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I was wondering why do heat pumps heat a place so slowly as compared to the resistive heaters?

(This thought just came to my mind when upon ironing my shirt the iron became hot quickly which is a resistive heater while the back of my refrigerator takes time to heat up which is a heat pump example)

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  • $\begingroup$ Get two examples that have the same heat output then compare... Think: your fridge is about 150W and the iron is 2000W. $\endgroup$
    – Solar Mike
    Apr 9, 2021 at 18:59

3 Answers 3

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Clothes iron

  • 2 kW resistive heater.

  • Aluminium, 0.6 kg.

  • Specific heat capacity of Al: 0.9 kJ.kg-1K-1.

  • $ \frac {\Delta T} t = \frac P {m \times SHC} = \frac 2 {0.6 \times 0.9} = 3.7\text {°C/s}$.

Room

  • 5 kW heat pump thermal output.

  • Dimensions: 3 × 4 & × 2.5 = 30 m3.

  • Air density: 1.27 kg/m3.

  • Mass of air: 30 × 1.27 = 38.1 kg.

  • Specific heat capacity of air: 1.02 kJ.kg-1K-1.

  • $ \frac {\Delta T} t = \frac P {m \times SHC} = \frac 5 {38.1 \times 1.02} = 0.129\text {°C/s}$.

Substitute in your own numbers as required.

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If I understood correctly the question, what you are asking is

Why does an iron get so much quicker hot and reaches a higher temperature, compared to the heat pump that has a higher power rating?

Although the technologies are quite different (i.e. the resistive elements transform electricity to heat, while the heat pump transfers heat energy from a cooler room to a hotter room), the main reason that you perceive this, is the compactness of the heat surfaces.

The iron has a much smaller area compared to the power, so it is able to heat the ironing surface much quicker.

At the same time, the heat pump of an aiconditioning, has to warm up/cool down all the masses before being able to transfer heat.

Another thing is that the iron has a guaranteed power output (depends only on the available electricity), while the air conditioning heat pump will also depend on the relative air temperatures.

However, for a given room a resistive element of 5kW and a heat pump of 5kW, they should warm up the space in the same average rate. The main difference will be that the resistive element at 5 kW will start warming up the corner where it is placed, while the heat pump will have better circulation and warm the place more evenly (but less perceivably).

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The reason is the temperature of the air that comes out of the air handler. Heat pumps product relatively cool air, so it takes longer to warm up a room.

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