What performance to expect from central AC?

Questions on how we spend our money and our time - consumer goods and services, home and vehicle, leisure and recreational activities
Post Reply
Topic Author
yosh99
Posts: 138
Joined: Thu Apr 19, 2012 8:44 pm

What performance to expect from central AC?

Post by yosh99 » Sat Sep 08, 2018 5:35 pm

We installed a “high velocity” central air conditioning unit for the first time a couple of weeks ago and suspect it is undersized. The AC installation company says not. They have a full guarantee of proper sizing, installation, etc for the first year and will even remove the unit and refund the price after a year if they cannot satisfy the customer. So far there has been no indication they will not honor this guarantee, but I’m getting concerned about their foot dragging.

Not ever having a central AC before I’m not sure what performance to expect. We’ve been diligently recording performance for the last few weeks. On one day, for example, the outside temperature was 80F at noon. The AC was set at 70F and after running for hours, the temperature at the thermostat was 73F. Three hours later, the outside temperature had risen to 85F and the inside temperature rose to 74F. The installer has double checked everything and all is running according to spec. They talk about the negatives of “oversizing” the AC and short cycle times, but currently the unit runs all the time without being able to bring the temperature down much more that 7-8F of outside temperature.

The problem now is that it is late in the season and temperatures won’t be high until next summer so we won’t be able to have one of the engineers actually experience the problem. I’m concerned about what this unit will do next July when it’s 95F. I’m documenting my concerns with the installer (photos of the thermostat, outside temp, etc) so that we have a case should the unit prove inadequate next year, but I’d prefer thay upgrade the unit now and be done with it.

What performance should one expect from a central AC? Any advice?

User avatar
BolderBoy
Posts: 4750
Joined: Wed Apr 07, 2010 12:16 pm
Location: Colorado

Re: What performance to expect from central AC?

Post by BolderBoy » Sat Sep 08, 2018 5:42 pm

yosh99 wrote:
Sat Sep 08, 2018 5:35 pm
What performance should one expect from a central AC?
Better than that, I would think. But there are other variables in play. How "tight" is your house? How well insulated? Where in the house - exactly - is the thermostat situated?

My house is very tight and very well insulated so my AC doesn't break a sweat keeping the house at 73 when the outside temp is 100. Sure, at that outside temp the AC runs more often but it doesn't run non-stop. One weekend a few years ago, some picky guests wanted the AC set to 68 when the outside temps were 100 and then the AC ran quite a lot, but it still kept the inside temp at 68. (my house is 2600 sq ft, two story + basement)
"Never underestimate one's capacity to overestimate one's abilities" - The Dunning-Kruger Effect

mouses
Posts: 4141
Joined: Sat Oct 24, 2015 12:24 am

Re: What performance to expect from central AC?

Post by mouses » Sat Sep 08, 2018 5:52 pm

You don't actually know the max temperature drop is 7-11 degrees; It may be that 73 or 74 is the lowest degree setting it can reach. My thermostat is set with limits. 73 or 74 is pretty darn nippy, to a woman anyway.

User avatar
ResearchMed
Posts: 10147
Joined: Fri Dec 26, 2008 11:25 pm

Re: What performance to expect from central AC?

Post by ResearchMed » Sat Sep 08, 2018 6:04 pm

mouses wrote:
Sat Sep 08, 2018 5:52 pm
You don't actually know the max temperature drop is 7-11 degrees; It may be that 73 or 74 is the lowest degree setting it can reach. My thermostat is set with limits. 73 or 74 is pretty darn nippy, to a woman anyway.
Who set the limits?

Is this a rental home where the owner pays for utilities? Didn't sound like that.

I've seen limitations in hotel room thermostats, for example (and we won't return to those).

But "setting limits" to a homeowner's own HVAC?

:confused
This signature is a placebo. You are in the control group.

mhalley
Posts: 8226
Joined: Tue Nov 20, 2007 6:02 am

Re: What performance to expect from central AC?

