Summary: If your air conditioner runs all the time, but your house still doesn’t get cool there may be a problem. The air conditioner isn’t low on refrigerant, so it may be the size of your unit that is inadequate to cool your home.
DEAR TIM: Help! We recently had a replacement air conditioning unit installed. It can’t seem to get the house cool enough and it always seems humid as well. The factory representative came out with the installer and verified that there were no coolant leaks and that the unit is functioning properly. I’m really “hot” under the collar about this. What could be wrong? J. K.
DEAR J.K.: Believe it or not, it is quite possible that you may have had the wrong sized unit installed. Based upon everything that you sent to me, I feel that this is the only explanation.
Most homeowners are unaware that air-conditioning units come in a wide variety of sizes. They can range in size anywhere from 1 ton to 7 tons, often in half ton increments. Don’t confuse these tons with weight. One ton of cooling is equivalent to 12,000 BTU’s (British Thermal Unit). It takes roughly 12,000 BTU’s to melt a ton of ice in a 24 hour period. Bear with me, I’m about to explain all of this.
Air conditioning equipment removes heat and humidity from your home. The size requirements of this equipment depend upon how much heat is in your home and how quickly it is generated. This heat generation is commonly referred to as “Heat Gain” and is measured in BTU’s. There are two types of heat gain, external and internal. External heat gain is that heat which comes from sources outside of the house (sunlight, hot air). Internal heat gain is that heat which comes from sources inside of the house (people, lights, cooking, appliances, etc.).
These sources of heat must be measured accurately. Tables have been developed for these specific purposes. These tables are based upon many variables, all of which must be taken into consideration. The proper use of these tables, in almost all cases, requires that the heating contractor gather specific data concerning your house.

The data which must be gathered includes, but is not limited to, many of the following variables: thickness of wall and ceiling insulation, square foot area of insulated ceilings and floors, square foot area of exterior walls and the compass direction they face, square foot area of windows in each of these wall and efficiency of these windows, shading of windows, average number of people occupying the house, and geographic location of the house.
When all of these measurements are made and the mathematics completed, the heating contractor will have arrived at a figure for “Heat Gain” for your particular home. The calculations are so specific, that two identical houses on the same street may have two different heat gain calculations. For example, the house with the greater number of windows facing west will have a larger heat gain.
The importance of performing the calculations should not be underestimated. An air conditioner that is too small will be unable to remove all of the heat and humidity. An air conditioner which is too large will “short cycle.” It will not run long enough to adequately remove excess humidity. Air conditioning units should be sized for the exact heat gain or slightly larger. Properly sized equipment runs at peak efficiency.
I suggest that you call your heating contractor and ask him or her to produce the complete set of calculations. Make the individual prove to you that in fact the proper sized equipment was installed. The air conditioning unit outside of your house will have a factory applied sticker or plate indicating how many BTU’s it is rated for. Check it yourself!

Summary: Calculating the proper size of your air conditioner is critical. If your HVAC system is not the proper size, your house will not be comfortable. Calculating heat gain can be done with the help of the Manual J. Get a copy for your air conditioning sizing.

My friends - the Gibson’s - purchased a tract home about 15 years ago. It was actually the sub-division model home. The front of the house has no less than 10 windows if I am not mistaken. One month ago their AC unit gave up the ghost. My HVAC man installed a new AC unit for them. The Gibsons had always complained about being uncomfortable. They thought the AC unit was low on Freon. It turns out the AC unit was never sized right from the get go. It was 1.5 tons too small!

To add insult to injury, the return air ducts from the second floor rooms were non-existent. There was just one central return duct in the second floor hallway. My HVAC man did a heat gain calculation to make sure that the proper sized unit was installed. He has a neat computer program that allows him to do this. You can do the same thing by hand - I have done it numerous times - and arrive at the same results.

One Size Doesn’t Fit All

Tract houses often have similar sized AC units. The builder or HVAC person simply does one calculation and often applies it to all houses that are similar in size. Guess what? This doesn’t work! In the case of my friends, the glass on the front of their house happens to face west south-west, the WORST possible direction! The blazing sun late in the afternoon cooks the inside of their house. I’ll bet that a number of you reading this bulletin have similar situations.

