Supplementing Central AC System


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Old 08-17-23, 10:49 AM
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Supplementing Central AC System

I have a 20-year-old system that has become inefficient. Over the last several years, I have noticed energy bills creeping a little higher each year in the hot summer months. It has been 100°+ for the most of the last 6+ weeks and my AC system is doing everything it can do, but cant keep the house below 78° in the afternoons. It runs about 18 hrs a day and doesn't cycle. It cools fine at night but still doesn't cycle as much as it should. Last electric bill was $850.

I plan to kick this can down the road until after the first of the year before replacing the system so here is my question...would adding a window unit in the living room allow the system to cycle during the day and use less electricity or would any energy savings on the central unit be negated by the energy usage of the window unit?
 
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Old 08-17-23, 11:02 AM
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I would have a good/reputable HVAC company come inspect your system. They might find that it's low on refrigerant or maybe a duct has come loose costing you efficiency.

In the mean time make sure you have a clean air filter and that all your registers/vents are open. Also make sure any rooms you are cooling have their door open for better ventilation.
 
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Old 08-17-23, 11:25 AM
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I had a tech come out and look at the system. I was hoping it might be a tad low on refrigerant, but the tech said pressures were good and he wouldn't add any refrigerant. He said the system is doing all it can do with it this hot. I'm just wondering if adding a window unit would help supplement the central AC system and allow it to cycle more throughout the day?
 
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Old 08-17-23, 11:39 AM
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suptupp - Generally, typical home HVAC systems are sized and designed to only maintain about a 20° lower indoor temperature compared to the outdoor temperature. So, if the outdoor temperature is say 100°, maintaining a 78° indoor temperature is actually to be expected. Attic insulation, roof color, west facing windows, open shade/blinds, brick that absorbs and retains heat, square footage, layout, etc. etc. all make a difference in how well a HVAC performs.

Your A/C running constantly during the day is also to be expected at 100°+ outdoor temperatures and it is designed to do exactly that... As I'm sure you can understand, it's typical for the A/C to perform better at night when it's not competing against the high daytime temperature and intense sun and its radiation.
 
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Old 08-17-23, 12:25 PM
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I'm not a pro but I was under the impression the 20 degree temp split (delta) was between the return and supply air. air. Too me there are to many variables to make any statement that relies on "outside" air as a primary factor. But if your an HVAC pro....(In Phoenix, So Cal, the middle east their homes are all 90-95 degrees inside during the day?)

To answer the OPs original question, I can give you my recent experience. I have an older R22 system in a 2 story (upper story only has a 200 sq ft bedroom and a bathroom/loft open to the downstairs - total about 400 sq feet). I recently (4 weeks ago) purchased one of those "new" u-shaped window ACs from Costco (12K BTU). While doing various tests with it over the last few weeks I've come to the following conclusions: My main AC use has been cut by about 1/3 on roughly like temperature days. The upper floor/bedroom is now a livable space (it functioned as an office when tolerable). The downstairs receives the "fall" of the cooler air, causing the downstairs to remain noticeable cooler throughout the day. Not sure but since my older 5 Ton 10 SEER R22 one speed HVAC is running less the energy used by the smaller 1 Ton 16 SEER can't be that much more. The bonus advantage is that the office upstairs is now functionally usable all the time (much to the appreciation of my wife). Hope this helps.
 
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Old 08-17-23, 01:09 PM
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Thanks for the input; I understand why the system is performing poorly. I live in Houston so it's not the first time we've experienced heat waves. The system used to perform fine in these conditions with the expected added energy consumption that goes with it. The only thing that has changed is the age of the system. The system is just not as efficient as it used to be and now it can't maintain a temp below 78° when it's this hot. I'm mainly looking for a way to supplement the system and ease energy usage to get through the remainder of the summer. I will probably replace the system in Jan/Feb.
 
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Old 08-17-23, 02:02 PM
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I was under the impression the 20 degree temp split (delta) was between the return and supply air. air.
It's actually both return air vs supply air as well as outdoor vs maintained indoor temperature during high summer temperatures - all depending of course on the specific HVAC system size and its quality of installation and the specifics of the house in which the system is installed.

What is The 20 degree Rule?

