Cold Weather Heat Pump Charging

Checking the refrigerant charge of a heat pump in cold weather can be confusing for many technicians, at least partly due to the many different methods recommended by different manufacturers. Although there are many methods, most are described as heating performance charts or heating cycle check charts. A check chart is just that, a chart for CHECKING the charge during the heating cycle. It is not intended to be used for actually ADDING refrigerant. Why? Lower outdoor temperatures cause a lower evaporator temperature and pressure. Lowering the evaporator pressure increases the compression ratio, so the compressor capacity is reduced. Less refrigerant is being circulated in the refrigerant system because of the diminished compressor capacity at low outside ambient temperatures. The refrigerant that is not being circulated must sit somewhere, usually in the accumulator. For this reason, a charge which is adequate to maintain correct pressures and temperatures at 25°F may not be adequate at 35°F. How? Suppose that the unit was undercharged according to the check chart. If you add just enough refrigerant to bring it up to the required pressures, you have only added enough refrigerant for correct operation at that one condition. When the outside temperature rises, the unit will be capable of pumping more refrigerant. However, since you didn't add any extra refrigerant, there won't be any more to pump. The system will be undercharged. Just adding a little extra is not a great ida either because you really have no way to determine how much extra to add. This is why manufacturers say that the check chart cannot be used to CHARGE the system, only to check its operation. If the system is undercharged, the recommendation from many manufacturers is to recover the refrigerant in the unit and weigh in a total system charge according to the manufacturer's instructions. 

Thanksgiving 2011

Once again, Thanksgiving is here and I have so much to be thankful for its hard to know where to start. My family is doing well, my children are achieving and prospering, and my HVACR program has so many students that we have had to work really hard to meet their needs. All the Air Conditioning instructors at Athens Tech have really stepped up their game. Dennis Payton, Coleman Simmons, Gary Manley, Jack Parsons, Gene Smith, Bill Ely, and Kenneth Harris have done a stellar job this semester. I know that our students are fortunate to have the dedicated and talented instructors that I am fortunate to work with. We are coming to the end of our first semester since our system switched from quarters to semesters, and the transition has been smoother than expected.

I know that there are still far too many people without work and that the hard times have boosted our enrollment as people look to invest in their future. Our job as teachers is to insure that they have made a wise investment. We are preparing our students to answer the call when opportunity knocks. How many stories have you heard about someone being successful because they were in the right place at the right time? There is another part of their success that often goes unmentioned: they were prepared to answer when opportunity knocked. That is what students all across America are doing today – preparing for opportunities to come. Schools are bursting at the seams. We are planting the seeds of tomorrow’s success. I truly believe that a better educated, better trained pool of workers will propel our country into a time of prosperity in the coming years. Times will get better, opportunity will knock, and our students will be thankful that they were prepared to answer the call.

80% Furnace versus 90% Furnace

This week I would like to follow up on the 80% versus 90% debate. The primary argument for installing an 80% furnace is the lower cost of the equipment. However, a colleague of mine suggested that there is really not much difference in the installed cost because of the difference in the cost of the vent pipe. Standard B-vent is far more expensive than the PVC that is used in 90% furnaces. If the furnace is in a crawl space or basement, the 80% furnace must be vented through a chase or closet while the 90% PVC vent can usually be run sideways out through the wall. This also saves on labor. His argument is that the extra cost associated with installing a Category I furnace vent eats up the difference in cost between an 80% furnace and a 90% furnace. 

For a changeout, the math will favor an 80% furnace because there is already an existing vent. However, check to see if the vent is adequate. You may need to make improvements to the existing vent for the furnace to operate safely. Often, a fan assisted replacement furnace will use a smaller vent than the older natural draft furnace it is replacing. It is possible for the old vent system to be too big, causing condensation inside the vent which leads to rapid vent corrosion. Many older natural draft furnaces are vented into masonry chimneys, a practice that is usually discouraged when applying fan assisted furnaces. Several manufacturers require a metal flue liner for the masonry chimney, eating up most of the cost savings. Another common practice in older furnaces is the use of single wall vent connector. You really don’t want to connect a new fan assisted furnace in a cold location to a single wall vent connector, especially not an old one. I have seen single wall vent connectors rust through and fall on the ground in less than a year after installation of a new fan assisted Category I furnace. Again, replacing the old vent connector eats up some of your savings.  

It is true that there is still quite a cost difference between an 80% single stage furnace with a PSC blower and standard controls versus a 90% two stage furnace with an ECM motor and communicating controls, but the cost difference has more to do with the improved blower and controls. For a true cost comparison, both furnaces should have similar blowers, staging, and control systems.