Some HVAC contractors try to size AC units by doing a simple square foot analysis. They use a gross multiplier that ranges from 400 (older homes) to 1,000 (newer homes) sq. ft. of floor space per ton to arrive at total BTU heat gain or total tonnage. This method is unacceptable. If your HVAC person proposes this, find a qualified technician who understands heat gain and knows how to use Manual J.

The Bible - Manual J

Calculating heat gain is really complex. You must use a booklet called Manual J. This booklet takes you step by step through the process making you account for each and every source of heat. Many scientists have worked over the years to formulate the tables, formulas, and values that combine to form this bible of the air conditioning industry.

You can purchase a copy - and I URGE you to do so - if you want to really understand how air conditioning works. The ordering instructions can be found if you keep on reading.

The manual is actually a rather technical booklet, but if you follow instructions well, you can easily work out a heat gain computation of your own. You will at least get close. Don’t get bogged down in all of the technical mumbo-jumbo within the manual. Simply take your measurements and crunch some numbers.

Finding a Professional

The same outfit that publishes Manual J also has a new program that will allow you to locate a professional who is dedicated to a high professional standard. They have a new certification program called the Air Conditioning Excellence (ACE) Technician Certification Program. Air conditioning contractors who are ACE certified meet a high standard because they have completed a comprehensive, industry education and examination process and have demonstrated technical expertise with numerous types of air conditioning (and heating) units in the field. They also have a thorough understanding of non-technical and safety issues that impact the equipment in your home. If you want a professional in your home, I suggest you look for someone who has this ACE certification.

Oversizing Equipment

Some people want to oversize their equipment so they are assured of being cool on those hot days. This can be a big mistake. You actually want to undersize your equipment ever so slightly. Researchers at Texas A & M found that air conditioners that were undersized by 10 to 20 percent are more efficient and more effective at removing water vapor (humidity) from your indoor air.

Now I will agree with them to a point, but I don’t know if I would undersize my own equipment to the 20 percent benchmark! I would go 5 percent at the most….a totally unscientific approach since I have always sized the equipment on my jobs to match or slight exceed the calculated heat gain.

Oversizing a unit causes problems because the unit doesn’t run long enough. You want your air conditioner to run for 15 - 20 minutes at a time. This allows the recirculating air to come into contact with the cold coil inside the air handler. This is how humidity is removed from the air. You will feel most comfortable when as much moisture is squeezed from the interior air as is possible. Short cycling air conditioners don’t remove humidity well at all.

Calculating Heat Gain

The first thing you need to do to calculate the heat gain of your house is to purchase a copy of Manual J. This booklet is very likely at your local library and/or you may be able to borrow a copy from a local air conditioning distributor. The point I am trying to make is that it contains 116 pages of tables, examples, and other valuable information that is physically impossible for me to include in this tiny Bulletin.

My copy of Manual J has an awesome example computation. You can see the floor plan of the house and the computation sheet. This allows you to see how they arrived at all of the individual heat gain BTU’s. The example computation will allow you to easily compute your own house’s heat gain.

The following instructions are meant to HELP you make sense of the rather technical Manual J. Without the following notes, I’m afraid that you might give up in frustration.

You can purchase your own copy of Manual J by contacting the following Association. The cost ($35) is steep, but there is no other booklet like it:

Calculate Temperature Difference

OK, it gets much hotter on average in Las Vegas or Dallas than it does in Chicago. This is important as you need to keep in mind the heat intensity when sizing AC. Manual J has extensive tables that will list your city or town or one very near you. There is a temperature listed that will usually only be exceeded 2.5% of the time. In other words, it can get hotter than that temperature but only for short periods of time. You don’t want to design your system for worst case scenarios. If you do this, it will be oversized.

75 to 78 degrees is the recommend interior temperature. You subtract 75 degrees from the temperature listed for your city. This is your design temperature. You will have to round this value to some factor of 5 for all subsequent calculations. In other words, if you arrive at 23 degrees, call it 25.

Heat Gain Through Windows / Glass Surfaces

The largest and most direct source of heat gain is glass. You know this anytime you have stood in front of a window on a hot sunny day. You feel like an egg in a skillet.

Heat gain through glass is affected by the compass direction the glass faces, whether or not the glass is shaded by an awning or trees, the layers of glass, and the total surface area of the glass. This aspect of your calculations is critical. If you make a mistake here, it can be big. Take accurate measurements and be sure of your compass directions.

Your BTU heat gain calculations start once you have measured all of your windows and noted all of the other data. I prefer to add up all of the square footages of the glass that face a certain compass direction. I then only have to go through the calculation one time.