Post by mhalley » Sat Sep 08, 2018 6:10 pm

How many stories, square footage and size of ac?

livesoft
Posts: 71379
Joined: Thu Mar 01, 2007 8:00 pm

Re: What performance to expect from central AC?

Post by livesoft » Sat Sep 08, 2018 6:19 pm

Here are a couple of possibilities, but maybe they were checked already:

1. We rented a home where the air return duct in the attic became detached from the AC unit, so the AC unit was taking in attic air at about 120 degrees, trying to cool it and sending it down into the living area. Of course, the house got hot. If any ductwork is compromised, then the leak(s) could cause the AC to run all the time and never cool low enough.

2. We have door in an upstairs unused closet that opens to attic space above the first floor. Someone left this door open, so 120 deg attic air was infiltrating the 2nd story living space. Not good.

Anyways, we have separate AC units for upstairs and downstairs. They can cool the home to 68 deg when the temp is about 100 deg outside. The home is pretty well insulated though.
Wiki This signature message sponsored by sscritic: Learn to fish.

User avatar
FrugalInvestor
Posts: 5427
Joined: Fri Nov 07, 2008 12:20 am

Re: What performance to expect from central AC?

Post by FrugalInvestor » Sat Sep 08, 2018 6:29 pm

It sounds like the unit is either not sized properly or not installed properly. I don't know how you're going to determine which it is as this should be the job of the company that sold it to you. You know that it's not cooling the house properly and that it needs to be fixed - that's all you need to know. If you haven't already, I'd take your concerns to an owner or high level manager of the company you purchased it from and hold their feet to the fire. They either put in an undersized unit or didn't design or carry out the installation properly.

(edited to add) You definitely don't want an oversized unit. It would cool quickly but cycle often. But you also don't want an undersized which it sounds like you have if they're right and it's properly installed.
Last edited by FrugalInvestor on Sat Sep 08, 2018 6:34 pm, edited 2 times in total.
IGNORE the noise!

goblue100
Posts: 1213
Joined: Sun Dec 01, 2013 10:31 am

Re: What performance to expect from central AC?

Post by goblue100 » Sat Sep 08, 2018 6:32 pm

I'd spend 20 bucks on one of these:
https://www.homedepot.com/p/Ultra-Perfo ... wAodMzgG3A

And I'd measure the temp coming out of your vents. To effectively cool your house it should be below 60. If it isn't that low, there is something wrong.
Financial planners are savers. They want us to be 95 percent confident we can finance a 30-year retirement even though there is an 82 percent probability of being dead by then. - Scott Burns

User avatar
RickBoglehead
Posts: 5608
Joined: Wed Feb 14, 2018 9:10 am
Location: In a house

Re: What performance to expect from central AC?

Post by RickBoglehead » Sat Sep 08, 2018 7:19 pm

What analysis did they perform, and then show you, to properly size the unit?

A properly running AC unit should put out air in the low 60s at least, and therefore be able to cool to close to that level.

https://www.probuilder.com/7-tips-accur ... lculations
Avid user of forums on variety of interests-financial, home brewing, F-150, PHEV, home repair, etc. Enjoy learning & passing on knowledge. It's PRINCIPAL, not PRINCIPLE. I ADVISE you to seek ADVICE.

Point
Posts: 196
Joined: Mon Jul 10, 2017 9:33 pm

Re: What performance to expect from central AC?

Post by Point » Sat Sep 08, 2018 7:39 pm

Get a digital laser thermometer and take readings around the house, including at all the outlet ducts. Make sure your filters are clean too.

You should be getting <60° from the ducts closest to the unit. If not, then you may have an issue with the temp not going low enough. Do not close any ducts— this can affect cooling.

Depending on your house setup, sun exposure, floors, you may get poor cooling in certain areas. Presumably they took this into consideration.when you have your data, have a meeting with the company and review the facts.

mouses
Posts: 4141
Joined: Sat Oct 24, 2015 12:24 am

Re: What performance to expect from central AC?

