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There are four ways to economically aircondition a house:
- Get an efficient central airconditioner.
- Design the house to stay cool even on hot days
- Move to Arizona and use a swamp cooler
- Carefully arrange unit airconditioners (window units) to keep the house cool.
This webpage addresses the first item. It's a companion to a webpage directed to item 4.
The following repeats general information which is also on the other page:(If you've already bored with that stuff, please skip to Modifying Central Systems)
- EER - Energy Efficiency Rating
- calculated in BTUs/watt.
Alternative calculation: BTU/(volts*amps). That works out as
(BTU*amps)/volts. (Taken at something like 95 deg.F outside and 80 deg.F, but BTU/watt is close 'nuff.)
- SEER - Seasonal Energy Efficiency Rating
- used for central airconditioners, apparently using a slightly different methoud of calculation, but somewhat comparable to EER. Some higher SEER systems tend to have EER ratings lower than their SEER ratings because SEER is an averaged figure and can adjust power usage under lighter load conditions. 10 EER for a 13 SEER central system is not unusual. (Water source airconditioners are available at 18-21 SEER.)
- BTU - British Thermal Unit
- measurement unit for cooling -- actually BTUs/hour. BTU is an English unit (actually it's BTUs/hour), and there isn't a direct metric equivalent. BTUs and EER seem common throughout the world, however. To get EER from "kilowatts cooling" the formula is:
(0.000293 x kilowatts cooling/watts electricity), but you won't find a bunch of used airconditioners where you are anyway! An older rating is "ton", also an English measurement.
In the past year (2002), energy requirements in the US resulted in airconditioners having a minimum EER or SEER of 10. Expect all US units to be 10 EER or higher, and most foreign units to meet these standards within a year or two.If someone trys to sell you a new 10 EER airconditioner as "high efficiency", you are being scammed!
Regardless, the information here is useful because used A/Cs can be expected to have a commercial lifetime of 6-12 years. High efficiency ACs are 14 SEER, with 15 SEER available from most manufacturers and 16 SEER mentioned. Water source airconditioners are 18-21 SEER.
If you're ready to get a central system, you can get a high SEER system and of-course forget about the steel window ornaments.The difference in cost between 12 SEER and 14 SEER is fairly low, and considering the cost of electricity, the 14 SEER system (e.g., Goettl) is worth the extra cost.
Typically building contractors select the cheapest unit possible because of limited resources, because they don't expect to see a return on their investment, or because they don't have to pay for the electricity.
If you are "spec-ing" a new house or remodel, then you have a choice of choosing a high efficiency small airconditioner or a low-end larger unit. There's another choice..
If you can get away with a couple of used unit airconditioners for a year, then you can add central air a year or so after construction, using an equity loan. (This doesn't work if you're using a heat pump, of course. Or maybe it does, but only if you can heat with bonfires.) You may or may not need to make provisions for central air during construction, but it's a good idea to do so.
If you're buying a central A/C, get a reset timer if there's any chance of someone cycling the thermostat. (Some of the high-SEER systems include these, often as part of the controller for the variable speed compressors used on high-SEER systems.)
But don't throw out that window unit. It's nice to have a "boost" unit. Besides, cooling a single room is cheaper than cooling the whole house.
In the US, all new split (central) airconditioners and heatpumps must be 10 SEER or greater. Usually 12 SEER is available for a slightly higher price, and up to 15 or 16 SEER may also be reasonable. ("Combined single systems", for trailers and some apartment-type buildings must be 9.7 SEER or greater, with close to 12 SEER available for a slightly higher price.) In 2006, the US will go to 12 SEER for new systems.Water source airconditioners are available at 18-21 SEER. These are true airconditioners; not "swamp coolers", but of course require a water source for cooling the outside coil.
Water source airconditioners (with corrosion resistant heat exchangers) should be used if you have a swimming pool or other water source.
Heat pumps work best in mild weather. The most efficient way to heat is with a heat pump in mild weather and a fuel furnace in sub-freezing weather. York apparently produces a highly efficient gas heat pump (York Triathlon) which is different from an electric/gas furnace/airconditioner combo.
Many installations do not combine airconditioning and heat. Obviously a house hot water or steam system would be separate from the airconditioner using conventional ductwork.The ducts on a discrete (separate) airconditioning systems are optimized for cooling. This is especially true in multistory houses, since not only are the airconditioner vents at the ceiling, but most of the air enters the top floor. In other words, air delivery to most of the downstairs would be poor if used for heat. Therefore a heat pump would not be very uselful in such separate installations.
