycling of the equipment.  Pools and spas tend to lose heat very quickly, and recovery can become impossible with excessive defrost cycles during cold days.

Finally, many heat pumps will feature a method to pair multiple units to cycle on and off together to provide extra capacity for large pools.  This helps keep equipment wear to the minimum as all heat pumps will be operating in concert during a demand cycle.

In Conclusion

While pool heating can be daunting at first glance, they are relatively simple machines that are usually easy to service with some prior exposure.  The basic refrigeration principles all remain the same.  The next time a customer asks you why their pool is cold, don’t shy away from getting some hands-on experience.  You may find a niché at your company!

— Zach

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

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