Post by mouses » Sat Sep 08, 2018 11:23 pm

ResearchMed wrote:
Sat Sep 08, 2018 6:04 pm
mouses wrote:
Sat Sep 08, 2018 5:52 pm
You don't actually know the max temperature drop is 7-11 degrees; It may be that 73 or 74 is the lowest degree setting it can reach. My thermostat is set with limits. 73 or 74 is pretty darn nippy, to a woman anyway.
Who set the limits?

Is this a rental home where the owner pays for utilities? Didn't sound like that.

I've seen limitations in hotel room thermostats, for example (and we won't return to those).

But "setting limits" to a homeowner's own HVAC?

:confused
The Hitlerian thermostat installer, no longer in my employ, environmentally oriented, and deaf to my information that older people feel temperature extremes more. However, it turns out that the range is okay, so I haven't had that fixed.

User avatar
Fletch
Posts: 752
Joined: Thu Jun 04, 2009 1:25 pm
Location: USA

Re: What performance to expect from central AC?

Post by Fletch » Sun Sep 09, 2018 5:26 am

AC should be sized to provide a 25 degree drop from outside temperature (give or take). Installer should have done a Manual J evaluation of your house that takes into account insulation, windows, climate etc. See: http://www.loadcalc.net for a simplistic version. Suggest you visit this forum and explore or post your questions: https://hvac-talk.com/vbb/
“Meaningless! Meaningless!” says the Teacher. Whoever loves money never has enough; whoever loves wealth is never satisfied with their income. This too is meaningless.

ponyboy
Posts: 900
Joined: Fri Feb 06, 2015 10:39 am

Re: What performance to expect from central AC?

Post by ponyboy » Sun Sep 09, 2018 5:40 am

Fletch wrote:
Sun Sep 09, 2018 5:26 am
AC should be sized to provide a 25 degree drop from outside temperature (give or take). Installer should have done a Manual J evaluation of your house that takes into account insulation, windows, climate etc. See: http://www.loadcalc.net for a simplistic version. Suggest you visit this forum and explore or post your questions: https://hvac-talk.com/vbb/
Ive been told about the 20 degree range. 90 outside, 70 inside. We recently had a unit installed on 2nd floor (attic.) A couple months ago when it was 100 degrees for a week...our ac had trouble keeping up. Wouldnt go below 76 degrees until night came. At the time we didnt have curtains (we were finishing up renovations.) Once we put up curtains...that kept the upstairs a lot cooler...just blocking the heat of the sun coming in.

If its 80ish outside and you cant keep a steady temp around 73 degrees...theres a problem. Ours doesnt run that much if its only 80 out...we keep ours at 74 during the day and 71 at night when we sleep. Downstairs is a separate unit and thats always set to 74 degrees.

Saving$
Posts: 1909
Joined: Sat Nov 05, 2011 8:33 pm

Re: What performance to expect from central AC?

Post by Saving$ » Sun Sep 09, 2018 7:08 am

approx 20 degree drop is the max you should expect.
The unit running all the time may be exactly what it should do. Some of the better systems modulate with a variable frequency drive so they never turn off. Since the compressor uses the most power when it turns on, this, makes them much more efficient. Also increases occupant comfort because it takes more humidity out of the air.

Installer should have run a Manual J calculation. Did they? Do you have a copy?
What type of unit is it? What type of system - you stated "high velocity" which makes me think it is NOT a mini split type (ducted or single head). Is this some type of standard split system? We need more info.

The suggestions to seal your home are spot on. This can make a huge difference. HUGE.
Another thing to consider is where is the unit getting its make up air? Is it pulling unconditioned air from the outside? 100% or less?

tomd37
Posts: 3231
Joined: Thu Mar 01, 2007 10:39 pm
Location: Middle Tennessee

Re: What performance to expect from central AC?

Post by tomd37 » Sun Sep 09, 2018 7:38 am

Yosh - I cannot find it now, but I just recently read a post on this site where a new hvac unit was installed and was not running properly. After many tests, and including getting another company to check things out, it was determined that the newly installed unit was over-charged with refrigerant. The new unit came with a certain amount of refrigerant in it and the installing company did not know that. :oops: So they added refrigerant as if none was in the unit and therefore had too much refrigerant. It took them several hours of bleeding and testing but as soon as they got it down to the correct refrigerant level the unit performed as it should.

