Heat Pump Performance - Power of The Triangle (Δ)
When it comes to heating systems, flow rate is king.
Too much flow? The heat source struggles to maintain temperature efficiently.
Too little flow? Not enough heat reaches the rooms.
Get it right, and everything works in harmony.
This isn’t unique to heat pumps. Long before renewables, I was fixing flow problems on oil and gas systems. Single-pipe systems extended like two-pipe. Combi boilers with flow rates so high the burner couldn’t hold temperature. Mis-wired valves. Stuck diverters.
Flow has always been the quiet troublemaker.
The difference is: traditional boilers are like sports cars with an on/off switch. Lots of power. When something’s wrong, they can often brute-force through it by dumping heat into the system.
Heat pumps can’t.
To achieve 300%+ efficiency (COP 3.0+), a heat pump must be correctly designed, commissioned and delivering the correct flow at the correct time.
When flow is wrong on a gas boiler, efficiency might fall from 90% to 70% and few people notice.
When flow is wrong on a heat pump, electricity bills make it obvious.
The Formula That Impacts Everything Heating System: Q = m × c × ΔT
At i-HEATPUMP, one of our most powerful tools is simple physics.
Q = Heat Output (kW)
How much heat a system, zone, radiator or UFH loop is delivering.
m = Mass Flow Rate
How much water is moving through the system.
c = Specific Heat Capacity
For pure water this is fixed. Add glycol and it changes.
ΔT = Temperature Difference
The difference between flow and return.
If you know mass flow rate and ΔT, you can calculate actual heat output.
No guesswork. Measured performance.
Why Heat Output Matters
To raise temperature, heat supplied must exceed heat lost. To maintain temperature, they must match. If there’s a mismatch comfort or cost suffers.
Mass Flow Rate: Transport for Heat
Heat needs transport.
You can move it quickly in small pipes.
Move the same heat more slowly in larger pipes.
Shift it with a higher ΔT over a short time.
Or a lower ΔT over a longer time.
Heat pumps are typically most efficient running “low and slow”: Higher flow, lower ΔT, longer run times.
Can modern heat pumps run at 55°C+? Yes — especially R290 (propane) and CO₂ systems. Is it as efficient? No.
There is no free lunch, designing a heating system to run warmer will always cost more. Saving money on smaller radiators and pipes = higher running costs (monetary and carbon). This will always be a balance - it’s up to the individual to decide what balance point they find acceptable.
Flow too fast and ΔT collapses. Too slow and emitters starve.
Correct pipe sizing, pump configuration and hydraulic design are heating fundamentals - not “heat pump issues”.
Heat pumps simply expose mistakes more openly.
What About the Triangle?
Δ (delta) looks like a triangle. It means “difference”.
Heat pumps rely heavily on ΔT in their control logic. It influences:
Compressor frequency
Energy output calculations
Fault detection
Defrost strategy
If ΔT is wrong (or if the controller thinks it’s wrong), efficiency suffers. Assuming a heat pump has the correct value/s to calculate ΔT can be the difference between paying for inefficiency long term and exposing a quick, cost-effective improvement.
The i-HEATPUMP Approach
We don’t assume. We measure:
Flow rate
Flow and return temperatures
ΔT across emitters
Calculated heat output (kW)
System performance trends
Using calibrated temperature probes, thermal imaging and specialist diagnostic tools, we verify real performance. Commissioning without measurement is configuration and hope. Verification is different.
Our systems convert the maths into meaningful numbers, helping identify underperformance instantly and drift over time.
Use the Flow Rate Calculator
Use the interactive calculator below to:
Estimate heat output (kW)
See how glycol concentration affects performance
Understand how flow rate limits maximum output
Visualise the impact of ΔT changes
Simple inputs. Powerful insight.
Heat Output Calculator
Whether you’re a homeowner, installer or asset manager, if you want to access a heat pump service contractor who measures & verifies rather than assumes, you can book an appointment here.