If you have a single story house with separate systems, a heat pump may be a reasonable option to supplement the furnace, but only if the additional cost is low. You'd still want the system optimized for best cooling efficiency. Heating efficiency would not be significant because the heat pump would only be used during mild conditions in any case.
I can't give a direct answer on this one. Higher efficiency airconditioners have a maximum fan speed approximating the "medium" fan speed of older units, but that doesn't explain much. Presumably in all but the hottest weather a low speed will provide sufficient airflow across both the indoor and outdoor coils to provide near maximum design efficiency. But cooling capacity (BTUs) doesn't equate to comfort.For whole house airconditioners, a higher fan speed may be necessary for delivering the cold air efficiently.
The real answer regarding fan speed is that if a higher fan speed makes the room more comfortable, then that is really optimum. If cycling the fan off or using a lower fan speed means that you need a colder temperature setting, then it's more economical to leave the fan on or use a higher speed.
(There are also reasons for running a fan at a slow speed. Besides personal preference, a slow fan speed can be used to increase moisture removal. If you have an oversized system, a two-speed fan switch will allow you to remove humidity on warm, humid days. Fan speeds are generally adjustable, so this should be a simple fix.)
For information on EER, SEER and using window unit airconditioners, please go to www.scn.org/~bk269/airconditioners.html This information below is directed to central systems which are modified or have major components replaced.Much of this is based on theory and has not been "field tested". I'm not an airconditioner manufacturer, and even if I were, I wouldn't be stupid enough to aircondition a field.
For the most part, new systems are purchased as a complete package from the manufacturer. Interchanges are possible by looking up part numbers used in complete systems. This makes sense for all-new systems, but doesn't accomodate changes later on. Most good residential HVAC people will follow these guidelines, which is "the way to go" for a new system. Exceptions are among those designing elaborate systems, such as systems for semiconductor fabrication facilities, and commercial "multi-split" systems.
If you are modifying an existing system, here are a few possibilites. These have not been thoroughly researched or tested, so YMMV!
- All components must be designed to either to the capacity of the compressor or larger. In other words, the compressor capacity should not exceed the capacity of any other part of the active cooling loop.
- A thermostatic expansion valve (TXV or TEV) is often necessary to compensate for mismatched components. The TEV is optimally sized to match the capacity of the compressor.
In general, modern systems are selected to be the smallest capacity believed capable of cooling a house in the area. This is fairly easy to estimate and affords optimum comfort and good economy of operation. So if a house can be maintained with a 24,000 BTU system, the recommended installation will be 24,000 BTU. Upgrades would be higher efficiency systems or systems perceived by the installer to be higher quality.
A system "matched" to the cooling requirements of the house results in better humidity control and presumably less cycling of the airconditioner. This is particularly ideal for humid climates where it is common to leave the airconditioner on for long periods of time.
In partial replacement of airconditioner systems, the tendency is to match components as much as possible. Again, this makes sense, since it is more likely that a matched system will "work".
Partial replacement of an existing system or modifying an inadequate system, presents a different scenerio.
Looking at replacement or modification, consider the components:
There are also several components outside of the active cycle:
- 1. compressor
- The compressor should never be rated higher than the capacity of the other active cycle components (outdoor coil, indoor coil, expansion valve).
- 2. outdoor coil (condensor on an airconditioner)
- 3. indoor coil (evaporator on an airconditioner)
- 4. expansion valve
- The compressor works against this valve to force the fluid between gas and liquid states.
- 5. thermostat
- 6. blower fan
- 7. ductwork
- 8. advanced compressor controls, if any
- On some systems, it's possible to adjust the speed of the compressor, and to adjust fan speed for optimum humidity control.
As mentioned, with a new system, someone (the manufacturer) has already matched the components according to capacity and SEER ratings. But taking these same components, here's some options:
Components Outside of the Active Cycle:
- 1. compressor
- As mentioned, the compressor should never be rated higher than the capacity of the other active cycle components (outdoor coil, indoor coil, expansion valve). There are different types of compressors, including variable speed compressors and scroll type compressors. Scroll type compressors seem to be favoured by installers as a better design.
If you have to replace one of these, consider a variable speed compressor or a smaller compressor if appropriate. Scroll compressors are considered more efficient. York is considered by many service people to be good quality.
Compressors are often protected by two fuses. Typically, there is one breaker at the electrical supply panel and a small fusebox adjacent the compressor outside. Both sets of fuses or breakers should be sufficient (small enough) to protect the wire, and one should be sufficent to protect the compressor (within the limits of the electrical codes and the manufacturer's recommendations, of course).
