Air conditioning and heat pumps

Here are answers to frequently asked questions about air conditioning.

How air conditioners work

Q: How does an air conditioner (AC) work?

A: Air conditioners perform two basic functions: heat removal and moisture removal. Even in Arizona, we have a monsoon season with higher-than-normal humidity levels. The lower the humidity level, the more comfortable you will feel at a given temperature. As your warm indoor air is drawn up through the filter, it passes over a very cold coil whereby the heat and moisture are removed. If you've ever noticed a PVC pipe running off your roof that drips water, that is the moisture removed from your home.

Q: Is it more economical to operate the fan on my air-conditioning unit continuously or just turn on ceiling fans in the rooms in use?

A: Unless you are using an electronic air filter that requires a continuous stream of air, you're better off setting your unit's fan on "auto" and using ceiling fans in occupied rooms.

Q: To save on cooling costs, is it recommended to shut the air-conditioner vents in rooms that are unused and closed?

A: If designed correctly, the air delivery system in your house has been sized according to the flow requirements of the unit and each room. Closing off vents can create a pressure imbalance within the system that can reduce the effectiveness of your cooling system. By closing off registers, the increased pressure within the ducts can also cause increased duct leakage.

Q: Can I allow the upstairs to heat up even more during the day and cool the downstairs less at night?

A: Anytime you increase the temperature on your thermostat, you save money. When a unit first turns on, it takes seven to 10 minutes of continuous operation to reach maximum efficiency. The longer it runs after reaching peak efficiency, the better it is for the unit. When you return home at night and the unit is forced to run longer to bring the temperature down, it is running at peak efficiency. If you are on the SRP Time-of-Use™ Price Plan , and run your unit during the off-peak period, you'll be taking advantage of lower-priced electricity.

Q: Is it more energy efficient for the air conditioner to come on for short bursts of cooling (running for five to seven minutes at a time) or to come on and stay on for longer periods? How about the wear and tear on the unit as well?

A: A heat pump's job is two-fold. Not only does it remove the heat from your home, it also must remove the moisture from the inside air in order for you to feel comfortable. When the unit first turns on, it will take about seven to 10 minutes for the unit to get to its peak efficiency and remove the moisture from your home. By allowing a unit to run five to seven minutes, you're not getting the most out of the system. The starting and stopping is also hard on the system.

Q: Should I leave interior doors open or closed during heating and cooling?

A: Because the air-conditioning systems in our homes are closed-loop system, they are designed to move a specific amount of air throughout the house. Closing bedroom doors causes the air pressure in those rooms to increase. The air-conditioning unit pushes air in but cannot get it back out, which causes the unit to force conditioned air out of the bedroom under the door and through cracks or leak around the windows, wall plates and ceiling fixtures and draw in hot attic or outside air through similar cracks and leaks in the rest of the house.


Q: We are new to Arizona and are wondering how to configure our air conditioning for summer. We have two units, one upstairs and one down. How do you recommend we set them up for comfort and energy savings? Should they be set on different temperatures? Do we redo the settings at night? What temperatures do you recommend?

A: Because heat rises and extra heat gain exists on the second floor because of the windows and attic, the upper level will require more cooling than the lower level. To maintain a constant temperature in your home, set each thermostat at the same temperature. However, to reduce energy costs, you can maintain a comfortable temperature on the floor you are occupying. If you're not using the upstairs, set the second-floor thermostat a couple of degrees higher. Reverse the process at night when you're upstairs.

Q: Is it better to set the thermostat to a high temperature (85° to 90°) while at work all day and cool the house (two-story, approximately 3,200 square feet) when you get home to a comfortable temperature (approximately 80°), or to keep the thermostat just a degree or two higher during the day so that the cooling unit doesn't have to work as hard during peak hours to cool the house?

A: You will save money anytime you can increase the temperature on your thermostat and cause the unit not to run. The old myth that says you will spend more energy bringing the temperature back down than you would have spent just leaving the thermostat alone is just that, a myth.

Q: Does setting the fan switch on my thermostat to the "on" position cause air imbalances in my home?

A: Assuming there are no interior doors closed in the home, there should be no air imbalance caused by running the air-handler fan. However, in typical Phoenix construction, with a central air-conditioner return, you can get significant pressure imbalances when interior doors are closed. Because the door undercuts are not large enough to allow for sufficient free flow of air, it is common to create imbalances.

