New central air unit not working properly

Old 07-03-14, 06:53 AM
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New central air unit not working properly

Hi,I had a new AC unit installed a month ago,when the temperature outside is above 87 it can't keep up.thermostat is set on 70 house is reading 76 .At night when it cools down a little it cools to 70 .The new unit is a 1/2 ton bigger than the old one .My old ac unit kept the whole house cool for 12 yrs until it broke.This is second service call in less than a month.The first call they said it wasn't fully charged when they installed it.Now it is still having the same problem.I am waiting for them to come service it again.Is there any questions I should ask them ?I don't want to go thru this every time it is above 87.thanks for any help
Old 07-03-14, 07:26 AM
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The indoor temperature split between the return-air at the supply air grille closest to the air handler should have around a 19 to 21°F ’sensible’ temperature drop with either an 80°F or 75°F indoor temperature @50%RH. This is also the indoor split with the higher SEER units at 13 SEER or above.

***Regardless of the indoor %RH you can ballpark figure the sensible Btu/hr by multiplying 1.08 (I use 1.1) *X the sensible temp-drop *X the CFM of airflow through the indoor coil.

IE; normal temp-drop of 20°F * 1.08 * say 850-CFM (a 2-Ton system) is 18,360-sensible * 1.28 is (2-Ton) 23,500 total, sensible + latent Btu/hr.

At 80°F & 50% indoors the sensible on a Goodman unit data is around 72%; 100 -72 is 28 or 1.28 * sensible of 18,360 is 23,500 total, sensible + latent Btu/hr.

If the indoor temperature split is too high it may have very low airflow which needs to be brought up to its normal CFM Rate of around 350-CFM for dehumidification, otherwise, 400 to 425-CFM per/ton of cooling; 2-Ton 800 to 850-CFM.

When the temperature reaches around 80 to 85°F outdoors and the indoor temperature is 78 or 80°F and the relative humidity indoors is right around 50% RH the outdoor condenser air-discharge temperature-split above the outdoor temperature, should be around 20 to 21°F or around 105°F. The new 13-SEER units will also usually be near this condenser temp-range; higher SEERs than 13 will be proportionately lower.

If the indoor temperature is 75°F and the indoor relative humidity is 50% then the air discharge air temp-split off the condenser should be around 17°F or 102°F off condenser.

If the outdoor condenser split is too high your air handler may be drawing hot high moisture content air into the return from the attic causing the high condenser discharge air temperature; this will also lower the indoor temp-drop.

If the temps are within plus or minus a degree or two, that air-conditioner is performing rather well.

On the new air-conditioning systems at 14 SEER & higher the outdoor condenser temp-split is lower than it is on the 10 to 12 SEER units; the ‘indoor’ temp-split is the same.

At 50% indoor relative humidity you can take the temp-drop number *times 1.08 *times the CFM of airflow to get the sensible Btu/hr absorbed by the indoor coil, because of duct temperature & air losses, that doesn’t mean all of that heat is being drawn out of the rooms.

A higher %RH will lower the temp-drop; let’s say mfg’ers sensible is 80% at those conditions you can multiply the sensible Btu/hr by 1.2 *times to get the total Btu/hr; at 75% sensible *times 1.25 equals total Btu/hr.

You can check test the CFM in the furnace heating mode; temp-rise above indoor return air temp * 1.08 / into furnace Btu/hr output. IE., temp-rise 60°F * 1.08 is 64.8; 76,000-Btu/hr furnace output / 64.8 is 1173-CFM air volume flow. You can use your home owner’s manual to get the cooling CFM.

The reason I alone provide these safe, easy, & effective ways to ballpark the performance of your Air Conditioning system is that very few HVAC service persons check the actual delivered Btu/hr performance of the system; the actual delivered to & from the rooms performance of all air-conditioning systems should be checked by competent service technicians with a permanent record left at the air handler in the home.

National tests have shown that the average air conditioning system usually only delivers 63% of its Rated Btu/hr due to an ‘inadequate/improper’ install & engineering-design setup. Therefore, it pays you big-time to easily check you air conditioner’s operating performance.

In a Video Experts Explain why you need to become HVAC energy efficient literate; IE; a 14-SEER becoming a 7-SEER after the install:
Old 07-03-14, 08:18 AM
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Did you replace the furnace and A coil too?

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