Air Conditioning

How do I know what size air conditioner to get?

To exactly size an A/C unit for a specific area requires a heat load calculation which is a bit complicated and is usually made by a licensed HVAC contractor who is experienced and qualified to make these calculations. A quick and easy rule of thumb to use for sizing an air conditioner for a room or house is the square foot method. This formula is used only for a quick approximate and is based on certain guidelines regarding insulation R values, shading, ceiling height, construction materials, windows, doors, purpose of use and occupancy. In layman’s terms 1 ton of cooling capacity is equal to 12,000BTU of cooling capacity and 600 square feet of residential living space requires 1 ton of cooling capacity based on adequate insulation, 8′ ceiling height and tight windows and doors. To calculate your requirement using this method get the total square footage of living space and divide that number by 600. The answer is the number in tons of air conditioning capacity required. To convert tonnage to BTU’s simply multiply by 12,000. This general rule of thumb method can be used for approximate sizing of both central and room air conditioning equipment.

Which manufacturer makes the best air conditioner?

Air conditioning is a matured technology so popular brands such as TRANE, AMERICAN STANDARD, YORK and LENNOX would be your best bet. Most other brands use parts made by the same manufacturers. The main considerations then would be the price, warranty, attractiveness, noise, etc. Some manufacturers offer a 5 year warranty on all parts while others offer only 1 year. Some offer 10 years on the compressor while some offer only 5 years. Some units are straight out ugly and will be of no compliment to your landscaping. Whatever you decide. You may buy the best system in the world but if it is not properly installed, you will actually be buying nothing but a big headache for years to come.

Why is my unit icing-up? Why am I getting such little air flow from my registers?

There are several things that can cause frost on your coil and/or reduced air flow. Anything that restricts the airflow through the inside unit will cause frost. As the frost builds up on the coil, the airflow becomes more and more restricted making the condition worse. When the frost is also on the outside pipes next to the compressor, you have a situation where damage to the compressor can occur.

1. Extremely dirty air filter restricting the airflow through the inside unit.
2. Extremely dirty (clogged) cooling coil restricting airflow through inside unit.
3. System low on freon, causing coil to freeze up.
4. Check your return grill to make sure that it is not being obstructed.
5. Blower motor overheats and “kicks off” on safety switch.

The following test is the most important test you can performed on a system that is running but not cooling enough. This test can help determine if your system is low on Freon or will uncover several other problems that can cause your system to malfunction. It will tell a lot about what is wrong with the system. Start by inserting a stem type thermometer in the air supply duct, record the temperature. Now take the temperature of the air entering the return air duct. Subtract the supply temperature from the return temperature. The temperature difference should be 17 to 22 degrees. Make sure there is no frost on the coil before performing this test. A dirty cooling coil is a common cause of “Not Enough Cooling or “Freezing Up” of the system.

You can check to see if the coil is dirty by removing the filter and looking at the underside of the coil with a flashlight. Sometimes a mirror will help you see under the coil. You can determine if the blower motor is kicking off on overload by setting the thermostat to “fan=ON” and listening to the blower run. If the motor kicks off after a while, then you have a bad motor. It may take a half-hour or more before the motor overheats and knocks on overload. In addition, you can feel the body of the motor to see if it is extremely hot. Be careful when you touch the motor as they get very hot. Also, turn the power to the unit off before performing this test.

There's water spilling out of my inside unit, what can I do about this?

If you do not see ice build-up on the larger copper tubing (covered with a black, spongy insulation) when you run your system, then you probably have a clogged drain line. A clogged drain line is usually caused by algae build-up inside the drain line. And yes, there is something you can do to prevent this condition. Algae is a living plant and will grow in your drain line until it clogs the line. The air handler provides a cool, damp environment for development of molds and mildew and if left untreated these growths can spread into your ductwork. If only moderate to light buildup is present then there are chemical disinfectants specifically designed for use in air handlers that will kill the existing mold and mildew and control new growth. These disinfectants are safe and very effective and are applied by simply spraying into the filter intake and by placing “Algae Strips” directly in the drain pan. If the coil has mold or mildew present then it also should be treated. Make sure that the face of the cooling or evaporator coil is clean so that air can pass through freely.

