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Old 07-27-2011, 01:39 PM   #1
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a/c runs for many hours maybe return vent issues


My house built in 67 have been doing upgrades for 5 years and bills are getting lower and lower. But a/c just keeps running to long.

Main floor of house 1800 sq ft
basement about 1600 sq ft

3 bedrooms up and 3 bedrooms down.

New 14 seer Heil 3 1/2 ton a/c unit with gas heater.
Unit located middle of basement

Attic has an R value of 60 in and around the middle.
House has hip roof and tappers off to maybe R 18 on edges.

Even on a hot 105 degree day attic temp is about 117 to 120 degrees at most. Most the time when I come home attic temp around 5 pm is 100 to 105 degrees. I have a temputure sending unit in attic to read temp at all times.

My house is very very air tight, I have caulked everything repainted and insulated walls in rooms I have remodeled.

I have full tinted windows (single pane)

Blinds and drapes are close full time on windows south and west side of home.

The main floor has 10 vents that blow 48 to 58 degree air depending on the time of day.

The main floor only has 7 returns.

All vents and returns are notched between the wall studs and cut down thru the floor. So not to take up floor space for vents and returns.

The basement is always cool and cold is summer time really nice down there. For the entire basement it has 4 vents and no returns.

I Have 11 ceiling fans in my home they are used while in the room to move air around. They are off if room not used.

My thermostate is energy star rated like everything else in my house.

I set it for 79 degrees at 330 in the afternoon. so by the time I get home about 4 or 5 house is being cooled.

My house does not get warm till about 3 pm, it stays pretty cool for us. the temp at 3pm is about 81 or 82 degrees and this is warm to us and we need a/c at this time even with house at 82 at 3 pm its well over 100 degrees outside.

So at 330 the a/c kicks on but it will run all the way to 11 pm and the temp will drop from 82 to about 80 degrees at 5pm and maybe by 7pm it might get down to 79 degrees and sometimes it might hit 78 degrees before 11 pm and kick off. This is set at 79 degrees. It will cool to 78 degrees before thermostate says to kick it off.


So you see my a/c runs and runs and runs only to move maybe 4 degrees at best I have seen that sucker sit at 79 degrees for hours and not move while the a/c runs.

I'm thinking my returns are not enough I'm running out of ideas, I did move one return vent up high towards the ceiling and made the room feel cooler but it didn't change the run time on my a/c.

Need some serious help. Thanks Dennis

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Old 07-27-2011, 01:54 PM   #2
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a/c runs for many hours maybe return vent issues


It takes a while to remove the heat that has been convected into the house all day while the AC wasn't running, not to mention the latent heat from high humidity.

If you want the house cool in the evening when it's been 100 outside all day you will probably need to start the unit sooner or let it run all the time. (I know, that's not what you want to hear.)

I didn't see any details in your post about the temperature difference between the supply air and the return air temperatures (Delta-T). This information would help to determine if your AC is working properly.

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Old 07-27-2011, 01:58 PM   #3
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a/c runs for many hours maybe return vent issues


You really have 48 degree air at some registers with 78 degree return? Sure doesn't sound right but I'm no expert. Ive always heard an 18 to 20 degree drop across an evaporator is normal. Not questioning your data but wondering what your measuring those temps with?
I had a similar problem years ago and it turned out that attic air was being pulled into the return air stream. As fabrk8r says, measure your delta-t but measure at the evaporator outlet and at the blower inlet, not at the return air registers.

Last edited by Mike in Arkansas; 07-27-2011 at 02:06 PM.
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Old 07-27-2011, 02:30 PM   #4
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a/c runs for many hours maybe return vent issues


My understanding is that your typical central HVAC system is not meant to be switched on and off on a daily basis. It's not unusual for a properly sized and functioning system to have a cooling capacity of 1 degree per hour on the assumption that the system will be left on and at most the user might set it a few degrees higher during the day when they are away with a programmable thermostat.

So perhaps your system is not performing poorly, particularly with the heat your structure is absorbing in 100 degree weather, but rather you are asking it to do something it was not designed to do.
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Old 07-27-2011, 03:08 PM   #5
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a/c runs for many hours maybe return vent issues


I find it interesting how low your attic temp is; I'd expect something >115* in the summertime in daytime unless it is shaded by trees? How low in the attic is your sensor?
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Old 07-27-2011, 04:35 PM   #6
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a/c runs for many hours maybe return vent issues


3 1/2 tons for 3400 square feet seems on the marginal side size wise considering the temperatures this summer. Also, sounds like a lot of your heat gain is in the late afternoon and evening so it may be the the west/southwest part of the house could use a little shade. you say your windows are tinted but is it a low E tinting? My home currently has inadequate attic insulation. I have thermocouples above and below the insulation (many other places also) being read by a data acquisition system (I do this for a living so the stuff is available). The data in continually graphed and it's interesting to see how the temperatures drop in the attic in the evening but the temperature under the insluation continues to go up until the late evening hours. This makes the ceiling temperature rise also and in affect it becomes a huge radiator. Hoping more insulation will help with that and also venting the attic better to drop the temps there. I get an average of 85 degree ceiling temperature and with an 78 degree room temp, that calculates to over 8000 BTU of heat being emitted by the approximately 1000 sq. foot ceiling. With a room temp of 75, that figure increased to almost 12,000 BTU. A full ton.
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Old 07-27-2011, 05:52 PM   #7
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a/c runs for many hours maybe return vent issues


Remember at 100 deg it's design temp , that's the temperature it runs the most. Every little thing can contribute to more run time, dirty filters, dirty condenser, dirty evap, high humidity, ductwork size, etc etc. The unit will probably run 16 to 20 hours on a 100 deg day. 100 deg day full tilt balls and all.

