Seattle Community Network
SCN Member Pages
(if that doesn't work, try http://www.scn.org/ or
http://www.scn.org/members/)
This could cut your electric bill in half.
Overview
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 explains the last item, carefully arranging unit airconditioners. This also works for houses with inefficient central air systems.
(skip to theory and technique for efficiency)
Terminology - "EER" and "SEER"
- 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.
- 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 central ACs are 14 SEER, with 15 SEER available from most manufacturers and 16 SEER mentioned. Water source airconditioners are typically 18-21 SEER.
Unit airconditioners are inherently efficient because:
- They don't have ductwork.
- They direct cool air where needed.
- They allow zone cooling, e.g. bedroom at night.
- It's possible to buy one or two small efficent units to do most of the cooling, so efficiency can be had at a low cost.
- Unit airconditioners are typically more efficient than central airconditioners even without counting loss through the ductwork.
This technique is very cheap because it only involves purchasing one very good airconditioner for the house. The rest of the house can be cooled with a combination of
- Older units already owned
- Purchased used airconditioners
- An inefficient central airconditioner
- A high-efficiency central airconditioner
In my previous house, I had one high efficiency unit in the bedroom, one fairly good unit in a computer room, and one lower efficency unit downstairs for occasional use. In other words:
- 12 EER 7500 BTU unit in bedroom upstairs
- This was purchased new. It runs most often, occasionally all night. (Panasonic, approx. $350. at Frys)
- 9 EER 8000 BTU unit in computer room
- Found thrown out on the sidewalk. It runs while I'm on the computer, and on hot weekend days it helps cool the whole house.
- 7.5 EER 5000 BTU unit in kitchen
- This would "cover" the kitchen in case the upstairs units weren't enough.
The house had separate circuits for a bedroom airconditoner and the kitchen. The computer room airconditioner draws 7.5amps, which is easily handled by the 15amp circuit there.
On particularly hot days, the high efficiency bedroom unit ran full-time. The airconditioner in the computer room ran when I was in the room or to "boost" the bedroom unit when cooling the whole house. The low efficiency unit downstairs was rarely run.
This means that most airconditioning is done with the high efficiency unit or with the high efficiency unit boosted with another good unit. The maximum cooling of the house is obtained by turning on the low efficiency units, but the total time of use of the lower efficiency units is low.
If the EER ratings were 12, 7.5 and 6, respectively, the average would be 12 with just one running, 9.7 with two running and 8.5 with all 3 running (assuming same BTU for each). Not bad for only one unit being high efficiency! (My own numbers are 12, 9 and 7.5, but the numbers in the example are more realistic.)
For houses with no central air:
- Buy the most efficient airconditioner you can find.
- Install that efficient airconditioner at a location where it will be used most of the time. Preferrably this is also a location which will tend to cool the rest of the house.
- Determine how many more units you need to keep the house cool.
- Use the second best airconditioner for another location with good air circulation to the house.
- Use any other units where needed.
This is only necessary if you need to figure out what power is available for different units.This describes North American wiring.
- In general, a bedroom or living area circuit is 15A.
- If you connect two 7A airconditioners to the same circuit, you will probably exceed the capacity of that circuit and so the breaker will trip.
- Each kitchen circuit is 20A. Refrigerators, microwaves ovens and other heating appliances may draw a large percentage of that 20A. Refrigerators will typically draw about 7A. So try to find a kitchen circuit without the high-power appliances on it.
- Dedicated airconditioner circuits are 20A. Most dedicated airconditioner circuits can be converted to 220v, and all dedicated circuits can be converted to 117v. An electrician converts 117 to 220 by connecting the "white" wire (common) to a second pole of a 220v breaker. The conversion from 220 to 117 is done by disconnecting the white or red wire from the second pole and connecting it to the common post. In both cases, the current rating remains the same, and of course the outlet must also be changed.
About the highest efficiency presently available is under 12 EER. Friedrich makes several high quality units, including large and small units. They also make high efficiency heat pumps and "through wall" units. Panasonic makes 11 EER units in 5800 BTU ($270) and 7500 BTU ($350).It's possible to find 10 EER units used, but most are lower, in the 7 EER range. NOTE In the US all new A/Cs are at least 9.8 EER or SEER (as of 2002). Most used ones are about 7 EER.