Ask your hvac company or another company to check the level of refrigerant now.
Tom D.

Auream
Posts: 482
Joined: Thu Mar 20, 2008 1:07 pm
Location: Raleigh, NC

Re: What performance to expect from central AC?

Post by Auream » Sun Sep 09, 2018 8:51 am

As others have said, an oversized unit is not a good thing to have from an efficiency and comfort point of view. And oversized units are installed much more frequently than undersized. So it makes me skeptical that the unit is undersized, particularly if they did actually do an accurate load calculation. It’s possible though if they screwed up the numbers (garbage in, garbage out).

My first thought though is incorrect installation, a detached duct, etc. causing the unit to waste a good portion of its cooling output.

Mike Scott
Posts: 1344
Joined: Fri Jul 19, 2013 2:45 pm

Re: What performance to expect from central AC?

Post by Mike Scott » Sun Sep 09, 2018 9:44 am

As mentioned, a properly functioning central AC should give you about a 20 degree temperature drop. Something is wrong if it won't do that.

criticalmass
Posts: 1497
Joined: Wed Feb 12, 2014 10:58 pm

Re: What performance to expect from central AC?

Post by criticalmass » Sun Sep 09, 2018 9:52 am

The only way to ensure unit is working properly is to measure refrigerant charge, subcool, and temperature drop. If it is humid, remember that dehumidifying the air is a function of cooling, and this consumes a lot of energy that would otherwise go directly to cooling air. As the house humidity falls, the cooling efficiency improves.

68 degrees inside when outdoor temps are 100 is a very agressive energy consumption. Personally I wouldn’t want an ac that is sized to do that.

adamthesmythe
Posts: 3400
Joined: Mon Sep 22, 2014 4:47 pm

Re: What performance to expect from central AC?

Post by adamthesmythe » Sun Sep 09, 2018 11:36 am

I have installed high-velocity systems in two different houses and have been very happy with the result.

Having said that- it does sound like you have a problem. Testing the output temperature seems to me the first step. I think the specs for this are actually lower for a high velocity system but I am not sure about this. It would be reasonable to request a check of the deltaT.

And I agree- you want neither too large or too small a unit.

Topic Author
yosh99
Posts: 138
Joined: Thu Apr 19, 2012 8:44 pm

Re: What performance to expect from central AC?

Post by yosh99 » Sun Sep 09, 2018 11:37 am

Thanks to all for your comments and advice. To answer some of your questions,

The house is a 50 year old brick Rambler with 1200 sq feet on the main floor and 1300 sq feet in a finished basement. Being in Minnesota it's well insulated with double paned glass windows. The unit was purchased from a large HVAC contractor and one of their engineers visited the house, took measurements, and sized the unit (2.5 ton) according to his assumptions and software. Their technicians have been in twice to confirm that everything is working according to spec. This is a high velocity system with the compressor (high end Carrier) outside and evaporator in the attic with 17 two-inch cold air ducts in the ceiling of various rooms. There are no ducts in the basement, but the basement entry is open and there is one opening with a grate providing an opening between the basement and main floor in the center of the house. (I don't think the engineer was aware of the grate and I wonder if this air flow between the basement and main floor is the reason for his calculations being off)

So the standoff is this: Their worksheets say the unit is right-sized and they confirm it's working properly. We say the unit has never been able to pull the temperature below 73F even when the outside temperature peaks at 80F. I ask what's going to happen when it's 95? They say over-sizing the unit is a mistake. We're going round and round and until it gets hot again next year, I don't see how either one of us can prove anything. They disregard the data we've collected this fall.

It is very helpful to have some of you confirm that an AC should cool 20F from ambient. That's all I ask of this unit.

