Low vs high-temperature heat pumps and the role they play in decarbonising heating and domestic hot water

Moving away from gas boilers is imminent, now more than ever before.
Decarbonising domestic heating is a key component of the government’s overall target to achieve net zero emissions by 2050.
According to the latest statistics, UK’s 28 million homes accounted for 18% of all UK greenhouse gas emissions in 2021, majority of which come from burning natural gas.

UK Government sees heat pumps as the main technology to decarbonise these 28 million homes over the next decade; however, the current heat pump uptake is about 11 times lower than the annual target, with just about 55,000 units installed annually. To better understand this challenge, it’s crucial to consider the types of heat pumps and the emerging technologies available – specifically low vs high temperature heat pumps.

Low vs high temperature heat pumps: what’s the difference and which are preferred?

The key differences lie in their operating temperatures and how they integrate into your home’s heating system.

 

Low vs high temperature heat pumps

 

 

Low-temperature heat pumps

Low-temperature heat pumps (LTHPs) typically operate between 35C and 55C (in terms of the water temperature they achieve) and are considered to be more efficient (lower flow temperature means low heat loss). While LTHPs can comfortably heat most homes, they perform best in well-insulated properties or for underfloor heating. As a result, a fabric-first approach is often recommended with LTHPs, along with fitting in larger, compatible radiators to compensate for low temperatures. These heat pumps also often need complex plumbing upgrades.

Which brings us to the major concern that prevents most UK households when transitioning from fossil fuel boilers to low-carbon heating systems: the idea that heat pumps may not provide the ‘same level of power’ as a gas boiler, or ‘heat their homes as effectively’.
LTHPs produce a relatively lower output temperature, which may not be suitable for much of the UK’s existing housing stock, where insulation is often not up to standard and conventional radiators are designed to work with higher water temperatures (60-80C) produced by gas boilers.

However, LTHPs can be suitable for well-insulated modern new-build homes, which tend to have newer, compatible radiators. In these homes, LHTPs can operate more efficiently due to lower water flow temperatures (<55°C), delivering reduced energy consumption as well. LHTPs are a good match for underfloor heating as it requires lower water temperatures, thanks to a more extensive surface area and uniform heat distribution.
In terms of refrigerants used, LTHPs traditionally included R410a refrigerant, but is currently being phased out of use on account of its high global warming potential (GWP). A more popular choice for modern LTHPs is R32 refrigerant, which has a much lower GWP. R32 not only reduces the environmental impact but also offers improved energy efficiency, allowing LHTPs to operate more effectively while consuming less power.

High temperature heat pumps

High temperature heat pump (HTHPs) systems are designed to deliver heat at higher temperature levels (60C-85C), the same at which oil & gas boilers operate, and can heat standard radiators and work with poorly insulated homes.
In short, they deliver same level of warmth for your home as fossil fuel boilers but are considerably faster.

HTHPs theoretically have lower efficiency compared to low-temperature ones because higher temperatures reduce efficiency. However, in practice, many high-temperature heat pumps perform just as well as low-temperature models. This is because our homes can actually work well with lower temperatures. So, even if a high-temperature heat pump is installed, it doesn’t have to run at a high temperature necessarily. As a result, these are often a better option for older buildings where complex retrofitting insulation is difficult, where installing larger radiators isn’t an option, or for properties with high demand for domestic hot water. HTHPs thus make it easier for a majority of dwellings (which are typically old housing stock) to transition to heat pumps from fossil fuels, allowing residents to benefit from efficient low-carbon heating without extensive modifications.

Moreover, while HTHPs are generally less efficient than LHTPs due to higher compressor workloads, modern innovative refrigerant technologies, such as R290 (propane) and R744 (carbon dioxide), along with various refrigerant blends are helping in narrowing the efficiency gap. These advancements enable HTHPs in delivering higher heat outputs while still optimising overall performance.

The question about domestic hot water (DHW)

“Heat pumps can provide domestic hot water. Usually, a heat pump will warm up water in a tank gradually over time. However, space saving technologies such as heat batteries are an option for consumers with limited space.”
– Richard Halsey, Innovation Director for Growth at Energy Systems Catapult

Both LTHPs and HTHPs need thermal storage (typically a hot water cylinder) to store DHW.
However, when a gas boiler is replaced by a low-temperature heat pump, it may need a larger cylinder with a much larger coil capacity to make up for the lower operating temperatures, as it potentially takes longer to heat water​.

