Heat Pump Work In Winter: 13 Important Concepts

How does a heat pump work in winter? It is always a curious question for all of us. We know that in winter the objective of heat pump is to heat the room. Let’s analyse the working of heat pump in winter.

During winter, the liquid refrigerant sucks heat from outside air and becomes vapour form; the vapour refrigerant is compressed to high temperature and pressure. Then the refrigerant is allowed to pass through the room. During that period, the refrigerant releases the heat to the room, by which the room temperature increases. 

This system is a good option for places where mild winter occurs, and it can save energy compared to conventional heating. However, for severe winter, the heat pump alone is not a good alternative. The hybrid system is used in that scenario.

There different type of heat pumps are available. The classification is based on from where the heat is taken, i.e. the location of evaporator. Some of the types are :Air source heat pump, Ground source heat pump, water source heat pump etc.

A schematic diagram of heat pump with main components are shown in figure below;

How Does A Heat Pump Work In Winter
The main components of a heat pump
The outside unit of a small air source heat pump Credit: https://pixabay.com/de/photos/kleine-luft-w%c3%a4rmepumpe-4963069/

How does a ground source heat pump work in winter?

The temperature below few feet from the ground is at a stable temperature of 55oF irrespective of the season. Let’s see how this fact is utilised to run a heat pump.

In ground source heat pumps, series of strong pipes are installed below the ground; this is the ground source heat exchanger. Coldwater from the heat pump is circulated through these pipes, and the water absorbs heat from the ground. Then, the water transfers this heat to the refrigerant in the heat pump. 

A schematic diagram is shown below.

Heat pump
Ground source heat pump Credit :https://www.flickr.com/photos/sagabardon/5086132916

The ground source heat pump eliminates the burning of fossil fuel; hence this is environment friendly. Germany, USA, Sweden, Canada, Switzerland are the main countries using this heat pump.

What temperature is a heat pump not effective?

The heat pump is not advised in all temperatures. The ambient temperature influences the effectiveness of the heat pump.

As the ambient temperature decreases the effectiveness of heat pump decreases, from the research the limiting ambient temperature is calculated as 40oF. Hence, a heat pump is advised to use when the ambient temperature is above 40oF. The heat pump becomes less effective than other heating options when the temperature reduces to 25 to 30oF.

Hence, we use an alternative system in those regions where the temperature falls below 40oF. Fossil fuel or any cheap fuel is burned to extract heat in these regions during peak winter.  

The heat pump is working between two temperature limits, room temperature and ambient temperature. Therefore, the performance of heat pump depends on both these temperature. One can assume that if the ambient temperature is low, the heat pump consumes more work to extract the heat; hence efficiency decreases.

Let’s analyze the effectiveness of heat pumps mathematically. The effectiveness of a heat pump is measured as Coefficient of performance (COP). COP is defined as

COP = (Heating effect)/(Work done to the system)

Let’s analyze the COP of a Carnot cycle. Carnot cycle is an ideal cycle that has the maximum COP.

The COP of the Carnot cycle is defined as;

COP = Thot/Thot – Tcold

Tcold is the ambient temperature, and Thot is the room temperature. Let’s assume that we set 68oF in heat pump, hence the room temperature is 68oF. Now let’s assume two conditions when the ambient temperature is 40oF and 20oF.

When these temperature conditions are applied, we get COP of 11.5 and 6.7 for the ambient temperature of 40oF and 20oF, respectively.

(Note: Care should be taken while calculating the COP, the temperatures should be in Kelvin scale or Rankine scale.)

Here, the COP is reduced when the ambient temperature is reduced. The calculated COP is for the maximum possible cycle. This COP cannot achieve in an actual cycle. Hence, we can conclude that as the ambient temperature decreases the COP of heat pump decreases.

What temperature should I set my heat pump in the winter?

This is always a concern for many of us while operating a heat pump.

The human comfort is the primary objective of a heat pump in home. From scientific researches, it is concluded that 680F is best for human comfort during winter. It is advised to reduce the operating temperature further when we use the heat pump continuously. 

Can heat pump work below freezing?

The question here is what happens when the ambient temperature is below freezing point. Is it safe to operate?

Yes we can use heat pump in freezing conditions. The freezing point of refrigerant used in a heat pump is far below the freezing point of water; hence the refrigerant in the heat pump will not freeze even though ambient temperature is below the freezing point of water.

If the question is “is it advised to use heat pump in extreme cold?” then the answer is “no it is not advised”

However, in extremely cold conditions it is not advised to use heat pump. We discussed the effectiveness of heat pumps in previous sections. When the temperature is less than 40oF, the effectiveness of the heat pump reduces; hence heat pump consumes more energy than simply burning fuel.

