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Unread 2016-06-22, 07:41 PM   #1
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Default Can your A/C Keep up?

On these hot days my A/C has a hard time keeping up. My house is bigger than the normal house...but I always have arguments with my wife about it. She thinks there is something wrong with the A/C and I say that it's probably that way for most people.

When its 100+ degrees out we may have it set on 70-73ish and its showing around 76-78 in our house. On days when its cooler it has no problems keeping up.

Thoughts?
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Unread 2016-06-22, 07:48 PM   #2
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My house is tiny but highly inefficient. So the air turns off, then turns back on in 5 to 10 minutes
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Unread 2016-06-22, 08:03 PM   #3
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Sounds like the unit (or A coil) is undersized for the house
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Unread 2016-06-22, 08:10 PM   #4
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I live in an older house, all brick in the city, older windows. Ac unit is about 6 years old. I set it at 70 and if we have consecutive days of 90+, it's not unusual to get up to 75.
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Unread 2016-06-22, 08:14 PM   #5
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I feel like I recall reading somewhere that A/C's lose a ton of efficiency and efficacy if trying to cool much more than 20 degrees below the outside ambient temp.

Trying to cool a house to 70 when it's over 100 outside is probably tough for most units.
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Unread 2016-06-22, 08:31 PM   #6
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My rental house in Manhattan can't on the really hot days. Landlords never replaced the old as all hell single pane glass that just bleeds the air. Glad I don't pay utilities.
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Unread 2016-06-22, 08:40 PM   #7
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My house does great. It's set at 70 and I can hit 60s all day long.


The Teggy on the other hand....
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Unread 2016-06-22, 08:50 PM   #8
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I will add that while we have a decent sized home, now that it's just myself and the wife living there we have the vents closed off in the basement as well as two of the bedrooms on the top floor.

Nobody ever uses those spaces.

That probably helps a lot.
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Unread 2016-06-22, 09:47 PM   #9
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Your AC sounds just about right. For maximum comfort and most humidity removal, you want an AC that is sized so that it's just about maxed out in the hottest of days. An AC that's is oversized will cool the air off quickly, but won't run long enough to remove the humidity.
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Unread 2016-06-22, 10:15 PM   #10
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Mine has been keeping up set on 73-75, we run the blower all the time and the AC only kicks on as needed. Seems to help keep the house temp more even.

Havent noticed much difference in electric costs since we've started leaving the fan on either.

2 yo unit in a 17yo house.
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Unread 2016-06-22, 10:24 PM   #11
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I'm sitting at a cool 70 degrees with the ac only on periodically. The beauty of a newer house with high efficiency and being a ranch helps tremendously.
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Unread 2016-06-23, 12:09 AM   #12
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Quote:
Originally Posted by jwdb1fish View Post
I will add that while we have a decent sized home, now that it's just myself and the wife living there we have the vents closed off in the basement as well as two of the bedrooms on the top floor.

Nobody ever uses those spaces.

That probably helps a lot.

You can actually reduce the efficiency and output of the motor by closing off to many vents. It can build up pressure, and slow the fan/motor down reducing the air flow. Also causes more strain on the motor and can burn them up quicker.
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Unread 2016-06-23, 12:57 AM   #13
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Originally Posted by DanenGraham View Post
You can actually reduce the efficiency and output of the motor by closing off to many vents. It can build up pressure, and slow the fan/motor down reducing the air flow. Also causes more strain on the motor and can burn them up quicker.

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Unread 2016-06-23, 05:26 AM   #14
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My finished basement and main floor keeps cool no problem but upstairs is another story and all our bedrooms are upstairs. We just bought the house 3 months ago. But found out we have a 2.5 ton unit when we really need around a 4 ton unit. It seems upstairs vents are not pushing hardly any air out at all. I just ordered a 8" 420cfm blower to put in the ductwork going upstairs to try to get more airflow in the bedrooms. It sucks working overnights and waking up in the middle of the day sweating your balls off because it's 80 degrees in the bedroom. hopefully this extra blower will help.
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Unread 2016-06-23, 07:50 AM   #15
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I have a Nest thermostat, so when I'm gone for the day it gets up to 80 on auto away. That means it barely runs at all during the day, but when 4:30 hits, it runs pretty much nonstop until it gets dark.