Note that SEER (central airconditioners, "seasonal energy efficiency rating") is different from EER. Central airconditioners have both SEER and EER ratings. The SEER rating is generally about 3 units higher than the EER rating.
There are some central systems which have particularly high SEER ratings because they can adjust parameters related to line pressure. Typically these use variable compressor. This is a definite savings and may be less costly to run alone than the combination of central air and window units..
On a new airconditioner, it's easy. Look on the box.Airconditioners made in the last 20 years have EER ratings somewhere on the data tag. Sometimes the data tag is inside the front, behind the filter.
If the EER isn't listed, calculate it by dividing BTU/watts. (see "terminology" above.)
A Common Malfunction
or How to Get a Free AirconditionerIt seems the primary failure mode of unit airconditioners is the thermostat, which simply goes out of range. The thermostat should "click" on or off so as to activate the compressor. Usually the thermostat range goes too warm, meaning the compressor never gets turned on. Check it by:
CAUTION: Do not cycle the compressor off-then-on in less than 3-5 minutes, because that will "smoke" the compressor.
- With airconditioner OFF, rotate thermostat until you hear a click.
- If you fail to hear a click, turn the fan on. Then try to turn the airconditioning on. You should hear the addition of the compressor noise.
So if you find a unit on the sidewalk (and it's not resting beneath a window on top of a dead pedestrian), and the thermostat doesn't click, then chances are the unit will work if you adjust the thermostat mechanism internally. (I've found sidewalk units which were fully functional, so perhaps they were thrown out because the owner just couldn't get electricity to the sidewalk.)
To Fix the Thermostat
As with all electrical projects, do not attempt this unless you are qualified. Making a mistake with electricity can kill you or start a fire. Remember, you are dealing with serious current and voltage - this is not like changing a network card on a computer. There are a lot of exposed wire terminals and connectors at line voltage, and if you mess with this hot, you will have a Palestinian peace demonstration.
- Make sure the airconditioner is unplugged, stupid.
- Open the inside panel. A 1/4 inch (6 or 6.5mm) nutdriver helps. Be careful with the vent cable and the thermostat tube.
- Unscrew the thermostat from the panel face.
- The adjustment is a small screw, usually to one side of the shaft. Turn counterclockwise to reduce temperature.
- If convenient, drill a hole in the panel between the shaft and the screw, so you can make further adjustments.
- If you break the thermostat, bypass it by locating the short connection between the thermostat and the fan/power switch, removing that connection and moving the other wire from the thermostat to that fan/power switch connection.
- Make sure there aren't any short circuits when you put the stuff back together. If there are any short screws, they go near the coils.
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.) You may or may not need to make provisions for central air during construction.
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 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 ordinary 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 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.
No. Go to a garbage sale (sometimes spelled "garage" sale but they seldom sell their garages) and buy a couple of fans and a fly| swatter.More to the point, when buying a high efficiency airconditioner, you are essentially buying the parts to a higher capacity system. Fortunately, you partially benefit from the higher capacity in that it becomes more economical to use the system a little longer. That means you can get away with less because you can run it longer on hotter days, and you are more comfortable because you can run it on days that are not very hot.
Also consider the payback period, and the fact that energy costs tend to increase substantially over time. What may be economical today may be considered costly to run after an increase in rates.
It's a bit of energy conservation. In the summer of 2001, I was helping a couple of people with their airconditioning, so I figured I could write up something on the web to help others.This primarily concerns window units. Central systems are theoretically professionally sized. Unfortunately there are far too many contractors out there who will sell a system with the lowest SEER rating available and call it "high efficiency" (without qualifying the statement).
Contractors: If it's the Low-Bid 10 SEER Special, then why not present it as such? A two-tiered bid will usually beat the competition anyway.
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.)
This is a separate subject. So I put it on a separate page, For information on EER, SEER and using window unit airconditioners, please go to www.scn.org/~bk269/centralac.html
- 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. If the aircondiioner uses a separate thermostat, match the make and type of the heating thermostat.
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.
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"
There is no minimum engine size for an airconditioned car. That's because it is not necessary to use the airconditioner at maximum acceleration. Most new cars have a shutoff device which turns off the compressor during maximum acceleration, so it isn't even necessary to manually do this.