Summer is well and truly upon us and temperatures have been sweltering in the last few weeks. Are you envious of the comfortable temperatures you find in buildings with a central air conditioner? If so, you might be wondering if you can cool your home with heat pump technology.
The truth is that this IS possible, but does require a special kind of heat pump installation. Read on to find out more.
The technology behind a heat pump
Although they are relatively ‘new’ to the home heating scene, at their core, heat pumps actually use quite old technology. They work very similarly to a fridge but in the opposite direction.
Whereas a refrigeration system takes heat from inside the unit and displaces it outside, a heat pump takes heat from outside your home and distributes it inside.
It does this by means of a refrigerant liquid which boils at a very low temperature. The liquid absorbs heat from the outside air and turns into a gas. This gas is then compressed further to increase its heat and is cycled into your home. Once inside, it releases its heat energy and returns to a liquid state. The process then starts again.
This principle of displacing heat using a liquid refrigerant is the same principle that an ac unit uses
The main components of a heat pump system – namely the compressor, evaporator coil, condenser coil and expansion valve are the same as an air conditioning unit. They all work together to extract heat from one area and put it into another. In heating mode the pump extracts heat from outside (even when it’s cold and moves it indoors). In cooling mode the cycle is reversed. Heat is taken from inside and displaced outside.
Reversible operation
Many modern heat pumps are ‘reversable’. This means that they can switch between heating and cooling modes. Typically this is done through a ‘reversing valve’ that changes the direction of the refrigerant flow and the heat exchanger
In cooling mode the indoor coil acts as the evaporator (absorbing heat from the room), and the outdoor coil becomes the condenser (releasing heat outside). This effectively turns the heat pump into an air conditioning unit.
Can all heat pumps cool?
In a word, no. In order to answer this question we need to remind ourselves about the different types of heat pumps on the market.
Air to air heat pumps
These heat pumps heat your house by blowing hot air into it. They are more common in the US and parts of continental Europe than The UK. If you have a system like this you will need a separate solution for heating your hot water tank
Air to water heat pumps
This type of heat pump heats water, which is then circulated in your home via radiators or underfloor heating. They will also heat water for domestic use such as showers and baths.
In general, air to water units are not reversable. There are some fancy systems that can cool through underfloor heating. But standard radiators are not able to cool (more on this below). Air to air systems CAN be reversable but not every system will be able to. Engineers design these systems with cooling in mind, including drainage for condensate, controls and insulation.
Some air to water systems have a separate air conditioning system which you can install as part of the set up. The Samsung TDM system for example, which can blow cold air into your property.
How effective are heat pumps at cooling?
When set up properly, heat pumps can be quite effective at cooling. If you’ve been around heat pumps for a while, you will know that we use the COP (Coefficient of Performance) and SCOP (Seasonal Coefficient of Performance) measurements to evaluate a heat pump’s efficiency at heating.
For cooling we use two numbers. One called the EER (Energy Efficiency Ratio) and another called the SEER (or Seasonal Energy Efficiency Ratio). Let’s look at what these numbers mean and we calculate them.
The Energy Efficiency Ratio (EER)
The EER is defined as the ratio of cooling output (BTUs per hour) to power input (watts) in a controlled and specific operating condition with a specified temperature difference between the inside and outside air. It’s broadly similar to the COP in heat pump terms.
To determine the EER the unit is normally tested at an outdoor temperature of 35°C (95°F) with an indoor temperature of 27°C (80°F) and a 50% relative humidity.
The formular to determine the EER is as follows:
EER = Cooling Capacity (BTU/h) ÷ Power Input (Watts)
As a reminder, BTUs stands for British Thermal Units. It’s a measure of heating energy. One BTU is 3414 kWh.
Let’s say you have a 17,070BTU/h unit using 1500 Watts ( 1.5KW), the calculation would be as follows:
17,070 BTU/h ÷ 1500W = an EER of 11.38
This means the unit will deliver around 11.37BTUs (or around 3.29Wh) of cooling for every watt of electricity consumed in test conditions.
The EER is quite a theoretical figure but it’s great for giving us an idea of how a unit will perform under extreme heat conditions.
The Seasonal Energy Efficiency Ratio (SEER)
The SEER is to EER in cooling what SCOP is to COP in heating. It gives a much more pragmatic real world idea of how your cooling unit will perform in practice.
The SEER is calculated as follows:
Total cooling output (BTUs) over a season / total electrical energy input (watt hours)
So if an air conditioning system provides 50,000 BTUs (17.6kWh) and uses 2500 watt-hours of electricity (2.5kWa) it has a SEER of 20
50,000/2500 = 20
Difference in the UK and US SEER
In the US the SEER is expressed as a raw number. In this jurisdiction most modern heat pumps offer a SEER rating of between 14 and 22, but high efficiency models can exceed 25 SEER.
In the UK and the EU the SEER is calculated under EU standard conditions. This accounts for indoor and outdoor fluctuations in temperature, part load performance and different climate zones. The result is that the SEERs are much lower for various models.
