Window units vs. Central

[Highsmith]

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?

[Bird]

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.

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] 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. [img]/forums/images/icons/tongue.gif[/img]

[Bird]

</font><blockquote><font class="small">In reply to:</font><hr />
The NEW multi-compressor spilt systems are a better way to go.

[/ QUOTE ]

That's something I've never seen, nor even heard of, but you sure do make it sound good.

[Pat]

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

[Bird]

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.

[Pat]

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

[Bird]

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.

[Bird]

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.

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] 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. [img]/forums/images/icons/tongue.gif[/img] 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. [img]/forums/images/icons/crazy.gif[/img] 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. [img]/forums/images/icons/smile.gif[/img] 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. [img]/forums/images/icons/smile.gif[/img]