AC not cold enough at registers


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Old 07-25-08, 04:51 PM
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AC not cold enough at registers

I am having a problem where temperature coming out of the supply register is not cold enough. As an example, after the AC is running for an hour, the temp of the air is 68 to 70F coming out of the closest register.

I checked the indoor coil (3 ton unit, about 1200cfm total) and the temp is around 52 to 54F, but the air coming out of the plenum is around 66F to 70F (two locations).

My question is: should air coming out of plenum be around low 60's?

Is this a symptom of not enough airflow because of an undersized unit?
 
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Old 07-25-08, 05:00 PM
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You checked the coil, how/where exactly?, when you got the low 50's read?

Where is the blower located, and where do the ducts run? (like, in a basement, through crawl space, or in attic?

And what are the ducts made of?

And have you ever felt the air blow out the registers before, enough to know if the blower speed is about the same? Or is it less than what you remember?

Is blower belt drive or direct drive?

Is it real hot where you live, and with the sun out (nice to know especially with attic installation, as they can get real hot)
 
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Old 07-25-08, 08:51 PM
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When I had a technician came to check the unit, he removed one of the 10" duct at the plenum, then we use an infra red thermometer and scanned the surface of the coil. This is also how we measured the temperature coming out of the 10" duct collar.

Not sure what kind of blower it is, the air handler is a 3-yr-old seer-13 Goodman unit.

The air handler is in the attic, I live in AZ, so right now it's 100+ everyday out there. At the time of the test, it's about 101F out there, and 110 to 115F in the attic.

We used R6-insulated flex ducts here, there are three main ducts connected to the plenum: 16" main duct that supplies air to 3 rooms (about 700sqft of space). 10" supplying 200sqft of space, and an 8" supplying powder room and laundry room.

I know it's hot in the attic, but my other unit which is a 4 ton, can supply air of 62F when it's 105F out there.

What baffles me is different duct outlet in the plenum has different air temperature. e.g. the 8" duct collar has air of 66F coming out, while the 10" duct collar has air of 70F coming out. 10" collar is located at the top of the plenum, while the 8" is at the bottom. It seems not right to have a differential of 4F within a small space of the plenum.
 
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Old 07-26-08, 05:05 AM
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Temps

Depending totally on RH and Return air temp.

The temperature of the (Actual evaporator) is variable depending on what the heat load (or properly named: heat gain) is at the time you check it but: if your home was cool at the time, lets say 75 degrees (that which would be refered to as, the "return air" temp). - The air coming out of the supply vents should be roughly 55-58. In order to have 55-58 degree's supply air to the vents - the actual evaporator must be 35-38 degree's. Below 32 degree's would indicate low airflow, or low heat gain. (A Cold house @ 65-70 degree's and low RH - the house A/C system cannot continue to cool beyond this because the Evaporator will Freeze and the house temperature will rise because there is no airflow to accept heat gain at the evaporator being now frozen solid)

If ...it is true...and you have an actual evaporator temp of 50-60 degree's (as you so indicated by lazer temp check on the body of the evaporator), you have a "starved evaporator" or...a super high heat gain (furnace is on at the same time as A/C! - unlikely).

Some likely causes are:

*Lazer Temperature check is inaccurate - A Digital temp check of evaporator copper line at the evaporator aft/outlet side by attaching sensor to line, is much more reliable.
*High RH (Relative Humidity), to much capacity of condenser being used to convert water vapor into water instead of removing the heatgain from the conditioned air (gives you a bad "Delta T").
*Low On Freon
*Overcharged unit (Too much refrigerant)
*Expansion Valve Bad - orfice restriction
*Restrictions in flex duct (90 degree turns or more - dips up and down over rafters/joists - smashed - longer than 12 foot - between supply plenum and registers.
*High Heat Load - Leaky supply ducts - loss of a portion of supply air before it reaches the register, therfore the load inside the flex duct adds too much heat to the remaining air getting to the register to be 55-58 degree.
*Return Air Plenum Leaks hot attic air into conditioned return air
*Restrictions in Liquid Line, kinks, too small of liquid line, (Filter Dryer restriction)
*Compressor with bad valves (Not Scroll)
*Electric elements or furnace running at the same time as the A/C.

Note - The difference in vent temps means that the (Heat Load being picked up in each vent is different - most likely the plenum or flex duct itself is adding the additional load merely because there is not enough conditioned air) mainly because the Evaporator does not have enough refrigerant boiling off inside to pick up the total load!

