Explaining Delta T for HVAC Systems And Air Balancing
Delta T, or the difference between return air temperature and supply air temperature, is one of the most commonly used measurements in the HVAC industry – and for good reason. Monitoring Delta T over time can clue you in to things like sub-optimal system performance, unnecessary energy usage or impending equipment failure, helping you address these issues proactively instead of reactively.
Calculating Delta T is simple: just subtract the return air temperature from the supply air temperature. The difference, or delta, between the two is Delta T.
Ideal Delta T limits for heating and cooling
On the cooling side, the ideal Delta T range varies depending on who you ask, but a good rule of thumb is between 16F and 22F. On the heating side, the ideal Delta T range varies by system, so check the data plate on the furnace to see the temperature rise minimum and maximum (it’s usually a 30-degree spread). Generally speaking, if Delta T is high, it can indicate poor airflow. If Delta T is low, it can indicate poor system performance or capacity.
Using Delta T to diagnose heating issues.
Low Delta T: If Delta T is on the low end of the recommended range for the heating system in question, the air coming off the furnace will likely feel cool to the homeowner, and they might complain about drafts. If Delta T drops below the low end of the recommended range, condensation could start to form in the primary heat exchanger, eventually causing rust and limiting its life span. Try slowing the blower down.
High Delta T: When Delta T is near the high end of the recommended range, the furnace can start cycling on the limit control, which can lead to overheating and component damage. The system might not be moving enough air, but that can be remedied with basic adjustments.
Using Delta T to diagnose cooling issues.
Low Delta T: Delta T below approximately 16F can indicate a number of potential issues, including:
- Undercharge or low refrigerant levels
- A malfunctioning metering device
- Too much airflow through the evaporator
- Abnormally high humidity
- Liquid line restriction
- A malfunctioning compressor
- Leaking reverse valves
- Leaking return air ducts
If you’re seeing a low Delta T and you’re not sure where the issue is, try checking the air filter, humidity levels and system charge for starters.
High Delta T: Delta T above approximately 22F can indicate a couple issues, including the following:
- Low air flow resulting from a dirty filter, evaporator or blower wheel, not enough supply ducts or a too-small return
- Abnormally low humidity
- A blower that’s not running the correct speed (or running backward)
- If you’re seeing a high Delta T and you’re not sure where the issue is, try checking the air filter, humidity levels or blower motor to ensure proper airflow.
An old rule of thumb requires there to be a 20-degree delta T, but that’s too simple to be a hard and fast rule. Generally, the delta T will be between 14 and 23 degrees, but that number can vary wildly depending on blower type and load conditions.
HVAC duct air flow balance means that we have adjusted the flow through the air duct system such that we get the desired quantity of cool or warm air in all of a building's occupied spaces.
If the duct air flow system is out of balance you will find that when heating, some rooms are not warm enough while others are too cool. While in cooling or air conditioning mode you'll find similarly that some rooms are not cool enough while others are too warm.
Mobile Home Air Balancing
So, like a mobile home the furnace is in the middle of the home the floor registers closest to the furnace should be about closed or just cracked open and the registers farthest from the furnace should be fully open. Air takes the path of least resistance so if it can’t get out one register it will go down the trunk until it gets to the next register.
In my home I adjust my registers by feeling if the room closest to the furnace is too hot shut down the register. If is a Cold day and the furnace has been running the rooms should never be more than 1 degree difference.
It’s OK to close registers completely in bedrooms not being used to help cut down on utilities but don’t close too many! Keep the air flow moving so it doesn’t over temp and kick out the furnace.
If you have any questions, please email me.
Coleman HVAC Parts
Technical Service Advisor
Author, entrepreneur, e-Commerce
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