Appliance Energy Efficiency Standards

The Federal Department of Energy, DOE, has issued a direct final ruling making on furnace and air conditioner efficiency. The regional efficiency standards ruling went into effect October 25. The country is broken into three regions: North, Southeast, and Southwest regions. A map can be seen at MAP. Read more about the ruling at RULING.

In a nutshell: in the Northern region furnace efficiency will increase while heat pump and air conditioning efficiency will remain the same; in the Southeastern and Southwestern regions furnace efficiency will remain the same while heat pump and air conditioning efficiency will increase.

The table below listing the changes comes directly from the final ruling.

Table I.1 Amended Energy Conservation Standards for Furnace, Central Air Conditioner, and Heat Pump Energy Efficiency

Residential Furnaces*
Product Class	National Standards		Northern Region** Standards
Non-weatherized gas	AFUE = 80%		AFUE = 90%
Mobile home gas	AFUE = 80%		AFUE = 90%
Non-weatherized oil-fired	AFUE = 83%		AFUE = 83%
Weatherized gas	AFUE = 81%		AFUE = 81%
Mobile home oil-fired‡‡	AFUE = 75%		AFUE = 75%
Weatherized oil-fired‡‡	AFUE = 78%		AFUE = 78%
Electric‡‡	AFUE = 78%		AFUE = 78%
Central Air Conditioners and Heat Pumps†
Product Class	National Standards	Southeastern Region†† Standards		Southwestern Region‡ Standards
Split-system air conditioners	SEER = 13	SEER = 14		SEER = 14 EER = 12.2 (for units with a rated cooling capacity less than 45,000 Btu/h) EER = 11.7 (for units with a rated cooling capacity equal to or greater than 45,000 Btu/h)
Split-system heat pumps	SEER = 14 HSPF = 8.2	SEER = 14 HSPF = 8.2		SEER = 14 HSPF = 8.2
Single-package air conditioners‡‡	SEER = 14	SEER = 14		SEER = 14 EER = 11.0
Single-package heat pumps	SEER = 14 HSPF = 8.0	SEER = 14 HSPF = 8.0		SEER = 14 HSPF = 8.0
Small-duct, high-velocity systems	SEER = 13 HSPF = 7.7	SEER = 13 HSPF = 7.7		SEER = 13 HSPF = 7.7
Space-constrained products – air conditioners‡‡	SEER = 12	SEER = 12		SEER = 12
Space-constrained products – heat pumps‡‡	SEER = 12 HSPF = 7.4	SEER = 12 HSPF = 7.4		SEER = 12 HSPF = 7.4

The increased standards will be phased in over a period of years. Compliance with the standards in the direct final rule will be required on May 1, 2013 for non-weatherized furnaces and on January 1, 2015 for weatherized furnaces and central air conditioners and heat pumps. Industry reaction has been split. AHRI helped to develop the standards and supports them. They want to avoid the confusion and cost that was associated with the 13 SEER efficiency change that gave the industry very little time to prepare. AHRI also points to the Canadian experience when they mandated 90% . Their sister organization, HRAI, has stated that there were very few problems with Canada’s implementation of 90% standards. Read the AHRI position at LETTER. 

ACCA opposes the standards, especially the 90% furnace mandate, concerned that the extra cost will drive customers to illegal contractors, who feel no compulsion to follow the rules and will cheerfully install equipment that legitimate contractors are not allowed to offer. read ACCA position at ACCA. 

HARDI also opposes the regional standards, citing the difficulty this will make for distributors who service two or more regions. Read HARDI position at HARDI. ACCA and HARDI also cite that there is currently no enforcement mechanism – no DOE police. Again, the problem is that contractors who don’t abide by the ruling achieve a competitive advantage over contractors who do. Regardless of how you feel about the ruling, this is now the law, and all of us in the HVACR industry should prepare ourselves. As far as the DOE police – be careful what you wish for! 

Southface

The Georgia State Consortium of Air Conditioning Instructors met on October 25 at Southface in Atlanta. Southface is a non-profit agency devoted to energy efficiency and sustainability in buildings and communities. In keeping with their ideals for sustainability, Southface does not allow bottled water, box lunches, or  syrofoam cups in their facilities. I must admit, I was a little put out with all the rules. “Who do these people think they are?” But then I realized, they are actually practicing what they preach. They know who they are. A sponsor, Technical Training Aids, graciously provided our lunch wthin the confines of all the rules and it was great. We gathered there to hear about what Southface could offer and tour their facilities. They provided an overview of the new residential energy efficiency code and its application in Georgia. All new residential construction in Georgia is required to pass a blower door test and all new duct systems must pass a duct blaster test for tightness. Those two requirements are the big ones for HVACR contractors. Further, the people performing the test must be state certified to perform it. They get certified by taking a relatively short course that lasts less than a week and then passing a performance test. So at least in Georgia, installing tight duct systems is no longer an “add on” for the better homes, but a requirement. Although there will certainly be many contractors that will regard all this as a pain, at least one in Athens sees it as an opportunity. Anytime he sells a high efficiency changeout, he quotes a duct seal and wrap upgrade. A seal and wrap consists of removing the old duct insulation, sealing the duct, re-installing the old insulation, and insulating over the old insulation. The duct blaster is used to measure the duct leakage before and after the job to document the improvement. So even if you don’t work in Georgia, you might consider promoting duct sealing and insulation. If your competitors don’t do it, that is a way to differentiate yourself from them. Most homeowners can easily understand that leaky ducts cost them money. For many systems, sealing and insulating the ducts will improve system efficiency even more than installing a new high SEER system.

