Split Between Supply and Return only 10 Degrees


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Old 08-21-15, 11:13 AM
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Split Between Supply and Return only 10 Degrees

I just had a new system put in my recently purchased home. I never really got a good measurement from the old 2 ton AC as I had it charged but do to a bad refrigerant leak and age I had it replaced with another 2 ton system. The old system fully charged keeps temps about the same as the new ones. Unit constantly runs even on more moderate 80s temp days to keep up with 75. On 90 and up days the system struggles to keep temps below 78.

House is an 1160 sq ft rancher on a crawlspace in Hampton, VA. System is downflow with supply ducts running under house and the return directly above the air handler.

Got an ir temp gauge and tested air at return and at supply vents in all the rooms. My return air is 75 and the vents are running between 64 and 65 degrees.

The system is charged properly and I know the issue is not at the return due to there being only a sheet metal transition to the return grill from the air handler.

My suspicion is that the metal ductwork under the house needs the insulation (1968 so barely anything) removed and be sealed with mastic. When I go in the crawlspace now when the AC has been running it is very cool down there.

Question #2 would be that if my above suspicion is correct would I be ok to just mastic and air seal all the duct work and wait till later to insulate it?
 
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Old 08-21-15, 11:25 AM
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I would certainly start with sealing the duct work and see what, if any difference that makes in your split.
 
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Old 08-21-15, 11:54 AM
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I agree with stickshift, but leaks in the supply duct won't effect the Delta T much.
How do you know it's properly charged, did you have someone come out and check the subcooling and superheat temp's?
 

Last edited by skaggsje; 08-21-15 at 11:57 AM. Reason: correction
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Old 08-21-15, 12:24 PM
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Just changed the batteries in the IR thermometer just to make sure Fluke brand so not a cheap one and getting the same thing in the house 75 return between 64 and 66 throughout the house 66 being in the bedrooms farthest from air handler.

And ive had the guy that installed it my neighbor come by and check a few times and the charge was good everytime.
 
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Old 08-21-15, 12:27 PM
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Because the ducts are in the crawlspace what is the chance that after sealed there is going to be an issue with heat gain in the summer or loss in winter if I DON'T insulate?
 
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Old 08-21-15, 12:32 PM
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Another quick question for you all could this possibly be a symptom of an undersized system?
 
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Old 08-21-15, 04:53 PM
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Your system may be slightly undersized for the insulation level in the house but I really question the temperature readings. You do not get good results in trying to measure air temperatures with an infrared thermometer. I suggest that you use a probe-style thermometer actually stuck into the supply and return grilles. A ten dollar instant read (digital) cooking thermometer should be sufficient.
 
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Old 08-23-15, 10:37 AM
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probably should have gone with a 2.5 ton

at 1160 sq feet a 2.5 ton would have been better. especially on 90 degree days. My house is 1200 and thats what i put in. Not an expert plus you are farther north but 90 degrees is 90 degrees whether its north or south. Also know its been getting hotter every year up north. I believe formula is 500 sq feet per ton. Oh and insulation will definitely make a differnce. Bought a house with a converted garage. It always felt like I was outside . It was cold in winter and extremely hot in summer. Opened the outside walls due to some rot. No insulation. Installed insulation and room was a lot more comfortable.
 
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Old 08-23-15, 01:48 PM
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You can verify the size by doing a heat loss/gain evaluation, but I favor bringing the house into line with a smaller ac unit as opposed to increasing the size. A smaller unit will run more which does better at lowering the RH.

A plus for air sealing those ducts as well as the house is, any pressure difference between inside and outside means air is escaping or entering. the resulting air that returns must then be conditioned. A house that hasn't been aggressively air sealed will exchange all of its inside air every 3 to 4 hours, more frequently is common.

As Furd stated, IR thermometers can be difficult, especially when pointed at metal surfaces. Plus those IR units can have a wide target area which dilutes the reading.

Bud
 
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Old 08-25-15, 09:05 PM
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at 1160 sq feet a 2.5 ton would have been better. especially on 90 degree days. My house is 1200 and thats what i put in. p.
No. Anything more than 2 tons for that floor area in a very hot climate or 1.5 tons in a mild, northern climate is likely oversized, possibly grossly.

A load calculation needs to be done to correctly size, and if the split is really only 10 degrees in typical operating conditions (50% humidity inside), something is wrong with this system be it significant return air leaks pulling in humid, unconditioned air from an attic or crawlspace, too much airflow, missing duct insulation or an issue with the a/c itself.
 
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Old 08-25-15, 09:09 PM
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I had it replaced with another 2 ton system. The old system fully charged keeps temps about the same as the new ones. Unit constantly runs even on more moderate 80s temp days to keep up with 75. On 90 and up days the system struggles to keep temps below 78.
Many things can cause this problem.

To start, the split should be checked at the air handler rather than the vents. Also measure the return air wetbulb (wrap the end of a digital stem/;ab thermometer in thin wet fabric and put it into the return duct at the air handler) and post.

Check the split of the condenser (air temp being blown out of the outdoor unit - outdoor air entering the coil) and post, along with the model number. We can use this to calculate heat rejection and see if the a/c itself is working right.

You should post the model number of the condenser
 
 

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