Some manufacturers are starting to transition from copper coils to aluminum coils for both condensers and evaporator coils for various reasons. When making a decision to purchase a new system you need to inform yourself about the pros and cons of purchasing a system with aluminum coils or copper coils. Traditionally, copper was the chief choice to use to manufacture evaporator coils and condenser coils because of heat transfer rate, cost, pliability (easy to bend and swage), and because copper line sets are used to join split systems. The cost of copper has skyrocketed and that was a game changer. Many manufacturers have started looking at aluminum because the cost of aluminum is cheaper than copper and aluminum has some of the same benefits of the above mentioned properties that copper has. Continue reading “Copper Versus Aluminum Coils – Condensers & Evaporator Coils” »
There are some problems with an air conditioner or heat pump system that can be avoided with proper maintenance. Other problems result in a mechanical or electrical problem with the systems that needs to be resolved. Compressor overheating is a serious issue that can cause serious problems with your air conditioner or heat pump system. A compressor that overheats can still operate and provide effective cooling or heating but if the temperature inside the compressor rises too much then the oil in the compressor can break down and reduce the lubrication and cooling effect the oil has inside the compressor. This leads to possible mechanical failure and other problems that
will occur to the compressor and in the refrigeration system that results from compressor overheating. When the oil becomes overheated it changes chemically and other a lack of lubricating and cooling effects it becomes like an acid. This eats away at the windings and will cause compressor failure.
The check for excessively high temperatures is to take a discharge temperature on the discharge line at least 6 inches from where it leaves the compressor. Temperatures over 300° Fahrenheit means there is a serious problem that needs to be addressed.
What are the causes of compressor overheating and how to fix it:
High Head Pressure – This can be caused by dirty condenser coils, a bad condenser fan motor, a recirculation of air back into the coils, improper rotation of the condenser fan, non-condensibles (air, nitrogen or other gas), and an overcharge of the refrigeration system. Another thing found that can cause this is any extra heat source near the condenser such as a drier vent which can introduce extra heat into the system.
- Improper Voltage – This can be a power company issue that needs to be corrected by the power company especially high voltage but low voltage can be a problem in the circuits providing power to the compressor. Lose connections and corroded wires or terminal connections can cause low voltage problems that result in compressor overheating. This can also be the result of a power company transformer issue along with the high voltage problem.
- High Superheat – this can result from a low refrigerant charge, a restriction in the refrigerant piping or a faulty metering device. It can also result in a liquid line too close to a heat source such as hot water piping. In that case the liquid line needs to be insulated from the hot water piping or relocated.
Short Cycling of Compressor – this can be caused by a low charge or faulty metering device. Air flow across the evaporator coil can also result in this issue and can be from a dirty air filter or frozen evaporator coil. A bad capacitor can also cause the problem.
- Other Possible Causes – A failure of the internal overloads inside the compressor or bad compressor valves can cause issues with overheating. These problems can be difficult to resolve in hermetic compressors without replacing the compressor.
It is important to resolve this matter as soon as possible to prevent further damage to the system. Some of the problems can easily be resolved while others will require major surgery. Happy compressor troubleshooting and good luck.
It was a very hot day and I got a ticket to respond to a call for no cooling. The customer said there was ice on the air conditioner. The secretary instructed the customer to turn the air conditioner off so the ice could thaw before I got there to figure out the problem. It needs to be thawed before any air conditioner troubleshooting can occur. There are two main causes why the evaporator coil will freeze and cause ice on the air conditioner. One is decreased or no air flow and the other is a low refrigerant charge.
For low air flow the homeowner can possibly fix the problem depending on how serious the issue is. This problem can be because of dirty air filters for the air conditioner or heat pump. The problem is easily solved by thawing out the evaporator coil changing the dirty filter for the air conditioner or the heat pump system. Air conditioner or heat pump systems equipped with a low refrigerant pressure switch will not freeze as the low air flow condition will cause a drop in refrigerant pressure and cause the low pressure switch to trip the compressor thereby shutting off the compressor and preventing any freezing until the pressure increase closing the low pressure switch allowing the compressor to run again. People with air conditioners or heat pumps that have low pressure switches will likely need to call a technician but only after checking the air filters.
