Heat pumps are energy efficient electrical appliances that take warmth from outside and bring it indoors.
They work a bit like a fridge in reverse - absorbing heat from the air, ground or water outside and moving it into your home. A refrigerant inside the system goes through four stages (evaporation, compression, condensation and expansion) to capture and release that heat where it’s needed.
Heat pumps are much more efficient than traditional boilers because they move heat rather than generate it, making them one of the most effective ways to make your home greener!
How heat pumps work - step-by-step
Step 1. Evaporation - collecting heat
The refrigerant liquid within your heat pump absorbs heat from the outside air, ground or water. As the refrigerant picks up this heat, it changes from a cold liquid into a warm gas.
Step 2. Compression - increasing the temperature
The warm gas is compressed, which raises its temperature. This step is powered by electricity, but it uses much less energy than traditional heating systems.
Step 3. Condensation - transferring the heat
The heated gas passes through a heat exchanger inside your home. This transfers the heat to your home's central heating or hot water system. As it gives off heat, the gas cools and turns back into a liquid.
Step 4. Expansion - cycle starts again
The refrigerant liquid flows through an expansion valve, which lowers its temperature further, and the process restarts until your home or water reaches the temperature set on your thermostat.
Types of heat pumps
While all heat pumps follow the same basic process, the key difference is where they collect heat from, either the air, ground, water, or through a hybrid system.
How does an air source heat pump work?
Air source heat pumps (ASHPs) absorb heat from the air outside. That heat is transferred indoors through the refrigerant cycle described above. There are two main types ASHPs:
- Air-to-water systems heat your radiators, underfloor heating and hot water. These are the most common in homes across Northern Ireland.
- Air-to-air systems use fans to blow warm air around your home. They can't be used to heat water.
Most homes in Northern Ireland use monobloc systems, where all the components are in a single outdoor unit. Split systems include an indoor unit as well, which can be more efficient but usually costs more and takes up additional space.
How does a ground source heat pump work?
Ground source heat pumps (GSHPs) collect heat from the ground. Pipes are buried in your garden and filled with a water and antifreeze solution. This fluid absorbs heat from the ground and transfers it to your heating and hot water system.
If you don’t have the space for horizontal trenches, you might be able to install a vertical borehole. These systems are very efficient but tend to cost more up front.
Ground source heat pumps are currently the most common type installed in homes across Northern Ireland.
How does a water source heat pump work?
Water source heat pumps work in a similar way to ground source systems but draw heat from a nearby body of water, such as a river, lake or stream.
A coil or pipe network is submerged in the water to collect low-level heat, which is then transferred to your heating system through the refrigerant cycle.
These systems can be highly efficient but are only suitable for properties with access to a suitable water source. They are much less common in residential areas but may be a good fit for rural homes in the right location.
How does a hybrid heat pump work?
Hybrid systems combine a heat pump with a traditional boiler - typically gas, oil or LPG. The heat pump handles most of your day-to-day heating, while the boiler provides a boost when temperatures drop or when hot water demand is high.
This approach gives you the energy-saving benefits of a heat pump while keeping the backup reliability of a boiler. In the future, some hybrid systems may also be compatible with hydrotreated vegetable oil (HVO) instead of fossil fuels.
The Northern Ireland Housing Executive has already installed over 300 hybrid systems in homes as part of a retrofit pilot scheme.
Heat pump pros and cons
| Advantages | Disadvantages |
|---|---|
Up to four times more efficient than gas boilers. | High initial costs to install a heat pump. |
Low-carbon form of heating. | Complicated installation process that may involve digging up your garden. |
Bring down your heating costs in the long term. | Takes up space in your garden and/or inside your home. |
Safer than a combustion heating system. | Lower efficiency in very cold weather. |
Long lifespan and low maintenance costs. |
|
Can be used to provide cooling as well as heating. |
|
Increase property value |
|
Is a heat pump right for your home?
Many homeowners ask “should I get a heat pump?” Heat pumps can be a great choice if you live in Northern Ireland, but they work best when your home is prepared. Here are the main things to look at before deciding.
Consider the costs vs savings
You also need to weigh the cost against potential savings on your energy bills. For households in Northern Ireland, this can be anything from a yearly saving of £2,300 to an extra £500 in costs, depending on the heating system you’re replacing.
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Make sure you’re on the cheapest tariff
If saving money is a key reason for switching to a heat pump, make sure you’re on the right electricity tariff. At Power NI, we offer competitive green electricity tariffs designed to make low-carbon heating more affordable.
Discover Eco Energy
Check property requirements
Most homes in Northern Ireland are suitable for a heat pump, but your property type and location will influence which system works best. Air source units need to be placed outside with at least 1 metre of clearance from neighbouring boundaries.
Ground source systems require either a large garden or a borehole. Planning permission isn’t usually needed, but it’s always worth checking with your local council before starting.
Account for hot water usage
Heat pumps usually heat water to around 55°C, compared with about 65°C from an oil boiler. Because the water isn’t kept as hot continuously, most heat pump systems include a built-in legionella cycle or immersion heater that raises the temperature above 60°C once a week to kill legionella bacteria.
If your household uses a lot of hot water, you may need to upgrade to a larger cylinder. Heat pumps heat water more slowly than boilers, so storage capacity is more important. Installers can recommend the right size tank, so you always have enough hot water when you need it.