Locate the proper table(s). You will need to know your design temperature, the compass direction, type of shading, and glass thickness. Once you zero in and find the right values, you will cross reference these to a number that is called the “Heat Transfer Multiplier” (HTM). You take this number and multiply it times the square footage of glass surface for that particular compass setting and shading requirements. The resulting number is the number of BTU’s that you are “gaining” through your glass. Write this number down as you will be adding up ALL of the BTU’s to get to your total Heat Gain. Note the vast difference in the numbers between similar windows that face West and ones that face North.

Heat Gain of Exterior Doors

You now proceed through the same exercise for your exterior doors. You will be asked to select a door type and then find your design temperature and cross reference once again. Use this HTM and multiply it times the total square footage of exterior door types. Once again the resulting total will be BTU’s of heat gain from the doors.

Heat Gain Through Walls, Ceilings and Floors

You will need to follow the same steps you did in the above exercises to get the BTU heat gain from all other surfaces that are in contact with the hot, humid outdoor air. This simply requires accurate measurements on your part and an investment of time. Do it on a step by step basis following all of the steps in Manual J.

Air Infiltration

All houses leak air. Old houses tend to leak more air than newer ones that are of moderate to high quality. There is a special calculation that you must do to find out the BTU’s of heat gain your house experiences from air infiltration. You basically have to calculate the total cubic foot area of your house multiply this times .40 and divide the result by 60. This gives you the cubic feet per minute (CFM) of air infiltrat

Summary: HVAC ductwork sizing is critical for proper heating and cooling in every room. Return air ducting is necessary in almost every room. Have to get the air back to the furnace. These ductwork tips will help balance your air conditioning and heating system.

HVAC Return Air Ducting
Air Conditioning Sizing - Tips on Ductwork

There is nothing more uncomfortable than lying in bed on a hot summer night sweating. Tossing and turning does no good. What’s worse, you know that the first floor or basement of your house is much more comfortable. Does it have to be this way? Absolutely, positively NOT!

My house and lots of others that I have built and remodeled are very comfortable, even in the hottest weather. The reason is simple. My HVAC contractor, Richard Anderson, knows the importance of return air ducting AND sizing correctly both supply and return air ducts. This is the key to comfort.

Existing Hot Houses

I’ll bet that 3 out of 5 people who read this bulletin have an existing home that has no or inadequate return air ducts. Don’t worry, we are going to solve those problems. It isn’t always going to be easy or pretty, but it can be done.

Return air ducts are necessary in just about every room. However, there are two rooms that I never place return air ducts - kitchens and baths. Let’s try to keep the garlic odors and hair spray in these rooms if you don’t mind! The reason for return air is simple. Your furnace is simply a recirculating pump. Instead of pumping water, it is pumping air. We need to get the air back to the pump. Furthermore, without a return air duct in each major room, the supply duct air has to “push” all of the air in the room out of the way as it makes its way towards you. The return air ducts help pull the air as it is being pushed.

Where Can You Put Them?

If you have an existing ranch style home, installing return air ducts in each room can be done fairly easily. You do what we do in new homes. You use the wall cavities as the duct. Think of it. Drywall that is nailed to 2×4s is really a duct - a box with 4 sides. All you have to do is cut out the floor immediately below the wall cavity as well as the bottom wall plate. Once you have created the opening, sheet metal will allow you to connect this to your existing return air system.

Two story houses can be more of a challenge. You have to figure out a way to get return air from the ceilings of the second floor to the basement or furnace room.

Do you have a laundry chute that you don’t use that often? Can it be sacrificed for the cooling cause? We have used these with tremendous success.

Sometimes you can use a kitchen pantry closet or a hall closet. We have often been able to put a duct in one corner of the pantry or closet. This duct extends up to the second floor where it often lines up with an interior hallway wall.

Often you can collect the ceiling air through a network of flexible pipes in an attic. These pipes join together in one central location that permits you to run a duct down to the basement or a second floor closet.

As a last resort, you may have to put a duct in the corner of a room(s). These can be drywalled easily. To effectively disguise it, you may build another one several feet down the wall. Shelves can be put between these two “stacks”. If they are in a child’s room, they can often be painted in a decorative manner as if it was intended for them to be there.

It was common in many older homes to have projections in rooms such as this. These bump outs often were fireplace chimneys as they rose through to the roof.