Other conditions that will cause low air flow include a bad blower motor, collapsed duct work, and a dirty evaporator coil. All three conditions will likely require the attention of a technician. The call I responded to on that day turned out to be a dirty evaporator coil. The customer failed to keep a filter in the system. The air handler was in a closet next to the kitchen. It turns out they kept the trash can next to the louvered door where the air handler was located. All the dust and dirt and some of the things that were thrown in the trash were found in the evaporator coil. That caused the reduced air flow and for the air conditioner to get ice on it.
Why does the air conditioner get ice on it when there is reduced air flow? The air in your home has moisture or humidity in it. Part of the process on air conditioning is to remove the moisture from the air. In air conditioning and heating engineers and engineering technicians use a psychrometric chart to determine the properties of air. This is the basis for calculating the size of air conditioning or heating system you need to for the load or demand. The moisture in the air contains heat along with air containing heat. The heat in the moisture in the air is called Latent Heat and the heat in the air is called Sensible Heat. Both forms of heat need to be removed from the air and that is what your air conditioner does. It removes the latent heat by dropping the evaporator coil below the dew point. When the evaporator coil is operating below the dew point the moisture in the air condenses. When moisture condenses it turns into a liquid and needs to be drained off through condensation piping or pump. This is why you see piping on your air conditioner that drains the water to the outside or see window units dripping water outside. It is the moisture in the air being drained outside after the evaporator coil condenses it. The heat is also removed from the air (the sensible heat) by being absorbed into the cooler evaporator coil. The refrigerant in the evaporator absorbs the heat and carries it outside leaving you and your home feeling comfortable.
When there is a lack of airflow the refrigerant temperature drops below freezing. The moisture in the air around the evaporator then begins freezing to the coil and any refrigerant piping leaving the evaporator going to the compressor (Technically the suction line). The solution to fixing the ice problem is to fix the air flow problem. The evaporator coil on that day was completely plugged up with dust and debris. When I arrived the ice was thawed and I quickly identified the problem. Because of how dirty the evaporator coils was I had to pull the coil from the air handler requiring a major operation on the air conditioner system. After removing the evaporator coil I took the coil to a car wash. I sealed off the refrigeration piping so prevent water or moisture from getting inside the piping. I then completely sprayed the evaporator with coil cleaner and got the nozzle for the car wash and spray the coil cleaned being careful to angle the spray as to not damage the coil fins. All the dirt and dust and trash was cleaned from the coil and the evaporator coil re-installed.
After it was re-installed everything was returned to normal. The air conditioner was started and it cooled the home as a normal air conditioner would cool a home.The customer was instructed as to the importance of keeping a clean filter in the air conditioner system. If you have ice on your air conditioner because of dirty evaporator coils it is likely because someone failed to maintain a good air filter in the air conditioner. There could possible be other reasons for ice on your air conditioner and it will most likely require the services of a good HVAC contractor. Before you call make sure you check the air filter. If it is dirty change it and thaw out the system by leaving the fan on but the air conditioner off for several hours.
Dirty Evaporator Coils and Ice On Air Conditioner
Installing a new air conditioning and heating system requires skill and knowledge of proper procedures in electrical wiring, controls wiring, and pipe work. On commercial projects a skilled electrician handles the electrical hook ups, a controls technician handles the control wiring, and a pipe fitter handles the pipe work including the refrigeration piping which can include chilled water and refrigeration pipe work.
Residential Installation Factors and Considerations
In residential installations two or three skilled HVAC technicians usually handle the complete installation except for running line voltage from a breaker panel to the air handler and the outdoor unit. By code that requires an electrician and an electrical permit (In many municipalities however code requirements vary). It is important that all the work be done correctly according to code and best industry practices following basic fundamentals. This is very important in many regards including correct installation, the finished job looking professional, and the reliability of the system for its operating life. This requires a skilled knowledgeable technician that can wear many different hats to get the job done right.
