While the acronym HVAC can stand for several things, in this context, it stands for Heating, Ventilation and Air Conditioning. Your HVAC system is the core of keeping your home comfortable in terms of heating, cooling, humidity control and air quality. I won't touch on the air quality and ventilation portions of HVAC, but I will touch on heating systems as they apply to the southern climate, and I will go into detail regarding air conditioning, SEER ratings and other relevant information.
Heating
Since this will be a brief discussion, I'll touch on it first to get it out of the way. In the generally hotter southern climates, winters are usually mild. While it can definitely feel bitterly cold here in the Houston area, the temperatures themselves typically do not fall below freezing for any length of time. Hard freezes are uncommon, happening only several nights per year. It feels colder than it really is due to the relative humidity. So while we do have a seasonal requirement (albeit short) for heating, it is definitely not where our major temperature control dollars are spent.
In this area, you never see oil-fired furnaces or boiler-based heating systems in residential buildings like you would in the northern, colder climates. Typically, we have two choices here for heating: electric resistance heat, and natural gas heat. Obviously, if you have a choice, natural gas heat is by far the better, more efficient, and more economical way to go. If electricity is your only choice (no natural gas in your neighborhood), when you replace your heater, be sure to buy the most efficient model you can. The extra expense will definitely be worth it in terms of long-term energy savings and reduced bills.
For natural gas heaters, there are typically two types available: 80% efficiency and 93% efficiency. I 'm not sure what the initial cost difference is between the two types, but obviously, the 93% model is more efficient than the other one. How much more? I'm not sure, but if you live, say, in northern Texas or the like where the heating season is longer than down here along the coast, it would likely be in your interest to pay the upgrade. Down here, I would say it is likely a toss-up unless you just have to have the most efficient model available.
Air Conditioning
For southern climates where it is hot and humid a large portion of the year, this is the big daddy. Depending upon where your thermostat is set, it is not unheard of for AC units to run 12 hours a day or longer on the hottest days in an older, leaky, poorly insulated home. At one of the homes I used to live in with zoned air (two separate units), the upstairs unit sometimes ran 18 hours per day and still couldn't keep up with the heat load placed on it!
Obviously, reducing the load placed on your units so that they run less while still keeping the house comfortable is extremely important. It is also a balance that is difficult to strike. Other sections of this litany on energy efficiency cover how you can approach doing that (sealing the home envelope, efficient attic, etc.). This section, though, deals directly with the air conditioning equipment itself.
Terms
First, I should get some technical term definitions out of the way. In terms of heating and cooling, system output is measured in BTUs, or British Thermal Units. The official definition of a BTU is the amount of energy required to raise the temperature of one pound of water one degree Fahrenheit at normal atmospheric pressure (which happens to be 29.92 inches of mercury). That doesn't translate well to heating and cooling loads. To make it simpler, the higher the BTU output of a unit, the faster (but not necessarily more efficiently) a unit can transfer heat into or out of a space. More on that as it relates to humidity later.
I'm sure you've all heard the term ton as it relates to the size of an air conditioner. One ton translates into 12,000 BTUs of cooling capacity. So if you have a 3 ton air conditioner, that means that AC is capable of producing 36,000 BTUs of cooling.
SEER ratings. The acronym stands for Seasonal Energy Efficiency Ratio and is used as a measure of cooling performance and efficiency of air conditioners. The ratio is calculated by dividing the total BTUs of cooling output during an average annual operating season by total kilowatt hours consumed during the same period. The higher the SEER, the more efficient the unit. Before 2006, the government mandated a minimum 10 SEER rating on all new equipment. In 2006, new standards went into effect requiring a minimum of 13 SEER. As of now, the highest SEER rating I've found is 20, and that comes in a top-of-the-line, high-dollar Trane system.
System Components
A full discussion of air conditioning systems would not be complete without at least a brief description of the components of a system and what those components do. To get us started, here is an excellent diagram. This image is reproduced from Insulate and Weatherize by Bruce Harley, The Taunton Press, 2002, ISBN 1-56158-554-8, page 133.
As you can see, central air conditioning systems are comprised of five main components: compressor/condenser, air handler/blower, evaporator coils, supply air ducting and return air chase/ducting. Although the diagram above indicates the house has a basement (which are not common at all in this neck of the weeds), the premise is the same: you have indoor components and outdoor components. Most commonly, the indoor portion of the equipment in this part of the country will be found in the attic, and in a few cases, in an inside closet within the conditioned space.