Balancing Return Air Flow

If you want to really have a way to get the most bang for your return air buck, think of installing grill covers on the wall that have operating louvers. This will allow you to choke down rooms possibly on the first floor that are drawing too much air. This in turn creates a greater suction in the rooms that really need the cool air. This principle is no different than dampers which are used on supply lines to regulate air flow.

The grills with louvers are inexpensive and effective. On an older existing home that is being retrofitted for central air-conditioning, they are a must.

Reversed Air Flow On Old Homes

Do you have a house that is approximately 80 years old? If so, your supply ducts may be on inside walls. You may have a single giant return air grill in the floor or at the bottom of a staircase. This is all wrong.

Supply ducts should be located on outside walls, preferably under windows and near doors. The intent is to wash the exterior walls with heated or cooled air. You are trying to combat the heat or cold at its source.

The single giant return air grill in the floor balances the load at the furnace motor, but does nothing to promote cross ventilation in each room.

What, you don’t think you can match the hardwood flooring? Rubbish! Go to a first floor closet and get the flooring from there. A good hardwood installer can make the repair and no one will be the wiser. Who cares if the flooring inside the closet doesn’t match.

I wish you luck in your effort to stay cool. Don’t hesitate to contact me if I can help.

Air Conditioning Sizing Considerations

Residential air conditioners, both central and window units, are like shoes. They come in various, different sizes. Buy the wrong shoe and you will be uncomfortable. It is no different with air conditioning.

Bigger Is NOT Better

So you really want your house cold. What are you going to do? Buy a monster 5 ton central unit? That could be a mistake. If you put in a unit that is too powerful (produces too much cooling), it will short cycle. It will turn on, blast out vast quantities of cool air, and then shut off. The result will be improper dehumidification. You will be cool and clammy. The air conditioner needs to run for a sufficient time to squeeze out the humidity that has seeped into your house. You achieve the highest comfort when the air conditioner removes as much humidity as possible from your interior air.

How Are They Sized?

To properly size an air conditioner, one has to perform a heat gain calculation. In other words, you must determine how fast heat is getting into your house and what things inside the house are contributing to heat gain. To perform the calculation, you need to know how much insulation is in your exterior walls, how much is in your attic, the size and type of glazing for each window, the type of construction of each window, extra special lights or cooking appliances that generate heat, compass direction that each wall of your house faces, etc. In other words, you have to do some work to get the equipment sized right. Tables and charts have been around for years that convert these measurements into BTUS. Once you know how many BTUs your house is gaining, you can purchase the correct sized air conditioner.

When purchasing a unit, ask for these calculations! Don’t just buy the same size unit you now have. Maybe it was not sized right. Maybe you have upgraded your windows or insulation. Maybe you added a small or mid sized room addition or remodeled an attic space.

Replacing An Existing Unit - BEWARE!!

If you have an existing house which needs a new central AC unit, be careful! For you to achieve the published efficiency and performance that is sold to you by the salesperson, the interior coil housed in the furnace must be compatible and matched for size! If it is not, the outdoor unit will struggle to keep your house cool. It is a question you must ask the installer. Make him/her prove to you that the interior coil will handle the outside unit. Remember, the inside coil could have been wrong from the beginning! The original furnace contractor could have downsized one size to save money. The AC will work, but not as well as it should have!

Ductwork Tips - Air Flow Problems - Tips

Static Pressure - Not Electricity

If you have an existing house that has cooling problems, there may be numerous things that are contributing to the problem. We have already discussed the sizing of the air conditioner and importance of proper return air. But there are other problems that can cause poor cooling. One of these is static pressure.

If you are healthy, did you know that your blood pressure is very nearly the same at your heart as it is in the last digit of your index finger? Why doesn’t the pressure drop as the blood goes way out to your finger? That’s easy. Your blood vessels get smaller and smaller the farther away from your heart. At each bifurcation ( a “fork” - where a single vessel splits into 2 vessels) the two new vessels are smaller than the original. This is because the energy to push the blood has now been split. If you kept one vessel the same size as the original, there would be less energy to push the same amount of remaining blood!

This is the same thing that must be considered with the air in your ducts. The air which is flowing from a duct near the furnace should have very nearly the same force as one which is far away. Take a look at your ducting system if it is visible. You will notice a large trunk line which comes off the furnace. Sometimes it will split and go two directions. These should be smaller than the original. Then, notice how every so many feet, a pipe is branched off to go to a room(s). Does the trunk line get smaller? If it doesn’t, especially after two branches, you could have a static pressure problem.

