central AC vs window unit

[Mongoose]

We have an opportunity to maybe get a (new) window unit AC for free. There is an opportunity cost in that we could alternatively get usable household items (about $320 worth of soap, garbage bags, paper products, cleaning supplies) instead.

Our current cooling needs are covered by a very old AC unit (20 year old central AC) that is...ok. It's hard for us to tell how expensive it is to run as our electric company doubles the rates in the summer when we run it. (It looks like it takes a ton of power based on the bill but we're paying $0.18/kwh summers as opposed to $0.093/kwh October-June)  I keep it set on 78-80 F. The summer temperature has been running between 90 and 100 F with 50-80+% humidity (June to September-ish).

I am not heat tolerant and won't become so (even without running an air conditioner...heat acclimation just isn't going to happen). We have considered switching to two window units (or maybe one?) instead of using the central AC. We turn off the central AC if/when it gets down to 75 and open the windows/use fans. This doesn't happen much during the hot parts of summer.

There is one 12'x12' bedroom where our family could sleep (and therefore put an AC). The other two bedrooms are near that but the kids don't care where they sleep and would prefer our room anyway. Then there is a hallway to a large living room (15'x30') open to the kitchen.  No windows in the living room open but there is one window in the kitchen that does. (Don't ask. The house predates us finding MMM and has a stupid set-up for heating/cooling/crossbreezes. And it's too big.) My thought is that the living room/kitchen area is too big to cool with a window unit but I don't know.

I did some calculations based on the "average costs" on the AC units at the beginning of summer and decided that, with the cost of the units plus the electricity to run them we would probably not break even compared to the costs of the extra electricity to run the central AC.

Options are:
1. Get a window unit for the bedroom. This will undoubtedly improve sleep quality/quantity as the bedroom gets pretty hot at night (keep in mind I sleep best at meat-locker temperatures...think windows open in the mountains or the heat set on 55 F). I don't know how much/if we could use the central AC less. This would leave us ~$180 for household consumable items.

2. Get two window units (~$150 out of pocket) and switch to those instead of the central AC. Put one in the kitchen window and one in the bedroom. (Not sure if the one in the kitchen would cool the living room...living room has 2 ceiling fans). I have no idea how much/if any this would save us on electrical costs. (Before we moved here, I lived a decade in a place where AC isn't needed and I don't remember how much it cost/what we used prior to that).

3. Keep using the central AC and use the $505 on other household consumables (only available in merchandise...limited selection but all stuff we would buy anyway). We could get a year's worth of stuff we normally buy for "free" this way.  This seems like the Mustachian/rational/frugal option as we're not sure of the efficacy of the window units in cooling our house adequately.

[pbkmaine]

Cost is less important than what is best for your health and well being. If it were me, I'd get the one window unit and run it in the bedroom so that you could all sleep well. Question: will you need to replace the central AC anyway when you move from that godforsaken place?

[Mongoose]

I don't know if we'll need to replace the central AC when we move. Is a newer AC critical on resale? It and the old furnace both work. I've never sold a house so I don't know anything about it.

I don't know how realistic we are (having zero experience) but our preference would be to finish the cosmetic stuff anymore sell the house without replacing anything (knowing we might get a long less for it but hopefully not caring).

[brokescientist]

I don't know if we'll need to replace the central AC when we move. Is a newer AC critical on resale? It and the old furnace both work. I've never sold a house so I don't know anything about it.

I don't know how realistic we are (having zero experience) but our preference would be to finish the cosmetic stuff anymore sell the house without replacing anything (knowing we might get a long less for it but hopefully not caring).

]

AC is not critical on resale, however an inspector will point out the age and condition of the current unit.  It would be a good starting point for the potential buyer to talk down the price.

That is me speaking too,  I am sure others will not even consider buying a home with a non-working/old/broken AC.

[Mongoose]

Another question is how much it helps/hurts to open windows at night during high humidity? For instance, based solely on temperature, it would make sense for me to open the windows tonight since it is 70 outside and 78 inside. However, the humidity is above 90%. It feels cooler with the AC on 78, I presume because it is lowering the humidity. Then again, my grandparents lived near here with no AC while I was growing up and we always had the windows open. I don't recall any mold or other humidity induced problems.

[Choices]

I'm not a parent, but training the kids to want to sleep in the same room with us seems like a potential landmine and a hard habit to break.

[Cranky]

Do you have a ceiling fan in the bedroom? We have an elderly central air system and keep the temperature at 78, and with the ceiling fan going at night, it is *plenty* cool for sleeping.

While the kids are probably fine with sleeping in your room for now, that's not a long term, big picture solution.

How long is your cooling season? We have maybe 9 weeks in the summer that we run the a/c, so while it does shoot up our electric usage, over the average of the year it's not too bad.

