17-20 degrees

 

18-20 is what we go by

 

depends on the unit the humidity and where you are taking temp measurements.

could be anywhere from 10-20 deg and still operating properly.

 

Exactly.

Depending on the location of the ducts and air handling equipment, the temperature at the grill and vents may vary a great deal.
I have seen duct with R-6 insulation in a vented attic gain 2-3 degrees every 10 feet of run, especially if it runs near the peak of the roof. In this situation, I check the temp drop at the unit.

If the ducts and air handling unit are in the crawl or a basement then the heat gain on the ducts are almost 0, and you can read at the grill and vent.

Also, if there is a high and low return, it is best to check at the indoor unit to get an average temp on the return side.

Another factor, as quoted above, is humidity. If there is a high amount of humidity in the home, usually from the unit being turned off or opened doors or windows, the temperature drop will be lower due to the refrigerents energy being used up to remove the humidity. The unit needs to run for 24 hours with the house closed up, except for normal traffic, and then tested.

I always expect a minimum of 15 degrees difference, but prefer 17 to 20.

 

As stated above.
Where those readings taken at the coil, or at the registers.
What were the conditons that day, when the readins where taken.
How long was the unit running before the readings were taken.

 

thanks for the responses.

the temp difference was taken at the unit.

i am not sure how long the unit had been running, but it has been tested twice and the difference was 13 and 12 respectively. neither day was overwhelmingly hot or humid, typical summer days in the 80's.

 

How much difference btween the return at the unit and the return grill in your home?

 

Might be a dirty coil.

Did your hvac guy check any thing other then temp difference.

 

We are taught (present student) 18 to 20 degrees is optimal.

 

i'm not sure what the difference is at the grill, don't think either HVAC contractor tested it there.

is there anywhere in writing that states it should be 18 to 20 degrees?

if the difference is not in that range, is it considered a non-functioning unit? i am asking because the contract said the HVAC must be functioning, and while it is theoretically running, the temperature difference is still too low.

 

if you wanted it to function like a new system you should have bought a new house. if the system is from the 80's i would say its "functioning"

 

Functioning is a broad term.
And your A/C is functioning. It may not be functioning 100% proper, but your contract may not specify that.

 

I am in the same situation with a home inspection. Inspector would run it for 1/2 hour and take reading diff of 65 out/73in 2nd floor main at registers with a infrared.only 8 degrees.I found return connected to a unfininsh area with out insulation in the addition all so very noisey returns. I sealed off return,sealed majer leaks at plenum,adjusted fan to medhigh, doublechecked condenser preasures and adjusted to specs.The realitor turned the systems on 3 days earler and set temp to 72degres. (( Out door temp 90 , indoor temp 72, temp at plenum 58, return 73 )) 15 degree differance. the masterbedroom zone was at 15 differance as well. My concern is the inspector will measure at the register and get below 14 degree differance and want more done to get better readings. 14 to 20 differance is what he wants. With it being 90 outdoors and 72 indoor even before i made repairs I would think was working properly the minute i walked in the door. this house has 5 tons down 3.5 ton and 2 up..

 

infrared thermometer




I don't have much faith in ifrared thermometers when checking TD.
They only read surface temperatures of the grills and I have seen the grill temp be a few degrees different than air temp taken with my K-type sensor on my meter.
Has anyone else had similiar findings???

 

So you're saying that an HVAC contractor came out and did nothing other than measure TD? Why didn't they just tell you what was wrong with it?

Odds are, that there is a problem with the system, but without several other readings it's impossible to say. From what you've given it seems that both the home inspector and the contractor are clueless about what the TD means. It cannot be used by itself to determine anything about system performance. If it's below 15 then that raises a red flag, but it still requires more measurements to be taken before the system can be determined to have a problem.

 

Double post removed.