Thanks,

User avatar
FrugalInvestor
Posts: 5427
Joined: Fri Nov 07, 2008 12:20 am

Re: What performance to expect from central AC?

Post by FrugalInvestor » Sun Sep 09, 2018 12:36 pm

Here's some good information on how and where the temperature drop should and shouldn't be measured. If you're working with the installer it may help you confirm that they are giving you valid measurements.

https://inspectapedia.com/aircond/Air_C ... atures.php
IGNORE the noise!

goblue100
Posts: 1213
Joined: Sun Dec 01, 2013 10:31 am

Re: What performance to expect from central AC?

Post by goblue100 » Sun Sep 09, 2018 8:26 pm

yosh99 wrote:
Sun Sep 09, 2018 11:37 am

We say the unit has never been able to pull the temperature below 73F even when the outside temperature peaks at 80F. I ask what's going to happen when it's 95?
It is very helpful to have some of you confirm that an AC should cool 20F from ambient. That's all I ask of this unit.

Thanks,
You really need to measure the temperature of the air coming from the vent when the AC is running. I don't agree with the people that say 20 degree's is acceptable. I live in Texas, and when it is 110 it is not 90 in my house. It is 78. I suspect the air coming out of your vents is like 70, and I don't see how the company could defend that.
Financial planners are savers. They want us to be 95 percent confident we can finance a 30-year retirement even though there is an 82 percent probability of being dead by then. - Scott Burns

User avatar
jabberwockOG
Posts: 2085
Joined: Thu May 28, 2015 7:23 am

Re: What performance to expect from central AC?

Post by jabberwockOG » Sun Sep 09, 2018 8:57 pm

Having lived in Texas and now Gulf Coast area I can attest that in a normally insulated house a correctly configured and installed HVAC system should be able to easily keep the house at 73-74 when it is 90-95 outside and not have to run continuously.

If you can't get down below 73 when it is only mid 80s outside you system is not running right, or it was incorrectly configured or installed.

I'd have them recheck and retest all AC settings first, then check all duct-work - cold air return and vent runs for breaks and leaks. And if necessary have them move you up to a 3 ton AC system - but my guess is that a 2.5 ton system for HVAC in that size house way up North would be plenty.

Turbo29
Posts: 863
Joined: Tue May 01, 2018 7:12 am

Re: What performance to expect from central AC?

Post by Turbo29 » Sun Sep 09, 2018 9:13 pm

Here in Phoenix I can get to 74F (maybe less but I don't try) when its 116F outside.
It is by the goodness of God that in our country we have those three unspeakably precious things: freedom of speech, freedom of conscience, and the prudence never to practice either of them. --M. Twain

random_walker_77
Posts: 954
Joined: Tue May 21, 2013 8:49 pm

Re: What performance to expect from central AC?

Post by random_walker_77 » Sun Sep 09, 2018 9:45 pm

yosh99 wrote:
Sun Sep 09, 2018 11:37 am
Thanks to all for your comments and advice. To answer some of your questions,

The house is a 50 year old brick Rambler with 1200 sq feet on the main floor and 1300 sq feet in a finished basement. Being in Minnesota it's well insulated with double paned glass windows. The unit was purchased from a large HVAC contractor and one of their engineers visited the house, took measurements, and sized the unit (2.5 ton) according to his assumptions and software. Their technicians have been in twice to confirm that everything is working according to spec. This is a high velocity system with the compressor (high end Carrier) outside and evaporator in the attic with 17 two-inch cold air ducts in the ceiling of various rooms. There are no ducts in the basement, but the basement entry is open and there is one opening with a grate providing an opening between the basement and main floor in the center of the house. (I don't think the engineer was aware of the grate and I wonder if this air flow between the basement and main floor is the reason for his calculations being off)

So the standoff is this: Their worksheets say the unit is right-sized and they confirm it's working properly. We say the unit has never been able to pull the temperature below 73F even when the outside temperature peaks at 80F. I ask what's going to happen when it's 95? They say over-sizing the unit is a mistake. We're going round and round and until it gets hot again next year, I don't see how either one of us can prove anything. They disregard the data we've collected this fall.