- 2. outdoor coil (condensor on an airconditioner)
- These can't be too large; however once the fluid is liquified, any additional cooling of the outdoor coil has minimal effect or efficiency or capacity. The only way to take advantage of a larger coil is to allow the compressor to do less work, either by advanced controls or possibly by use of a thermostatic expansion valve (TXV).
If you have an unlimited water source, obtain a water cooled system. If you have an intermittent water source, consider a "desuperheater", which is a heat exchanger for water. The "desuperheaters" are also used to provide free heat for a domestic water heater or swimming pool. The desuperheater is normally placed between the compressor outlet and the outdoor coil inlet, but if the hot water from the desuperheater isn't being used place it downstream of the coil, at the outlet of the air cooled outside coil. The desuperheater will have more effect at the outlet of the outside coil. This is backwards from a standard setup. Technically, a water heat exchanger downstream of the evaporator is not a "desuperheater" at all; it's a "supplemental condensor".)
- 3. indoor coil (evaporator on an airconditioner)
These can also be oversized to gain more efficiency.
- 4. expansion valve
- This is the most significant part of any mismatched system. Matched systems use properly sized fixed oriface or piston-type expansion valves. "Thermostatic expansion valve" (TXV) cost more (about $115 for a small system) but are able to adjust for differences in component sizes. More below.
- 5. thermostat
- The obvious -- a programmable thermostat. These are also very convenient. If the heat thermostat is separate, match the brand and type.
- 6. blower fan
- It is often possible to use a fan speed switch to take advantage of the multispeed fan. This is generally unnecessary for an optimally system, but in the case of an oversized system, reducing fan speed can be used to reduce humidity when necessary.
- 7. ductwork
- Not much to be said here unless the ductwork is inadequate.
- 8. advanced compressor controls
- Some systems can accept sensors which adjust the system output. For example some systems accept an input which allows the system to be adjusted to reduce humidity, using a humidity sensor. That helps reduce necessary cooling on warm (but not excessively hot), muggy days.
One very simple control is a timed reset switch. If the compressor is turned off, the timed reset switch prevents the compressor from attempting to restart for a predetermined time, typically 5 minutes. This is a lot more effective in preventing compressor damage than a thermal overload switch. (I have seen cases where a realtor attempted to sabotage a system by turning it off and on several times. She reported that the system wouldn't work because the compressor wouldn't go on. This was before timed reset switches were common, so I had a pretty good idea what she was doing to make that determination. I just replied that, "The airconditioner works.")
What to do
Obviously, if the system is working, there would be nothing to change. If you are replacing failed parts, try to select components which will give you better efficiency. This is often possible if the original system had a high BTU output compared to what is needed for the building. Thus, keeping the larger components with a smaller compressor may increase efficiency.One problem is that older systems used more efficient coolants (freon) and so what would have otherwise been oversized may turn out to be sized for the smaller system.
In general, the compressor should be sized at or below the capacity of the other components.
A thermostatic expansion valve (TXV) is particulary advantageous in a mismatched system or a system which is potentially mismatched as a result of field adaptation of components.
Will it work?
Well, probably. Like I said, there's a lot of theory here. At the end of the day, the things that control efficiency is the power (watts) to the compressor and the amount of cooling. Theoretically, one could reduce the pressure in the system, but that reduces the effectiveness of the system, and places the system outside the design parameters for that system. So the idea is to reduce the load on the compressor to reduce the power without going outside the compressor's design specifications.If the compressor works against a TXV which adjusts pressure according to supply and the indoor and outdoor coils have greater capacity than the compressor, the compressor should have a lighter load while still producing good output.
The change in efficiency would theoretically be dramatic. Consider a "basic" modern system with 10.0 SEER. If the same compressor were installed in a water source system, with a large indoor coil, then the system would have between 16 and 21 SEER; more like 18 SEER. That efficiency should apply even if the system is a hybrid water/air system. If that's true, that's an 80% increase in efficiency.
Thermostatic Expansion Valves (TXVs or TEVs)
All single fluid cycle airconditioners use an expansion valve. That's how the fluid goes from one state to another. All work by restricting ("metering") fluid in some way. The typical location is at the inlet to the evaporator. When fluid leaves the expansion valve, it is at lower pressure, and therefore evaporates. This is the refrigerant cycle.There are two basic types of expansion valves used for refrigeration; a fixed type and a modulating type. The fixed type used in the HVAC industry would be a capillary tube or a metering piston. (These are sometimes called "oriface" types but a TXV is also an oriface.) The modulating or regulating type would be the TXV.