The heart of the system is the compressor, also known as the condenser coils. This is the part of the system that sits outside, and it is responsible for compressing and circulating the refrigerant through the system. As the refrigerant expands in the evaporator coils, it cools off - much like those cans of compressed air used for computer dusting - and therefore cools the air flowing over the coils. The refrigerant also absorbs heat from the air passing over the coils, and as it circulates back to the condenser/compressor, releases that heat to the outside environment. In essence, the refrigerant transports heat from the inside to the outside, cooling the indoor environment as a result.
The air handler - which I also call the blower - is usually a multifunctional unit, because it generally also houses the furnace's burners/heat exchangers (natural gas) or heating elements (electric). Its primary function is to draw air from the interior of the house through the return air chases/ducting (the air filters will be in there somewhere), circulate it over the heating/cooling devices, and supply the conditioned/heated air to the living space.
For air conditioning, the "cooling device" in the air handling system is the evaporator coil. As mentioned, this is where the refrigerant circulating through the system performs its heat exchange duties. As the refrigerant expands inside the coil it becomes cold, thus cooling the air flowing over it. The evaporator coils aren't called that just because it sounds cool, though (pardon the pun). Another extremely important function the evaporator coils perform is dehumidification. As air quickly cools, any water vapor condenses out of the air and forms water droplets. An analogy is the water droplets that attach themselves to your glass of cold ice water as it sits on your table. Why they call it the evaporator coil rather than the condenser coil, I'm not sure. Maybe it was because they already called the outdoor unit the condenser coil! Or maybe it is because the compressed refrigerant, which until this point has been in a liquid form under pressure, evaporates into the gaseous form in this lower pressure region of the system, thus providing the cooling effect. Anyway, water vapor in the air turns into water droplets and is drained to the outside through plumbing attached to the drip/drain pan. Some of the higher efficiency units specifically designed for higher humidity climates can remove around 200 pints of water from the air in a 24 hour period. That's roughly 25 gallons per day, and some serious dehumidification power!
The other components of the system - the return air chases/ducting and the supply air ducting and registers are the final components of the system, and they are responsible for transporting the air through the house and over the heating and cooling devices. I go into more detail on these important pieces of the system in the sealing section.
My Systems
In my new house, I inhereted two 10 SEER American Standard units rated at 4 tons each. The folks installed those abound 2004, so there's still plenty of life left in those. They're not as efficient as I would like, but as you'll see, replacing simply to get higher efficiency isn't always the best move - especially when large outlay capital projects are involved. What I'll discuss below are the systems and quandry I faced at my previous Katy home. I decided to leave this information here and intact as it does contain some good information and thought processes.
After doing some research, here is what I found out about my Katy home equipment. I had zoned air - one unit dedicated to the downstairs, and one unit dedicated to the upstairs. My downstairs unit serviced a lot more square footage than the upstairs unit, so it was naturally larger. The downstairs compressor unit was a 5-ton Goodman manufactured in 2001, and from what I could tell, the attic equipment (which includes the heater, air handler/blower, and evaporator coils) was manufactured in 1992. I'm not sure who the manufacturer of that equipment was. I replaced the heater and blower for the upstairs unit in April of 2005, so it was an almost brand new Trane at the time I originally wrote this. The idiots from ARS Service Express (who were sent by my home warranty company) didn't bother to tell me that my evaporator coil was original to the house and thus 25 years old and very inefficient. I would have paid for the replacement, but they didn't even offer that option - they just stuck a 25-year old piece of equipment back into the stack and didn't make a peep about it. So my evaporator coil is old and on the short list for replacement. The outdoor compressor for the upstairs was a 2-ton Goodman manufactured in 2002.
I'm not a fan of Goodman products. They do not manufacture very efficient systems, and they over-rate their systems' tonnage capacity. For instance, their 2-ton systems - which should have 24,000 BTUs of capacity according to what I mentioned in the Terms section above - actually only have about 20,000 BTUs. That's really a 1.625-ton unit, not a 2-ton unit. Their 5-ton units, which should have 60,000 BTUs, really have only 50,000 BTUs, just slightly more than a true 4-ton unit.
The other thing I found out about my Goodman units: they were only 10 SEER units. Granted, that was the minimum required by the government at the time they were produced, but more efficient systems were (and are) available.
System Replacement
All of this mixed-and-matched equipment got me to thinking: what about replacing my existing equipment? After lots of online searching, I found that nobody lists their prices, so I had to invite an air conditioning contractor out to the house to estimate the cost of system replacement.
For the second time in three days, I had to go on life support when I heard the costs.
