Cooling season is here, and when it gets really warm outside, you may wonder if your air conditioner is working correctly. Here is how you can check for yourself to know if your AC unit is blowing as cold as it should be.
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Delta T
An AC system, whether it is AC only or a Heat Pump that can both cool and heat, will draw in the air inside your home, run it through the system to remove heat from it, thereby cooling it, and then blow it through the vents as cooled, “conditioned” air. The air going into the system is called “return air”, and the air coming out of the system is called “supply air”.
How Many Degrees Can An Air Conditioner Cool Down
The difference (Delta) in the temperature (T) between the return air and the supply air is called, “Delta T”. In a properly functioning AC system, it should be approximately 15-20 degrees Fahrenheit (F). Some HVAC experts may vary these numbers slightly, (14-20 or 16-21), but 15 to 20 is a good range to use for our purposes here.
How Delta T Works to Cool
Here’s a common scenario: You come home on a hot day and the room temperature on the wall thermometer reads 80F. You set the temperature to 76 and the AC system turns on. It will begin drawing that 80 degree air into the system where it cools and removes some moisture from it, then sending it back through your room vents at about 60 to 65 degrees (15-20 degrees cooler than 80).
As the rooms begin to cool, the return air will become lower than 80, so the supply air will also be lower than 60-65. This process will continue with the rooms getting cooler and cooler until the thermostat detects 76 degrees and shuts off the AC system.
Checking Your Delta T
From this, you know that the answer to the question, “What temperature should air be coming out of the vent when cooling?” will vary depending on the temperature of the air in the room. But here is how you can check it to make sure that your AC system is functioning properly. NOTE: You will need a thermometer for this test. It may be an infrared thermometer or other digital thermometer that you can hold in your hand.
After the system has been running for at least 2 – 3 minutes, check the room air temperature. You can either check it near the wall thermometer (which will also show if your handheld thermometer agrees with the wall thermometer) or where the air is being drawn into the system in a return vent. Return vents are usually larger than supply vents, they do not have louvers that can be closed, and they will pull on a piece of paper held near it when the system is running.
*Write down the temperature of the air near to this vent.
Then check the temperature of the supply air coming from your AC system by holding the thermometer close to a supply vent. Supply vents will have louvers inside that you can see and can be adjusted by a small lever next to the openings in the vent. This air should be cooler, of course, than that which was being drawn into the return vents.
*Write down this temperature.
Now subtract the supply air temperature from the return air temperature. This is your Delta T. It should be somewhere close to 15 to 20, which will mean that your HVAC system is running at peak, or close to its peak efficiency.
Here is a simple chart that shows what you should expect to read at the supply vents with the AC running.
AC Vent Temperature Chart
| Room Temperature | Optimal Supply Vent Air Temperature |
|---|---|
| 70 F | 50-55 F |
| 72 F | 51-56 F |
| 75 F | 54-59 F |
| 78 F | 57-62 F |
| 80 F | 59-64 F |
What’s The Room-To-Room Temperature Difference Of Air Coming Out Of Vent
In a normally functioning HVAC system, the room-to-room difference should not be more than 6 degrees. That seems like a lot, and it is.
*Rooms with sun pouring into them will be hotter than rooms on the other side of the house, for example. This is especially true if the doors to the sunny rooms are closed.
*Rooms closest to the air handler will be cooler than distant rooms, especially if the ductwork design is poor, so most of the cooled air comes out the closest vents.
*Rooms upstairs will be warmer than first-floor rooms because heat rises.
What Causes Your AC to Have a Low Delta T?
If you have checked both the return air and supply air temperatures and found that your Delta T is too low – maybe only about 10 degrees instead of 15-20 – here are some possible reasons:
1. Dirty or clogged air filter. Many people simply don’t think about changing their home HVAC air filter. This will cause poor air flow in both summer and winter. Change (or at least check) the air filter at least once every 3 months. Clean or change it if needed.
2. Blocked return air intake. Sometimes the return air intake vent is low on the wall where it can be blocked by furniture. In older homes, it might even be in the floor where a rug or furniture might be preventing good air circulation. Allow at least 6” of space between the return air vent and any furniture. Never cover it with anything.
3. Leaking or blocked ductwork. If your air ducts were sealed with duct tape when they were installed, the tape may have deteriorated and come loose. This may allow some of your conditioned air to leak into an attic or crawl space instead of coming through your supply vents. Obviously, even a partially blocked air duct will prevent the proper flow of air in your system as well. Remove any blockages that may be present.
4. Uninsulated ductwork in a hot attic or crawlspace. If the air that was cooled has to travel through very warm (or even hot) ductwork, it will not come out the supply vents as cool as it otherwise would. This will result in an AC system that may struggle to cool your house to the desired temperature. The solution to this problem is to insulate the ductwork in these areas with a quality insulating product, either foam or fiberglass.
5. Issues that probably require professional attention. These might include (a) low refrigerant, which may also mean that there is a leak in the system, (b) dirty evaporator coil, which is located inside the plenum of your furnace, or (c) any of several more serious problems that should only be addressed by a competent, licensed HVAC technician.
What Causes Your AC to Have a High Delta T?
The primary cause of a difference in return air vs supply air temperature, or Delta T, is an AC that is much too large for the space.
If your central air conditioner system is too big, then it will remove heat from the air too quickly. The result will not only be a high Delta T, but two negative issues:
1). The air won’t get properly dehumidified, so it will become cool and clammy.
2). The AC will run on short cycles, with lots of starting and stopping, which is much harder on it than running cycles of normal length.
Final Thoughts
If you are wondering if your home AC system is blowing cool enough air, it is quite possible that it was hot weather that caused you to think about it. Most of today’s AC systems are designed to operate efficiently in temperatures up to 95F-110F (based on different locations). But there are other factors that can still cause your air conditioner to struggle to keep up in heat that’s near or over 100F.
Many south or west-facing windows will allow a lot of solar energy to invade your rooms, making them difficult to cool. This can vary greatly from one room to another, depending on their orientation to the sun. Good window blinds or insulated drapes can help to minimize this.
Many homes simply don’t have adequate insulation, and this will make keeping them cool in very hot weather almost impossible. The cost of having good insulation added to your home may be offset by energy savings in the long run.
If you have done all you can to check the Delta T and the other items listed above and you still do not have good, cool air coming from your air conditioner, it’s time to contact a reputable HVAC company and have them check out your situation.