In the 1970s, fixed orifaces became popular as a cost savings measure. In the late 1980s, there was a resurgence of TXV use as energy consumption became an important issue.
A TXV, like all expansion valves, is located at the inlet of the evaporator (indoor coil for cooling). It has a temperature probe positioned at the evaporator outlet, and works from a combination of outlet temperature and inlet pressure. They're marked with different colours according to the coolant used, apparently because each coolant has a different operating pressure. The maximum capacity is determined by the compressor size.
The TXV has the capability of regulating refrigerant flow over a wide range of load conditions. Ideally the TXV should be large enough to accomodate the capacity of the compressor. It should not be oversized although a TXV supposedly has an ability to shut off refrigerant flow entirely. Fortunately it is possible to adjust a TXV, which accomodates variations in the capacity of the system.
Because the TXV can increase or decrease refrigerant flow according to load, it can raise or lower the amount of energy used by the compressor. Another benefit is that it can control the temperature of the compressor and keep it in a closer range through a wide load variance. Both of these are great benefits for energy consumption and compressor life. Also, the 100% shut-off valve can increase SEER by up to .50 because the liquid line is pressurized and ready to supply liquid to the valve the moment the compressor starts.
Adjustment of the TXV is determined by the amount of superheat in the refrigerant gas. Superheat is the temperature of the refrigerant gas above the temperature given by the pressure and temperature chart for the specific pressure. In air conditioning, most valves are set to 10°F (6°C) superheat. The superheat determines the amount of liquid refrigerant mixed in the return gas. Too little superheat will cause the liquid refrigerant to damage the compressor. Too much superheat will cause poor evaporator coil performance and compressor overheating.
This is actually a lot less complicated than it looks. In the field, this is done by use of a thermometer. If the same thing were to be done with a fixed oriface valve, one would have to calculate the proper cooling pressures for the combination of components.
(This may be the only place where hybrid air/ground source systems are described, so you may wish to bookmark this page.)If you have a potential for a source of groundwater, even in the future, it is possible to convert your installation to a hybrid air / ground source airconditioner. Advantages are:As with pure water source systems, this presumes you have a water supply. If you are in an area where the cost of obtaining water for cooling is prohibitive, this would not be useful. Cooling water sources include such things as shallow water wells. A swimming pool makes an excellent cooling water source.
- It's relatively inexpensive
- It does not require sensors for the water. The water is caused to flow (pump or valve) whenever the outdoor fan, compressor or other device is running.
- The groundwater supply need not be reliable
- The system need not be protected from interruptions in the water supply
- It defers costs of a water supply installation
- When connected to a water supply, efficiency (SEER) is significantly increased
For this to work, you need
When to Do It
- a thermostatic expansion valve (TXV or TEV)
- a scroll compressor or other "high efficiency" compressor (preferred but not required)
- a water heat exchanger connection
The water heat exchanger is usually available as a "desuperheater" or "desuperheater coil". These small heat exchangers are intended to provide domestic hot water from airconditioner systems. (The term, "desuperheater" is derived from the entropy heat given off by the compressed fluid.) When a desuperheater is used for its original purpose, it is installed at the inlet to the outside coil. When used for converting an air source airconditioner to a hybrid system, the desuperheater coil goes at the outlet of the outdoor coil.
- When installing a new system
- When "opening up" the refrigerant system for repair
The desuperheater coil is the most commonly available heat exchanger. Other refrigerant/water heat exchange coils can also be used.
Heat exchange coils (desuperheater coils) come in two varieties:
The choice depends on the anticipated source of cooling water. Normally copper is sufficient for noncorrosive water. (Nomel is generally used in association with geothermal water, which often contains sulpher. Swimming pool water could theoretically be corrosive, but this is not likely.)
- copper
- nomel or corrosion-resistant
Why This Works
The efficiency of an airconditioner is achieved by increasing the heat discharged by the outside coil (evaporator), but only to the extent that the compressor does less work. In order for this to happen, three things must occur:Items 2. and 3. are achieved by a TXV and a scroll compressor. Lower efficiency compressors will work to some extent, but without the TXV, the system will retain a high evaporator pressure no matter how cool the discharge is. The TXV senses temperature and therefore only closes enough to maintain its supply in a liquid state.