R-22 to R-410A Retrofit Factors and Considerations
When manufacturers began selling the newer R-410A refrigerant in air conditioners and heat pumps because of government regulations it forced HVAC contractors and homeowners to not only replace the condenser when the condenser failed but also the indoor evaporator coil simply because the old R-22 refrigerant coil and the newer R-410A refrigerant will not work together plus if the refrigerants mix then it will cause the compressor to fail prematurely. One systems uses a natural oil and the other system uses a synthetic oil. When the oil mixes it will create a toxic sludge that is very bad for the compressor. It is possible to reuse the old refrigerant pipe and possibly an old R-22 evaporator coil with a new R-410A condenser (if the coil matches the capacity and technical requirements) but the system needs to be flushed thoroughly to rid the old pipe and/or evaporator coil of all residual oil left in the system. Recommendations are that the old system be completely replaced simply to prevent cross contamination and failure of the new system. It is best to safe than sorry later. It will cost more to replace everything but the cost of replacing a new compressor that failed prematurely due to cross contamination is more costly than simply replacing everything. Replace the refrigerant pipe and the evaporator when retrofitting R-410A with an old R-22 system.
Other fundamentals for refrigerant pipe design and installation include:
- Ensuring that new system is clean and free of debris especially in the piping. When finished brazing the pipe joints always use caps or plugs on any piping exposed to the atmosphere. Keeping trash and moisture out of the system is very important and will save time later on when the system is permanently sealed and ready for evacuation procedures.
- Proper pipe support is essential in the design and installation of the piping. Lose piping will vibrate and move. This will eventually eat a hole in the piping leading to catastrophic system failure in an air conditioner or heat pump. Ensure equipment has spring isolators or rubber vibration isolators to reduce vibration in the line set and equipment.
- Do not install the piping where it will be subject to accidental tripping or piercing. Anything that can potentially damage the piping needs to be addressed in the installation and design. Protecting the refrigerant pipe from any future damage is essential and can also make the job look professional when complete. That will make the customer happy also.
- If running the refrigerant pipe for an extended distance and keep refrigerant pipe within the manufacturers recommended limits. Always keep in mind the risers and possible pressure drop in the refrigerant piping. Oil traps should be used as recommended by the manufacturer. Keeping liquid refrigerant from reaching the compressor is essential to protecting the compressor especially reciprocating compressors. Proper refrigerant pipe is essential and recommended by the manufacture. Refrigerant pipe sizing charts are available in the installation instructions. All these things must be factored into the design and proper installation of refrigerant pipe work.
Avoid solder joints when necessary using home run piping from the condenser to the evaporator when possible. After brazing is complete proper refrigerant pipe evacuation and pressure testing is necessary to purge all non-condensing gases from the refrigerant piping. Dry nitrogen should be used for the pressure testing and purging and then a proper vacuum of the system should be done. Pressure testing includes test that the system will hold a charge of nitrogen (300 to 400 p.s.i.g but follow manufacturers instructions for maximum pressure testing limitations – remember the evaporator coil pressure limitations also) for at least 15 minutes and the same with the vacuum pressure testing. A micron gauge should be used and a 500 micron vacuum should be held for at least 15 minutes with the refrigerant piping system in a vacuum. Proper pressure testing ensures no leaks in the refrigerant piping or the evaporator coil. It tests any and all connections between the condenser and the evaporator coil. When the refrigerant piping is pressurized with dry nitrogen it is good practice to inspect all the solder joints and any other connections such as metering devices, filter-driers, solenoid valves, and flare connections with soap bubbles to ensure integrity of the refrigerant piping.
- Proper charging methods need to be used when charging the system. Most condensers come pre-charged with enough refrigerant with enough refrigerant for the condenser and a refrigerant line set of 25 feet. Other condensers come pre-charged with dry nitrogen and have weigh charge instructions with the new unit. The weigh in instructions should be followed and additionally proper super-heat and sub-cooling charging methods should be done to make sure the final charge is correct. Additionally some manufacturers also include a pressure temperature chart for various outdoor ambient temperatures and indoor load temperatures. It doesn’t hurt to use all the tools available to ensure the system is properly charged. Many HVAC companies use senior HVAC technicians to perform start up of any new installations. The final sign off on the finished installation is this senior HVAC technician who goes over the final installation with a fine toothed comb to make sure everything is correctly installed and operating as designed.