Comfort = Electric bill. Hot sweaty = less electric bill
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Old 07-27-2011, 06:33 PM   #8
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a/c runs for many hours maybe return vent issues


Ok see if this helps and I can answer some questions.

Just got home 430 central time

This is measure by two gauges.
Temp outside 109 degrees hottest day of the year so far.


Temp in house 81 degrees a/c been running since noon wife said she heard it kick on so that means it was 83 degrees in here. House warmed up faster then normal. Of course hot day.

Attic temp is 109 Degrees and this is 2 feet off the blow in insulation. Attic fan running.


Next reads are taken from a Raytek temp gun (not no cheapy)

Ceiling temp 81-85 degrees

floor temp 78-80 degrees

Basement floor 72-74 degrees

Bare basement ceiling 74-75 degrees
Finished area ceiling 73-74 degrees

Air intake return vent 79-81 degrees
Forced air side 54 degrees

Hope this helps with some questions, it's now 530 and its 80 degrees now at the thermostate.

My tint is a plain jane cheap tint from Lowe's it has helped much to cool the south and west rooms for sure.

To let the rest of you know I keep everything clean, I spray down my condensor every week keep it clean and clear, filters are cleaned monthly I have two of them one on the bottom of my unit and one on the side which is feed by a 10" tube back around to the side of the air return trunk.

Thinking of adding another to the other side of my up flow unit.

I have 10 vents on main floor and ONLY 7 return vents is this an issue?

Thanks everyone for the reply's it just seems this should cool down faster and cycle more often.
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Old 07-27-2011, 11:03 PM   #9
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a/c runs for many hours maybe return vent issues


Here I am 10 pm and a/c still running, 79* in the house. 10 hours straight now and the temp has moved from 83 to 79 degrees.

It's 92* outside, and 92 in the attic, oh wait my gosh the a/c just turned off!!!!! it's 78 degrees in the hallway.

This still can't be right 10 hours straight, I need to fix something

I can put more vents in with ease to the main floor. I can add more returns too.
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Old 07-27-2011, 11:20 PM   #10
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a/c runs for many hours maybe return vent issues


A 25-27 degree temp drop across the coil indicates not enough air flow. Take the blower door off and check again. If the drop goes to around 20 degrees then you're short return, no change would be short on supply.
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Old 07-28-2011, 07:43 AM   #11
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a/c runs for many hours maybe return vent issues


rooms aren't cooling....sounds like you have too many returns the air is being supplied but returned to quick to the unit try blocking some returns to get it to pull somewhere around the stat location and catch the load in the rooms..you need to pressurize with the supply gain heat and return back at 79/54 and you cant do 70F forget the register readings take then down at the furnace return and right on top of the A coil...question why isn't 54F catching the load where is that constant load coming from... is your house tooo tight?
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Old 07-28-2011, 12:20 PM   #12
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a/c runs for many hours maybe return vent issues


Don't worry about the attic temperature just worry about the conditioned space. You getting any water out of the condensate drain line? a lot or little?
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Old 07-28-2011, 12:47 PM   #13
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a/c runs for many hours maybe return vent issues


Quote:
Originally Posted by Marty S. View Post
A 25-27 degree temp drop across the coil indicates not enough air flow. Take the blower door off and check again. If the drop goes to around 20 degrees then you're short return, no change would be short on supply.
I couldn't agree more!!!!

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Old 07-28-2011, 06:55 PM   #14
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a/c runs for many hours maybe return vent issues


Quote:
Originally Posted by Marty S. View Post
A 25-27 degree temp drop across the coil indicates not enough air flow. Take the blower door off and check again. If the drop goes to around 20 degrees then you're short return, no change would be short on supply.

I went out and bought new temp prob. I poked a hole in the return duct just before the filter near the bottom of the unit just before the air enters the blower area.

77.5 degrees


I then shoved the probe in a hole just above the A coil about 6 inches above it.

64.8 degrees

temp in house

81 degrees

temp outside

106 degrees

humidity in home

43%

humidity is always around 42 to 50%


O.K. what does this mean now?

At the end of posting this the register at the end of the trunk reads 63.1 degrees it's always my coldest register.
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Old 07-28-2011, 07:07 PM   #15
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a/c runs for many hours maybe return vent issues


Quote:
Originally Posted by ddeennis View Post
I went out and bought new temp prob. I poked a hole in the return duct just before the filter near the bottom of the unit just before the air enters the blower area.

77.5 degrees


I then shoved the probe in a hole just above the A coil about 6 inches above it.

64.8 degrees

temp in house

81 degrees

temp outside

106 degrees

humidity in home

43%

humidity is always around 42 to 50%


O.K. what does this mean now?

At the end of posting this the register at the end of the trunk reads 63.1 degrees it's always my coldest register.
Clean Coils, time to hook up the gauge and check SH and SC.....I guess AC is under charge

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