- the refrigerant must be cooled to the maximum extent practical;
- the expansion valve must allow the evaporator pressure to drop when liquid is fluid is available at the lower pressure; and
- the compressor must be able to take advantage of the reduced load.
The first item is achieved by the heat exchange coil (the desuperheater coil). By placing the desuperheater coil at the output of the outside air coil, the air coil discharges the bulk of the heat, and allows the water to achieve maximum heat discharge. This allows the most effective heat discharge component to discharge heat from the refrigerant last. The water source heat exchanger brings the temperature to the low point. If the water is cooler than the ambient air (which is always the case in the summer), then the water source heat exchanger (the desuperheater coil) can start where the air coil left off.
This allows the water in the outdoor coils to condense at a lower pressure, which means less work for the compressor. The pressure in the outdoor coils is reduced by the TXV, and as the pressure drops, the more effective heat discharge maintains the refrigerant discharge in a liquid state.
If the desuperheater coil is installed at the inlet to the outdoor coil, the desuperheater extracts heat, but the outside air coil does the final cooling.
Hybrid Air/Water Source Airconditioner HOWTO
- Install a TXV in the system as the expansion valve.
- If you have a choice, use a scroll compressor.
- Install a desuperheater coil at the output of the outdoor coil; i.e., between the outdoor coil and the expansion valve.
- configure the desuperheater so that water will completely drain from the coil in the winter. If you install the water connection, make sure the water can completely drain from the outdoor connections and any other exposed connections.
- provide an electrical connection to operate a pump or valve when the compressor is running. The outdoor fan power supply is ideal for this.
Hybrid Air/Water Source Heat Pump Modification
This is a little more complicated whein applied to heat pumps. In many systems, the fluid reverses in the outdoor coil. This means that desuperheater has been moved to the inlet of the outdoor coil. The desuperheater coil will serve to supply heat to the outside coil but will not present efficiencies comparible to a water source heat pump. The water-warmed fluid will affect the system because less frequent defrosting will be required.
To show that I'm not original in my work, I copied more stuff from my www.scn.org/~bk269/airconditioners.html (The main webpage below; that webpage related to economical use of window "unit airconditioners" with and without central systems).
- Insulate
- especially the attic. Also get storm windows if you don't have insulated windows.
- Get a programmable thermostat.
- These things are much more useful for heating systems, but the ability to lower temperature pre-dawn and let temperature raise during the day should help costs a little.
- High Performance Filters
- These aren't really an efficiency item, but there are high performance paper and electrostatic filters available. Typically these are 10 cm (4") thick paper filters, and electrostatic filters of similar dimensions. The paper filters seem to work well. The paper ones are similar to the design of HEPA filters used in cleanroom filtration. I figure if that design is superior to electrostatic filters when used in cleanrooms, it's probably superior to electorstatic filter when installed in my house.
Avoid "anti-bacterial" or "anti-microbe" filters. The best thing that can be said about these is that the chemicals used in them might be safe. So why pay extra for toxic materials? (I also avoid the supposed high-performance filters listed by their own manufacturer as inferior to their electrostatic filter, but that's just my personal prejustice.)
Most insulation effect is had through the roof and windows. Therefore after-construction wall insulation, may not be cost effective in all cases.Storm windows give an existing house the most "bang for the buck". The insulative value of storm windows is better than many ordinary double pane windows. So even if you install new windows, keep the storm windows too.
Attic insulation is usually a bargain. Go for an "excess" amount.
Fiberglass is much better then cellulose (although to see some of the claims of the c"ellulose manufactures on the web, you'd believe that fiberglass is essentially scraped up pig manure). Cellulose tends to settle, especially in walls. Once installed, fiberglass is cleaner.
Another important factor is that cellulose requires mold inhibiters and flame retardants. I'm sure the manufacturers claim their stuff doesn't leak out, but there is none required in fiberglass. Current formulations include boric acid, sodium borate (borax) and sulfates. If you are able to determine that the insulation contains boric acid and borax, and no sulfates or other additives, it should be safe.
Once installed, fiberglass is cleaner than cellulose.
If you have knob-and-tube electrical wiring do not use cellulose. That combination is not safe and may be a fire hazard. Incidentally, romex (PVC shielded wire) should not rest against polystyrene insulation because the polystyrene makes the PVC brittle.
If you're D-I-Y'ing insulation, find out about moisture barriers. The moisture barrier should be on the "indoors" side of insulation. Some people use visqueen (plastic sheeting) as a supplemental moisture barrier. If you do so, punch holes in it with a knife so as to permit the insulation to "breathe".