First, the downstairs unit. The replacement equipment would include an 18 SEER American Standard compressor and matched evaporator coil, new furnace and variable-speed blower, and a new specialized programmable thermostat required by 18 SEER equipment. The cost: $10,100. To go with a 14 SEER unit and replace the same equipment: $8,340.
For the upstairs equipment, I had a couple of options. I could replace all the equipment, including the relatively new Trane furnace and blower. To do that with 18 SEER American Standard equipment would run us $8,780. Do do the same with 14 SEER equipment would run $6,785. My other option was to keep the Trane furnace/blower and just replace the compressor and evaporator coils. To do that with 15 SEER would run $4,430; 14 SEER would run $4,010.
MEDIC!!!!
Folks, that is a huge investment! Especially for replacing some relatively new and almost brand-new equipment. In short, I don't think the worst-case $18,000 investment is worth it when just considering energy savings alone. Even with 14 and 18 SEER equipment being roughly 23% and 36%, respectively, more efficient than 10 SEER equipment, you could never hope to recoup that investment in energy savings during the roughly 15 year life of the equipment.
An important side note here regarding mating some new high efficiency equipment with older existing equipment (such as my upstairs Trane unit). The effective SEER rating depends upon system components matching up. You can buy a 14 or 15 SEER compressor, but if you mate it up with older 10 SEER equipment, your effective SEER rating is definitely NOT going to be 14 or 15 (it won't be 10, either, but somewhere in between). To get the full benefits of the efficient systems and obtain the rated SEER, you must mate the proper equipment. Also, many of the new high-efficiency units require variable-speed blower fans in them in order to achieve maximum efficiency. My Trane upstairs is a single speed (full-out all the time), so no matter what compressor or coils I attach to it, without the variable speed blower, I will never achieve the rated SEER of the compressor and coil mating. Something to keep in mind.
Now, it will be a completely different story when the current equipment breaks and requires replacement (or if you're doing new construction and get to choose your equipment). Notice the cost differences between the 14 and 18 SEER units. No more than $2,000 per equipment set. When the time comes, will I pay the difference between the 14 and 18 SEER units? Again, it comes down to how long I think I'll be in the house when that time comes. Should I build a new home at some point, the decision will be a no-brainer - I'll be putting in the highest efficiency stuff I can get my hands on. But if my current equipment died next summer, the answer would be a definite yes, because in terms of annual savings, an 18 SEER is 13% more efficient than a 14 SEER. That's roughly a 5 year payback on the $2,000 premium, and worth it in my book when you take into account the overall life of the system.
With system replacement out of the question for me, at least for now, the next step: how can I make what I have more efficient from an equipment-only standpoint?
System Upgrades
As mentioned, my upstairs equipment consists of a 2002-model compressor, a 2005-model furnace/blower, and a 1981-vintage evaporator coil. Gee, where do I start?
The first item up for replacement is definitely that ancient evaporator coil. An estimate I've gotten indicates that will be about $1,600. It will be a somewhat longer payback in terms of efficiency savings, but worth it comfort-wise due to the extra dehumidification power of the newer units. So that's item number one on the list.
Update 9/25/06: I talked this morning with my AC contractor. After some discussion, he informed me that replacing the evaporator coil on the upstairs unit would cost $1,815. He also said that the efficiency gains the new coil would provide would be minimal, and even though it is a new unit, the gains in dehumidification would be insignificant. This is due to the older-type compressor the coil is attached to (an older-style reciprocating piston compressor rather than a newer scroll compressor), and those units definitely work in concert. So this unit was taken off my list of upgrades. I will wait for one of those system components to die, and when it does, I will go ahead and replace the entire system (compressor, coils and furnace/blower) with a high-efficiency unit.
Regarding major equipment purchases for the downstairs units, my plan right now is to wait until something dies before I consider doing something major. In other words, except for the items listed below, I'm not going to mess with the downstairs unit.
The second item: TXVs, or thermostatic expansion valves. If you'd like the details on exactly what they do and how they work, check out this site. The nuts and bolts of it: these valves regulate the amount of refrigerant entering the evaporator coils. In doing so, it allows the refrigerant that is in the coils to "boil off" or evaporate more thoroughly - that is, change from the compressed liquid form into a gas - without also pushing extra refrigerant still in liquid form through the coils at the same time. T his allows the refrigerant to be more efficient and absorb the maximum amount of heat from the air passing over the coils, thus cooling more air for less refrigerant compression and circulation.