It is very helpful to have some of you confirm that an AC should cool 20F from ambient. That's all I ask of this unit.

Thanks,
Offhand, 2.5 ton doesn't sound crazy, if it's working right. How hot is it in your basement? If it's underground, it should be pretty cool a/c, or no a/c, right? 2 ft below the ground, the dirt is pretty cool and, on a hot day, ought to stay pretty cool, so your basement gets natural cooling (which is why basements sometimes have moisture problems from humid air condensing.)

2.5 tons for 1200 sq ft and then some: https://www.acdirect.com/ac-package-uni ... calculator

By way of reference, in hot 105 degree central texas, we've got a 4 ton unit for downstairs and 3 tons for upstairs, each responsible for about 2000 sq ft of high ceiling space, and to keep at 78 when it's 105 outside is no problem. The a/c runs maybe 20-30min per hour. So 2.5 tons for a 1200 sq ft w/ seepage into a basement sounds quite reasonable assuming you don't have massive air flow around your windows/doors (and you'd definitely feel the chill in winter where you live!!!).

You want to think about this in two ways:
1) Is my a/c properly cooling air? This speaks to whether the a/c unit is working properly. Measure the temperature of the air at the intake vents and at the air outlets. You should expect to see a 15-20 degree drop. Check multiple vents -- poorly insulated ducts might warm up the air on long duct runs, and unbalanced poor airflow could also cause some vents to have marginally cool air, but some of the ducts had better see 15 degrees.
As your house cools, the temp of the air coming out of your ducts should get colder. i.e. if it's 73 in the house, you're expecting to see 58 degree air coming out of the ducts.

2) Is my a/c creating enough cold air to overcome the heat entering my house? Now as your house gets colder, the temperature delta between inside and outside gets greater. The bigger the delta, the more quickly heat gets into your house. So if it's 85 outside, it's easy to cool from 85 to 80, but harder to cool from 65 to 60, since at that point, you're taking in a lot of heat from conduction and your insulation only helps so much. This is where the "manual J" calculation comes in to make sure your a/c is able to pump enough heat out of your house to keep it comfortable. This takes into account how much sun you're getting, how much heat comes in through the walls/windows/doors/etc. Basically, when it's 85 outside, and it's 70 inside, your a/c needs to be able to pump out all the heat that's coming in (which would keep the temp constant) and then some more to lower the temp further. This gets at the a/c sizing, and the volume of cold air that your system can produce.

When it's 73 in the house, how cold is the air coming out of the vents when the a/c is running?

Jim180
Posts: 430
Joined: Wed Jun 26, 2013 9:47 pm

Re: What performance to expect from central AC?

Post by Jim180 » Sun Sep 09, 2018 10:05 pm

First check the temperature coming out of your vents. If the inside of your home is 73-74 degrees then the vent temperature should be around 53-54 degrees. If the vent temperature is much warmer then either your unit is not fully charged or there is an air circulation problem. If the vent temperature seems OK then your unit is simply undersized. There's no way your unit should struggle to keep up when the outside temperature is only in the 80's.

WaffleCone
Posts: 128
Joined: Fri Oct 23, 2015 11:48 pm

Re: What performance to expect from central AC?

Post by WaffleCone » Sun Sep 09, 2018 11:30 pm

Interested in what the solution ends up being. Hopefully you'll have one more heat wave before the end of summer.

A few quick comments:

We have a ranch style home similar to yours except slightly bigger, 1600sqft main floor plus we condition about 400sqft in the basement off a 2.5 ton conventional unit. Located in PA where humidity is more of a problem than heat. We typically keep the stat at 77-78 and it runs often but not constantly on very hot days.

Your opening to the basement may be the issue. All the cold air may be sinking to the basement pushing the warmer, more humid basement air up. This not only warms up your main floor but also increases the humidity, which translates into both higher cooling and dehumidfication demands.

