Window A/C units? (I'm in the mid-atlantic.)

mtn said:

ProDarwin said:

You guys speak about 'the north' like its so much cooler than 'the south'.  In the hottest months the difference is small.  Chicago averages in the summer are a couple degrees lower and a couple points less humid.  Obviously the summer is much shorter there.

Its nothing like the winter where there is a 30 or 50 degree difference.

And many times it is much more humid, thanks to that thing called "Lake Michigan"

I think my first datapoint was wrong on humidity.  To answer your question, almost never:

Chicago

Charleston

Even where I live is vastly different, which may or may not be 'deep south' depending who you talk to

Temperature (note the dotted line for 'percieved temp' is about 15 deg higher in Charleston in the middle of summer)

Chicago:

Charleston:

SVreX said:

You and I agree completely..  closing off a room reduces convection, and therefore has a great deal of effectiveness. 

But effectiveness and efficiency are not the same thing. 

I have no problem with small window units in a Northern house. It’s quite effective. 

But it is not efficient. The conduction thermal loss to adjoining spaces is pretty significant. 

I can put a window AC unit in an uninsulated garage and leave the doors open. Standing in front of it is still effective, but definitely not efficient. 

A well insulated house is a different story. The thermal envelope is utterly important, and maintaining equal temperatures throughout the enclosed space whike minimizing transfer or loss to outside the envelope requires very little energy. But it has to be centralized. That’s efficiency. 

Are you saying it's impossible for a window ac to be efficient? Cooling one room with a small window ac will cost less than cooling an entire house with a central AC. I'm confused as to how that's not a more efficient use of money and electricity. And thinking much beyond those two is maybe a bit more philosophical than a discussion of air conditioning needs to be.

In reply to dculberson :

I think it depends on how you define "effectiveness" and "efficiency".  Like you said, this E36 M3 could get philosophical quickly.  

Put it this way: a 454 Big Block is a very effective way to move a 1968 Camaro from Point A to Point B, a quarter of a mile away.  But it's not a terribly efficient way to do so.  

Back to A/C- centralization of the system using ductwork is a fairly inefficient way of transferring heat.  This is where mini-splits come in.  The HVAC occurs at the point of use.  Maximum efficiency.   And yes, cooling a 200 sq foot bedroom using 1000 watts of window units will be less costly than cooling 2000 sq foot of house using 8000 watts of central A/C.  But the central A/C wins out on cost/ sq ft.  

In reply to dculberson :

Again, that’s not efficiency. That’s cooling a significantly smaller volume inefficiently, and therefore cheaper. 

Efficiency is using fewer BTUs to cool and maintain a fixed space a specific delta. 

volvoclearinghouse said:

In reply to dculberson :

I think it depends on how you define "effectiveness" and "efficiency".  Like you said, this E36 M3 could get philosophical quickly.  

Put it this way: a 454 Big Block is a very effective way to move a 1968 Camaro from Point A to Point B, a quarter of a mile away.  But it's not a terribly efficient way to do so.  

Back to A/C- centralization of the system using ductwork is a fairly inefficient way of transferring heat.  This is where mini-splits come in.  The HVAC occurs at the point of use.  Maximum efficiency.   And yes, cooling a 200 sq foot bedroom using 1000 watts of window units will be less costly than cooling 2000 sq foot of house using 8000 watts of central A/C.  But the central A/C wins out on cost/ sq ft.  

Completely correct. 

Although, ductwork is only inefficient if it is outside the thermal envelope. If it is inside the thermal envelope, no efficiency is lost. 

The efficiency of an AC unit = how well it cools. 

The efficiency of a structure= how well it retains conditioned air. 

Cost to cool a structure= how many BTUs are burned. (That’s actually the one most of us care about)

Mini splits are SIZED smaller. They are designed to cool a smaller area. They are very inefficient when used to cool an entire structure. 

However, they do enable cooling smaller parts of the structure inefficiently, which means lower electricity costs. 

If the structure’s thermal envelope was compartmentalized, Mini splits could be very efficient. 