The system is running but the air is not very cold, what's wrong?

If some cooling is taking place but not enough, then the chances are that the system is low on Freon. The only reason a system would need Freon is because the Freon leaked out. If the leak is not sealed before recharging with Freon, the Freon will eventually leak out again. Leak repairs are expensive so if it’s a small leak it may be more cost effective to recharge the system every two or three years. Freon recharging is not a do-it-yourself job. As per federal law (EPA), only people certified to handle refrigerant (Freon) are allowed to recharge an air conditioning system.

Inside unit is running but the outside unit is not, what's up?

Tripped A/C breaker: Check this first. Find the breaker for the air conditioner. Turn it completely “off” even if it appears to be “off”, then turn it back on again. If it trips again, leave it alone and call National Air 504-341-2822. Thermostat wiring broken: Check the thermostat wires by the outside unit. This is the small cable with small wires going into the outside unit. Many times I have seen this cable broken or cut by lawn mowers or kids playing, or even by the dog.

The system will not run at all, what do you think?

The most common reason that a system will not run is because of a loss of power. In most every situation an air conditioning system is protected electrically by a breaker or fuse which is located somewhere in the power supply lines upstream from both the air handler and condenser units. This breaker is designed to provide over current protection and prevent electrical damage to your equipment. Find this breaker, turn it completely “off” even if it appears to be “off”, then turn it back on again. If it trips again, leave it alone and call your contractor. The second most common reason for a system not to respond when called for, is problems in the low voltage (24v) control circuit. This circuit is comprised of the controllers and relays that send signals to the components in your system to perform specific functions like heating, cooling and fan only. The most common problems are found in the thermostat connections and with failure of the transformer. You can change your thermostat yourself in most cases because the various wires coming to your thermostat are color coded and you simply hook up the appropriate color wire to the terminal for that color on the thermostat.

Procedure for cleaning the drain line

The drain should be made of plastic. If it’s black plastic, then you can pull it apart at a connection point. If it’s PVC (white) then you may have to cut it and after cleaning, re-connect it with a coupling. You can get a coupling at any hardware store. Whatever you do, DO NOT TRY TO DISCONNECT ANY COPPER LINES! Contractors usually use high pressure gas to blow out the drain line. But, you can clean the drain yourself by disconnecting the drain line at the unit and forcing water, from your garden hose, through the drain line. When disconnecting the drain line, keep a pale nearby as trapped water may come out of it. Don’t worry, It will drip out with very little pressure. Or, if you have a shop-vac, you can connect it to the drain line where it terminates outside the house, next to the outside unit. Attach the vacuum’s hose to the drain line using tape and vacuum for 10 or 15 minutes. After cleaning the drain line, drop several Algae Strips in the drain pan to prevent any further growth.

The outside of my unit is dirty or the air around the outside of my unit is blocked. What should I do?

Visually check the coil of the outside unit to make sure that it is not excessively dirty or blocked. If the coil is dirty you can flush it with a garden hose. Keep water clear of electricals.

My compressor is burned out or locked, or is not running for some other reason. What should I do?

If the fan on the outside unit is running but you do not hear the sound of the compressor running, then the compressor is malfunctioning. This would be a very serious problem requiring an a/c contractor.

Toggle titleMy air return is sucking in hot air. What should I do?

My air return is sucking in hot air. What should I do? If you have an air return duct in a hot area such as an attic or garage, make sure that this duct is not broken, split, or disconnected and sucking in hot air.

General Questions

How do I choose a contractor?

Some states require contractors to be licensed. Check with your state government, and if the state requires licensing of air conditioning contractors then make sure the contractor you use has a license.Make sure the contractor pulls a permit. A good contractor will have no objections to pulling a permit, and with a permit you will have the benefit of a city or county inspection. If a contractor tells you that you do not need a permit, check with the local building dept. This is a big purchase, so I recommend you get at least three estimates. This may take a little work but it will be well worth it. Before the contractor you choose begins work make sure you get it in writing. Be sure you get the warranty in writing, not just for parts but also for labor. Usually, the parts are warranted by the manufacturer while the labor is warranted (if any) by the contractor.