Q: If all things are equal, is it better to set your thermostat at a certain temperature all the time or use a programmable thermostat in conjunction with the Time-of-Use Price Plan?

A: Using a programmable thermostat to control the operation of your air conditioner will provide you the greatest savings on the Time-of-Use Price Plan . Not only will scheduled operating schemes reduce the unit's operating time during on-peak periods, but they also will increase the run time of the unit during off-peak periods, and this increased run time will allow the air-conditioning unit to run more efficiently.

Q: I want to replace our thermostat with a programmable one. Which do you recommend?

A: Rather than selecting a certain brand, make sure you select the right programmable thermostat for your system. There are different models for single-stage heat pumps, multistage heat pumps, straight air conditioning, air conditioning with gas heat, etc. Ask your service contractor what type of system you have and what model thermostat you need. After you know what model to get, determine your programming requirements:

  • 5-2 programming maintains two temperature schedules — one for weekdays and one for weekends.
  • 5-1-1 programming provides the added flexibility of setting different programs for Saturday and Sunday.
  • Seven-day programming lets you have different settings for every day of the week.

Air-conditioning size and rating

Q: I am buying a new air conditioner and have the option of purchasing a system that uses R-22 or a system that uses R-410A refrigerant. Will R-22 refrigerant be available in the future?

A: Even though R-22 will not be manufactured after 2010, the supply will last well beyond the life of a new system installed today. The efficiency of the two products is similar. However, R-22 tends to be more stable. R-410A is a mixture of refrigerants that has to have the proper amount of each to be efficient. Any leaks in the system could cause an imbalance and reduce the efficiency of the refrigerant.

Q: What is variable speed?

A: Variable-speed blower motors are designed to provide greater comfort through reduced initial air velocities and noise. When the unit first turns on, the blower operates at low speed, which not only provides less noise than a single-speed blower, but also allows the compressor and coil to ramp up before the unit begins moving large volumes of air through the system.

Q: What size air conditioner should I install on my 2,000-square-foot house?

A: It is important that new or replacement equipment not be sized by "rule of thumb" or by the existing equipment. The only accurate way to determine the correct capacity of heating and air-conditioning equipment for your home is to have a heat-load calculation performed on your home. This will give your dealer the heat gain and heat loss design data for your home. These figures can then be used to select the appropriately sized cooling or heating system for your home based on equipment performance data: The SRP Certified Contractor Program connects SRP customers with heating and cooling contractors that are able to perform Manual J calculations, which will determine the correct sizing for a new air conditioner or heat pump.

Q: What is SEER and ton?

A: SEER (Seasonal Energy Efficiency Ratio) is a measurement of a unit's efficiency, and ton is a measurement of a unit's size.

Q: Could you please let me know what SRP recommends on how many tons of air conditioning you should have per square footage of home?

A: It is important to make sure your new unit is properly sized. The best way to determine the proper size is to request an evaluation by an SRP Certified Contractor. SRP Certified Contractors perform Manual J or equivalent load calculations on installations of replacement and new heat pumps. A Manual J calculation determines the size of the air-conditioning unit required for your home.

Heat pumps

Q: How does a heat pump work?

A: Heat pumps take heat from the outside air and move it inside the house. The mild winters in Phoenix provide plenty of heat in the outside air that you can use. In summer, the heat pump moves heat from inside the house to the outside, providing efficient cooling.

Q: We have a heat pump that was installed in 1998. When installed, the thermostat was set to take into account a 1° temperature difference (more frequent cycling on/off of unit). We've recently been trying a 2° temperature difference (less frequent cycling on/off). Which is best for the most efficient operation of our heat pump?

A: When a unit first turns on, it typically takes about seven to 10 minutes of continuous operation before it reaches the steady state and highest efficiency. The longer the unit runs after it reaches its steady state, the longer the unit will be operating at its peak efficiency. Anytime you can reduce the number of cycles and increase the run time, you will save money and increase the comfort level of your home.

Q: Does the design of new residential heat pumps cause lights to flicker in the house?

A: New high-efficient heat pumps often cause lights in homes to flicker. The motors used in high-efficiency heat pumps draw more in-rush current (amps) when they first start (compared with older, less efficient units). This will cause the voltage to drop at your service panel for a fraction of a second. This can sometimes be observed as flicker (dimming of the lights).

