Central Air Conditioning running all day


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Old 03-12-11, 02:25 PM
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Central Air Conditioning running all day

I'm writing to see if someone can answer my questions regarding
central air-conditioning or refer me to someone who can. Our new central air conditioner runs all day and the company that installed it says that it's normal.

We live in a 14-year old R-2000 certified home in the country. The house is approx. 1,500 sq. ft per floor and there are a number of large argon-filled windows. After getting a number of quotes,
we chose Sears to install a 2-ton, 14.5 SEER unit in June 2010. Once we started using the unit, we noticed that it was running all day. I started keeping notes and found that it would typically
start around 9:30 in the morning and run all day if the temperature outside was anywhere above 27 degrees Celcius. The thermostat was set to 22 degrees Celcius. The company salesman came and measured the house to ensure we had the right size unit and he had their service people check the system out as well. Finally, the company service manager spoke to the manufacturer who replied that these new types of air conditioners focus more on the humidity and will typically run all day. The temperature in the house might even go up slightly in the afternoon depending on how
high the temperature gets outside. (I had central air in a previous house 20 years ago and if I turned on the AC when I got home from work, the house was cool within a few hours at most.)
The service manager told me that I wouldn't be able to get an AC to operate the way I wanted. Getting a larger unit wasn't recommended either because the fan and ducts might not be able to handle it and the AC coil in the furnace might freeze up completely.

My question is this - is it now typical for today's new AC units to run 10-12 hours a day without stopping or am I being fed a pile of baloney?

My basement ducts are already all closed. Is there anything else I can do?

I find it hard to believe that the AC must run all day, even when the temperature is below 30 degrees Celcius!
 
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Old 03-12-11, 03:55 PM
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you may have an undersized system or it may be low on charge. There are a lot of questions to ask such as, location, brand and model numbers and even serial numbers. Was there a manual J heat load calculation done on your home? How old is your home? What size was the old system?
 

Last edited by hvactechfw; 03-12-11 at 04:17 PM.
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Old 03-12-11, 06:45 PM
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A properly sized system will run all day when it is hot out If it cycles during the hottest days it is over sized
 
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Old 03-12-11, 07:46 PM
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Did they replace the evaporator coil when they replaced the outdoor unit?
If not, did they clean your old evaporator coil?
Was your old system a 2 ton? How well did it maintain desired temperature?

What is the difference in temperature between the supply and return vents?
 
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Old 03-13-11, 08:36 AM
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NO, it is not right ! Like the others said, a lot of questions here. Start with the temperature split first.....

Airman1994, I remember you said several times that a system should run all day if outside is hot. Is this theory documented in anywhere ?
 

Last edited by hvactechfw; 03-13-11 at 08:52 AM.
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Old 03-13-11, 08:54 AM
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I remember you said several times that a system should run all day if outside is hot. Is this theory documented in anywhere ?
A system has a design temp when a manual J was done to size the equipment. At the design temp it should run non stop.
 
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Old 03-13-11, 09:24 AM
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Quaildog,
We don't know what the design conditions are for your area. For example if the outdoor/indoor design temps are 27 & 22 your equipment should be able to maintain 22 when it is 27 outside. Under design conditions outdoors, the equipment will run constantly but should hold the indoor design temp. If it is warmer outside, you are going to loose ground.
 
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Old 03-13-11, 09:24 AM
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Hvactechfw. Got it answered for me! Long runs = better efficiency better rh removal less wear! Hardest thing is start up
 
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Old 03-13-11, 10:05 AM
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Thumbs up Welcome to our forums!

Summer conditions where I think you are:

"summers are warm, at times hot and humid with average high temperatures of 26C (79F) and low of 16C (61F), temperatures frequently exceed 30C (86F). Summer humidity levels can average 80% rh in the warmest months."
Is this close?
You need to be sure as others have suggested that your unit is working properly.
If it is you could do a load calculation yourself.

It appears to me that if your humidity is as high as it looks your salesman was spot on with the comments he made.
Humidity removal can take as much or more cooling power than lowering temperature in a dry climate.

We can't know for sure but he also could be correct about the design of your heating system.
It is common that when you install a/c as an afterthought you sometimes will need more airflow for the cooling load than what the heating system is designed for, especially if your house is a two story.
There are other things we can't know, like how many occupants, how often the doors are open and closed, what are your typical activities that could generate heat.

You could check their load calculations and some of the suggestions others have offered but I don't think anything is wrong.
 
