The pressure is on
By IAN MCTEERCooling
On a much smaller scale, a far more practical thing (I thought as a young technician) would be to include a set of factory-installed pressure gauges in every residential condensing unit. Consider the benefits of pre-installed pressure gauges:
• Fewer worries about refrigerant release to the atmosphere
• Prevent the introduction of contaminants like air, moisture, acid, and junk from other systems
• Likely yield more accurate information as some field provided gauge sets are in rough shape and the data is suspect
• Easier access to information promotes better practices such as proper commissioning and data collection
WHERE ARE WE TODAY?
I do not recall ever seeing a residential condensing unit sporting a set of factory installed pressure gauges. While it may have happened, the reality is that manufacturing cost versus benefit (can you sell it?) typically determines what features comprise a finished product. Since only the technician seems to benefit in this case, a salesperson cannot justify the extra cost of a unit so equipped to an end user who believes the unit will never break down anyway. However, some progress has occurred on the data collection front in the last few years.
HVAC manufacturers are producing outdoor units with advanced controls such as pressure transducers, temperature sensors and circuit boards that monitor these devices and report errors to a thermostat or computer. Admittedly, such units fall into the “high end” category with a hefty price tag. These products also have specialized sales, installation, commissioning and troubleshooting requirements so contractors must have the necessary training before attempting to sell them.
PRESSURE SWITCHES VS PRESSURE TRANSDUCERS
Pressure switches have been used in residential cooling and heat pump systems for many years. A high pressure switch is mandatory in an R-410A refrigeration system. Typically, this switch is wired in series with the 24 volt contactor so that a dramatic increase in system pressure will open the switch and break the electrical circuit to the contactor, thus stopping compressor operation. Once the compressor stops, system pressure will fall to a pre-determined point allowing the switch to close, thus restarting the compressor. Some systems have a low pressure switch used to prevent compressor operation due to loss of refrigerant or problems with the air handling side.
Pressure switches are “dumb.” A high pressure switch has no way of “knowing” that the condenser fan motor has failed and the lack of air flow over the outdoor coil is preventing the system from rejecting heat. Excessive heat, or a massive overcharge, causes the switch to do its duty and open the 24-volt circuit, thus protecting the system from catastrophic failure. Because system pressure will continue to rise and fall when the condenser is compromised, the compressor will cycle on the pressure control until the problem is properly identified and repaired. When a safety control operates continuously, it will eventually fail causing an expensive repair. (See sidebar notes below on high pressure switch replacement.)
A pressure transducer is a component that can detect fluid pressure then produce an electrical (or mechanical or pneumatic) signal related to the pressure. Pressure transducers use several types of sensing strategies: piezoresistive, strain gauge, magnetic, resonant and capacitive are typical. A capacitive transducer, also known as a variable capacitance transducer, is used by at least one HVAC manufacturer in premium residential air conditioners and heat pumps. This type of transducer uses two parallel metal plates separated by a dielectric material (such as air) so that the distance between the plates is variable. When acted upon by the fluid being measured, the distance between the plates will change causing the value of the capacitance to change. Since this capacitance value can be measured and compared against a known quantity, it is easy to directly measure the value of the input pressure. The transducer will report its measurements to a control board using a computer chip programmed with the manufacturer’s algorithm, which, in turn, tells the machine what to do with the data. A typical capacitance transducer is very accurate: expected accuracy is +/- 1 per cent within a temperature range of 55F to 130F. Outside of that range the transducer can maintain two per cent accuracy.
INTERPRETING THE DATA
On machines so equipped, the pressure transducer’s data can be used in several ways. Working together with a temperature sensor, the transducer can spot trends like increasing or decreasing pressure, superheat and subcooling. So instead of simply cycling a compressor on and off, the system controller will prevent system operation and send an alert via a flashing LED, or perhaps send an error code to a remote thermostat. Some systems will then send an e-mail or text message to the homeowner alerting them that the room temperature is trending out of range. The same combination of controls can help an installer to properly charge a system or to verify the existing charge is correct. It is also possible for a technician to measure a pressure transducer’s output to the control board using a DC volt meter. The measured voltage can be cross referenced with a chart of pressure equivalents provided by the equipment manufacturer. Thus, an external connection to the refrigeration system is not required: no refrigerant loss, no oil loss and no contamination.