Q: With the commercialization of variable-speed heat pumps, have the benefits of a programmable thermostat been reduced or eliminated? It seems to me that with variable-speed equipment, the correct operating strategy would be to set the thermostat at one setting and let the unit choose its own speed instead of using a "set up, set down" strategy that could cause the unit to run at high speed more of the time.

A: You would get better humidity control by setting the thermostat and leaving it. However, if the contractor oversizes the unit, there may still be a reason to set the temperature higher while you are away. Correct system sizing still has to be the No. 1 issue, and then nearly everything else will fall into place.

Air filters

Q: Do high-efficiency filters harm my air-conditioning system?

A: The use of high-efficiency filters won't necessarily harm your air conditioner; however, they can cause the unit to pull hot attic air into the ductwork, reducing the overall efficiency of the system. By adding a high-efficiency filter to your system, you add resistance to the air moving across the filter. Because the fan in your system is rated to move a certain amount of air, it will get this air from anywhere it can. If the added resistance of the filter makes it easier to draw air into the ducts through leaks in the ductwork, then it will do so.

Aside from the added resistance of a high-efficiency filter, this scenario is exactly why it is so important to replace your filters on a regular basis. As the filter becomes dirty, the resistance increases and the unit begins to pull more and more air in through the leaks in the ductwork. If your ducts are tight and in good shape, the use of a high-efficiency filter is fine. However, to avoid possible airflow problems, I recommend the 3/4-inch to 1-inch pleated filter that usually costs $4 to $6.

Q: I have never changed my filters. Every time I check them, they look like the day I bought them. Is that good? My house is 2,400 square feet, and my electric bill is subnormal.

A: Never having to change your filters is unusual, especially with Arizona's fine dust. If you are using very inexpensive filters, they may be too loose to catch any of the fine dust. This allows the dust to accumulate on the unit's evaporator coil, which could cause problems down the road. I recommend trying the 1-inch pleated filter (typically $4 to $6). More expensive filters may be too restrictive and could cause problems for your air conditioning unit.

Air-conditioner maintenance

Q: How do I balance the air being supplied to each room of my house?

A: The only accurate way of determining how to set your supply registers is to have a contractor complete a load calculation on your house. As part of this process, the calculation software will identify how much air should be delivered to each room. Once this is known, the contractor will measure and adjust the amount of air being delivered at each register. The "do-it-yourself" approach is to experiment with different settings over the course of a couple weeks until you get an even comfort level throughout the house.

Q: Are there advantages to shading your outside air-conditioning unit if you provide for proper air circulation?

A: Shading the coils of an air-conditioning unit has the potential to modestly reduce energy demand. However, improper shading techniques can result in a net increase in energy consumption by blocking proper airflow or trapping radiant heat. In many cases, a simpler approach is to plant trees strategically around a building to reduce indoor cooling loads or pursue other, more surefire air-conditioning efficiency upgrades.

Q: My landscapers built a three-sided wall around my ground air-conditioning unit in the backyard, and I am concerned that there may be restricted airflow to the unit. The wall is made of concrete block covered with stucco and painted on the outside. It is as tall as the unit and surrounds it on three sides. There is a break of about 24 inches on one side, and the wall is built about eight to 12 inches from the unit itself. How will this affect the life of the unit, cost to run, etc.? What options do you suggest to vent the wall ,or should I totally remove it?

A: The outdoor unit must be positioned a minimum of 12 inches from any wall or surrounding shrubbery to ensure adequate airflow. A 30-inch clearance must be provided in front of the control box (access panels) and any other side requiring service access to meet National Electrical Code. The top discharge area must be unrestricted for at least 5 feet above the unit.

Energy costs

Q: How can I estimate how much I'm spending on air conditioning?

A: To do this, take your March bill, which is typically a month when you do not operate either cooling or heating equipment, and subtract it from your summer bills (May through October). After the subtraction, what is left is an estimate of how much you spend on air conditioning. You can retrieve your past bills via My Account.

Find an SRP Certified Contractor, or call the Arizona Electric League's Heat Pump Council at (602) 248-7767 or the Energy Management Council at (602) 266-7283 for a referral.

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