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Highsmith
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Window units vs. Central
12/22/07 05:45 PM
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I know in years past the window units were terribly inefficient and central was by default better. But now I see window units with the Energy Star seal of approval and I wonder if there is much difference in electricity usage when cooling the same space. Anyone done the math?
Best Regards,
Highsmith
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Bird
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Re: Window units vs. Central
[re: Highsmith]
12/22/07 06:46 PM
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I thought in years past, it was the other way around. The window units were more efficient and cheaper to operate than centrail air, but they were noisier and you frequently had a stronger breeze blowing right on you than with central air.
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CJDave
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Re: Window units vs. Central
[re: Highsmith]
12/28/07 07:28 AM
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 There have always been built-in deficiencies that the required shape of window units just cannot overcome. More efficient compressors and coils have helped, but the game still tilts in favor of central air. NOW..... here is where it gets tricky..... Central Air actually has more opportunity for screwups. Bad ducting, plugged filters, slipping drive belts, ducts placed in the wrong spot; the list goes on and on. So a WELL MAINTAINED central air is more efficient, but once one little thing goes awry, the advantage goes away. The NEW multi-compressor spilt systems are a better way to go. Each room has it's own evap unit and they are very quiet because all you have is a coil fan. There are no ducts, only small refrigerant tubing from room to room, and the rooms have individual thermostats so zone cooling is easy. It literally is the best of both worlds.
CJDave
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Bird
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Re: Window units vs. Central
[re: CJDave]
12/28/07 07:53 AM
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In reply to:
The NEW multi-compressor spilt systems are a better way to go.
That's something I've never seen, nor even heard of, but you sure do make it sound good.
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Pat
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Re: Window units vs. Central
[re: Bird]
12/28/07 11:23 AM
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Bird, The reason window units could be cheaper to run is you only conditioned the rooms where they were. That and a poorly installed central unit with just one big zone is not particularly efficient.
I have a central unit, well 3 actually but each is zoned so you only heat or cool the parts of the house you intend to occupy at the time. Even if my units were not high SEER (which they are) just cooling part of the house at a time would give savings in operating costs. Years back I recall seeing lots of houses with multiple window units. If you try to condition the whole house then a central unit would probably be better. If you run just the window units for where you are you'd have to have a zoned central system to match or better the economy.
Pat
"I'm not from your planet, monkey boy!"
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Bird
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Re: Window units vs. Central
[re: Pat]
12/28/07 01:43 PM
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Well, Pat, I'll take your word for it, and that of the other guys. I just always heard that the window units used less electricity and were more efficient. Maybe I heard wrong. The first little house we bought had a sorry heating system and no air-conditioning except a 6500 BTU Sears window unit in the living room. Our bedroom was in the southwest corner of the house, so it got the afternoon sun. I worked two-thirds of the time on a late night shift and a third of the time on an evening shift, so I moved that 6500 BTU unit to the window in that bedroom so I could close the bedroom door to sleep during the day and put a new 18,000 BTU unit in the living room. They did a fine job; but were so noisy compare to central air, which we'd had in an apartment and which we had in the houses we bought later.
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Pat
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Re: Window units vs. Central
[re: Bird]
12/28/07 02:06 PM
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Bird, It is likely you were told wrong instead of hearing wrong. A moderate window unit can be cheaper to operate because they literally can't use as much electricity as a whole house central unit which is probably oversized for the house. It may be more economical but less efficient.
You may recall me impugning the HVAC industry with aspersions of how they typically oversize A/C units because 1. they make more profit on a big one and 2. they don't know how to properly calculate the right size so they oversize to be sure the unit is big enough and they don't get called back by a sweating homeowner.
Humidity often does not get proper credit for its influence on comfort. Dehumidifying is as important as cooling to achieve comfort. One of the most efficient dehumidifiers is an undersized A/C unit which runs all or nearly all the time trying to satisfy the thermostat. Oversize central units (a good estimate is that 3/4 of the central air units installed are oversized) do not dehumidify nearly as well as would be desired and consequently run more to cool more because wet air needs to be cooler before you feel comfortable. This is because the evaporative cooling of perspiration on your skin (when the air is properly dried) helps a lot with achieving comfort. Dry air will feel comfortable at a higher temperature whereas damp air must be chilled more.
The cost of cooling a house is mostly proportional to the difference between the inside and outside temperatures (the amount of heat you have to pump.) If the air is dried sufficiently you don't have to cool the house as much to feel cool.
An oversized unit knocks the air temp down and satisfies the T'stat quickly before much air gets passed through the unit so dehumidification is minimized. Oversize units run in short bursts with a fairly low duty cycle. This is more wearing on the equipment, and is the least efficient run pattern so it cost more per month to be comfortable than if the equipment were sized better. Properly sized units run a larger percent of the time and pas the air through the coil several times before the T[stat is happy. Each trip through the coil takes a little more water out of the air.