We are in the process of renovating, basically opening up the basement to the main floor, and the HVAC people can't agree on how to handle it. Some want seperate units altogether, others say we just need a good dehumidifier in the basement with a few more vents off the existing unit balanced appropriately.

I'm not sure what the answer is but having the basement open to the main floor significantly changes the calculations and they need to be aware of it.

One more thing-- check that your attic duct work is insulated (which it probably is) otherwise you're blowing cold air through a 110+ degree oven before it enters your home.

criticalmass
Posts: 1497
Joined: Wed Feb 12, 2014 10:58 pm

Re: What performance to expect from central AC?

Post by criticalmass » Mon Sep 10, 2018 9:23 am

Jim180 wrote:
Sun Sep 09, 2018 10:05 pm
First check the temperature coming out of your vents. If the inside of your home is 73-74 degrees then the vent temperature should be around 53-54 degrees. If the vent temperature is much warmer then either your unit is not fully charged or there is an air circulation problem. If the vent temperature seems OK then your unit is simply undersized. There's no way your unit should struggle to keep up when the outside temperature is only in the 80's.
Be careful jumping to conclusions. There are for more potential issues that can lead to bad cooling performance than undersizing.

Based on the general location and building information given, 2.5 tons may easily be the correct size. I would want to ensure it is operating correctly.

This means a competent technician who is skilled with high velocity systems, not someone who just knows how to read thermometers.

new2bogle
Posts: 1532
Joined: Fri Sep 11, 2009 2:05 pm

Re: What performance to expect from central AC?

Post by new2bogle » Mon Sep 10, 2018 9:40 am

goblue100 wrote:
Sat Sep 08, 2018 6:32 pm
I'd spend 20 bucks on one of these:
https://www.homedepot.com/p/Ultra-Perfo ... wAodMzgG3A

And I'd measure the temp coming out of your vents. To effectively cool your house it should be below 60. If it isn't that low, there is something wrong.
Exactly. I would add to say that it shouldn't necessarily be below 60.. What it should be is a 20 degree difference between cool air register and the a/c intake. Easy to check when you get the product mentioned above.

new2bogle
Posts: 1532
Joined: Fri Sep 11, 2009 2:05 pm

Re: What performance to expect from central AC?

Post by new2bogle » Mon Sep 10, 2018 9:57 am

goblue100 wrote:
Sun Sep 09, 2018 8:26 pm
yosh99 wrote:
Sun Sep 09, 2018 11:37 am

We say the unit has never been able to pull the temperature below 73F even when the outside temperature peaks at 80F. I ask what's going to happen when it's 95?
It is very helpful to have some of you confirm that an AC should cool 20F from ambient. That's all I ask of this unit.

Thanks,
You really need to measure the temperature of the air coming from the vent when the AC is running. I don't agree with the people that say 20 degree's is acceptable. I live in Texas, and when it is 110 it is not 90 in my house. It is 78. I suspect the air coming out of your vents is like 70, and I don't see how the company could defend that.
It's 20 degrees between intake and output. The intake is not the outside temperature, it's the intake vent inside your house... which should not be too hot (i.e, 110) in the first place if the a/c is working properly.

I am also in TX. My upstairs a/c is 2.5 tons and the upstairs is around 2,000 sq ft. It also has opening to the first floor so part of the upstairs is cooling the first floor. This a/c is able to keep the upstairs cool to 74 easily when it is 100+ outside. Haven't tried lower than that since 74 is cool enough for us even in TX July weather.

jehovasfitness
Posts: 553
Joined: Mon Jan 22, 2018 2:26 pm

Re: What performance to expect from central AC?

Post by jehovasfitness » Mon Sep 10, 2018 10:05 am

FWIW we had an older home (1960s) and likely leaks in the ducting where it would take forever to cool the home.

We now have a new home built in 2016 where no matter the temp outside, it will drop the temp inside by 1* every 30 mins.

Not sure how efficient that is, but it's way better than our last house.

On days where it was 100*+ out the unit would run non-stop at our last house. Now it keeps up no problem.

goodlifer
Posts: 518
Joined: Sun Aug 16, 2015 9:50 pm

Re: What performance to expect from central AC?