Gentle Turns

While you are looking at your ductwork, pay attention to how the branch lines come off the main trunk line. Are they abrupt 90 degree bends? Or, do they resemble an exit ramp from an expressway? You are looking for exit ramps or wye fittings. These fittings allow the air to change direction in a gentle fashion. Abrupt fittings absorb energy and cut down on the air flow.

Are there lots of turns or 90 degree turns in a branch? Remember, each bend absorbs energy. Turns or bends in ductwork should be minimal.

The A Coil - Dirt Catcher

We discussed earlier that you really have two coils as a part of a central AC system. One is outside you house. It dissipates the heat into the atmosphere. The other coil is inside your extended plenum at your furnace or air handler. This is the coil that gets cold. It cools down the recirculating air inside of your house. Well, guess what? Most people can’t see this coil. It is hidden behind the plenum sheet metal. What happens if you still use those worthless fiberglass filters? That’s right, this hidden coil can get covered with dirt. If it does, its ability to cool the air is significantly diminished. Buy, use, and regularly clean either high quality paper air filters or a rinsable polyester filter like I have in my furnace. Electronic air filters work very well too.

Helping Your Air Conditioner

The next time hot weather hits, I want you to touch the ceiling of your second floor rooms at about 5 o’clock in the afternoon. Pretty warm aren’t they! In fact, these ceilings can radiate vast amounts of heat into your house. Why not reduce the amount of heat in your attic? This will really help your AC.

Install one or more thermostatically controlled attic fans. These devices turn on and off automatically. They exhaust hot air from your attic space. If you install some excellent soffit vents so that cooler (90 degree vs. 150 degree or more) air is being introduced into your attic, you will go a long way in helping your air conditioner.

Reflective Foil Sheathing

Have you seen the foil faced foam sheathing that we builders use on new homes or room additions? Did you ever think how that stuff could work to bounce the radiant energy back towards your roof? Trust me, it works well. You can buy this stuff and cut it into 2 foot by 8 foot sheets that possibly can be inserted through your attic access panel. Staple or nail these to the bottom of your attic roof rafters. If you have little gaps here and there, it is not important. The more aluminum that faces towards the roof, the better.

Continuous Fan Movement

Several readers have contacted me about success they have had by allowing their furnace motor to run continuously. They indicate that this seems to balance the room temperature. No doubt this is true. The air movement also helps to evaporate your own perspiration. Evaporation is a cooling process in and of itself. That is why you feel more comfortable when a breeze blows. Look at your thermostat. There is a little switch that says either “On Off Auto”. Switch it to On for continuous fan blowing.

Summary: HVAC systems are very expensive. What is the payback period for the higher efficiency unit? Projected energy savings? Consider these factors and others when purchasing a new air conditioner and/or furnace.

Heating Equipment Life Cycle Costs
Air Conditioning Purchasing Tips

Don’t ask me why they do it. I don’t know! Furnaces seem to have a service life of 20 to 25 years, whereas air conditioning condensing units (that big box outside your house) seem to only last for half that amount. This wasn’t such a big deal 25 years ago. Energy conservation was not a big concern. Fuel costs were moderately low. Well, times have changed.

Thousands of people will be pondering what to do this year. Their air conditioning unit will give up the ghost. The salesperson will tell them how they should switch out that older 75 percent efficient furnace. Heck, the new furnaces operate at 90 percent or better, the salesperson will say. In fact, the furnace I’m going to let you have operates at 95 percent efficiency! Fuel saving will be enormous. Happy days are here again! ….. I can hear it now.

The Illusion

Here in Cincinnati, we have a 5 month heating season, November 1 through March 31. Some years, it stretches a little each way on chilly mornings. The point is this. To determine whether it is a good idea to switch out a furnace, you need to see if it makes sound economic sense.

Sometimes furnace and air conditioning people lead you to believe that you can save big bucks by switching out a furnace and/or air-conditioner that is still working fine. The numbers don’t always add up. You need to do some math.

Let’s look at a heating calculation. The first thing to do is to calculate what it actually costs you to heat your home. Select two bills from your last year’s records. One should be from the early fall and the other from the late spring. These bills will show what your base load is for your utilities. Base load simply means all other costs less heating and cooling costs. If you add the two bills together and divide by two you will get an averaged value.