How can I make my heat pump more efficient in the winter?

Some tips to improve the efficiency is given below.

  • Clean the filter frequently.
  • For fast heating of the room, do not set the heat pump temperature very high.
  • Don’t heat the spaces which you are not using.
  • Perfectly close all the ventilations in the room.
  • Always provide enough space in indoor and outdoor unit of heat pump for free flow of air.
  • Only put emergency heat mode when it is an emergency.
  • Make sure that the outdoor unit is easily accessible for cleaning.

Why is my heat pump blowing cold air when the heat is on?

There are mainly three reasons that you may feel that your heat pump is blowing cold air.

  • The heat pump is working correctly, but you are feeling it cold.
  •  The heat pump started working on defrost mode.
  • The heat pump is not working correctly.

Let’s discuss each point separately.

  • The heat pump is working correctly, but you are feeling it cold.

The heat pump is working correctly; however, when the ambient temperature is very low, the heat pump’s effectiveness and the ability to increase the temperature reduces. In these situations, the heat pump is heating the air, but you do not feel it as the temperature of heated air is far below your body temperature.

Generally, the electric heating starts automatically in these situations.  

  • The heat pump started working on defrost mode.

When a heat pump is working at a very low ambient temperature, water may freeze around the outdoor unit’s coils. The complete covering of the coil with ice should be avoided. The heat pump works on reverse mode to remove this frost, i.e., it starts cooling inside and heating outside coil.

After 1-2 minutes of operation, the heat pump starts working properly when the frost is completely removed.

  • The heat pump is not working correctly.

This is a serious issue, and you should contact a technician. There are many possibilities like leakage of refrigerant, damages in valves or reduction in heat pump efficiency, etc.

 Should I run my heat pump on auto or heat?

 There are three modes in a heat pump “Heat”, “Cool,” and “Auto”. 

It is advised to set “Heat” mode rather than “Auto” mode in the winter season. This is because the “Auto” mode may cool the room on a sunny winter day, which is unnecessary, i.e., the heat pump automatically gets reversed its operation, which should be avoided.

Should I turn my heat pump off in extreme cold?

The extreme cold situation may occur in winter in many countries.

It is advised to stop using the heat pump in an extreme cold situation as the effectiveness of the heat pump decreases, which leads to increased energy consumption, as discussed above.

Usually, the heat pump comes with an electric heating facility. Hence, in an extreme cold situation, the heat pump gets switched off, and electric heating starts automatically.

How long should a heat pump run per day?

We know that the old furnace heating technique won’t run continuously for a long time. What about the heat pump?

The heat pump can run continuously throughout the day if it is required. The advanced heat pumps come with automatic sensors, which allow the heat pump to stop operating when the required temperature is achieved; it starts automatically when the temperature drops. Hence, you should not worry much about energy consumption.

However, you can reduce energy consumption by manually setting the off time in a heat pump.

How do I know if my heat pump is defrosting?

Defrosting is very common in a cold situation. Defrosting cycle may be required for the efficient working of the heat pump.

You can know that the heat pump is working on defrosting cycle if the following is observed.

  • The indoor fan of the heat pump turns off
  • The heat pump stops heating the room
  • The defrosting indicator light blinks
Frosting in outdoor unit of heat pump Credit: https://commons.wikimedia.org/wiki/File:Heat_pump_model.jpg

How do I keep my heat pump from freezing up?

The outdoor coils of the heat pump may freeze while operating.

The defrost cycle is to avoid the freezing up of the heat pump. The operation of the heat pump reverses in defrosts cycle, and during that period, the heat pump cools indoors and heat outdoor so that the ice melts. This cycle operates automatically. Within 2-3 minutes, the heat pump starts operating normally. 

How do you unfreeze a heat pump in the winter?

You can unfreeze the heat pump in the following ways,

  • Defrosting cycle. In heat pumps defrosting cycle operates automatically. 
  • Remove the frost manually; you can pump water to the frost until it is melted. Or you can chip the frost with a tool

How much frost is normal on a heat pump?

We cannot say the normal frost quantitatively.

There are two conditions when you can say that the frost is too much in a heat pump.

  • When the frost prohibit the flow of air to the heat pump
  • When the frost is staying on the coils for more than 2 hours.

If these conditions are observed, it is advised to contact an operator as the defrost cycle is not running in your heat pump.

How does a pool heat pump work in winter?

The working of a pool heat pump is similar to the air heat pump.

The pool heat pump is used to heat the water to the swimming pool. In this, the condenser transfers heat to the cold water. The other processes are similar to the room heat pump. Hence, the condenser is dipped inside the swimming pool, and the remaining unit is outside the pool.

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