Replacement Guidelines

BTU – Before you replace that burned out furnace or boiler with a new one, make sure it is the right size to begin with. Don’t take for granted that the original installer didn’t make a mistake. Especially where forced air furnaces are involved, the sizing of the ductwork is critical to the heating capacity of the entire system. A 150,000 btu furnace feeding ductwork sized for 1000 cfm does not deliver 150,000 btu to the dwelling. Most likely, the burner kicks out on high limit due to over heating. Which is what it should do if there is not enough air flow. In such a case, this furnace has never produced at full capacity, and it’s longevity was shortened by the steady over heated condition. All furnace manufacturers match BTU and CFM output; if the ductwork cannot support the CFM requirement, the furnace will not perform to it’s potential. When replacing a burned out furnace, match the BTU to the ductwork as best you can. Breech is the venting of gas and oil burning systems. The smoke pipe, or connector from the combustion chamber to the chimney must run uphill. Before you purchase a replacement boiler of furnace, make sure the breech on the new unit will be lower than the opening in the chimney. For warm air furnaces, it is also necessary to match the style of furnace with the ductwork. Furnaces are produced in different configurations for different applications. There are counter flow models with a downward flow, low-boy models and high-boy models with an upward flow, and horizontal models with a sideways flow. Some models also come in front or rear flue arrangements which must also be matched. Make sure you equal or improve the venting of any replaced unit. Blower – In response to the increased popularity of central cooling in the last twenty years, many furnace manufacturers raised the air flow capacity of their warm air furnaces by converting from belt driven blowers to variable speed direct drive blowers. This change not only increased the air flow capacity of the system, but the noise factor as well. These blower motors run at higher speeds than the old lumbering belt driven versions, and can therefore produce more air noise. Extra precautions, such as canvas or flexible connectors are needed when installing a direct drive blower in place of a belt drive.

Forced Warm Air - Radiant & Central

Radiant in each room can be electric, wood stove, gas heater , kerosene, coal, a fireplace, etc. Central systems can be hydronic, steam or forced warm air. A forced warm air system uses ductwork to distribute heated air from a source (furnace or air handler) to each room. The furnace can produce heat from any number of fuels; gas, oil, electricity, wood, or coal, or a combination of any fuels. An air handler will use a hot water coil to produce heat. Unless fresh air is piped in from outside of the home, the system will re-circulate 100% of the air it supplies. This means it must obtain air from the home by way of a return air duct or ducts. Properly installed, a warm air system becomes a loop by which air is drawn from the living space through return ducts to the furnace, heated, and sent back to the same space through supply ducts. The advantages to this type of heating system are numerous. The air can be heated, cleaned, sterilized, humidified, or cooled (central air conditioning). If return air ducts are strategically located, the will reduce heat loss by recycling the warmest air back in to the system that collects at upper areas of the house. Supply ducts located around the outside walls of the rooms will temper the cold air as it infiltrates the home and reduce any discomfort from the air flow to a minimum. When installed by an experienced contractor, the ductwork will take up minimal or no extra space, and will literally disappear into the framing of a house.


What are the different types of heating systems?