Pat
"I'm not from your planet, monkey boy!"
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Bird
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Re: Window units vs. Central
[re: Pat]
12/28/07 04:08 PM
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Yep, Pat, I know about the effects of humidity. In some homes, the condensation removed by the HVAC system runs out on the ground and you pretty well see how much moisture is removed. In other homes, such as this one, the condensation drain is routed into the washing machine drain, so you never see the water. There is, of course, a sort of "trap" in the line (not quite as drastic as most P traps, but serves the same purpose) and I pour a little bleach through that line once a month.
Now with window units in the past, and with the roof mounted RV air-conditioners, we had quite a bit of water run from them, but oddly enough (to me, at least), I've never seen any water drip from the window unit I put in my shop/game room out back.
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Bird
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Re: Window units vs. Central
[re: Bird]
12/28/07 04:11 PM
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And, in this thread, we've talked about the difference in "efficiency" of window units vs. central air and I said I'd heard the window units were more efficient; poor choice of words. What I've actually heard is that the window units are more economical; i.e., cheaper, both to buy and to operate. And while I've "heard" that, I don't have reliable evidence or data to either prove or disprove it.
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CJDave
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Re: Window units vs. Central
[re: Bird]
12/28/07 07:20 PM
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I'm going to step in here and restate that window A/C units are at a disadvantage from an efficiency standpoint by virtue of their shape. Central air condensers, especially split systems, have way more opportunity to get rid of heat more efficiently because they can have an unlimited coil area. As I mentioned previously the opportunity for screwups in central air is huge; including design errors, installation errors, and lack of maintenance. A condenser full of leaves isn't going to be very efficient, and a plugged filter won't let much air circulate. As Pat pointed out, few A/C dealers are well trained enough to design and install a perfect system, so efficiency is sacrificed even as the system is built. There is a great deal of difference between "efficient" and "low cost". A window air could have low input AMPS, but if there are three window units going, you may have as much current draw as a whole house unit, and you are not removing moisture as well as central does.  I really like the new multiple evap coil central air systems. Each big room has it's own thermostat and evap unit. The coil fans are very low noise, and the multiple units give you true ZONE air conditioning. Each evap unit has it's own little compressor housed in a common condenser. The same system is used for heating.  I like to collect our condensate and pump it out to the back yard for the critters to drink. Guess where a big percentage of the fresh water used on cruise ships comes from? Yep, you guessed it; condensate water off the A/C. Many of the new window units don't pee any water because the condensate is channeled to a pan that abuts the hermet ball and the heat from the compressor evaporates the water. By doing it that way there is no problem with the water trickling down Mrs. Finklestein's window two floors below.
CJDave
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Bird
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Re: Window units vs. Central
[re: CJDave]
12/29/07 08:16 AM
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In reply to:
Many of the new window units don't pee any water because the condensate is channeled to a pan that abuts the hermet ball and the heat from the compressor evaporates the water.
Thanks for the information. I never heard of a "hermet ball" and don't know what that is, but I reckon that's what's happening with the condendsate from my window unit.
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Pat
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Re: Window units vs. Central
[re: Bird]
12/29/07 08:57 AM
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Bird, A design innovation that prevents water runoff from A/C units while increasing their efficiency is this:
Let the water pool up at the bottom of the unit till it is deep enough to be hit by the blade tips of the fan blowing on the condenser coils. This fan may be improved and have a continuous ring around the circumference. Anyway the fan picks up the water and slings it into the air stream going through the coils. The water is evaporated by the hot coils. The heat required to evaporate the water cools the coils quite a bit, increasing the efficiency above just a dry air flow. Unless you have TREMENDOUS humidity you may never see liquid water come out of the unit.
If you stand in the hot air flow out the back of the unit you may feel little drops of water once in a while after it has run enough to have enough water to touch the fan.
This is a really slick trick and was long overdue to be employed.
Pat
"I'm not from your planet, monkey boy!"
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Bird
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Re: Window units vs. Central
[re: Pat]
12/29/07 09:04 AM
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In reply to:
This is a really slick trick and was long overdue to be employed.
Yep, sounds like someone came up with a good idea.
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egon
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Re: Window units vs. Central
[re: Pat]
12/29/07 10:13 AM
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This is not new and may not be so slick. 
In some of the plants I worked at water was sprayed on air coolers during hot days. It took many years but finally it was determined that solids in the water precipitated out on the coils changing the heat exchange effects. Did more harm than good!
Egon
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Bird
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Re: Window units vs. Central
[re: egon]
12/29/07 12:41 PM
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I've probably mentioned this before, but many years ago, there was a product on the market that got a lot of publicity for awhile, then seemed to just disappear. It was the tubing, nozzles, etc. to spray a fine mist of water on the condensor coils during hot weather. It initially sounded like a good idea, but maybe it went away for the same reason you mentioned.