Most air conditioning systems can only handle a 20-degree difference between the outside and inside air temperatures. If it’s 80 degrees outside and your thermostat is set to 70, your unit should have no problem. But if it’s over 90 degrees at that same setting, your unit will have to run almost non-stop to get your inside air temperature to 70 (and it still might not make it).

 

Last edited by Kooter; 08-17-23 at 03:40 PM.
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Old 08-17-23, 03:56 PM
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It's currently well over a hundred outdoors here but 75° to 77° indoors, which actually can feel a bit chilly.

My thermostat setpoint is 76° which provides an indoor temperature of 75° to 77°, which is about ~30° lower than the triple-digit outdoor temperature. The system is perfectly designed and setup but that's about the maximum indoor vs outdoor difference I can expect with such excessive outdoor heat and radiation from the intense sun. Most systems can only expect 20° to 25° maximum difference.

Any outdoor temperature below 100° or anytime after sunset I can easily get the inside temperature cold enough to become a meat storage locker!




 

Last edited by Kooter; 08-17-23 at 04:14 PM.
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Old 08-17-23, 04:07 PM
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Last electric bill was $850.
​​​​​​​I plan to kick this can down the road until after the first of the year
At $850 a month a new high efficient system is going to pay for itself, do not walk but run and get some quote!

BTW, our GEO system costs us around $175 per month average, it's about the most efficient system you can get!
 
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Old 08-17-23, 05:27 PM
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Most air conditioning systems can only handle a 20-degree difference between the outside and inside air temperatures.
Like I said, I feel sorry for "most" everyone in Phoenix/Las Vegas this year living with 90-95 degree temperature in their houses and offices. Must suck.
 
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Old 08-17-23, 06:22 PM
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I feel sorry for "most" everyone in Phoenix/Las Vegas this year living with 90-95 degree temperature in their houses and offices.
Tumble - In the world of air conditioning, a 'hot day' actually has a specific definition. There’s this thing called the 'summer design temperature', which is the temperature that your location goes above only 1% of the time. Out of the 8,760 hours in a year, the 'summer design temperature' is the temperature exceeded only 87.6 hours, on average. This design protocol is used for the cooler northeast region as well as the boiling hot Las Vegas and Phoenix area. But obviously the two regions have drastically different temperature inputs.

Most people don't realize that an air conditioner should be sized to run almost continuously - or literally 24 hours a day continuously for days - at the 'summer design temperature'. That’s the idea behind a 'Manual J heating and cooling load calculation', a protocol developed by the
Air Conditioning Contractors of America (ACCA). But there’s some slack built into it, so you don’t need to worry about being hot on a day when the temperature is a few degrees over your 'summer design temperature'. If you’re getting runtimes of 45 minutes or more per hour when it’s near your 'summer design temperature', your AC is probably doing a pretty good job.

Needless to say - if the 'summer
design temperature' is missed the system will either be undersized and can't keep up - - or it will be oversized too much and not be efficient or not run long enough to remove the humidity.

The trick is to install a HVAC system that isn't too oversized so it will purposely have long runtimes to remove high humidity and keep the indoor rooms cool and dry. So, the 20° difference between outdoor and indoor temperature is a more appropriate difference for a typical 'summer design temperature' for most of the country. The summer temperatures of the deep south and desert west is not what most of the country experiences in the summer. Their 'summer design temperature' is markedly different.
 
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Old 08-17-23, 08:27 PM
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I'm just taking your adamant proposition that most ACs can only provide a 20 degree difference between inside and outside and applying it to millions of people that live in areas that routinely have temps in in excess of 110. Based upon what has been conveyed as factual, that implies millions of people are living in 90-95 degrees inside their residence. Maybe I'm doing the math wrong. Or maybe the proposition needs a lot more caveats or less of a blanket statement.

I apologize to the OP for sorta hijacking the thread, my non pro answer to them would be that I'm convinced a highly efficient window AC would readily take some of the load of an older inefficient HVAC. I hate the looks of a window AC but in my case the benefits easily outweigh the negative. Whenever I have to replace my main AC I'll address all its shortcoming at that time so I can eliminate the window AC.

Enjoyed the discussion, however I'll drop out now....
 
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Old 08-18-23, 08:49 AM
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I'm no pro but will add my 2 cents........

I have, in the past used a window AC to minimize the central AC run time, efficiency & inside temp. Personally, I've had good results using an 8,000 BTU window unit in addition to the central unit.
 
 

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