Post by goodlifer » Mon Sep 10, 2018 10:18 am

Our problem is our thermostat. The builder put it in the worst location possible, near a large, sunny window and on the other side of the wall from the oven. It is in the least used room in the house, on the opposite side of the family room and bedrooms. The thermostat temp never reflects the temperature in the rest of the house, so we wind up readjusting the temp frequently. Could your thermostat be part of the problem?

random_walker_77
Posts: 954
Joined: Tue May 21, 2013 8:49 pm

Re: What performance to expect from central AC?

Post by random_walker_77 » Mon Sep 10, 2018 10:27 am

goodlifer wrote:
Mon Sep 10, 2018 10:18 am
Our problem is our thermostat. The builder put it in the worst location possible, near a large, sunny window and on the other side of the wall from the oven. It is in the least used room in the house, on the opposite side of the family room and bedrooms. The thermostat temp never reflects the temperature in the rest of the house, so we wind up readjusting the temp frequently. Could your thermostat be part of the problem?
If it's only 80 outside and the a/c is running continuously and struggling to maintain 73 inside, then the thermostat is not the main problem here...

Chip
Posts: 2916
Joined: Wed Feb 21, 2007 4:57 am

Re: What performance to expect from central AC?

Post by Chip » Mon Sep 10, 2018 11:05 am

WaffleCone wrote:
Sun Sep 09, 2018 11:30 pm
I'm not sure what the answer is but having the basement open to the main floor significantly changes the calculations and they need to be aware of it.
Very true. We had friends with a ranch/finished basement with a huge opening to the basement (6 ft wide open stairway). The upstairs got a lot of afternoon sun. It was all served by a single A/C unit.

So much cold air migrated to the basement that a barely comfortable, on the edge of too hot upstairs resulted in a downstairs so cold it required a coat. That was with all downstairs vents closed off and all upstairs vents wide open.

The only solution I could see was to install some sort of system to circulate the cold air back to the upstairs. To do it well would have been very expensive.

It's no longer an issue since they moved. :P

goblue100
Posts: 1213
Joined: Sun Dec 01, 2013 10:31 am

Re: What performance to expect from central AC?

Post by goblue100 » Mon Sep 10, 2018 11:13 am

new2bogle wrote:
Mon Sep 10, 2018 9:57 am

It's 20 degrees between intake and output. The intake is not the outside temperature, it's the intake vent inside your house... which should not be too hot (i.e, 110) in the first place if the a/c is working properly.
Ok, my misunderstanding. OP, I still suggest you measure the air temp from the vents to make sure you are getting something close to that drop in temperature. My guess is you are not.
Financial planners are savers. They want us to be 95 percent confident we can finance a 30-year retirement even though there is an 82 percent probability of being dead by then. - Scott Burns

Smoke
Posts: 487
Joined: Thu Aug 30, 2018 12:45 pm
Location: Saturn

Re: What performance to expect from central AC?

Post by Smoke » Mon Sep 10, 2018 1:28 pm

If the unit is not keeping up with the demand at 80 deg or 85 deg, you are doomed when the temp goes to 95 Deg. Been there done that.
When the installer evaluated your house for the new unit they should have done a "manual J" engineering study on your home, which takes into account all your homes variables, insulation, windows, amount of vents and size of duct work, sun exposure, location in the country etc etc.

Not just a best guess on personal experience. Sounds very undersized to me. Or something wrong with the unit possibly.
I am in Tennessee, if the unit is an outside "air" source heat exchange, it will start to not keep up with demand above 95 deg outside between 4pm and 8 pm. 100 deg will be worse of course with keeping up. Not much you can do about it at those temps. You will be trying to transfer heat to very hot outside air, the hotter outside the less transfer.
Your unit seems to not keep up with demand at 80 deg.
Something is very wrong with either the size of the unit, or it is malfunctioning due to defect or poor installation. imo
Arguing for the sake of arguing is something I am not going to engage in.

Post Reply