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Old 03-14-11, 09:03 AM
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havctechfw, I think now I understand the theory behind it, sounds right. but the question is what is the design temp. In Houston the highest temp I remember was in 1995-6 & 2001, there was a week logn temp was at 104-108F. most other years the temp does not even go above 100F. So if the set temp is 108F which means that unit should not run non-stop after 2001 (for the last 10 years). Anyway, theory is theory, but I do have a couple of concerns: (1) if the unit run non-stop, don't you have to pay a lot for electricity ? (2) If the unit run non-stop, isn't that shorten the unit's life (2 fans and compressor) ? I bought 3 new houses in Houston the the last 20 years. all 6 A/C units never run more than 1hour (after first start up) before they stop. In fact the last house I bought was their Energy-star house, which means, If I pay more than $160 a month on electricity(for the first 3 years), the builder will pay the differences. So I don't think they will allow the unit run too long (may be their design temp is 120F). You may think it is not easy to figure out how much electricity that was used by A/C, but A/C is certainly use the most in the Summer.
 
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Old 03-14-11, 01:35 PM
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Clocert, you brought up a good point, what is the design temp? I don't know this information because i did not size the unit for the home. That information would go back to when the load calculation was done.

System run time.... Some new units are 2 stage units. These tend to run longer because they are not running a full capacity unless the demand or load is there. A unit that turns on and off or starts and stops more often is actually going to have the shorter life span. Start-ups are harder on the system then anything else.

In Houston you probably have a heat pump which will run much more often than an A/C because it is used for heating as well.
 
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Old 03-14-11, 06:17 PM
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Thanks, havctechfw, next time when I see a builder (I am a realtor, so I see them all the time) I'll ask. I doubt if they do manual J load calculation at all, For the last 10 years, I have never seen any builder did that (everytime you ask a technical question, they always said all designs are handed down from the corporation, they only follow the rules). Thanks again. BY the way, Homes in Houston do not use heat pump simply because gas (most homes here use gas heat, gas water heater, and gas stove) is much cheaper here.
 
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Old 03-14-11, 06:40 PM
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I agree, natural gas is Much cheaper here.
 
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Old 03-14-11, 09:25 PM
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Only way to size a unit is to do a heat load! If this is not done and the unit is over sized then the bigger unit will pull more amps than the corect sized unit
 
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Old 03-18-11, 08:18 PM
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Do Research; I believe it should cycle

Originally Posted by quaildog View Post
I'm writing to see if someone can answer my questions regarding
central air-conditioning or refer me to someone who can. Our new central air conditioner runs all day and the company that installed it says that it's normal.

We live in a 14-year old R-2000 certified home in the country. The house is approx. 1,500 sq. ft per floor and there are a number of large argon-filled windows. After getting a number of quotes,
we chose Sears to install a 2-ton, 14.5 SEER unit in June 2010. Once we started using the unit, we noticed that it was running all day. I started keeping notes and found that it would typically
start around 9:30 in the morning and run all day if the temperature outside was anywhere above 27 degrees Celcius. The thermostat was set to 22 degrees Celcius. The company salesman came and measured the house to ensure we had the right size unit and he had their service people check the system out as well. Finally, the company service manager spoke to the manufacturer who replied that these new types of air conditioners focus more on the humidity and will typically run all day. The temperature in the house might even go up slightly in the afternoon depending on how
high the temperature gets outside. (I had central air in a previous house 20 years ago and if I turned on the AC when I got home from work, the house was cool within a few hours at most.)
The service manager told me that I wouldn't be able to get an AC to operate the way I wanted. Getting a larger unit wasn't recommended either because the fan and ducts might not be able to handle it and the AC coil in the furnace might freeze up completely.

My question is this - is it now typical for today's new AC units to run 10-12 hours a day without stopping or am I being fed a pile of baloney?

My basement ducts are already all closed. Is there anything else I can do?

I find it hard to believe that the AC must run all day, even when the temperature is below 30 degrees Celcius!
Sounds like you might live in the Toronto area, if so: summer design is: 87-dry bulb (29-C), 72-Web bulb or 52% Relative Humidity.
That is the warmest city in Canada.

I believe there could be numerous possible problems casing the unit not to ever cycle.
Set the Room-TH at 25-C & see if it will cycle; if not I'd say there could be numerous things causing the system not to absorb enough heat from the rooms; or there is far too much air infiltration.

Find someone that knows how to check the duct system & air flow!

The indoor coil must match the new 2-Ton 14-SEER condenser & I'd go to a matching 3-Ton indoor coil with a TXV refrigerant metering device.

The larger coil will flow, the possible, higher heating airflow requirements better - in a cold Canada.

Do Google Internet searches for airflow; the indoor coil may not be absorbing enough heat.

Check the condenser discharge air to see what the temp-split is; if split is too high the Return could be pulling hot air from the attic; if too low a split, the indoor coil isn't absorbing enough heat! Record the outdoor temp & the temp-rise split here on this forum.

Also, get something to record the indoor humidity level & also the temp-split between supply air & return air.

In that climate, it seems a 2-Ton unit running properly would cycle at those temps.

In an old farm home loaded with windows, a little half-Ton window unit using a floor fan cools over 800-sf; that's over 1600-sf per Ton of cooling. It gets it to 76-F even when its 110-F Heat Index.

Your 1500-sf / 2Ton= is only 750-sf per Ton! Do a lot of Internet Research! - HVAC Retired
 
 

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