Update 9/25/06: I also discussed these units with my AC contractor. Turns out most new coil systems have the valves built into them. But I cannot install retrofit valves onto my current systems. Why? Because both of my compressors are reciprocating piston compressors, not scroll compressors. Scroll compressors are definitely more efficient than reciprocating compressors, but they're also able to handle the pressure differentials on the compressor side of the TXV. Reciprocating compressors cannot handle that differential, and the TXV valves can actually kill them. So I will not be installing TXVs on my current equipment.
The third item: compressor starter assist units, which are essentially capacitors. Capacitors are sort of like batteries, except they don't store a charge for long periods of time, nor do they provide continuous power with no external power feed like batteries do. Their purpose in life is to store energy for a peak demand, and then deliver that power when needed without placing an extra demand on supply power. Once the surge has been satisfied, the capacitor regarges over a period of time (albeit short, but still longer than the time required to satisfy surge demand). When an air conditioner's compressor unit starts up, it draws a huge amount of power, primarily in amperage. Sometimes during startup, a unit will almost tap out the 30 or 40 amp circuit that it is connected to before settling back down into a lower consumption mode during normal operation. This can place a strain on the compressor unit, causing its start cycle to take longer and draw more power for a longer period of time. Longer start cycles equal more wear and tear on the unit, more current draw, and less efficiency. Enter the starter assist capacitor. When the compressor goes to start, that extra starting power needed to get up and running again is immediately available from the capacitor. The end result is less startup current draw, less wear and tear, better efficiency, and longer unit life.
Update 9/25/06: While talking with my AC contractor this morning, he quoted me $178 per starter kit, so that will be $356 for both units. These will be installed on my systems on 10/3/06.
Return Air Chases/Ducting
When talking about HVAC equipment as a complete system, you can't forget the systems that supply air to the the units and then deliver that conditioned air back to the living space. I talk about sealing the ducting in another section, but here I want to mention the return chases/ducting. This is where your blower units breathe and take in air from the living space to alter (cool or heat). Ensuring the blowers have an adequate supply of air is extremely important to the overall efficiency of the unit and can greatly affect how much air those units can move and cycle over the heating and cooling elements.
I inhereted a proper system (in fact, the same contractor that made all of these recommendations for the Katy house did all the work for my folks on my current house) when I bought my new home. Again, in the interest of conveying as much information as possible, I decided to leave the information as it relates to the Katy house.
As it turns out, I had several problems with my return air chases. They both followed the same path as they entered the attic. When the downstairs chase met up with the upstairs chase, they were back-to-back. To make matters worse, they weren't sealed. That means that air was being exchanged between the return air chases, and the chases were also allowing unfiltered air from unconditioned space into the air flow. Not good.
My second problem was inadequate surface area at the return air grills. My upstairs return air vent was almost the right size. It originally had an 18x30 vent, which comes out to 540 square inches. In order for maximum efficiency, I needed 600 square inches for the size of my equipment (2 tons). To get that, I replaced that return air vent with a 20x30 grill. I know it doesn't seem like that big of a change and a lot of hassle for an extra 2 inches of width, but it made a big difference in air flow.
The downstairs return air vent was grossly undersized. It also had a single 18x30 grill. T his unit should have roughly 1,200 square inches of area based on tonnage, and it originally had only 540. Because of space limitations at that return duct's location, I couldn't install a larger return air grill without turning the entire wall, floor to ceiling, into a return air grill. Didn't really want to do that - primariliy for aesthetic reasons. Instead, I installed two new return air vents that measure 12x24. The placement, though, was a little unorthodox: upstairs at the top of the stairs. This turned out to have multiple benefits. First, the downstairs unit, which was over twice the size of the upstairs equipment in terms of tonnage, assisted the upstairs equipment by pulling air from that area. Secondly, the downstairs until helped remove some of its own heat load from the upstairs by intercepting the heat rise from downstairs as it made its way up the staircase. This made a huge impact in making the upstairs more comfortable and reducing the load on the upstairs unit, thereby causing it to run less.
These modifications were not cheap, though. Total cost for installing new return air grills and modifying and sealing the return air chases was $2,640. But I decided the investment is worth it, maybe not so much in efficiency as comfort. Also, the engineer that inspected the house when we bought it made mention of the leaky chases, so that was another incentive to have the repair work done. That work was performed on 10/3/06.
That pretty much does it for my discussion about the HVAC system itself. As mentioned, I talk more about ductwork and sealing in other sections of this writing, so be sure to check those out as they are all interrelated and directly affect efficiency.