For me, if the central a/c died now, and I didn't have the cash to replace it, I'd buy a couple of window units. My house isn't that big, and we don't need to cool it for a long stretch. The central a/c is nice, but it's definitely a big investment. (I plan to replace it next summer, along with the furnace.)

[Mongoose]

Yes we have a ceiling fan but with that running it isn't cool enough for sleeping for *me* at 78. I wish it was. That's what we've been doing but I get poor sleep during the summers. I have never dealt well with high humidity + heat. During the day I am fine with the AC set to 80.

We're not interested in a long term solution as hubby has promised we can move in two years or less (we really want to move for other reasons). We don't want to replace the central air or furnace if we can help it because we don't plan on being here long enough to for a new system to pay off in terms of lower bills. Our goal is to relocate back to where we don't need AC.

We need cooling quite a bit June through September. I do shut the system off and open windows if it drops to 70 or below at night. Those nights I sleep better (fan in windows). It does get more humid in the house though (last night's humidity was >90% but I opened the windows anyway; AC hasn't turned on yet today). My strong preference is for outside air over a shut up house. Averaged over the year, the electric bill isn't too bad. It would be nice to get the bills down over the summer when the rates double.

I'm curious as to how much/if a higher central air setting plus window unit would help with the bills. Or if there's an AC use strategy that helps minimize usage/costs while preserving a reasonable level of comfort (defined as 75 or preferably much lower at night and 80+ with lower humidity during the day).

[Spork]

From my own anecdotal data: Some of those window units are just awful from a power consumption point of view.  In particular, the ones sold at the big box stores are really cheaply made and generally run on 120v. 

Now... whether it's "more efficient" to run a crappy window unit on a single room vs a more efficient HVAC system running on 240v... that's probably something you'd have to experiment with.

In my case 2 crappy window units cooling 600sqft cost more in electricity than 2 large central air units cooling 2400sqft.  (There are other factors here besides the window units... but the window units were kilowatt suckers and struggled to keep up with even the small apartment.)

[Mongoose]

Thanks for the anecdote Spork. This would likely be the cheap unit described as it would come from a big box store. My HVAC is old but we can deal with the costs for two more months and then won't need it until next June.

[ohsnap]

...
Our current cooling needs are covered by a very old AC unit (20 year old central AC) that is...ok. It's hard for us to tell how expensive it is to run as our electric company doubles the rates in the summer when we run it. (It looks like it takes a ton of power based on the bill but we're paying $0.18/kwh summers as opposed to $0.093/kwh October-June)  I keep it set on 78-80 F. The summer temperature has been running between 90 and 100 F with 50-80+% humidity (June to September-ish).
...

No advice for you, but I'm extremely jealous of your electricity rates.  Our marginal Kwh cost in the summer is >$.30!  (the first kwh we use is $.15 and it goes up on a tiered basis very quickly from there!)

[Mongoose]

Holy hats ehallison! I'm unhappy with this rate but egads...yours would be horrific! I double checked my math and it's correct. We seem to have rates tiered by usage (lower rates for more usage September through May and flat rates in the summer). It came as a shock to me because I had always assumed my AC was a horrible energy hog (I'm not saying it isn't) but hadn't accounted for rates doubling during the summer. My bill was $223.54 last month but would "translate" to $117.42 for their non-summer rates. It would be $439 with your rates. Not great but it means that my AC isn't quite as bad as I thought. (Not a fan of AC but I don't tolerate heat and humidity well).

[AMandM]

Your electric bill should show the number of kWh used each month, so assuming your other usage is roughly constant you should be able to calculate roughly how many kWh the central AC uses.  If you know the wattage of the window unit you're looking at, and how many hours a night you'd run it, you can calculate an upper bound for how much it will use.  (The actual usage will be lower, since it won't be running at full power all night long.)

Here's another option:  Spend the whole $505 on household goods, and buy a cheaper window AC unit with some of the cash you saved.  I don't know how big your bedroom is, but looking at Walmart I see you can get a unit to cool 150 sq. ft. for less than $120, or one to cool 350 sq. ft. for $210.

[Mikila]

I would go with the window a.c. units.   Our experience has been that they are cheaper to run than the HVAC.  Two window a.c.s in our house, run on low 70% of the time, off the rest of the time, uses 8-10kwh/day less energy than setting the HVAC to 76/78 degrees.  The two window a.c.s keep it in the same  temperature range. 

However, our house is small and outside temps are 96/73  ish this time of year.  If you live in a 2000 square foot house in Phoenix, I doubt you would be happy with the results of using this approach.

[JLee]

It may be worth having someone come take a look at your central AC unit - it may not be operating properly.