Personally speaking, I tend to more or less play with lazer type temperature guns when I have one near me - I dont use or trust them beyond a smile. Its the physics and dynamics of how it works that makes it so variable in its readings and therfore inaccurate. I tend to also want to say in closing... that since the sole information here is based on that single check you did using the lazer type temp checking device you mentioned, then my synopsis of the problem would also be inaccurate and just a samatic in theory

Final Note - Airconditioning contains 3 major fields of study. That being Electrical (Hard) - Mechanical (Easyest)- And of course Physics(Hardest). In most cases, a technician will be strong in one and maybe two of these fields with professional schooling and much much study... and half a life of hands on experience. To be strong in all three means you have a really good technician before you. Unfortunately seldom, is that the case. The understanding must be in Theory....and hands on real world. To be weak in either means to be substandard.

For this reason, you (The homeowner) will have most success in finding the answers and correcting some of your A/C problems here on this forum in the mechanical/Electrical areas of this field (something that is "Broken or burned out"). However, the Physics (The most difficult in understanding) part of this field mostly cannot be corrected by a homeowner. When dealing with the "Physics" side of this field, it requires a very good technician (licsensed) to recognize and ability to fix the homeowner problems. It is chemicals and how they react to various pressures and temperatures I am eluding to here. I suggest that when these types of problems arise that truly the homeowner needs a qualified technician in place of advice on any forum.
As you can see, the physics side of this field, my comments assumptions here in this post is rather complicated just in explaining it...even to lessor skilled technicians much less a homeowner with no experience at all.
 

Last edited by The Real Deal; 07-26-08 at 07:55 AM.
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Old 07-26-08, 09:36 AM
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Additionally

Additionally to my last post.

*Airflow/Ductwork dynamics and fluid dynamics (Condensate), is also considered physics but bridges a positive - The ability to explain and repair in a forum in some cases to a homeowner DIY unlike refrigerants/chemicals and how they react under pressures and tempertures.

*Accurate information about the existing duct system and condensate being the key.
 
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Old 07-26-08, 09:59 AM
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Originally Posted by johndoe74
When I had a technician came to check the unit, he removed one of the 10" duct at the plenum, then we use an infra red thermometer and scanned the surface of the coil. This is also how we measured the temperature coming out of the 10" duct collar.

Not sure what kind of blower it is, the air handler is a 3-yr-old seer-13 Goodman unit.

The air handler is in the attic, I live in AZ, so right now it's 100+ everyday out there. At the time of the test, it's about 101F out there, and 110 to 115F in the attic.

We used R6-insulated flex ducts here, there are three main ducts connected to the plenum: 16" main duct that supplies air to 3 rooms (about 700sqft of space). 10" supplying 200sqft of space, and an 8" supplying powder room and laundry room.

I know it's hot in the attic, but my other unit which is a 4 ton, can supply air of 62F when it's 105F out there.

What baffles me is different duct outlet in the plenum has different air temperature. e.g. the 8" duct collar has air of 66F coming out, while the 10" duct collar has air of 70F coming out. 10" collar is located at the top of the plenum, while the 8" is at the bottom. It seems not right to have a differential of 4F within a small space of the plenum.
What SEER Rating is the 4-ton unit?
The lower EER & SEER Rated units had a larger BTUH capacity compressor to the area of the coils.

This provided the capacity to pull the evaporator pressure & temperature lower, resulting in higher Supply & Return air splits off the evaporator.

It is also probable there could be numerous problems regarding the installation & set-up of the 13-SEER system! HVAC RETIRED - udarrell
 

Last edited by HVAC RETIRED; 07-26-08 at 10:03 AM. Reason: Readability corrections
  #7  
Old 07-26-08, 10:04 AM
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Thanks for the reply! It certainly helped me learn something new about A/C problems.

I agree with every word you said, I've had 3 technicians came over and none of them can improve my cooling. All of them checked the pressure at the outdoor condenser and found the freon level to be ideal.

It's difficult these days to find a tech that's willing or have the knowledge to troubleshoot all the potential problems you mentioned.

Anyone can recommend a reputable and excellent A/C service company in the Phoenix metro area?
 
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Old 07-26-08, 10:07 AM
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Complicated indeed

All true.... Very good point indeed.
 
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Old 07-26-08, 10:10 AM
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Reputable company

Originally Posted by johndoe74
Thanks for the reply! It certainly helped me learn something new about A/C problems.

I agree with every word you said, I've had 3 technicians came over and none of them can improve my cooling. All of them checked the pressure at the outdoor condenser and found the freon level to be ideal.

It's difficult these days to find a tech that's willing or have the knowledge to troubleshoot all the potential problems you mentioned.

Anyone can recommend a reputable and excellent A/C service company in the Phoenix metro area?
Tri City Mechanical - One of the largest companies in Phoenix - 98 million a year. I worked there at one time.
 
 

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