- Finally, make sure to use all the proper and rated materials for the installation. Silver solder for brazing and only piping for refrigeration should be used. When in doubt about any technical details, installation methods, or quality and rating of materials always call the technical representative for the manufacturer.
Conclusion for Proper Installation and Design Practices
By following all the fundamentals and doing quality work the new installation will have reliability and keep working reliably for years to come. Proper installation also ensures you will get the efficiency the unit is rate at. Another reason why it is recommended people hire qualified HVAC contractors when purchasing and installing new equipment.
High Performance HVAC Refrigeration Piping Design & Installation Practices
For many years HVAC manufacturers used two compressors in their higher efficiency equipment to stage the air conditioner or heat pump for added efficiency. They would use a small compressor for the lighter loads or demands and then when the air conditioner or heat pump needed more demand the small compressor would shut down and the larger compressor would kick on to satisfy the demand. The controls would were often complicated and the equipment was difficult to troubleshoot when there was an air conditioner problem. There were all sorts of ways to accomplish staging including one manufacturer that would sell one whole unit that was actually two condensing units manufactured as one piece of equipment. The air handler for that condenser has a dual slab coil for each stage of the air conditioner. A two stage cooling thermostat would kick the second compressor when the demand called for additional cooling. Then new technology became available that changed how staging was done for higher efficiency models.
Then HVAC manufacturers began using scroll compressors and later an engineer figured out that the design of the scroll compressor can easily be modified to allow for a simple way to stage the compressor for low and high demand situations whenever it is needed. A standard single stage compressor will kick on when the thermostat calls for heating or cooling. Heating for a heat pump in heat mode and cooling for both the air conditioner and heat pump for the cooling season. The controls are not complicated and a single compressor can now be used for staging the air conditioner or heat pump. An engineer figured out that you can easily put a plate in the compressor. The plate has two holes drilled in it. One big hole for high demand and one small hole for low demand. A solenoid is used to shift that plate for whatever mode is called for from the thermostat depending on the demand. So now nearly every HVAC manufacturer is using the two stage compressor for staging in their higher efficiency equipment.
Life After Two Stage Scroll Compressors
There will come a day soon when modulating compressors are the king of the day just as two stage scroll compressors are now. The compressor will have an ECM motor controlling the speed of the compressor and the modulation will be somewhere from 25% all the way to 100% of capacity. The whole HVAC system of the future will be able to modulate based on demand. These types of systems are already available in commercial refrigeration and in mini-split air conditioners and heat pumps. The systems will be very efficient and save lots of money on utility bills which what the two stage scroll compressors do now. The modulating systems will offer more efficiency than ever in the future.
Depending on the type of HVAC air conditioning or heat pump system it is and the efficiency range of the system will depend on the type of metering device the system has installed by the HVAC manufacturer of the air conditioner or heat pump system. Lower efficiency HVAC air conditioner and heat pump models have fixed orifice types while higher efficiency systems have thermostatic expansion valves installed in the HVAC air conditioner or heat pump system. The thermostatic expansion valve is far more complex than the fixed orifice metering device as the thermostatic expansion valve metering device modulates the refrigerant flow based on the temperature of the refrigerant temperature in the evaporator coil. This allows a specific amount of refrigerant, based on demand, to be metered into the evaporator coil while the fixed orifice metering device allows the same amount of refrigerant to enter the enter the coil no matter the conditions or the demand. As the liquid refrigerant enters the metering device it changes temperature and pressure. A partial amount of the liquid refrigerant flashes into a refrigerant gas or vapor as it leaves the metering device and enters the evaporator coil.
