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Old 01-29-20, 05:02 PM   #1
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Default Could a window A/C be more efficient than my heat pump?

Maybe January is a weird time to be thinking about A/C, but I'm in North Carolina and it will be on in a month or two. Last summer I bought a house, and it has 5 ton heat pump that I think is a bit oversized. On top of that, I mostly keep the vents closed in 3 out of 4 bedrooms, which makes the heat pump even more oversized for the rest of the house. It seems to cycle a lot.

The center part of my house is hard to heat and cool due to having 18' ceilings in the kitchen/living room area. Humidity is also a killer here.

Here is what I am thinking: I could install a cheap 5k BTU window unit in the living room area, and simply let it run 24/7. Obviously this would not cool the entire house, but would cut down the number of cycles on the main heat pump. I think this might do a better job of removing humidity, and maybe reduce the temperature swings between cycles for better comfort.

Am I crazy? Do you think this would cost less to run than just letting the main heat pump do everything? The window unit would literally run non stop for 6 months or more, no cycling. I have a great place to install the unit through the wall where there is currently a dog door, I don't have a dog. I need to plug the hole either way...

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Old 01-29-20, 10:07 PM   #2
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Depending on the size of the rooms you actually use you could install a mini split in the bedroom you use. Then install a mini split in your main living room.

Mini splits in SC do a great job cooling. I had a 4 ton AC and switched to mini splits with a good savings. Make sure you get very high SEER rating my 9K units are 29 SEER they make even better ones now.
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Old 01-30-20, 12:14 AM   #3
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I thought about that, but the initial cost would be significantly higher. I don't think it would be worth the energy savings - I only plan on staying here a few years. If the main heat pump ever needed to be replaced, I would definitely go with mini splits.

One of the main reasons I'm thinking about the window unit is how totally dirt cheap they are in the spring time. And I already have several of them piled up in the shed...

I do plan on remodeling the master bedroom and making it a bit larger with it's own separate heating/cooling, I think I probably will use a mini split there. I kinda want to play with the window unit as a proof of concept.

I'm also a bit concerned about power outages, I'm in a very rural area and it would be death to go without A/C for very long in the summer. If I just ran the one 5k unit it would be really easy to run from a very small generator. Wouldn't totally cool the house but definitely would help humidity and make things bearable.

29 SEER is pretty freakin' amazing though! I think my current heat pump is 15 SEER...does that mean a newer mini split would cost about half to operate? Currently about 75% of my electric bill goes to the heat pump.
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Old 02-08-20, 07:50 PM   #4
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Ok so the dirt cheap twisty knob window units are all over the place because they don't work too well. Nearly all of them have the lowest efficiency ratings ever. For your goals they don't have enough surface area to dehumidify well either.

My advice would be to find a 9k to 12k window unit with a digital control and energy saver / energy star functionality built in. These units are usually physically larger than the equivalent capacity cheap units and have bigger heat exchangers in the box. The control board has different programs that would suit your needs better.
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Old 02-22-20, 08:30 PM   #5
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Quote:
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Ok so the dirt cheap twisty knob window units are all over the place because they don't work too well. Nearly all of them have the lowest efficiency ratings ever. For your goals they don't have enough surface area to dehumidify well either.
I disagree.

First, the particular unit I have makes 5,000BTU at about 350-400 watts depending on temperatures. This gives a COP of roughly 3.75. No, not as good as a mini split, but not bad...better than my heat pump. (which is only 6 years old) How long would I have to run a more expensive unit to save several hundred dollars in electricity? Also do not want to run an expensive air conditioner 24/7 and burn it out in a couple years. That would negate any energy savings.

Secondly, I've used this exact air conditioner as a dehumidifier. (Not mounted in a window) I don't have access to my data at the moment, but it was equal to or more efficient than most dedicated dehumidifiers on the market based on the amount of water removed and amount of electricity used. It really removes a massive amount of water when it does not cycle on and off.

The goals here are:
1. Reduce the number of cycles on the main heat pump. It will definitely do this. Excessive cycling causes higher electricity use and higher humidity. Also go from hot/cold/hot/cold all day on my body, I hate this.

2. Remove more humidity than the main heat pump. The fact that it would be running continuously is the biggest factor for this. That is what gets a lot out of the air. When you have a heat pump with a short cycle time, like I do, it doesn't have time to dehumidify before it finishes it's cycle and turns off. Last summer it hovered around 60-70% humidity inside.

The big question is would this use more or less electricity than just leaving the heat pump as is? And will it make the house more comfortable with fewer temp swings and less humidity? That is something that is going to be hard to determine without testing.

I don't know what you mean about the control board "suiting my needs better". The only thing I "need" is an on/off switch. It doesn't even need a thermostat. If it never cycles off then there is no big current rush when it starts. From a humidity perspective I don't think a unit this size will ever get below about 40% when it is 90%+ outside, simply from the outside leakage into the house.

Another thing I wasn't thinking about, my heat pump is rated at a particular efficiency, but my duct work and air handler are all in a very hot/humid crawl space. Even the cheapest window unit is probably more efficient if you account for all the duct leakage in a 25 year old house...
(Sealing/insulating/conditioning the crawlspace is on my list for the future)
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Old 02-22-20, 11:14 PM   #6
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Any window unit you buy that has a standard rating is done with a 10 degree Delta T between indoor and outdoor dry bulb temperature on high speed.

