A/C unit not bringing temp down

christians1July 6, 2010

Hello, I had a new trane XR 13 unit installed in 2008.

It seems to have been working great. but now, the temp out side is hitting 100deg and my a/c will only bring the temp inside down to about 78deg during the day. It will run all the time and then over night it will bring the temp to the setting 74 or 75 deg.

Does anyone think there is something wrong with the unit? It blows cold error out the vents.



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22 degree temp difference sounds pretty good. Just a homeowner, but I suspect the pros will say you have a properly sized system that is simply being overwhelmed by a rare extreme weather event, unless 100 degree days happen normally in your area.

    Bookmark   July 7, 2010 at 6:53AM
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What you need to check is the temperature of the air going into the system and the temperature coming out. There should be a difference of at least 15F.

Indoor to outdoor temperature difference does not matter.

    Bookmark   July 7, 2010 at 9:16AM
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If you have the 15 degree difference or better that WM mentions, I would suspect insulation or infiltration/exfiltration issues.

If you don't have much of a temperature differential, it will likely be an equipment issue.

    Bookmark   July 7, 2010 at 9:22AM
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WM, how does indoor outdoor not matter if you are doing a Man J to determine cooling load for a specific temperature in a home? That seems to break a few laws of thermodynamics. When my contractor showed me the numbers he stated what the design temps would be based on the Man J. He then went on to explain that if outdoor temps went beyond the calculated max degrees that the system would not be able to keep indoor temps as low due to the cooling requirement exceeding the system design. (His caveat was that this is assuming a perfectly sized system)

    Bookmark   July 7, 2010 at 9:51AM
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I think emarston is correct.

I would measure your supply and return temps to get the temp differential.

is system running continuously at the high temp of the day or cycling?

post back.


    Bookmark   July 7, 2010 at 10:03AM
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Would it make sense to use a separate dehumidifier? The short cycles won't do well in removing humidity as I understand it, so why couldn't you buy an inexpensive dehumidifier?

    Bookmark   July 7, 2010 at 10:39AM
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"how does indoor outdoor not matter if you are doing a Man J to determine cooling load for a specific temperature in a home?"

Sizing the system is NOT the same as determining if it is working correctly.

The drop tells you if the system is operating correctly.

If it is not sized correctly it will not cool to the desired set-point.

    Bookmark   July 7, 2010 at 4:25PM
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Units are designed to perform @ 95 degrees. Go over 95 and your unit will begin to lose capacity because it has a harder time discharging the heat from the outdoor coil into the air.

Measure the temp drop as close to both sides of the coil as possible. The temp drop is just one tool used to diagnose a system. Low airflow, etc could cause a low temp drop.

    Bookmark   July 7, 2010 at 10:02PM
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Having a txv helps with the humidity also which makes the higher set temps a lot more comfortable. A variable speed air handler would be better yet.

    Bookmark   July 7, 2010 at 11:13PM
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My suggestion to measure the temperature drop across the evaporator has nothing to do with a ManualJ. Not directly, anyway.

If he says he has 80F going in and 60F coming out, then that means one thing. If he says he has 80F going in and 75F coming out, that means something else.

    Bookmark   July 8, 2010 at 12:40AM
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Just to ask the question about checking the temp going into the intake and coming out of the vent should be a 15 to 20 degree difference. So if you have the A/C set on 74, and it's 74 degrees in your house, the temp coming out of the registers should be 15 to 20 degrees colder? So if it's 110 degrees outside in Phoenix AZ, a 20 degree difference would be considered good? So that would mean it would be 90 in the house? UGH.....I'm glad I don't live in Phoenix!

    Bookmark   July 8, 2010 at 4:16AM
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Mark - I think you are getting confused. There are two delta Ts going on here. Inside to outside and intake and output.

In the majority of the country, a properly sized a/c system should maintain 20 or so degree differential from outside to in. In Pheonix, the goal might be 30 degree difference (maybe more).

Inside the house, the difference between intake and output temps are still 15-20. So if the house was 80 degrees inside (outside could be 110), then the register temp should be 60-65. So you can live in Pheonix if you want.

It sounds to me like the OP has a properly sized system. If he likes it colder, then sun blocking, reducing infiltration, radiant barrier or more insulation is the most economical solution. He could replace the a/c with a larger unit but the downside (other than cost) is more humidity on cooler days and higher energy costs.

A stopgap measure is to set your thermostat really low so that all night the a/c runs and it might knock down the daytime temps a few degrees into the comfort zone. You can check the forecast and if it is going to be higher than 95 tomorrow, turn down the stat tonight.

    Bookmark   July 8, 2010 at 7:05AM
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Mark, you are misreading the post, I think. If the set point is 74 and the system is maintaining the set point, the air coming out of the supply registers will be 15 to 20 degrees lower than the set point. In other words, 54 to 59 degrees F.

It is the difference between the ambient air temperature and the supply air temperature, not the outside temperature. The 15 to 20 degree difference is an indicator that the HVAC equipment is working properly, not that it is sized correctly, nor that the house's thermal envelope is performing properly.

If the system can't maintain the set point yet does have an adequate differential, it is an indicator that it is either sized improperly or the house's thermal performance isn't what it should be, or a combination of those.

    Bookmark   July 8, 2010 at 7:17AM
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I had the the same exact problem with my 22 year old 2.5T Trane system. It was worse in the winter when the temp went below 20 degrees, the sucker would run forever and never keep up with the stat set on 69---we ended up using some portable room radiators and shutting the heat pump down to 64! The summers were not as bad.