Now, take your bills from last year when you know the furnace was running for any period whatsoever. Subtract the averaged base load amount from each bill. This will give you the extra each month you are paying to heat your house. This is not an exact science, as your electric bills will tend to be higher as you get deeper into the winter, less daylight you know.

If you total the overage for each month, you will arrive at your annual heating cost. Let’s consider this number. For sake of discussion, let’s assume it is $800.

The Real Savings

Remember our salesperson? He talked about enormous savings. Is it true? Not really. Your existing 10 -12 year old furnace is possibly operating at a 75 percent efficiency level. The new furnace will operate at 95 percent. This is a 20 percent difference. The actual savings is 21 percent because you divide the 20 percent by 95 percent.

How does that relate to our example? Well, let’s do the simple math. $800 times 21 percent savings per year equals a whopping $168. This isn’t bad, but what does it really mean? You have to look a little deeper to find the answer. Interest Income - Significant

So what happens if you don’t switch and get the new furnace? Will you go bankrupt? Will the utility company stockholders love you? How are you going to pay for that new furnace? From your savings? I thought so.

If you buy the new furnace your income stream gets slightly interrupted. In other words you loose the earning capability of the money you just gave to that salesperson!

Let’s assume the new furnace will cost $3,500 (a reasonable amount). If you put that money in a certificate of deposit now, it will generate approximately 5 percent per year. That is $175.00 before tax. Accckkkk! You will loose money (before tax) if you buy the furnace!

If you have any investment success whatsoever (stocks, mutual funds, etc.), you can actually make significantly more money by not buying the new furnace, Many people fail to perform this vital second step in the analysis.

Another common mistake is to fail to recognize the payback time. In other words, how many years does it take to pay off the new system? For this calculation, you need to consider the rising cost of fuel. Assuming a 2 percent per year inflation in fuel costs, you need to multiply your annual cost by 11.07 to arrive at the total cost for the next 10 years. In our example, our fuel costs over the next ten years would be $800 times 11.07 which equals $8,856.

Our savings, remember, was 21 percent. So, we should expect to save $8,856 times 21 percent or $1,859.76. Yikes!!!!! In ten years, we have barely paid for half the cost of the new furnace. Not to mention the fact that we haven’t offset the extra money we still have in the bank plus the interest it generated. It doesn’t appear that switching a furnace that still works at a higher level of efficiency really pays.

Alternatives

OK, so your furnace has 10 - 15 years of useful life. Does that mean you do nothing to save money? No. There may be some options available. For example, you may choose to install a flue damper. These devices automatically shut off the flue trapping residual heat left in the furnace after the burner shuts off. These help save money and energy.

What about replacing that old standing pilot light with a spark ignitor? This might pay for itself. If you have an oil furnace, you might think of installing a flame retention burner. This can sometimes improve the efficiency of your oil furnace by 10 - 15 percent.

Talk to your heating contractor. See what options they recommend. Do the math. It is simple and only takes a few moments. You might be surprised by the figures.

A Quick Sample Calculation

OK, Ok, I’ll show you how really simple it is to do one of these calculations. For your own situation, you need to just follow along. You will see just how simple it is to save money!

You can take it from here. To finish the calculation, simply obtain a quote from the heating salesperson for the cost of the new furnace. Then find out what your local bank is paying for their CD’s. Or, use the rate of return you have been averaging on your stocks, bonds, or mutual funds. Do you make more money in interest each year than what you will save? I thought so!

Tips on Purchasing a New Air Conditioner

Efficiency Ratings - Watch Out!

Air conditioners are not all created equal. Some perform much better than others. They are rated using a numeric system referred to as a SEER rating. The higher this number, the better the machine is at converting electricity to cooling. SEER ratings of 12 to 14 are not uncommon.

However, if your air conditioner fails and you replace it, the new unit may not deliver this promised efficiency. The SEER rating is calculated using specific inside matched coils and air velocity. It is very complicated. In order for you to achieve the high efficiency performance promised on the yellow sticker, you may have to make modifications inside your house. Discuss this possibility with your heating/cooling contractor. If this person looks puzzled or says “Huhhhh?” when you bring this issue up, I suggest you call another contractor immediately.

Extended Warranties - Be Careful!