Specific Types of Metering Devices used in HVAC Refrigeration:
- Thermostatic Expansion Valve – Also referred to as the TEV or TXV for short the thermostatic expansion is used in many air conditioning and heat pump applications including use in chillers for chilled water systems. TXVs respond to the temperature of the refrigerant leaving the evaporator coil or evaporator barrel. The TXV has a sensing bulb that holds a slight refrigerant charge inside the bulb. The TXV bulb is remote from the TXV and is attached to the TXV via a capillary tube or cap tube. As the temperature increases and decreases the refrigerant inside the bulb responds by expanding and contracting based on the temperature pressure relationship of refrigerants. As the refrigerant expands and contracts causes a bellows to move in and out which causes a piston to open and close precisely based on the leaving temperature of the refrigerant in the evaporator coil. This allows the TXV metering device to feed the evaporator coil the precise amount of refrigerant it needs to maintain a specific superheat. Because the TXV precisely meters the refrigerant the TXV is used in many air conditioner and heat pump systems that need a higher efficiency. A properly engineered and installed system that uses a TXV will only give the evaporator coil what it demands and nothing more or nothing less.
- The other type of expansion valve used in HVAC is the fixed orifice. The fixed orifice is simple and can be either a piston type or utilize a distributor that feeds capillary tubes which terminate in the evaporator coil. The piston type comes in various sizes and is interchangeable with the capacity or tonnage of the condensing unit. Always follow the manufacturers instructions for piston size if using the fixed orifice for HVAC refrigeration.
- Capillary tubes (cap tubes) are also used to meter refrigerant to the evaporator coil. These are considered fixed orifice since they hole in the tube(s) is a fixed size.
- There are other non-conventional types of metering devices used for HVAC refrigeration including the float type and the electronic type however these are not commonly used in standard HVAC equipment.
There are other types of metering devices used in HVAC vapor compression refrigeration but these are the most common types are used in HVACR in both the commercial market and residential markets. As systems grow more sophisticated and complex that is likely to change. Electronic types are the next big step in refrigeration used for air conditioning and heat pump systems.
The Evaporator Coil
Evaporator Coil – the evaporator coil is responsible for absorbing heat into the coils and the refrigerant. The evaporator coil is an essential component of vapor compression refrigeration. This is usually done as result of passing air or water over the evaporator coil where a heat exchange process takes place. Heat leaves the air or water (whatever medium is used) and is absorbed into the coil and the liquid refrigerant. As more and more heat is absorbed by the evaporator coil and the refrigerant again changes state from a liquid to a refrigerant vapor. By the time the refrigerant leaves the evaporator coil the refrigerant should be all vapor and ready to be received again by the refrigeration compressor.
The evaporator coil is typically inside the air handling unit or it can be installed in the duct work near the air handling unit. When the air handler blower turns on and the condenser is running refrigerant flows from the compressor through the condenser coils and then through the metering device. and into the evaporator coil. The refrigerant changes state in the evaporator coil and again in the condensing unit. In the evaporator coil the change of state is from a liquid to a vapor while in the condenser the change of state is from a vapor to a liquid. This is all necessary for the process of refrigeration to work properly. The typical evaporator coil is constructed of copper with aluminum fins surrounding the copper coils. The aluminum adds surface area to the copper coils and enhances the heat exchange rate making the coils more efficient.
Components of Vapor Compression Refrigeration – The Evaporator Coil
The condenser coil receives the high pressure high temperature refrigerant vapor from the compressor and immediately begins to remove heat from the refrigerant vapor. As the refrigerant vapor makes its way through the condenser coil more and more heat is removed and the refrigerant vapor changes state from a refrigerant vapor to a refrigerant liquid. While the liquid refrigerant changes temperatures from a higher temperature to a slightly lower temperature the pressure remains constant. As the refrigerant vapor leaves the condenser coil it makes it way to the metering device.
Most condensing units utilized in HVAC whether it is the commercial HVAC field or the residential HVAC field are constructed of copper with aluminum fins. The aluminum fins are mechanically attach to the copper tubes that make up the condenser coils or the piping that channels the refrigerant from the beginning of the condenser coils all the way to the end or exit of the condensing unit where the refrigerant makes its way to the metering device. The aluminum fins attached to the copper pipe enhance the heat exchange process in the air source condensing units. A few manufacturers use only aluminum coils in their condensing units to take advantage of the efficient heat exchange properties of aluminum. The aluminum coils also have fins on them and these are usually referred to as spine fin coils. The spine fins extend the surface area of the aluminum coil enhancing the heat exchange process for efficiency purposes.
