Delta-T (ΔT) is the temperature difference between two points in a hydronic system, most often between flow and return. Together with flow rate it defines transferred power: kW ≈ flow (m³/h) × ΔT (K) × 1.16. Heat pump circuits are designed around a ΔT of 5–8 K; conventional boiler circuits around 15–20 K; solar collector loops typically 10–15 K.
How is Delta-T used to calculate heat output?
Delta-T converts a flow measurement into power. Because water carries roughly 1.16 Wh of heat per litre per kelvin, the transferred power is: kW = flow (m³/h) × ΔT (K) × 1.16, or equivalently kW ≈ flow (L/min) × ΔT × 0.07. A circuit moving 1.5 m³/h with a 5 K difference between flow and return is therefore delivering about 8.7 kW. Installers use the same arithmetic in reverse during commissioning: measure flow and return temperatures, read the pump's flow rate, and you know — without any manufacturer software — how much heat the generator is actually putting into the building.
What Delta-T should a heat pump run at?
Most heat pump manufacturers design around a flow–return ΔT of 5–8 K at nominal conditions. A small ΔT means a high flow rate for a given power, which keeps the condenser working efficiently and the flow temperature as low as possible — and low flow temperature is where the efficiency lives: COP falls by roughly 2–3 % for every kelvin the flow temperature rises. If the measured ΔT is much wider than design, flow is too low (blocked strainer, undersized pump, closed valves) and the unit compensates with hotter flow water; if ΔT is very narrow, the pump may be wasting electricity on excessive flow. Target flow temperatures themselves are covered in What flow temperature should I run my heat pump at?
Why do boilers and heat pumps use different ΔT?
A boiler is indifferent to high temperatures, so classic radiator systems were designed around large temperature drops — commonly 15–20 K (for example 80 °C flow / 60 °C return) — which allows small pipes and low flow rates. A heat pump generates heat most efficiently at low temperature, so the design logic reverses: keep flow temperature down, accept higher flow rates, and take a narrow 5–8 K drop. This is why reusing an old radiator circuit for a heat pump often requires larger pipes or pumps: at the same power, halving ΔT doubles the required flow.
| Circuit type | Typical design ΔT |
|---|---|
| Heat pump, radiators or fan coils | 5–8 K |
| Heat pump, underfloor heating | 5–7 K |
| Conventional / condensing boiler radiators | 15–20 K |
| Solar thermal collector loop | 10–15 K |
| DHW cylinder coil (charging) | 5–10 K |
What does ΔT mean across a solar collector?
In solar thermal, two different ΔT values matter. The first is across the collector: the solar controller compares collector and tank temperatures and starts the pump once the collector is a set number of kelvin hotter, stopping when the difference collapses. The second is the collector-to-ambient difference (Tm − Ta) used in the collector efficiency curve: the hotter the collector runs above outdoor air, the more it loses, which is why collector efficiency falls steadily as operating temperature rises. Keeping the loop ΔT moderate — around 10–15 K — balances useful stratified heat delivery against pumping energy.
What is "low Delta-T syndrome"?
In larger and commercial systems, low ΔT syndrome describes circuits that return water almost as hot as it left — because of overpumping, bypasses or fouled heat exchangers. The symptom is high pumping cost and generators that never reach their rated capacity, since power = flow × ΔT × 1.16 and the ΔT term has collapsed. The cure is balancing: trimming flow at each emitter until every circuit achieves close to its design temperature drop.
Frequently asked questions
What is a good Delta-T for a heat pump?
5–8 K between flow and return at design conditions is the range most manufacturers specify. Check the installation manual for the exact figure; a much wider measured ΔT usually signals insufficient flow rather than good performance.
How do I calculate heat output from Delta-T?
Multiply flow rate by temperature difference and by water's heat capacity: kW = m³/h × ΔT (K) × 1.16, or roughly kW = L/min × ΔT × 0.07. For example 20 L/min at ΔT 6 K is about 8.4 kW.
Is a bigger Delta-T better?
Not in itself. Wide ΔT reduces pumping energy but forces higher flow temperatures in heat pump systems, costing roughly 2–3 % COP per extra kelvin of flow temperature. Each technology has its design window: stay near it.
Why is my radiator Delta-T so small?
The circuit is likely overpumped or unbalanced: water rushes through emitters too fast to release its heat. Balancing valves throttled to the design flow at each radiator restore the intended temperature drop.