Heat Pump Defrost Cycle | mysimulator.uk

Heating

Weather

Outdoor air temperature 2 °C

Relative humidity 80%

Controller

Defrost trigger (frost thickness) 3.0 mm

Simulation speed 6×

Live Readouts

Frost thickness

0.4 mm

Heating COP

3.10

Cumulative defrost energy penalty

0.0%

How it works

An air-source heat pump absorbs heat from outdoor air at its evaporator coil. Whenever the coil surface runs below 0 °C — which happens once ambient air drops below roughly +5 °C, because the evaporating refrigerant sits well under the air temperature — airborne moisture condenses and freezes onto the fins as frost.

Coefficient of performance (Carnot limit):
  COP_carnot = T_hot / (T_hot − T_evap)      [temperatures in kelvin]
Real system:
  COP = η · COP_carnot                        η ≈ 0.45 (compressor + duct losses)
Frost penalty:
  T_evap = T_out − ΔT_approach − k_frost · d_frost
           (frost insulates the fins and blocks airflow, pulling T_evap down further)

Frost accumulates fastest not at the coldest temperatures but in a narrow band around 0–4 °C, because very cold air simply holds less moisture to deposit — this simulator's growth curve peaks near +2 °C and fades out below about −10 °C, matching real frosting data for air-source units.

Once frost thickness crosses the trigger threshold, the reversing valve flips the cycle: hot, high-pressure discharge gas is routed backward to the outdoor coil to melt the frost, while the indoor coil briefly stops delivering useful heat. This reverse-cycle defrost costs energy twice — the compressor keeps drawing power with no heat delivered, and (in real units) a booster or resistance element often runs to offset the cold blast indoors. The tracked "cumulative defrost energy penalty" is the running share of total compressor + auxiliary energy spent on defrost cycles rather than on space heating.

Outdoor temperature / humidity — set the weather; together they drive how fast frost grows and how low the COP is even before frost forms.
Defrost trigger — the frost thickness (mm) that forces a defrost; a lower threshold defrosts more often (cleaner coil, more energy lost to defrost cycles) while a higher one lets COP degrade further between defrosts.
Force defrost now — starts a defrost immediately, useful for comparing thin-frost vs thick-frost defrost cost.

Hot discharge refrigerant Cold suction refrigerant Frost on outdoor coil

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