Since most structures do not have compartmentalized thermal envelopes, increased delta= less efficiency. In other words, Southern homes are harder to cool with mini splits because the cooling load (delta) is bigger. Each of those unused warm spaces around a cooled space makes the mini splits more inefficient. 

It works in reverse for Northern homes during heating seasons. Mini splits are to cooling as electric baseboards are to heating. Sure you can heat only one room at a time, but everyone knows you are gonna pay more if you try to heat a Northern house with electric baseboard heaters. Because they are inefficient. (But they are effective for localized heating)

Ductwork is horribly inefficient.  Not because of thermal losses.  Because of the fan required.  Go down to the average central a/c unit and look at the size of the fan running that is adding heat to the system.  Fan wattage is reduced like tenfold from central a/c to minisplit/window unit. 

SVreX said:

Efficiency is using fewer BTUs to cool and maintain a fixed space a specific delta. 

Alternate definition:  Efficiency is using fewer BTUs to keep occupants cool and comfortable.  I don't have A/C because I want my living space(s) cool, I have it because I want to be cool.

To use an engine analogy:  Some container ships burning bunker fuel can approach 50% thermal efficiency.  It would still be very inefficient for me to drive an empty one across the ocean to go to Asia.

Paul, you're arguing semantics here. In this situation, efficiency means "how much money does it cost to keep the rooms we are in cool, while we are in them".

In reply to mtn :

Nope. No argument. 

volvoclearinghouse said:

In reply to dculberson :

I think it depends on how you define "effectiveness" and "efficiency".  Like you said, this E36 M3 could get philosophical quickly.  

Put it this way: a 454 Big Block is a very effective way to move a 1968 Camaro from Point A to Point B, a quarter of a mile away.  But it's not a terribly efficient way to do so.  

Back to A/C- centralization of the system using ductwork is a fairly inefficient way of transferring heat.  This is where mini-splits come in.  The HVAC occurs at the point of use.  Maximum efficiency.   And yes, cooling a 200 sq foot bedroom using 1000 watts of window units will be less costly than cooling 2000 sq foot of house using 8000 watts of central A/C.  But the central A/C wins out on cost/ sq ft.  

RE; costs. Compare the cost of buying 3 smallish A/C units and putting them in your windows yourself versus having a central A/C installed.  

Then compare the costs of maintenance.  When the central units fail a service call is likely greater than the cost of a new replacement  window unit. 

Finally, there is the cost of replacement to maintain efficiency.  Since a new central A/C unit is so expensive  they tend to remain in place long after more modern efficient units are available.  Whereas smallish window units can be replaced in a rotational fashion as more efficient units become available.  

The great thing is the older inefficient units retain some value either as spares or on Craigslist.  Whereas a 20 year old central A/C unit will cost to be properly disposed of. 

One more point, Window units do not have to be put in windows.  Cutting a hole in the wall up high anyplace the unit will be effective.  And  is less than an hours work. The only cost is a short 2x4 (or 2x6 ) to properly frame around the opening. The unit can stay in place year around and just be covered in the winter with an enclosure painted the same as the rest of the house.  Come summer cooling time the enclosure comes off  and returns in the fall.  

With a remote control, The whole thing works like a mini split at a tiny tiny fraction of the cost. 

SVreX said:

The efficiency of an AC unit = how well it cools. 

The efficiency of a structure= how well it retains conditioned air. 

Cost to cool a structure= how many BTUs are burned. (That’s actually the one most of us care about)

Mini splits are SIZED smaller. They are designed to cool a smaller area. They are very inefficient when used to cool an entire structure. 

However, they do enable cooling smaller parts of the structure inefficiently, which means lower electricity costs. 

If the structure’s thermal envelope was compartmentalized, Mini splits could be very efficient. 

Since most structures do not have compartmentalized thermal envelopes, increased delta= less efficiency. In other words, Southern homes are harder to cool with mini splits because the cooling load (delta) is bigger. Each of those unused warm spaces around a cooled space makes the mini splits more inefficient. 