However, the truth is this: current economic conditions combined with a largely 13 SEER market means this kind of control refinement will not find its way down to entry level conventional spilt air conditioner and heat pump equipment in the near future. A transducer is a relatively inexpensive part but you must also consider the cost of its “delivery system.” For a manufacturer the cost of related components like a control board, power supply, refrigeration system access fittings, wiring harnesses (not to mention soft costs like production line task times, assembly line operator training, engineering time to design, test, and implement control algorithms, and technician training) translates into more expensive equipment. Additionally, providing features that allow data to be monitored and reported through the internet will push the installed cost beyond the reach of the 13 SEER market. Darn, I really want pressure transducers.
WHAT TO DO?
The simple answer is: try to sell more premium equipment. For many people technology is an important convenience and its benefits should be part of the kitchen table discussion. No need to dwell on the aspect of utilizing such technology to forewarn about potential breakdowns. The important idea is this kind of equipment will be using two-stage or variable refrigerant flow compressors, electronic expansion valves, variable speed fan motors, automatic commissioning assist, and complete system monitoring. All these refineme
nts help to provide maximum comfort at the lowest possible operating cost.
Technicians: since electronic monitoring is not coming to the garden variety residential split air conditioner anytime soon, your best course of action is to avoid accessing the refrigeration system unless absolutely necessary. When commissioning a new system or when your investigation tells you a system analysis is necessary, I highly recommend the use of a more sophisticated gauge set known as a “refrigeration system analyzer.” This tool typically features a four valve manifold with a ” vacuum port, electronic pressure gauges (transducers), and two live temperature monitors. An analyzer provides superheat and subcooling data for a variety of refrigerants. In my view there is no easier way to gather this important information. Whatever your gauge set: handle it carefully, check it regularly for accuracy, test for leaks, and keep the internals clean. Valve rebuild kits are available from the manifold manufacturers. Test and charging hoses must be in excellent condition otherwise your safety is at risk so treat yourself and your manifold to a new set of hoses this year.
On a service or maintenance call there is nothing more important than first inspecting the air handling system with a detective’s eye and a bright flashlight. Filters, blower wheels, secondary heat exchangers and evaporator coils must be clean. When the system has operated for at least 15 minutes in cooling mode, use an accurate wet bulb thermometer to take the entering and leaving wet bulb and dry bulb air temperatures. A convenient smartphone app I like to use is called HVAC Psych, it quickly converts wet bulb temperature to enthalpy. The difference between air entering and leaving enthalpy is called delta enthalpy (H). Plugs in the basic air conditioning formula:
HT = CFMT x 4.5 x H
For this formula to be useful in the field, accuracy is paramount. Actual airflow must be known, otherwise the formula is completely useless. Suppose a two-ton system (nominal 24K Btuh) was moving 786 cfm with a H of 6.5 Btu/lb. Thus, 786 x 4.5 x 6.5 = 22 990 Btuh. I think this system is working well. In view of my results using nothing more than an accurate thermometer there is no need for more invasive testing. Emerson has a new iPhone App called Check & Charge including a “Proper Airflow Range” calculation that I am interested in evaluating this summer.
Advanced technology has a way of quickly becoming the norm in our industry so it is reasonable to say that widespread use of electronic monitoring controls is just around the corner. Then again, there may be a day in the future when a young technician says to me, “How come I still can’t read suction pressure on-line?” I wonder if that day will be before or after the Leafs win the Stanley Cup again. <>
High Pressure Switch Replacement
Test Point (see Figure 1) – pressure transducer reports to this plug on the control board. Technician tests between +VP pad and Test Point Common (not shown) with DC volt meter. DC voltage reading is compared with a chart to determine system head pressure.
Suction Pressure transducer (Figure 2-foreground) along with low pressure cut-out switch.
Some R410A high pressure (HPS) safety switches are brazed into the system (see Figure 3 ), others may have threaded fittings. Should the HPS ever require replacement, system charge must be recovered first. Never put a secondary valve like a Schrader or ball valve between the switch and the system. Failure of the secondary valve means the safety switch would not operate and catastrophic failure will occur.
Periodically check gauge accuracy (see Figure 4):
• Place a cylinder of the appropriate refrigerant in a room with stable temperature for at least four hours
• Then, with an accurate thermometer, measure the temperature of the air around the cylinder
• Pressurize the gauge set
• Compare the pressure reading with a temperature/pressure card or an App like Emerson’s PT-Pro
• At 72F ambient, pressure gauges should be reading 207 psig
• Follow gauge manufacturer’s instructions to reset indicator needles as necessary