All I can tell you is they put in a new heat pump last year. Its a 3T trane. It keeps the house both warm and cool now...though I have been setting the stat at 76-77 during the day and dropping it to 73-74 at night--around 9pm. It cycles on and off and at 76-77 and while a little uncomfortable upstairs (79 degress), there is no sense overworking the thing since I am not home during the day. Now I usually just leave it 74 all day and bump it 73 at night because it always a couple of degrees warmer in our bedroom above the garage. These 100 degree days are extreme though!

    Bookmark   July 8, 2010 at 4:48PM
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" the sucker would run forever and never keep up with the stat set on 69"

Undersized system or leaky house.
Older systems often were so oversize the leaks did not matter.

We cut the designs a little tighter now, and AC has to be correct or you end up cold and damp from lack of humidity control (an oversize system that does not run long enough to remove enough moisture).
Residential AC is not normally controlled by humidity.

Commercial systems routinely cool the air to set humidity, then heat it back up to get temperature control.
They run this way year round.
It cost more but avoids problems in the spring and fall when the system would otherwise be to large.

You can size a system to make whatever inside to outside drop you want, it just gets to be a problem when the load is not at maximum and humidity is not well controlled.

There are large systems that use an accumulator (really just a tank) to hold refrigerant.

The compressor is controlled to keep liquid in the tank, and the thermostat (and/or humidistat) then control the air handlers and evaporator valves for temperature and humidity.
They are complex and expensive systems.
Way above what a house needs.

    Bookmark   July 8, 2010 at 5:06PM
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after all these posts from people who have taken the time to reply, wouldn't it be nice for the OP to provide some feedback?


    Bookmark   July 8, 2010 at 5:15PM
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Ok....Gotcha....So if it's set on 74 degree inside and maintaining that set temp, the cool air coming out of the registers should be 15 to 20 degrees colder than that. I did check the air coming out of the registers and it was 53 degrees.....Although it definitely doesn't feel as cold as a car a/c....The car a/c feels like it's coming straight off an ice block

    Bookmark   July 8, 2010 at 8:21PM
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What the majority of consumers don't understand is that air conditioning doesn't so much *make* cold as it *moves* heat ... from inside the house to outside. The capacity (BTU) rating is the amount of heat it can move in a stated period of time, typically one hour. A 36,000 BTU unit can move 36,000 BTUs of heat in one hour. One BTU (British Thermal Unit) is the amount of heat required to raise the temp of 1 pound of water by 1°F. A 36,000 BTU air conditioner can drop the temperature of 36,000 pounds of water by 1°F in an hour. Water weighs about 8-1/3 pounds per gallon, so that's about 4,337 gallons. It's a little more complicated than that, since some of the heat reduction is used to condense moisture out of the air.

The temperature drop across the evaporator (air going in the return and blowing out the registers) can perhaps be thought of as the amount of heat removed in one "pass" through. The system continually recirculates the inside air through the evaporator to continually remove heat and lower the indoor temperature to the setpoint. If more heat is coming INTO the house ... from air leaks around doors and windows, opening of exterior doors, appliances and lights operating, the body heat of people, and radiation/conduction through walls and ceilings and windows and doors ... than the system can remove, then the indoor temp may either hold steady instead of dropping, or even slowly *increase*.

Regarding Mark40511's scenario of 110°F outdoor temp in Phoenix -- If the indoor temp is 74°F and the outdoor is 110°F, and the drop is 15°F to 20°F, then the air blowing out of the registers will still be 15°F to 20°F lower than the 74°F return air ... not 90°F (20°F drop on 110°F outside temp). However, if the system is designed for an indoor temp of, let's say 74°F at an outdoor temp of 98°F (based on construction of the building, insulation properties, occupant count, windows, doors, sun exposure and so forth), then at an outdoor temp of 110°F there's MORE heat from outside coming into the house than the system is able to combat, and it won't be able to drop (or hold) the indoor temp to 74°F.

    Bookmark   July 8, 2010 at 8:22PM
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If the ducts are not sealed you can have a whole other situation. I had issues with my old system satisfying the stat and checked my supply and return temps in the basement at the furnace. I had a return temp over 80 degrees and a supply temp around 15 cooler. All of the returns are on inside walls and nothing is in the attic. I was getting a good portion of return air from the attic.

    Bookmark   July 8, 2010 at 8:46PM
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Yes I read on another post where this couple went into their attic in the middle of the afternoon in FL and their attic was cold.....(Leaking ducts air conditioning their attic)..

I have one of those weather station sitting on my computer desk in the living room that shows the outdoor/indoor temp etc.....I have my heat pump set on 73 and the indoor temp on my weather station says 70.2 degrees and it feels like it! But the A/C thermostat says 73...Could the a/c thermostat be off?

    Bookmark   July 9, 2010 at 3:43AM
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As per me if the temperature difference is much you have a cause to worry.You should get your unit checked.
Air conditioning florida is offering wide range of air conditioning services, hvac repair, air conditioning maintenance,and much more......

    Bookmark   July 9, 2010 at 7:43AM
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Kathysierra, are you advertising here?

    Bookmark   July 20, 2010 at 12:18AM
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Great info! We have a 1910 house that has an old house smell and a few bats. We put in a basement window fan and opened up another window in the oppisite end of the house (both North facing) to exhaust the smells. Now that it is so hot. The duct are all dripping and the AC can't keep up with the 100 plus heat. Am I right that we can't bring in the air from out side into the basement to exhaust the smells and keep the AC working properly. Thanks!

    Bookmark   July 19, 2011 at 12:08PM
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Fran67, start a new thread.

    Bookmark   July 19, 2011 at 1:37PM
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