There are many different systems in place today to heat a home. To make things simple we will look at heating systems as a combination of two separate components, the distribution method and fuel. The different combinations of fuel and distribution systems make for the wide variety of systems on the market. Heating systems come in all sizes, types and fuels. In residential systems there are basically two different distribution systems, forced air (ductwork) and radiation (baseboards). These systems can be fueled by one or more of oil, electricity, natural gas, propane, solar or wood. In most cases you can categorize home heating systems into two broad categories, furnaces and boilers. Generally speaking furnaces heat air and use a system of fans and ductwork to move that air around the house. Boilers heat water and use a circulator pump and piping to move the water through radiators, thus heating the home. Furnaces come in all shapes, sizes, efficiencies and fuels. The most common furnace in urban areas is a forced air, natural gas unit. In rural areas, or areas that do not have access to natural gas pipelines, furnaces often burn oil, propane or electricity to produce heat. Regardless of the fuel all residential furnaces work on the same principle. A fuel is burnt in a heat exchanger to produce heat. Air is then passed over the heat exchanger where it picks up heat, the air is then delivered to the house through a ductwork system. Furnaces are often classified according to efficiencies. You’ll often hear furnaces being referred to as Standard, Mid and High Efficient units. The efficiency is determined by the furnace’s AFUE . According to the Ontario Ministry of the Environment and Energy a Standard furnace is one whose AFUE is below 70%, a Mid Efficient furnace is one whose AFUE is between 71% and 82% and a High efficient furnace is one whose AFUE is above 90%. If you are considering the purchase of a new furnace, do not go by the label the manufacturer has assigned, compare AFUE ratings. When a furnace is installed in a basement it is considered an “Upflow” furnace, meaning the cooler air from the home enters the base of the furnace, and exits out the top of the furnace. The furnace is connected to a series metal boxes and pipes, which is the ductwork. The very fist metal box on the top of the furnace is called the PLENUM. If you have air conditioning the evaporator coil is installed in this plenum. If a furnace is installed on the main floor of a home and the heat comes from floor registers, it is a “Downflow” furnace. In a downflow furnace the cool air from your home enters the furnace at the top and the warmed air exits at the bottom.

How do furnace manufacturers compute an AFUE?

An AFUE (annual fuel utilization efficiency) rating is assigned to every furnace using a very complicated formula that takes into account many factors, such as fuel use, electricity use, and the fans used in the furnace.

How can I tell if the heat exchanger in my furnace has cracks?

You can’t. Most cracks in the heat exchanger occur in areas that are hidden away. Even if the cracked areas of the heat exchanger were visible, the cracks themselves are extremely difficult to detect by anyone without a trained, experienced eye. Contact National Air to perform a “co” Carbon Monoxide test and keep your family safe.

What causes the cracks to form in a furnace's heat exchanger?

In the normal course of furnace operation, the heat exchanger experiences repeated heating and cooling, which causes a corresponding expansion and contraction of the metal. Cracks will tend to form in areas of the heat exchanger that are weak, such as the areas that have been welded together at the factory. More weak spots and holes will form as a result of corrosion. Corrosion of the heat exchanger often signifies problems with the exhaust venting. What happens is that water in the exhaust, which contains carbonic acid, cools too quickly and condenses out, draining back down the chimney and onto the heat exchanger where the acid eats away at the metal.

My compressor is burned out or locked, or is not running for some other reason. What should I do?

If the fan on the outside unit is running but you do not hear the sound of the compressor running, then the compressor is malfunctioning. This would be a very serious problem requiring an a/c contractor.

Is there anything special I need to do to get my heating system ready for winter?

Most heating systems are quite reliable and will provide you with quick, comfortable heat when you need it, providing you take good care of the system. We recommend that you call National Air at 504-341-2822 check out your furnace BEFORE the first cold night hits. If you have air conditioning you should shut it down and cover it for the winter. Follow these easy steps: · Clean or change your furnace filter · If your furnace or boiler has a pilot light, make sure it is on. · Some homes are equipped with a “FRESH AIR INTAKE” that brings in fresh air from the outside for combustion. If your home has one, make sure it is not blocked and clean the outside lint trap. · Set your furnace to the “heat” position and turn up the thermostat. · Allow your furnace to run through a couple of cycles to make sure it is working properly. · Do a quick visual inspection of the furnace area to make sure there are no items that could interfere with air flow or combustion. · Call a qualified professional and arrange for a professional cleaning and inspection.

Troubleshooting & Routine Maintenance

Why bother with a yearly Preventive Maintenance Service?

