Saturday 15 April 2023

Air source heat pumps heat the outside air even when heating your home

Standing in front of a heat pump when it is working hard you will feel an arctic blast of cold air. So you would think that if every house has one the outside air will get colder. After all, we know that cooling buildings with air conditioning (A/C) adds to the urban heat island effect. Since cooling our homes makes the outside air warm, you would think that heating our homes would make the outside air colder. In fact, using an air source heat pump to heat our home heats the outside air as well as our home. Read on to find out what happens to the air outside when we use air source or ground source heat pumps in heating or cooling mode, compared with a gas boiler for heating.

1 Simplified heating scenario.

The diagram above shows a simplified heating scenario. Your house is losing 1000 W of heat through the walls, roof etc. When the house has a steady temperature the heat gains must equal the losses, so 1000 W heat has to be replaced to keep your house warm. You use an air source heat pump with an efficiency of 400% so you need 250 W of electricity and the rest of the heat comes from the air. When you use electricity it is converted to heat and ideally this is all retained within the house. (In practice there will be at least some used outside for the fan but for simplicity we ignore that.) The diagram shows the heat flows. For every 1000 W lost from the house we take 750W from the outside air. The outside air has a net gain of 250W.

2 Heating scenario with other gains.

In practice the situation is bit more complicated than this, as there are various other sources of heat in the house. People breathing, using appliances and solar gains through windows could offset half the heat loss. That means we only need to replace another 500 W of heat and the outside air has a net gain of 625 W. 

3 Cooling scenario - hotter outside than inside.

Here is the same house using A/C to cool down – also a heat pump. This time we assume that the heat pump power is dumped outside the house, otherwise we need even more cooling. Also we have increased the efficiency of the heat pump to 500% since cooling is normally a little more efficient than heating. When the air outside is hotter than inside the house we have heat gains through the walls etc. but the temperature difference is normally small; it might be 4°C hotter outside whereas in winter it could be 20°C cooler. So we have reduced the building fabric heat transfer to 200 W but the other heat gains in the house are the same. This time the outside air has a net gain of 640 W. 

4 Cooling scenario  - slightly cooler inside than outside.

Even if it is cooler outside we may still need A/C to cool the house down. This example shows 200 W lost to outside but since there is 500 W other gains we still need to reject 300 W heat from the house. The outside air has a net gain of 560 watts – less than before but still a lot.

Avoiding A/C for cooling.

As long as the air outside is cooler than inside, we can use extra ventilation to cool it instead of A/C. This can be as simple as opening windows but there needs to be air flow through the windows to get this to work. If there is a breeze you can open windows on two sides of the house for cross ventilation (and keep internal doors open so that the air can flow). If not, then you really need to open windows downstairs and upstairs to get a breeze from the warm air rising. This is very hard if your home only has one storey, such as a flat, which is one reason why flats are more prone to overheating.

You can also make it easier to keep the house cool by reducing the other gains. That means switching appliances off unless they are needed, making sure hot water pipes are well insulated, keeping solar gains out with blinds, and being as lethargic as possible. If you must get some exercise, do it outside!

A ground source heat pump is different, also a gas boiler

Most of use are still heating with a gas boiler. This takes no heat from the air and further heating comes from the flue gases. Considering the scenario 2, if the boiler efficiency is 90%, then the outside air gains 1000 W from fabric loss plus 55 W from flue gases. This heat gain for the outside air  is a part of the winter-time urban heat island effect (link to

When a ground source heat pump is used for heating, the effect is similar to the boiler as the heat pump takes heat from the ground instead of the air.

In a cooling scenario with a ground source heat pump, the air loses or gains heat depending on the difference in temperature but overall the effects are smaller than with an air source heat pump. In scenario 3 the air is slightly cooled (140 W added, 200 W taken away) while in scenario 4 the air is warmed but only by 260 W compared to 560 W with an air source heat pump.

If you have a ground source heat pump used for both heating and cooling then the ground acts for you as an inter-seasonal thermal store. However, in most cases even if we have A/C we take out more heat in winter than we add in the summer. Over the year the ground has a net loss which needs to be replenished. For shallow GSHP this comes mainly from the sun; for borehole heat pumps we get heat from geothermal energy. 

This table compares the overall effect on the outside air in the scenario 2: heating with 1000 W heat loss and 500 W other gains.

Table of heat gains to the air from heating mode (scenario 2).

Air exchange-37500
Flue gases0055
Net heat gain to outside air62510001055

In summary.
An air source heat pump can be used for either cooling or heating but either way it heats the air outside. The air moving through the heat pump becomes cold, which is why it is important to put it in an area with plenty of air flow. However, we can have as many heat pumps in a city as we like and the net effect will increase the heat in the atmosphere. This is because the heat losses through walls and roofs etc are greater than the heat taken from the air. 

On the other hand, our cities will be cooler in winter than they are now because the urban heat island is reduced compared to with a gas boiler. 

In summer, using A/C has a strong heating effect on the air and hence a heat island effect. Heat transfer through walls and windows is small but we need to remove heat from solar gains, use of appliances and people breathing. Minimising these gains makes it much easier to rely on natural ventilation instead of A/C. However ventilation is very hard for some homes, especially flats.

Ground source heat pumps are a bit different. In winter the net heat gain for the air is similar to with boilers. In summer, using a GSHP for A/C reduces the urban heat island effect considerably. 

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