Window unit AC

I live in a 14x74’ mobile home. It has a 3-ton package unit outside for heating and cooling. We keep it 75° during the day (cats at home) but we like 69° at night to sleep. My question to you is…. Would we save money by just getting a window unit (8000 BTU or so) for the bedroom instead of cooling the whole place down at night? We could leave the central unit on 75° all the time and just lower the bedroom. But I don’t know if that would really save money or not. We’re only cooling about 850 square feet in total. (We have one bedroom closed off to use as storage).
Thanks for any advice!!!
Chuck.


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in my old house, pre civil war, that's all I have is one 5000 btu window unit. even in hottest summer my electric bill might go as high as $75, but we have inexpensive TVA COOP electricity here. and this little thing will freeze us out if turned down all the way and put on to run constant instead of thermostat setting. a little fan to blow the air around to back bedroom gets the cool air around. the newest models of ac units are so much more efficient than just a decade or so ago its not even funny.

Save your money and run you big AC. The wall unit will suck up more energy on the 120V. Plus your big unit will work harder to cool off the rest of the house in the morning. Plus the excess humidity in the rest of the house ain't good during the night. I'm in Mississippi and can say how troubling humidity can be.

Think in the terms of thermal mass, where inherently everything (walls, furniture, appliances etc) is maintained at your base desired temp of 75, you need to use additional power in the bedroom to drop the temp there to your desired 69deg or it would also need to run continuously to maintain that desired temp and not need to run hard to get the temp down. Additionally if there is a return (or even gaps around the door) in the bedroom your 69 deg air is drawn off and distributed to the rest of the house.

Overall though given the efficiencies of the newer window units the cost may not be that great.

I have a 1200 square foot story and a half house. Of which the upper level is only used for storage. Is not heated or cooled.

When I bought the house 24 years ago I put in a new high effeciency forced air furnace and central air.
After 10 years I started having to call a service company to get the central running ever spring.

Eventually they gave up, suggested replacing it.

So for 1/4 of the cost of replacement I went to Lowes and bought 2 window units. A larger 8k btu for kitchen living room area.
A smaller 5k btu for bedroom. Normally the living room unit is set to 72 degrees and left alone. The bedroom unit we would turn on around 9pm and by 10 it was cool in there. Shut it down at 6-7.

My electrical costs dropped to 1/3rd of what they had been. Mostly because we are only cooling the area's we are in the most.

I'll never go back to central. In fact this spring my furnace finally died and I had the company pull the central.
This is what works for me, but YMMV.

That package unit is probably 80% 12 sear. Not very efficient if you are not shaded well.

Check your room size versus a/c manufacturers charts. Don't go to big or it won't have enough air to work with in a small room.

Got 2 window units to help out the minimal size central AC unit that is installed when we bought this new house.
These keep up just fine after a hurricane takes out the grid.
Wifes happy, Me happy

If you live in a place with high humidity, as I do, keep the central unit going. I could do what you consider, but the house would mold over in a short time. The mildew would smell the place up.

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Originally Posted by Electrod47

Save your money and run you big AC. The wall unit will suck up more energy on the 120V. Plus your big unit will work harder to cool off the rest of the house in the morning. Plus the excess humidity in the rest of the house ain't good during the night. I'm in Mississippi and can say how troubling humidity can be.

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I wouldn't bet on it. My 4.5ton/54000BTU rated heat pump system died several years ago. House had never really been comfortable. We replaced it with 5 5000btu window units, and cut our electric bills in half. A couple of years ago, we put in a 18000btu mini-split system, which isn't enough for the whole house, but three more 5000btu window units do most of what we need. I'll be putting in one more for my workshop, since I'm finally getting where I could work in it. After this past winter's major hike in electric bills, my bill is still falling. We had to use electric heat then, but it should be a lot better before long.

Bill in OKC

We had a central air unit installed in our very pre civil war home in 1997 when we moved in, on a very old furnace utility bills here are combined gas/electric. The furnace was replaced in 2005, that year utility rates nearly doubled here and our bill dropped substantially. In 2015 we had what the weather service called a straight line wind, it looked to me like a tornado (my third) our outside central air unit was fried. We had a new efficient one installed and again with a rate hike our bill dropped. I like my central unit as opposed to a, or more window units. Again, to each his own.