It works in reverse for Northern homes during heating seasons. Mini splits are to cooling as electric baseboards are to heating. Sure you can heat only one room at a time, but everyone knows you are gonna pay more if you try to heat a Northern house with electric baseboard heaters. Because they are inefficient. (But they are effective for localized heating)

That’s very clearly said.  I agree with your descriptions and most of your conclusions.  

Now this may be nit picking but I honestly believe it has some value to the discussion. 

Regarding night cooling.  Let’s assume 90 degree day temps, with 75 degree evening temps. Not uncommon, and not linear in that those are peaks.  

The difference between a cooled bedroom in the middle of the night and an uncooled house gets smaller.  So the delta you mentioned is reduced. 

So much so, that my master bedroom cooled by a small unit in the wall above my bed  keeps us comfortable  until well into the summer heating season. 

 Neither my wife nor I are trim and fit, in fact with her diabetes and my age we suffer greatly in the heat.  Without A/C  it could be life threatening. 

Yet one tiny A/C  keeps us comfortable until well into the summer heat. 

You're arguing (ok, stating?) that the only measure for efficiency is watts per btu/hr of cooling. That is not the case. Efficiency is using the least of the available resources to accomplish your goal.

I agree that a central air conditioner will beat the pants off of most window air conditioners on a SEER rating scale. But in this context that does not mean it is the most efficient.

No. 

I am not arguing, and frankly don’t give a damn what we call it, or what definition we decide to make up for whatever word we choose. 

I don’t care. 

I was trying to share my knowledge and experience as a Certified Energy Rater in plain language, for those who might care.

I agreed with everyone that window AC units are the best option for the original poster. 

They are not, however, the best  (nor cheapest) way to cool a structure with a more significant load on it than his.  

I’m out. 

Thanks for all the replies guys. It made an old English t.v. series pop into my head. 

Necro'd.

I have a house with central A/C, it's glorious. Although the third floor master bedroom can get toasty. I have an attached rental/motherinlaw that is not connected to the central air system. I know summer is drawing to a close but I really like my current tenants and was considering buying them a window A/C unit. Most PNW residents aren't used to 80, let alone the 90+ it's been. The rental is very small, 950sq ft or so. With a pretty open floor plan. The living/dining/kitchen/entryway are all essentially one room, with a bedroom and another no-specific-use-room off that. I had been thinking a big 15kbtu unit for the living room window as it's in the shade and would shoot it across the most lived space. Reading through this thread make me think that maybe two smaller 5k btu units would do better? (one in the bedroom and one in the living room?

Thoughts?

We are running window units this summer because our central ac took a dump and due to covid, we don’t have the discretionary income to fix this right now. Or we do, but are pinching our pennies. 

We are running 5ks in the bedrooms (2), upstairs, and an 8k downstairs. That cools the dining room, kitchen, and family room. Probably 800 square feet. 

One thing I really like is that the downstairs one is WiFi controlled. Blah blah blah security, yeah, I know. But it is really nice to set it at 85, leave for a week, and set it to 69 a few hours before we get home. I wish I ponied up and did that in the nursery too. Bedroom for us, adults, I’m happy with the cheapest 5k mechanical available. 

For the rental, I’d probably say go with an 8k in the living room if budget allows, 5k if it doesn’t, and a 5k in the bedroom. 

Amazon has midea U shaped window units now. The compressor is outside, there's no accordion shades on the side, so they seal better, and you can still use the window. I ordered one before my hospital stay, it arrived while I was inpatient and my neighbor installed it for me prior to my getting home. I can't speak on install, but he said it took about a half hour and it's honestly quieter than my central AC unit is. In addition to remote and programming, there's an app, so I can turn it on when leaving PT or work and have it cool when I arrive home. I ordered it to assist my weak central unit and because with the window lock and smaller window opening needed, I'd trust it more from a security standpoint in a detached garage later on down the road. 

Here's my only hint on window units. Buy them from costco. They have them at the beginning of the cooling season. When they go bad in 2 years, take them back to costco and they will return it or exchange for a new one. So therefore you can  buy once but keep cool for years.