Problem?

[blazenut]

I have read that you should ideally have a 14-19 degree temperature difference between the registers and returns while testing the A/C. Today i inspected a home and the difference was over 25 degrees in some locations. Is this a problem? The compressor was 7 years old.

[Mark P]

I usually go with 14 -21 difference, but anyway a delta t above the upper limit may be a result of insufficient air flow across the evaporator coil due to heavy icing of the coil.

[blazenut]

What would possibly cause that, and what could you check at the inspection to verify this? The filter was new, some of the registers were a little beat up but im fairly sure that air was flowing ok. The compressors fins had some damage, but not that much out of the ordinary. How would you report this?

[Mark P]

Low refrigerant level can cause the coil to ice up. If you can see the coil it may look something like the picture below. Which I took a few weeks ago. Most of the time the coil is not visible.

In your case I would report a high Delta T and recommend it be checked out by a HVAC contractor.

Click to Enlarge

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[Terence McCann]

I have read that you should ideally have a 14-19 degree temperature difference between the registers and returns while testing the A/C. Today i inspected a home and the difference was over 25 degrees in some locations. Is this a problem? The compressor was 7 years old.

It can be caused by a few things.

Where did you measure supply air temp and discharge air temp? When I check temperatures I measure return air temperature at the thermostat and supply air temp as close to the air handler/furnace as possible.

Low air flow is the main cause for higher than normal delta T. This can be caused by the fan speed being incorrect on a multi-speed fan, dirty filter, dirty coil, lack of return air etc.

When a coil ices up, like the one Mark was showing, it could be due to improper refrigerant charge, sever lack of air flow due to dirty filter, dirty coil or indoor blower not working etc. Typically when a coil is iced that badly you won't have much air flow at all and, depending on where you measure discharge air temperature, it may be warm or cold. It would be comparable to trying to blow air though an ice cube. If I saw a coil iced like that I wouldn't bother checking temperatures. I'd shut the unit off and inform the owner that there is a serious problem with their AC and they need to have it serviced before running it again. You can bring back a slug of liquid which will ruin a normal recip compressor.

[caryseidner]

Was it Space Pak? I have never tested a Space Pak system that had less than a 25 degree temp. difference. At first I wrote them up, but after the 10th (or so) I stopped calling them out.