I think a modern 8,000 is about the biggest than can still run on a 15 amp 120 circuit,
but you're up pretty close to the max. power it can handle.

If you can run a 220 circuit for a bigger unit, it tends to work out better, and maybe a little cheaper for some reason on power usage.
It doesn't have to be the size of a dryer cord, but a smaller one that uses those outlets that look like "- l" .

BTW, 8 or so years ago, I had about $700 of insulation blown into my attic. Dropped my electric cost about 10-15%. I understand from the OP, this is not feasible for you. Back about 5 years ago, I replaced my A/C (42 yoa) with a 2 stage H/E unit. Average electric bill is $150, hottest of months. Keep humidity about 50% in winter, slightly lower in the summer.

As Bill stated, take a look at the mini split. It may work for you and they are easy to install. I've installed four of them and they came precharged. No idea on prices nowadays as everything has gotten ridiculous.

I put a mini split in my basement for mainly heating purposes this year. It does work well, super quiet. You can get units that are multi stage, one outdoor unit and up to 5 indoor units attached. Easy to install and you still have windows.

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Originally Posted by Winger Ed.

I think a modern 8,000 is about the biggest than can still run on a 15 amp 120 circuit,
but you're up pretty close to the max. power it can handle.

If you can run a 220 circuit for a bigger unit, it tends to work out better, and maybe a little cheaper for some reason on power usage.
It doesn't have to be the size of a dryer cord, but a smaller one that uses those outlets that look like "- l" .

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I thought an 8500 btu was the maximum, since that was the biggest I could get about 10 years ago when I installed one in my insulated garage. Fast forward to this year and it finally went out. It had always been a bit undersized and I was amazed that I could get all the way up to a 14,500 btu in 120 volt at my local Walmart. So I did, and for a very reasonable price. My garage work area is wonderful now.

I would suspect that improvements in efficiency have allowed manufacturers to increase the output of these units.

Thanks for the replies everyone. I appreciate the information!
Chuck.

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Originally Posted by Beagle333

I live in a 14x74’ mobile home. It has a 3-ton package unit outside for heating and cooling. We keep it 75° during the day (cats at home) but we like 69° at night to sleep. My question to you is…. Would we save money by just getting a window unit (8000 BTU or so) for the bedroom instead of cooling the whole place down at night? We could leave the central unit on 75° all the time and just lower the bedroom. But I don’t know if that would really save money or not. We’re only cooling about 850 square feet in total. (We have one bedroom closed off to use as storage).
Thanks for any advice!!!
Chuck.


Sent from my iPhone using Tapatalk

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Depends on your big unit.
That's exactly what we did when we lived in the trailer with a 3 ton AC unit(same size as yours) since you're only keeping the single room cooler than the rest of the house it will save you some money (assuming your thermostat isn't in your room) My electric bill usually stayed under $200 in the summer. This was an all metal trailer in pretty much full sun

The house we're living in now (2 miles away), I had bought from foreclosure and completely redid, including additional insulation. I put in 2 heat pumps 3ton ea (1 for each floor, multistage/variable capacity) to replace an old 5ton package unit the house is just shy of 1700sqft (~840sqft per floor). Last summer the upstairs unit was out for 3 months (couldn't find the part) so I used a window unit upstairs to keep it tolerable. My electric bill got around $250 per month, normally it is $150 per month or less in the summer

So depends on what your central unit more than anything else

I have wondered if it would benefit the efficiency of my heat pump, since the condenser unit is in direct sunlight.

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Originally Posted by fixit

I have wondered if it would benefit the efficiency of my heat pump, since the condenser unit is in direct sunlight.

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in cool mode I doubt it'll have much effect since whatever the outside is, the coils will be hotter, and the fan forces the same temp air (ambient) over it regardless.
However, being in the sun might actually help you in heat mode since the sun will give it more "heat to move inside" I don't know how to calculate how much it would help though

If you are on a well, using a sprinkler or drip hose on the coils can make a huge difference. You can actually run a condescending unit without a fan if you have a sprinkler going nonstop.

It never seems to fail if I get a no cool call in the afternoon the unit sits on the west side of the house.