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CJDave
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Re: Window units vs. Central
[re: Bird]
12/29/07 06:26 PM
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The condensate water that is used to help cool the condenser coil has little or no impurities in it so build-up on the coil is not a problem. As soon as you try that trick with tap water, you are basically distilling the water and that leaves the impurities, whatever they may be, on the coils. You sometimes see evaporative coolers used as a "booster" to help lower the temperature of the air that is used to cool the coils of the condenser. That works well in very arid climates like FRESNO CALIFORNIA for example. That's an old Fresno trick; using an evap cooler to boost the condenser. After all, when it's 115, you got to do SOMETHING.
CJDave
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CJDave
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Re: Window units vs. Central
[re: Bird]
12/29/07 06:30 PM
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I call the modified spherical-shaped housing that surrounds hermetically-sealed compressors the 'hermet ball". Don't touch one while the unit is running unless you touch it real close to the low side inlet.
CJDave
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Re: Window units vs. Central
[re: CJDave]
12/29/07 07:08 PM
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Dave, I never lived or worked or went to school in any building with refrigerated air until I was nearly 20 years old. I bought a 7 year old house trailer (later called mobile homes and now manufactured housing). At that time, all of them that I knew of were metal skinned, and mine was 8' wide by 28' long; yep, smaller than modern RVs, but also at that time, the 10' house trailers were the top of the line. Anyway, the bedroom was in the back end, but I put a Fedders air-condtioner in the front wndow and it moved enough air to cool the bedroom in the back when I worked nights and slept during the day.
But my parents only had a big evaporative cooler in a living room window of their house. I think they were usually called swamp coolers back then. So I'd seen homes with evaporative coolers and homes with refrigerated air, but I had never seen homes with both until 1990 when we spent a month in Phoenix, AZ.
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CJDave
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Re: Window units vs. Central
[re: Bird]
12/30/07 06:11 AM
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I can still remember the day in the summer of 1947 when my Dad got up from his sweaty attempt at taking a nap, ....he had a dairy and milked cows and delivered milk in the pre-dawn hours each day. ....and went to Azevedo Hardware in Gustine, California where he bought an Essick Evaporative Cooler. The copper tubing that was used to take water to the cooler from an exterior faucet was "war copper" a tube that was slightly undersized, ... not a full 1/4".... and which would not fit the regular fittings so you had to solder a transition sleeve to use it. We had that cooler as our only climate modifier up until the time I left home to go away to college (1961) and the apartment we had in college was my first-ever use of an air conditioner. A swamp cooler will make 70 degree air, IF it is in an arid climate, and IF the pads are in good shape, and IF the water distributor tubes are nice and clean.
CJDave
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Re: Window units vs. Central
[re: CJDave]
12/30/07 06:53 AM
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That pure condensate water will have an affinity for any airborne particles and soon have lots of impurities in it.
Egon
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Bird
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Re: Window units vs. Central
[re: CJDave]
12/30/07 07:27 AM
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Dave, in the late '40s and early '50s, I had an old army cot; not sure just where it came from, but I'd put it out in the backyard and sleep outside a lot in the hot summer time. And I sometimes helped my younger sisters move their standard double bed out in the yard to get out of that hot house.
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Pat
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Re: Window units vs. Central
[re: egon]
12/30/07 08:02 AM
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Egon, Water with dissolved solids (minerals) sure would coat the condenser and reduce its functionality.
Condensate is essentially distilled water which is why it is OK to put through the condenser as I mentioned to Bird. If you had mineral free water available spraying it on the condenser would be OK. Just using typical tap water is NOT GOOD.
Pat
"I'm not from your planet, monkey boy!"
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Re: Window units vs. Central
[re: Pat]
12/30/07 05:21 PM
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Pat:
The condensate starts out fine but due to particulate matter in the airflow things may change. Chances are the condensate may also have sub 7.0 PH levels but this will depend on your geographic location.
Egon
Egon
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CJDave
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Re: Window units vs. Central
[re: Bird]
12/30/07 06:19 PM
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As kids we used to play in the upper deck screened in "sleeping porches" that some houses had. That's where the beds went all summer in hopes of catching a breeze. Ohterwise it was lay there and sweat. 
CJDave
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Re: Window units vs. Central
[re: CJDave]
12/31/07 06:42 AM
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Yep, we weren't fortunate enough to have something like that, Dave, although my paternal grandparents did. They had a huge screened in back porch all the way across the back of the house, so that's where they slept in the summer. In fact, before television days, I guess they spent most of the day out there, too. I know they had rocking chairs, a sofa, and my grandmother's sewing maching that stayed out there. Then in later years, it was completely enclosed and a window unit air-conditioner was installed.
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