A yearly Preventive Maintenance Service on your system is a wise thing to do. Note the words, Preventive Maintenance. When done properly, maintaining your system will save you money on energy bills, but just as importantly, a yearly preventive maintenance will help your system last longer. This is important because heating and air conditioning systems are very expensive. Another benefit is that problems can be discovered before serious damage is done, and they can also be discovered and appropriate measures taken before a complete breakdown in the middle of winter or on a hot, humid day.Dirt is the biggest enemy of electro-mechanical systems. During a Preventive Maintenance, certain parts that can cause trouble when dirty, are cleaned to keep the system running smoothly and efficiently. Some of these parts are, the cooling coil, condensate drain, burners, pilot lights, and most importantly, if needed, the flue. The flue is that part that conveys poisonous combustion by-products (gases) out the chimney. Dirt is the biggest enemy of your air conditioner! Giving your air conditioning system a good electrostatic air filter is the best thing you can do for your air conditioner. A good air filter will extend the life of your air conditioner because the important parts, like the blower assembly, the cooling coil, and other inner parts will stay cleaner, operate more efficiently and last longer. And, as a side benefit of having a good air filter, your family and you will enjoy breathing much healthier indoor air. Air that will be free of dustmites, mold, mildew, dust, pet dander, etc. etc.


1. Be sure to turn the water and humidistat off before any type of service.
2. Special chemicals can be used to clean mineral deposits, but one of the safest is vinegar. A 50% mixture of vinegar and water will usually dissolve or help clean away deposits.
3. If the humidifier has a drum with a belt, replacement of the belt at the beginning of the heating season can be the easiest fix if the water has a high mineral content.
4. If the humidifier has screens or is the wick type, some corrosion can occur as well as build up. Heat pumps and electric furnaces usually have steam humidifiers to increase output. They are line voltage powered and must be unplugged or disconnected before any type of service.
5. With the exception of some mist type humidifiers, all have a reservoir. Stagnant water sitting in the reservoir during the off season can be a breeding ground for bacteria. Turn the water off and clean the humidifier if possible when heating season has ended. Other than mineral deposits, the most common problem is sediment in the water supply. The float inlet valve has a tiny hole that easily clogs. The float is usually made of plastic, and if cracked will fill with water and sink. The valve will be stuck open and the reservoir will overflow. Use caution when scraping deposits off the float. If the deposits cannot be removed, replace the float or it will sink and cause an overflow.


1. Check that the thermostat is set higher than the actual room temperature and the selector switch is set to “heat” (cold weather months) or check that the thermostat is set lower than room temperature and the selector switch is set to “cool” (hot weather months). If the thermostat is programmable, ensure it has fresh batteries.

2. Test for power to the air handler by moving the fan switch from “auto” to “on”. If the blower runs, the air handler is operational. If nothing happens when you move this switch, check the circuit breakers on the air handler cabinet and/or the breakers or fuses in the main panel. If any breakers are tripped, reset them once. If they trip again do not reset them. FATAL HIGH VOLTAGE conditions exist inside the air handler cabinet and inside the access panel of the condenser. Only a qualified professional should access them.

3. If the air handler runs constantly but cannot satisfy the thermostat setting, it is possible the backup heat is running but the condenser is not. Some condensers will have the high pressure cut-out switch accessible externally. Look for a button sticking out of the cabinet in the vicinity of the refrigerant pipes. Press it in. If the machine starts up, the head pressure got too high, possibly from turning on and off too quickly, too much or too little refrigerant, or an electrical interruption. As the unit ages, the switch can weaken and pop easily. If the condition repeats itself often, have a qualified professional check it.

4. Because all the controls are internally mounted, and FATAL HIGH VOLTAGE wiring is exposed, only a qualified professional should open these panels.


1. Check that the thermostat is set higher than the actual room temperature and the selector switch is set to “heat” (cold weather months) or check that the thermostat is set lower than room temperature and the selector switch is set to “cool” (hot weather months). If the thermostat is programmable, ensure it has fresh batteries.