I have a 2 stage system and it will run the second stage for most of it then switch to the first stage.

After reading this, I guess taking 5 hours to drop 6 degrees when it's 103 out isn't bad.
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Unread 2016-06-23, 08:01 AM   #16
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I keep my house at 68, and have to otherwise my reef aquarium will overheat and die.
I have seen the house run at 70 these past few days though, which is still acceptable.
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Unread 2016-06-23, 08:13 AM   #17
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Normal units will only be able to cool off your house by 20 degrees from the outside temperature. So if it is 100 degrees outside, if you seeing below 80, you are doing ok.
Also, it depends where your unit is located in reference to the sun...if it gets direct sunlight, it will struggle more - add poor insulation and other factors, it could be 15 degrees.
My unit is almost 20 years old just like the house, but my house is fairly small, only about 2,700 sqft. Every 3 years, I have to add about 2-3 lbs. of R32 which costs me about $ 150-200. It also seems that every other year, I have to replace a coil or a capacitor or something else. I think on the average I am spending about $ 150/year keeping that old dinosaur running. However, it has no problem keeping the house at 30 degrees cooler than the outside temperature.
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Unread 2016-06-23, 08:13 AM   #18
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Quote:
Originally Posted by Rob View Post
On these hot days my A/C has a hard time keeping up. My house is bigger than the normal house...but I always have arguments with my wife about it. She thinks there is something wrong with the A/C and I say that it's probably that way for most people.

When its 100+ degrees out we may have it set on 70-73ish and its showing around 76-78 in our house. On days when its cooler it has no problems keeping up.

Thoughts?
Ive never lived in a house that couldnt keep up. All the way from a 1968 ranch to my current house built at the end of 2010.

Id have it looked at. You should really have it checked on once or twice a year. Regular maintenance helps stave off having to have the thing replaced. Plus having it working optimally means not only does your house stay colder but it also means it runs less, saving you $$ on electricity.

The only exception would be a retrofit on a 1930s house or something, lord knows there.

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Normal units will only be able to cool off your house by 20 degrees from the outside temperature. So if it is 100 degrees outside, if you seeing below 80, you are doing ok.
Not really. Its not working on outside air, its working on inside air. Or it should be. If your house is sealed but started at 90 then it would intake 90 degree air and pump out 70 degree air, that is how its designed. But eventually it would lower the overall temp of the air in the house and no longer be taking in 90 degree air. If its taking in 75 degree air then it would be pumping out 55 degree air, at that point then it switches to cycling to maintain a relatively consistent temp. That is the only measure they have to regulate temp, its not like it adjust the temp its putting out, its just built to pump out 20 degrees less than its taking in no matter what.

If its running properly and the house is sealed then it would reach any comfortable temp in any environment commonly seen in the US. You just adjust the size of the evap/condenser to achieve that. Its why people in Arizona can still have cool homes in 120 degree heat. Its also how your freezer (which uses the same system) can get sub zero in a 70 degree house.
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Unread 2016-06-23, 08:31 AM   #19
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My thermostat is set at 72 and yesterday evening it was reading 76 inside the house. Not terrible for an approx 1500 sq ft split level home, and my units are only like 3 years old.
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Unread 2016-06-23, 09:48 AM   #20
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Quote:
Originally Posted by Ryan Stewart View Post
Ive never lived in a house that couldnt keep up. All the way from a 1968 ranch to my current house built at the end of 2010.

Id have it looked at. You should really have it checked on once or twice a year. Regular maintenance helps stave off having to have the thing replaced. Plus having it working optimally means not only does your house stay colder but it also means it runs less, saving you $$ on electricity.

The only exception would be a retrofit on a 1930s house or something, lord knows there.


Not really. Its not working on outside air, its working on inside air. Or it should be. If your house is sealed but started at 90 then it would intake 90 degree air and pump out 70 degree air, that is how its designed. But eventually it would lower the overall temp of the air in the house and no longer be taking in 90 degree air. If its taking in 75 degree air then it would be pumping out 55 degree air, at that point then it switches to cycling to maintain a relatively consistent temp. That is the only measure they have to regulate temp, its not like it adjust the temp its putting out, its just built to pump out 20 degrees less than its taking in no matter what.