Many condensing units also house the compressor and condensing unit controls. The condensing typically utilizes its own dedicated circuit from the main circuit breaker panel. In a split system the condensing unit will have a line set or refrigeration lines that run from the condensing unit to the evaporator coil inside the building. The condenser is an essential component of vapor compression refrigeration.
Components of Vapor Compression Refrigeration – The Condenser
In a typical HVAC air conditioner or heat pump there are four major components that make the process of refrigeration work. Basically, this is how air conditioners work. And how heat pumps work. The first component in the process is the compressor or the heart of any air conditioner or heat pump system.
HVAC Refrigeration Compressor – The HVAC refrigeration compressor receives the refrigerant vapor from the evaporator coil and compresses the refrigerant vapor. The vapor enters the refrigeration compressor at a low pressure and a low temperature and leaves the refrigeration compressor at a higher temperature and a higher pressure. The lower temperature refrigerant vapor is responsible for providing cooling for the refrigeration compressor and it also carries with it a small amount of oil which migrates from the compressor throughout the system. After the refrigerant vapor is compressed it is discharged to the condenser coil. The compressor is an essential component in the vapor compression process of refrigeration.
It is necessary for the evaporator to supply the compressor with vapor refrigerant as it is impossible to compress a liquid. Compressors that receive liquid refrigerant can fail mechanically. In heat pump applications the system is equipped with a refrigerant accumulator. The accumulator allows liquid to boil into a vapor before entering the compressor thereby preventing any liquid refrigerant from entering the compressor. The term liquid slugging is used for compressors that receive liquid refrigerant. A properly installed, maintained, and engineered air conditioner or heat pump system will never experience liquid slugging so it is especially important that qualified personnel install and service air conditioning and heat pump systems.
Components of Vapor Compression Refrigeration – The Compressor
Refrigeration Pressure Switches
Richard – What are refrigeration pressure switches used for in refrigeration circuits and how do refrigeration pressure switches protect the system from further damage?
Thanks for the question and for being a reader of High Performance HVAC! First there two types of basic refrigeration switches in the typical high grade residential system. I say high grade because not every refrigeration system in air conditioners and heat pumps have refrigeration switches to protect the refrigeration system.
High Pressure Refrigeration Switch – The high pressure switch will kill the compressor circuit if the pressure exceeds the rating on the pressure switch. These pressures will vary depending on the type of refrigerant the air conditioner or heat pump uses in the system. On some air conditioners or heat pumps this switch is manually reset only simply because if it trips there is definitely a problem that needs to be resolved before the system should run again. On Rheem and Ruud heat pumps and air conditioners the reset button is usually red on located near the control panel. A simple push of the switch will reset the switch and the operation of the air conditioner should return to normal until the pressure exceeds the rating of the switch. Things that can cause the pressure switch to trip include:
- Bad condenser fan motor
- Coils plugged up with dirt and debris
- Fins on the coils bent
- Blockage in the refrigeration system
These issues, if they exist, should be taken care of before before the switch is reset. The high pressure switch prevents the system from operating at extremely high pressures.
Low Pressure Switch – The low pressure switch is typically an automatically reset switch that will reset itself when the pressure on the low pressure side of the refrigeration systems returns to normal or above the setting on the switch. When a refrigeration system operates at lower than normal pressures the system can freeze up with the freezing beginning at the evaporator coil and working its way back to the compressor via the suction line. Freezing of the refrigeration system is bad and lead to liquid refrigerant slugging in the compressor which can lead to compressor failure. Things that can cause the low pressure switch to trip include:
- Low refrigerant charge from a refrigerant leak
- Blower motor failure in the air handler
- Plugged evaporator coils or a blockage of air flow
- Failure of a refrigeration component in the refrigeration system
Again, if these issues exist in the refrigeration system they need to be addressed to prevent the low pressure refrigeration switch from tripping.