2. An simple test for power is to move the fan switch from “auto” to “on”. If the blower runs there is power to the unit. Check your circuit panel for blown fuses or tripped breakers. If you find a blown fuse, replace it or reset a tripped breaker once. If the problem continues, call a qualified professional.

3. If water is coming from a well, check the water supply. A clogged filter or strainer may trip safety switches that will not allow the unit to run without water. The controls are mounted internally and FATAL HIGH VOLTAGE connections are exposed once the cabinet is opened, only a qualified professional should access them. Do not attempt to adjust these controls under any circumstance.


1. The thermostat should be set higher than the actual room temperature. If you have a programmable thermostat, ensure it has fresh batteries.

2. If your system is central air, or the system is zoned, make sure the selector switch is set on “heat”.

3. Check the emergency switch which is usually a red switch plate on the side of the furnace or somewhere near the location of the furnace. Ensure it is on.

4. Check your circuit panel for blown fuses or tripped breakers. If you find a blown fuse, replace it or reset a tripped breaker once. If the problem continues, call a qualified professional.

5. With standing pilot furnaces, the burner will not light if the pilot has gone out. If you are not familiar with lighting the pilot, call a qualified professional.

6. Furnaces with electronic ignition have a sequence of safety steps the burner takes to light. Information is usually attached to the inside of the burner or blower access door. Follow the procedure according to the manufacturers guidelines to avoid the risk of fire or explosion.

7. Do not for any reason disconnect any piping to check for gas supply. A special instrument is used to check this and should be done by a qualified professional.

8. If the furnace is vented through PVC pipe, examine the ends of the pipe or pipes outside. Blockage of any kind to these pipes will cause the system to shutdown.


1. Grid failure Excess dirt or a any large objects stuck in the grids will cause an electrical short. Sometimes a loud snapping noise will indicate this. If cleaning does not stop the snapping and the grids are dry, inspect the fine wires stretched across the terminals. Often oxidation will make them white, gently run an old soft toothbrush up and down them once, then rinse and dry again.

2. Power failure The majority of electronic air cleaners will have a light to indicate operating power. If the light is not on when the system is running and the switch is on, there are two possibilities for power not being received. The power pack has either burned out or the power connection to the power pack has been disconnected. Another reason is the unit has a pressure switch that has failed. This pressure switch senses air flow and charges the grids. Either repair entails opening up the power pack and exposing FATAL HIGH VOLTAGE. This should be left to a qualified professional.


1. Prior to any performing any maintenance such as removing the screens and grids, it is imperative to shut the power off and wait a few seconds to allow the grids to lose the static charge they contain.

2. The screens are dishwasher safe. If you choose not to use a dishwasher, slosh them around in a tub of hot soapy water. This will loosen the dirt so that they can be washed off with a garden hose.

3. Use extreme caution when handling grids. The plates are very sharp, and can cause injury to the person handling them.

4. Take care that you do not bend or distort the grids or the fine wires. Correct spacing between the grids is instrumental to proper functioning.

5. When you reinstall the grids, do not turn the power on until they are completely dry. The water will short the grids out and damage to the power pack can occur. If you are not sure about the dryness, it is ok to leave the power switch off for a day or two; air flow will dry any residual water.

6. The grids have arrows that will display the correct direction for air flow. Improper installation will render them useless. If the air cleaner was correctly installed in the return air duct originally, air flow will be going from the duct toward the appliance. The arrows should point in the same direction.


1. The thermostat must be set higher than the actual room temperature. If you have a programmable thermostat ensure it has fresh batteries.

2. If your system is central air, or your system is zoned, make sure the selector switch is on “heat”.

3. Check your circuit panel for blown fuses or tripped breakers. If you find a blown fuse, replace it or reset a tripped breaker once. If the problem continues, call a qualified professional. The controls inside the cabinet contain FATAL HIGH VOLTAGE. Do not attempt to service the controls inside the cabinet, call a qualified professional.


1. Replace the air filter monthly or wash the electronic air cleaner grids if under constant use.

2. Clean the humidifier. Some humidifiers are the steam generating type and have line voltage power. This type should be shut off prior to any servicing.