If its running properly and the house is sealed then it would reach any comfortable temp in any environment commonly seen in the US. You just adjust the size of the evap/condenser to achieve that. Its why people in Arizona can still have cool homes in 120 degree heat. Its also how your freezer (which uses the same system) can get sub zero in a 70 degree house.
You have a good point. This is however, how it was explained to me by 2 friends that own A/C HVAC businesses in the area. Heck...one of them has a 4,000 sqft house and has 2 huge units in his house - when we got to 102 here, they could only get their house to read 80 inside.
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Unread 2016-06-23, 10:24 AM   #21
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Quote:
Originally Posted by ForcFed93 View Post
I have a Nest thermostat, so when I'm gone for the day it gets up to 80 on auto away. That means it barely runs at all during the day, but when 4:30 hits, it runs pretty much nonstop until it gets dark.

I have a 2 stage system and it will run the second stage for most of it then switch to the first stage.

After reading this, I guess taking 5 hours to drop 6 degrees when it's 103 out isn't bad.
Your away temp should never be more than 5 degrees higher than your home temp.
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Unread 2016-06-23, 10:49 AM   #22
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Your away temp should never be more than 5 degrees higher than your home temp.
Source? I've never heard that.
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Unread 2016-06-23, 10:55 AM   #23
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Source? I've never heard that.
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Unread 2016-06-23, 11:26 AM   #24
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Quote:
Originally Posted by Rob View Post
On these hot days my A/C has a hard time keeping up. My house is bigger than the normal house...but I always have arguments with my wife about it. She thinks there is something wrong with the A/C and I say that it's probably that way for most people.

When its 100+ degrees out we may have it set on 70-73ish and its showing around 76-78 in our house. On days when its cooler it has no problems keeping up.

Thoughts?
What is the age of the house, how many tons is the unit, what is the ft2 of the house, is it a 2 story or split?

A lot of houses are built with the smallest unit they can get away with. They usually size it for 70-80% of max load so on those warmer days they can't keep up. Builders also use cheap inefficient units.
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Unread 2016-06-23, 11:27 AM   #25
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Not really. Its not working on outside air, its working on inside air. Or it should be. If your house is sealed but started at 90 then it would intake 90 degree air and pump out 70 degree air, that is how its designed. But eventually it would lower the overall temp of the air in the house and no longer be taking in 90 degree air. If its taking in 75 degree air then it would be pumping out 55 degree air, at that point then it switches to cycling to maintain a relatively consistent temp. That is the only measure they have to regulate temp, its not like it adjust the temp its putting out, its just built to pump out 20 degrees less than its taking in no matter what.
Not really. It's working with the outside air AND inside air. The air conditioner moves heat from the inside of the house to the outside of the house. Assuming 100% efficiency for conversation sake, if you are cooling a mass flow of air inside your house 20 degrees, you are heating the same amount of outside by 20 degrees.

The air conditioner works off the principal that if you compress a fluid, it heats up, and when you decompress a fluid, it cools off. And in addition to pressure alone, there is a phase change of the refrigerant from liquid to vapor, which traps heat energy. Ever notice how cold a can of compressed "air" duster gets cold as you let pressure off it? Same principal (and the "air" in the duster is actually a refrigerant).

So, when you take a refrigerant vapor and pressurize it, it will get hot. That's the compressed vapor goes outside to your condenser. That's the outside unit. What that unit does is cools off the hot vapor with outside air with a fan to cool the vapor enough to turn it back into a liquid. The cooler the outside air, the more refrigerant will turn back into a liquid. A condenser sitting outside in 100 degree heat with the sun beating down on it will not condense as much refrigerant into a liquid as a a condenser sitting in the shade on a 80 degree day. That's where the outside temperature comes into play in air conditioning operation.

And on the inside, the opposite is going on. Your coil is getting fed the ambient air cooled off liquid refrigerant from the condenser outside. There is a valve that lets the pressure off the compressed liquid, and right away the temperature drops just like the air duster can. The now lower pressure liquid/vapor refrigerant mixture enters the A coil (evaporator) inside the house. Inside air is passes over the coil and causes the refrigerant to boil off turning it back into a vapor. That phase change from liquid to gas, along with the decrease in pressure causes the temperature to drop drastically. The hotter the inside air passing over the coil, the more refrigerant gets turned back into a vapor to be re-compressed to start the cycle all over again.

Sorry if I went a little too mechanical engineer there.
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