While air conditioning systems in the US might have a SEER of between 14 and 22, in the UK and EU they would typically range from between 4.5-6.5. For this reason, the efficiency is often expressed as an Energy label as follows:
| SEER Rating | Energy Label | Efficiency Label |
| 4.1 – 4.6 | A+ | Minimum for new models on the market |
| 4.6 – 5.1 | A++ | Above average |
| 5.1 – 6.1 | A+++ | Highest efficiency models |
What do these numbers mean in reality?
It’s all very well to understand how the SEER and the EER are calculated. But what do these numbers mean in the real world.
The EER will tell you how well your air conditioning system will perform at peak conditions and under fixed controls. But in reality, and especially in the UK, the SEER is a more useful number. It will tell you how well your air conditioning unit will perform over the course of a summer and, more importantly, how big your monthly bills are likely to be.
Does a heat pump cool well in the UK?
This will largely depend what kind of cooling system your heat pump has.
Underfloor based cooling systems
Let’s deal first with an underfloor cooling system. In a normal wet underfloor heating system, a network of pipes is laid into the concrete screed in your floor. In the winter, hot water is pumped through these pipes to provide a gentle heat to your home.
With an underfloor cooling system it’s the same but in reverse. Instead of hot water being piped through your underfloor system, it’s cold water (typically around 16-20 C). The floor acts at a large radiant surface, gradually cooling the air above it.
These systems won’t work as efficient as air based systems, but they can lower indoor temperatures by around 2-4°C on average. As with air source heat pumps themselves, these systems work well in well insulated homes with little air leakage. The system is not designed to remove condensate (like traditional air conditioning). So it is also important to keep the floor above the dew point (normally around 16-18°C). If this is not done, water can form on the floor causing other issues.
Even though we have a lot of cold weather, these systems are gathering popularity in the UK. With our milder climate and lower cooling needs, we’ve traditionally not needed widespread air conditioning. Although global warming and climate change maybe changing this. But if you are only looking for the occasional bit of cooling on the hottest days of the summer months this could be a good option for you.
We’ve summed up the pros and cons below:
| Pros of underfloor cooling | Cons of underfloor heating |
| Silent (no fan noise) | Limited cooling capacity |
| Even temperature distribution | Slow response time |
| Low energy use | Condensation risk (floor must stay above the dew point) |
| One heating/cooling solution (with air to water) | Expensive to retro fit. |
Air based cooling systems
You may have a reversable air to air heat pump. Or you may have an air to water system with a separate air conditioning unit. In this case you’ll be looking at an air based cooling systems. As we already stated, in cooling mode, these systems function just like normal air conditioning.
These systems perform much better than underfloor systems in terms of their cooling power. The best reversable air to air air-source heat pumps can perform as well as dedicated air conditioning units. This will keep you nice and cool in the summer months.
However there are some disadvantages. As above, we’ve summed up the pros and cons below:
| Pros of air based cooling | Cons of air based cooling |
| More effective than underfloor based systems | Can be noisy |
| Very responsive, can cool in minutes | You may need unsightly indoor units. |
| Easier to fit and to retrofit | with air to air you will need water heater |
| No condensation risk |
What are the advantages of using your heat pump for cooling?
The main advantage of a reservable heat pump is cost of installation. Although the initial install cost might be a bit more than a straightforward heat pump, you won’t have to install two systems. In the end this will be cheaper than separate air conditioning alongside your heat pump.
Your monthly utility bills and energy costs are also likely to be lower and using two separate systems alongside each other. You’ll also be using less fossil fuels to acclimatise your home.
What are the disadvantages of using your heat pump for cooling?
As we’ve stated above, the main disadvantage is the cost of installation. Whether you plump for an air based cooling system or an underfloor cooling system, the initial outlay is likely to be higher than just going for a heating system alone.
If you live in the UK (or temperate parts of the US), the performance you get from a reversable system is likely to be perfectly adequate to keep you warm during the summer months. If you live in a more extreme climate however, with higher outdoor temperatures, you may need an additional cooling option to keep your house comfortable.
I want my heat pump to cool, where do I go from here?
If you already have an HVAC system installed then you should check whether it’s reversible. With an air to water heat pump (the most common types in the UK) this is unlikely to be the case, as reversible air to water systems are still quite rare.
If you have an air to air system, you may find that you can reverse your system to function as an air conditioning unit. But remember, not all systems can be reversed. If your home has a non-reversible air to air system, you may be able to change it for one that is reversible without too much hassle.
If you currently heat your home with a gas boiler (gas furnace) and you are thinking of installing a heat pump then now is the time to figure out if you want a reversible unit. Most house holders replace their gas boilers with an air to water system. These units will work with an existing radiator or underfloor heating network. They will also provide hot water for domestic use. As we’ve mentioned, if you have underfloor heating then you can achieve a very limited cooling capacity with a reversible air to water system. But if air conditioning is important to you, then you may want to plump for an air to air system. This will be much more effective in air conditioning mode.
And don’t forget, with some air to water systems, you can also buy a separate air to air air conditioning unit. It’s a more expensive installation but you do get the best of both worlds.
For more information about air source heat pump installation don’t forget to check out some of our other articles. We also have information about the cost of installation and the likely running costs of your unit.
Tim Bichara. Tim is a tech and property entrepreneur who is passionate about modern construction and energy saving technologies