Assess your current heating system
Heat pumps work at lower flow temperatures than oil or gas boilers, so your existing radiators may need to be upgraded. Aluminium radiators, or those with a bigger surface area, are usually best, because they heat up your home quickly and efficiently.
Underfloor heating also works well with a heat pump, because it operates at lower temperatures. Considering all this, it’s best to get a technical assessment of your current heating system to see what upgrades you need, if any.
Ensure your home is well insulated
Investing in a heat pump can be wasteful if you don’t first improve your home’s insulation. To maximise the impact of a heat pump and minimise its running costs, you want as much of the warmth generated as possible to stay in your home.
That might mean upgrading to double-glazed windows, adding more loft insulation, and cavity wall insulation. Draught-proofing your home and sealing up any cracks will also help.
How to use a heat pump efficiently
Hopefully now you’re a little more familiar with how a heat pump works, and if it's going to work with your home. If you decide to install a heat pump, these tips will help you get the most out of it.
Tip 1: Choose the right installer
Use an accredited heat pump installer from the Microgeneration Certification Scheme (MCS) or Competent Persons Register. They’ll size and design the system for your home’s layout and insulation, which is key in many older homes in Northern Ireland.
Ask for a heat loss calculation and a clear user guide as part of the installation.
Tip 2: Keep on top of maintenance
Routine maintenance helps your heat pump work efficiently all year round, and a well-maintained heat pump can last for 20+ years. You'll need to:
- Book a professional service once a year
- Keep outdoor air source units clear of debris, snow or overgrown plants
- For ground source systems, check antifreeze levels and pressure every few years
Tip 3: Learn how to set your thermostat correctly
Heat pumps work best when they maintain a steady indoor temperature. Instead of switching them on and off, set your thermostat to a comfortable temperature.
In summer, reduce the heating schedule if it’s not needed, and ask your installer if your system has a cooling mode you can use in warmer months.
Tip 4: Make sure you’re on the right electricity tariff
Because heat pumps run on electricity, it’s important to be on a tariff that suits a low-carbon heating system, especially if you heat your home for long periods.
Make sure your electricity tariff includes stable rates, supports higher usage, and takes advantage of renewable electricity - just like the Power NI Eco Energy Tariff.
Switch to Power NI Eco Energy
Our Eco Energy tariff provides 100% renewable electricity, designed to work seamlessly with heat pumps. It helps your system run efficiently, cuts your carbon footprint, and comes with Power NI Perks, giving you discounts on energy-saving products.
Discover Eco Energy
Heat pump FAQs
Still have some questions about heat pumps and how they work? Here are answers to some of the most common questions we get:
-
How do heat pumps work in cold weather?
The refrigerant used in most heat pumps is significantly colder than outside temperatures in Northern Ireland, even in cold weather. That means the heat exchange process used by heat pumps works in temperatures as low as -10°C for air source heat pumps and -8°C for ground source heat pumps temperatures (bearing in mind the ground temperature is usually significantly higher than the outside temperature). -
Can heat pumps operate during freezing temperatures?
The refrigerant used in most heat pumps is significantly colder than outside temperatures in Northern Ireland, even in cold weather. That means the heat exchange process used by heat pumps works in temperatures as low as -10°C for air source heat pumps and -8°C for ground source heat pumps temperatures (bearing in mind the ground temperature is usually significantly higher than the outside temperature). -
Why are my air source heat pump radiators not hot?
If there’s a fault with your air pump, the system will tell you. This could be due to debris or damage stopping the outside unit from working properly. Check for any blockages outside. If the system doesn’t notify you about a problem, it’s probably unrelated to your heat pump. This could be a problem with your central heating system or your radiators needing to be bled.
Keep in mind that air source heat pump radiators never get as hot as those heated by gas or oil-fired boilers. They operate more efficiently to maintain a steady, comfortable temperature and don’t need to be as warm to the touch. -
Should I leave my air source heat pump on all the time?
Yes, generally you should leave your heat pump running all the time. It's more efficient to have your heat pump on at a lower temperature constantly, rather than turning it off and on, because it takes more energy to start it from a cold baseline.
If you're not at home, some heat pumps have settings like "set-back" or "holiday" mode, where you can set a low temperature like 15°C. This prevents your house from getting extremely cold and protects pipes, all while using less energy than it would to turn it on and off completely. -
Do you need planning permission for an air source heat pump?
Most air source heat pump installations are a permitted development and don’t require planning permission. But it's always best to check with your local planning office first because some properties - especially listed buildings and those in conservation areas - may need planning permission. -
Should I turn my heat pump off while on holiday?
Heat pumps have a holiday mode that performs the same job as turning your thermostat down slightly. It’s best to avoid turning it off entirely because big drops in temperature will force your heat pump to work harder when it’s turned back on. -
What can cause a heat pump to break?
Frequently switching a heat pump off and on again, extremely cold temperatures, faulty pumps, broken valves, debris in the system, poor quality antifreeze and incorrect pressure could all cause a heat pump to break. Regular maintenance will help you to avoid most of these problems. -
What do I do if my heat pump is not working?
If your heat pump is not working, it’s best to contact the manufacturer or installer for advice on what is causing the problem. Most systems have displays that show warnings if something goes wrong. It might turn out to be a problem with your heating system, not the heat pump itself, which can be easily resolved.