Since LTHPs operate at around 35C-55C, they’re efficient for space heating (with adequate insulation and larger radiators) but may not be enough to produce higher temperatures needed for DHW, which often needs heating water at >60C. So achieving higher temperatures often requires using immersion heaters, and this could impact overall system efficiency. This eventually makes them ‘less ideal’ for direct DHW delivery.

HTHPs, on the other hand, are designed to produce higher temperatures, which makes them more suitable for delivering DHW without the need of an external immersion/backup heater. Thanks to modern refrigerants, such as R290, HTHPs can even maintain efficiency while delivering required heat levels.

Comparing low vs high temperature heat pumps for legionella protection

Legionella protection is often a critical factor in choosing DHW systems. Legionella bacteria thrives between 20C and 45C but cannot survive over 60C. LTHPs struggle to provide this level of protection, and may need additional heating systems like electric immersion heaters to carry out energy-intensive legionella protection cycle.

HTHP systems using natural refrigerants such as R290 and R744 can easily reach temperatures high enough to suppress potential legionella growth.

The need for thermal storage for domestic hot water

Despite the ability to reach high temperatures, HTHPs still need additional thermal storage, such as a hot water cylinder, for a constant, on demand supply of DHW, particularly in peak demand periods. By storing excess heat from HTHPs, hot water cylinders can enable households to have immediate access to DHW, which not only improves overall heating system performance but also ensures the comfort of hot water.
However, traditional hot water cylinders come with their own challenges, most of which affect a vast majority of existing UK households – such as high heat losses over time (which offset the HTHPs efficiencies), big footprint (most households don’t have the cupboard space and complex plumbing necessary for installing a hot water cylinder when replacing a gas boiler), and long reheat times to manage greater demands.

How do Sunamp’s Thermino heat batteries help in maximising high-temperature heat pump performance?

Using the patented Plentigrade phase change material technology, Sunamp’s Thermino heat batteries store and release energy at a much greater density compared to hot water storage systems.

Here's how the heat batteries make a difference compared to a hot water cylinder:

Sunamp thermino vs hot water cylinder
 Sunamp Thermino vs hot water cylinder

 

Compact size: Sunamp’s heat batteries take up significantly (up to 4x) less space compared to bulky hot water cylinders, making them ideal for space-constrained homes or massive retrofitting projects.

Minimal heat loss: Unlike conventional cylinders, which experience significant heat loss over time, Sunamp’s heat batteries are packed with vacuum insulation, retaining stored energy far longer, and ensuring that the heat generated by HTHPs is used more efficiently.
Watch how a homeowner was able to enjoy hot water for up to 4 days from a fully charged Thermino battery despite power outage.

Instant mains pressure hot water: The high-powered heat exchanger or heating element immersed in our PCM rapidly charges the Thermino battery and enables heat extraction to provide fresh, mains pressure hot water (up to 10 bar or 1 MPa) at a constant temperature whenever needed. To get a peek into the water pressure performance, watch the detailed pressure and hot tap tests conducted by a homeowner in Bohemia.

Balancing demand & efficiency: By integrating Sunamp thermal batteries with HTHPs, households can better balance energy generation and demand, making the most of off-peak tariffs. These batteries can store surplus energy generated during off-peak times by HTHPs or by pairing with solar PV, and release it as needed, optimising both energy consumption and cost savings.

Long-lasting and no G3 compliance necessary: Unlike hot water cylinders, which can suffer from scale buildup & corrosion over time, Sunamp heat batteries contain very low water storage (<7L), which not only reduces maintenance costs but also eliminates the need for G3 Building Regulations related to water safety. This makes them a more sustainable choice for long-term use with high temperature heat pumps.
Thanks to the Optimino technology, Thermino xPlus batteries are compatible with a growing list of leading heat pump manufacturers, which you can view here.

If you’re interested in learning more about myths associated with heat pumps, we’ve covered that in detail here.