Just about every manufacturer of air conditioners offer extended warranty programs on parts and labor. However, you must be careful. Often a third party company handles this ‘insurance’. That’s what extended warranties really are. If you make a claim, it is no different than any other insurance loss. A manufacturer who backs its own warranties may be a better choice. Look before you sign!

Sizing Air Conditioning Equipment

Did your last air conditioner not work too well? Don’t blame it on low efficiency! It may not have been sized properly! A new unit sized the same will perform poorly no matter how efficient it is! I covered this topic extensively in Builder Bulletin 31. You may wish to send in for it if you don’t already have it. Heat gain calculations are a must! Make your contractor perform these important calculations! It is not uncommon in new subdivisions for all the houses of a similar size to have the same sized air conditioner. Did you know this is possibly a mistake? The size of the air conditioner is a function of many thing, including but not limited to, the number of occupants, the compass direction the house faces, the number of windows, and types and quantity of interior lighting. If you have made changes in your windows and have new Low-E glass, your cooling requirements may even be less! All of these things need to be calculated. Computer programs do it very quickly. Ask for proof of these calculations before you sign the contract for the new unit. Remember, the first guy could have goofed. If they simply put in the same sized unit as you have, it could be wrong.

Projected Energy Savings Table

This table says it all in my book. I have already shown you how to determine your real annual heating costs. See where your costs plug into the table in the first column. The second column shows you the savings the HVAC sales person tells you that you will achieve should you switch from a 70 to a 95 percent efficient furnace. They quickly say that within X years you get a payoff and start saving money. Not so fast! What happens if gas rates go down? The payoff takes longer. Well, gas rates doubled in some areas this past winter. What happens if they are cut in half and drop down to where they were last year? The payoff time could double or triple OR extend beyond the life of the furnace!

Summary: A HVAC, or climate control system, makes your home comfortable by removing humidity. Your HVAC unit must run long enough to dry and cool the air. Get your HVAC sized correctly. Too large a unit will not run long enough to remove the proper humidity, while a too-small unit is unable to cool and dry enough moisture from the air.

Air Conditioners

Residential air conditioning is a very complex and complicated trade. Many homeowners do not realize the amount of technology and training it takes to properly calculate and install air conditioning equipment. Heating and air conditioning is by far the most technically advanced trade in residential construction. Technology advancements are common. Professional HVAC contractors must constantly attend seminars and classes to stay current.

However, even though HVAC technology produces high-tech equipment on a daily basis, some things do not change. One of the constants in residential cooling is the necessity of performing specific calculations to accurately determine the ‘heat gain’ of your home or business. It is impossible to properly size air conditioning equipment without performing these calculations. The recent advancements in personal computers have made this a much simpler task, however the calculations must be performed either by hand or by computer.

Another constant in residential air conditioning is humidity. Humidity is the moisture that occurs as water vapor in the air we breathe. The higher (more water vapor) the humidity in warmer air, the more uncomfortable we become. This principal is very easy to understand. The evaporation of water from a surface actually is a cooling process. It actually lowers the temperature of the object. Normal body perspiration is the way in which our bodies cool themselves. As the perspiration evaporates from our skin it cools it. However, the slower the rate of evaporation, the slower the cooling process.

When the humidity is high, the air has less ability to accept additional moisture. In other words, it is harder for water to evaporate. Air conditioning equipment has the ability to lower the humidity of air in your house. However, it can only do this while it is running. The moist air in your house passes through an evaporation coil inside of your furnace. This coil is cold, much like a cold can of soda or beer. Just as water droplets form on a cold can, the water from the air does the same thing on this coil. As long as you do not introduce any additional outside air or have numerous air leaks, the process happens within a few hours. The air in your house becomes cooler and drier. The perspiration on your skin can more readily evaporate and the net result is that you feel very comfortable.

However, if the air conditioner does not run long enough, it can not extract enough moisture from the air. The air will get cooler, but it will feel damp. You will feel cool but clammy.

The point of all of this is that the air conditioning unit outside of your house must operate long enough to properly ‘cool’ your house. It can only do this if it is sized properly. And, as I mentioned earlier, it can only be sized properly if you do the calculations. If a unit is sized too small, it simply won’t have the ability to cool the house when it gets hot, even if it runs constantly. Whereas, a unit which is too large (too much capacity) will ’short’ cycle. It will not run long enough to remove enough moisture from the air.