Making Connections

Most curriculums, courses, and books are organized in a manner that resembles house construction. You lay the foundation and then add to that foundation as the material gets progressively more complex. The idea is to present the information in an orderly manner so that complex concepts can be understood. I believe that teachers have been using this general scheme for centuries. I also believe teachers have been frustrated for centuries when they discover that the reason the students don’t understand a higher level concept is that they really don’t make the transition from one conceptual level to another. The problem is that learning is really not too much like stacking bricks, but more like wiring circuits. We have to make connections to learn. It is not enough to present material in an orderly and logical fashion, we must also make connections between different pieces of information. Every chance you get, you need to show how one piece of information connects to another. The more mental connections the students make, the more likely they are to remember the information. Students sometimes ask how a particular piece of information is relevant. I believe what they are trying to say is that they don’t have anything to tie the information to. Random, disconnected facts are quite difficult to remember, and are not normally particularly useful. I like the Bing commercials where people blurt out a series of largely unrelated facts that are essentially useless because there is no logical connection between them. To students, I am sure our lectures can sometimes sound like one of these commercials. I try to connect ideas together – showing how one part of a system affects the others, how pressure is related to temperature, how resistance is related to current – you get the picture. Relationships and connections between ideas are just as important as a carefully crafted sequence of information. This is not to suggest that building a foundation of information is wrong, I would just use the analogy of a framework instead. A framework can allow connections in more than one direction. Understanding a few fundamental physics concepts provides a framework for understanding both refrigeration and electricity. Taken by itself, physics can be pretty dry stuff until you realize that it describes the world around us. Once the students start making connections linking concepts together the whole process can go viral – they start finding their own connections and asking their own questions. This can be a bit scary because you no longer have total control of the flow of information or questions, but it produces an energized, lively class. More importantly, when the students start connecting concepts they are taking an active role in their learning and are far more likely to retain the information.

Electric Heater Resistance - A Moving Target

Most people know that inductive loads such as electric motors draw more current when they start. Did you know that this is also true of most electric strip heaters? The resistance of a conductor increases as its temperature increases. This is true of the nichrome wire used in electric heaters. The heater resistance is less when the heating coil is cold than after it heats up. The increase in resistance causes a decrease in current. In short: an electric heater draws a little more current when it starts than after it heats up. The difference for an electric strip heater is nothing like the huge surge that a motor has, but it is measurable. For incandescent light bulbs, the difference is huge because the temperature difference between hot and cold is much greater. An incandescent light is basically a heater that gets white hot and gives off light. Lights don’t work for Ohm’s Law experiments because their resistance hot is around 10 times as high as their resistance cold. Many years ago I tried having students do Ohm’s Law calculations with lights – it just does not work. The readings were so far off, that the results did not make sense.  I tried to put the difference down to measurement error, but the readings were always similar. Screw in cone heaters work better, but if you put enough current through them for them to get red hot, their resistance will increase and your readings will still be a little off. At least they are in the ballpark. It is something of a catch 22. To measure resistance an ohm meter puts a small amount of voltage across a resistance and measures the current, and then determines the resistance based on the amount of current that flows through the resistance. The problem is that the resistance changes when different amounts of current flow through the same device, based on the temperature change of the device.  Remember that the resistance is turning the electricity into heat, and the heat changes the resistance of the device. Fortunately, the relatively small difference we see in electric strip heaters does not create a substantial problem for us, but it does raise questions when trying to demonstrate Ohm’s Law. If you want to reduce the amount of error introduced by the temperature change, just work with lower voltages and current levels. Use a typical electric strip heater, but only put 50 volts through it. The current and voltage are still measureable and the results will be much closer to what Ohm’s Law predicts. I know there are probably many skeptics reading this, so I challenge you to prove it to yourself. Take a heater, a variable voltage source, a voltmeter, and an ammeter. Read the current draw of the heater at several voltages ranging from just enough voltage to get a current reading to the full voltage the heater can take. Then calculate the resistance using Ohm’s Law by dividing the measured current into the measured voltage. You should see that the resistance is higher at higher voltages because the heater is getting hotter. Have Fun!