Delta T (ΔT) in HVAC Systems – Complete Explanation

 Delta T (ΔT) in HVAC Systems – Complete Explanation

In HVAC, Delta T (ΔT) means: Difference between two temperatures Formula: ΔT=T1−T2​ Where: ΔT = Temperature difference T₁ = First temperature T₂ = Second temperature Why Delta T is Important in HVAC Delta T tells us: • How much cooling or heating is happening • Whether equipment is working properly • Energy efficiency of system • Water flow and air flow condition • Coil performance • Chiller efficiency In HVAC mainly two Delta T are used: • Water Side Delta T • Air Side Delta T 1. Water Side Delta T (Chilled Water / Hot Water) This is the temperature difference between: • Supply water temperature • Return water temperature Usually used in: • Chillers • FCU • AHU • Heat exchangers Chilled Water Delta T In cooling system: • Chiller sends cold water → called CHWS (Chilled Water Supply) • Water absorbs room heat • Warm water returns → called CHWR (Chilled Water Return) Formula: ΔTwater = Treturn−Tsupply​ Example 1 – Normal Chilled Water Delta T Suppose: • CHWS = 6°C • CHWR = 12°C Then: ΔT= 12°C − 6°C = 6°C Meaning: •Water absorbed 6°C worth of heat from building •System is working normally Typical HVAC chilled water Delta T: • 5°C to 7°C common in many buildings Low Delta T Problem (Very Important in FM) Example: Supply = 6°C Return = 8°C Then: ΔT = 2°C only This is called Low Delta T Syndrome Causes of Low Delta T • Excess Water Flow • Water moves too fast. It cannot absorb enough heat. Example: Valve fully open Pump speed high Result: Small temperature rise 2. Dirty Cooling Coil Air cannot transfer heat properly. Result: Water absorbs less heat Low ΔT 3. Low Air Flow Due to: Dirty filter Fan problem Closed damper Less warm air reaches coil. Result: Less heat transfer Low ΔT 4. Oversized AHU/FCU Coil area too large for load. 5. Bypass Valve Open Cold supply water directly mixes with return water. High Delta T Problem Example: Supply = 6°C Return = 16°C ΔT = 10°C Possible reasons: Low water flow Coil partially blocked Strainer dirty Pump issue Water stays too long in coil and becomes very warm. HVAC Heat Transfer Formula (Water Side) Cooling load formula: Q=mCpΔT Where: Q = Heat transfer m = Water flow rate cₚ = Specific heat of water ΔT = Temperature difference Meaning: Higher Delta T = More heat transfer (if flow is proper) 2. Air Side Delta T This is temperature difference between: Return air / entering air Supply air / leaving air Formula: ΔTair=Tentering air−Tleaving air​ Low Air Delta T Causes Example: Return air = 24°C Supply air = 20°C ΔT = 4°C only Possible causes: Dirty coil Low refrigerant Compressor issue High air flow Expansion valve issue High Air Delta T Causes Example: Return air = 26°C Supply air = 8°C ΔT = 18°C Possible causes: Low air flow, Dirty filter and Frozen coil Air stays too long on coil and becomes too cold.