View Full Version : how cold at vents
07-17-2006, 07:00 PM
newbie.. first post..
How cold should the air be coming out of the vents?? I see a lot of people saying their AC "does not blow cold".. can i measure this by putting a $35 digital thermometer on top of a vent for a few minutes?? Is there a reasonable standard to go by??
07-17-2006, 07:08 PM
From a consumer standpoint...good question. But aren't we all "consumers" in this economy(CPI, consumer spending, etc.).Thank you
07-17-2006, 07:27 PM
The temperature of the air coming out of the supply depends on the temperature of the air going into the return. So the number you are looking for can vary.
The warmer it is in your home, the warmer the air coming out of the supply registers. Make sense?
An acceptable temperature drop is 15-20 degrees across the evaporator. So, if the temperature going into your return (intake in layman's terms) is 80 degrees then the air coming from the supply should be approximately 60-65 etc.
Integrity of duct insulation, duct leakage, thermal regain or loss, air flow, equipment operating condition etc., can have a large effect on these numbers.
So, there is no magic number but a measurement of the equipments ability to remove heat.
I think you are asking this question in order to identify if your AC is putting out enough cooling. I submit that a technician can measure temperatures closer to the air handler to see if temperature drop is OK, and a few other things that are over my head but key to knowing if the machinery is working OK.
Once the machinery is working OK, I submit the more important question for a homeowner is, does the system maintain the thermostat setpoint that you want and need? If yes then the whole system is working OK and is sized big enough for your house (you want to avoid oversizing but that's another thread).
I myself have been taking temperature measurements at the supply vent using a temperature data logger. There are two systems in my house, one rather old and one relatively new. On the old system temperatures were running 71-73 Fahrenheit in 2005 when it was blowing 465 cfm/ton due to a technical mistake. But it was keeping the house at setpoint during some 100-degree days. This year the airflow is reset to 400 cfm/ton and the temperatures are in the 65-67 degree range. On the newer system I am seeing temperatures in the 58-62 degree range, it appears to be oversized, is running perhaps 320 cfm/ton using a dehumidistat and variable speed blower.
Hope that helps -- Pstu
07-17-2006, 11:29 PM
you are looking for as close to an 18 degree split as you can get. A split is the difference between the temp going into your return and the tempcoming out of your vents. of course,there will be a slight difference in the temp of your vents the farther they are from your unit.So i always try to measure my split at the vent closest to the unit.
07-18-2006, 04:44 PM
thanx for the answers.. i put a digital thermostat on the vent on the ground floor and after about 10 minutes it was down at 59 degrees so i guess that is ok.
thanx again for the replies.
07-18-2006, 10:02 PM
The air coming out of my vent about 20feet away is 43.3 degrees. The air going in the return is 71.9. That is a 28.6 degrees temp drop. My unit is running 325cfm/ton recovering from high humidity caused open windows after the storm.
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