So, your 5kbtu 9.8 EER happens when it's 75 inside and 85 outside on high speed. Plus or minus 1 EER is normal. Also, this only holds true after the outdoor fan starts slinging indoor moisture on the outdoor heat exchanger. Without the "swamp" cooling effect, performance is closer to that of a dehumidifier. So, on those swampy 100 degF days, your actual EER is lower, probably closer to 8.

However, on these "lower efficiency" days, the unit is actually moving more BTU than a cooler day. Since the condenser pressure is higher, the cap tubes flow more mass into the evaporator. Considering that the humidity is higher on hot days, the capacity increase will follow the humidity load.

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Old 02-23-20, 06:43 PM   #7
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I'm not talking about what the window unit is actually "rated", but rather measurements I have taken myself. I've never seen it go over something like 412w on a hot day, though I have not done extensive testing on this. I'm just assuming it is actually doing the 5k BTU of cooling it claims because I can't really measure that.

My temps aren't that extreme though, summer highs are usually in the mid 80s, but lows often don't go below upper 70s. Humidity is the big killer. I live next to a lake and it might as well be like an Amazon rain forest in the summer. I'm hoping if I lower the humidity inside I can keep the A/C a little warmer and still be comfortable.

I'm super curious about playing with a liquid desiccant for dehumidifying and drying it using heat pump reject heat...but that's a whole different topic...
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Old 02-23-20, 08:08 PM   #8
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In your application, the little 5k unit will pull lots of latent heat out of your house with the water. Plus some cooling effect, but mostly condensate. This will help your main cooler move the temperature faster and more evenly, if you lower the dewpoint enough.

What I'm saying about the digital control units is that they have a dehumidify cycle where the compressor cycles on and off with the program so it uses less energy to move the same amount of water out of the air. With one of the newer models running this mode, you might only average the same power draw as the 5k unit, but pull out more water. Just saying, they're available everywhere second hand pretty cheap.

If you are trying to get humidity under control, it would be super cost effective to do a blower door test and seal up the biggest leaks. I usually seal up the window around the ac unit with heat shrink film, double sided clear tape and clear packing tape. I usually glue the fresh air flap shut on the smaller ones to keep them from leaking air.
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Old 02-26-20, 11:22 PM   #9
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Quote:
Originally Posted by jeff5may View Post
What I'm saying about the digital control units is that they have a dehumidify cycle where the compressor cycles on and off with the program so it uses less energy to move the same amount of water out of the air. With one of the newer models running this mode, you might only average the same power draw as the 5k unit, but pull out more water. Just saying, they're available everywhere second hand pretty cheap.
Hmmm...why would cycling the compressor make it more efficient? It seems that most energy upgrades these days are about cycling things as little as possible.

I found my notes from testing the A/C as a dehumidifier last summer. Test was over 2.5 hours outside where the temp and humidity stayed at 73F and 89%. During this time, it collected 4.4kg of water. This works out to an "energy factor" of 4.4L/KWH. From what I can tell most energy star units are rated at less than 2L/KWH. I'm not sure why there is such a big discrepancy, maybe the official test is at lower humidity, but it seems like they try to use the best possible conditions.

Pretty close to a gallon per kilowatt seems pretty darn good! I was actually using it to make distilled water to flush out a radiator...

My house is pretty well sealed except for the floor and crawl space. It was built in '83 but every window and door was replaced a few years ago with modern vinyl dual pane, and they are all sealed well. They also put on new siding and house wrap. Unfortunately I just have a lot of glass letting heat in and out. I plan on spraying the walls in the crawl space with spray foam and putting a better vapor barrier down, before installing a dehumidifier down there. My duct work is definitely not in the best shape.
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Old 02-27-20, 12:31 PM   #10
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Ok so I'm going to run through a couple of quick suggestions not related to the proposed solution and quit talking about them. Since it's winter, find heat leaks. The cheapest infrared cameras on the market work better when it's cold outside. Turn on a range hood vent or a bathroom fan or two and take a tour.

Same thing for the ductwork, turn the heat up to 85, wait a few minutes, and go scope out the flaws. I see it this way: you'll be paying for something; you might as well be paying the for the cure and not recurring medication. Same thing with glass holes: window tint or screen on the outside if heat gain is a factor, clear film or something else on the inside if heat loss is a factor. It makes little sense to improve the heat retention and rejection of the rest of the place if you have hundreds of square feet of glass holes in it. Might as well get some super effective medication.

Ok I'm done.

Your example of the operation of your mighty mite is typical of the industry. If you ran the unit perched indoors, it would only let you down after most of the moisture was wrung from indoors. Then, it would probably still break even as far as latent moisture removed versus added powerline heat. Your central unit would still have an easier life. Best possible water removing action due to zero Delta T and Delta H, your sensible compressor heat is swallowed by water down the drain. Same setup and argument as the desiccant fountain, only the desiccant fountain has a brine pump somewhere.

Once you have split the air conditioner airflow, the only time you get the same effect is late summer evenings. The removed water helps to subcool the condensed refrigerant and forces the evaporator temperature down. The little contact patch at the bottom of the condenser really does a lot. Besides chilled water feed, it enjoys wet bulb temperature due to the slinging water. (Not outdoor wet bulb, condenser wet bulb.)

As for dollars per comfort? It all costs extra. The key is balance of the system. If the mighy mite saves money is very subjective, way too many factors to consider. It will increase comfort if it's able. Run what you bring, see if mother nature beats you. If she does, trial and error baby.

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