Best Hot Water Temperature for Heat Pumps (UK Guide That Actually Makes Sense)

Hot water is one of those things most people don’t think about until something goes wrong. It runs out halfway through a shower, takes ages to recover, or the electricity bill lands and suddenly it becomes very interesting.

With heat pumps, hot water is one of the easiest parts of the system to get right, but it’s also one of the most commonly set up badly without anyone realising. I see plenty of systems that are technically fine, nothing obviously broken, but the cylinder is running too hot, reheating too often, or just left on whatever settings it came with out the box.

Everything works, but it’s not working well.

And over time, that’s what costs money.

So what temperature should it actually be?

For most homes, you’re aiming for:

45°C to 50°C

That’s the sweet spot.

It’s hot enough to give you usable hot water for showers, baths and general use, but low enough to keep the heat pump working efficiently.

Start pushing much higher than that and you’re asking the system to do more work than it needs to. Drop much lower and you’ll start noticing it in day-to-day use.

It’s not about chasing maximum temperature. It’s about finding the point where comfort and efficiency meet.

Why hotter isn’t better

This is where a lot of people get caught out.

It’s easy to assume hotter water is safer or better. In reality, it usually just costs more.

Heat pumps are at their most efficient at lower temperatures. The higher you push the cylinder, the harder the system has to work and the more electricity it uses.

Then there’s heat loss.

Every cylinder loses heat. Even a good one.

The hotter the water inside it, the faster that heat escapes into the surrounding space. That means the system has to keep reheating it just to maintain temperature.

It doesn’t look dramatic in the moment, but over time it adds up.

Why hot water often drives running costs

A lot of people focus on heating when they talk about efficiency, but hot water is often the hidden contributor to higher bills.

Your heating system can run at relatively low temperatures, especially if it’s been designed properly. That’s where a heat pump performs best.

Hot water is different. It requires higher temperatures, which means lower efficiency.

If the system is constantly reheating the cylinder, or maintaining unnecessarily high temperatures, it spends more time operating in that less efficient range.

In plenty of homes, when someone says their heat pump is expensive to run, the issue isn’t the heating at all. It’s the hot water setup.

What about legionella?

This is where most of the confusion comes in.

You’ll often hear that hot water must be stored at 60°C. That’s only part of the picture.

The real objective is to control the risk of bacterial growth, particularly legionella.

Legionella bacteria tend to grow in water between roughly 20°C and 45°C. Once you get above 50°C the risk drops significantly, and at around 60°C the bacteria are effectively killed.

That’s why higher temperatures come into it.

The Health and Safety Executive (HSE) guidance is clear that systems should be designed and operated to control this risk, and that temperature is one of the main methods used.

But that doesn’t mean your cylinder needs to sit at 60°C all day.

How heat pump systems actually deal with it

Most heat pump systems use a periodic disinfection cycle.

This means the cylinder is run at a normal, efficient temperature most of the time, and then periodically heated above 60°C to kill off any bacteria.

In most cases, manufacturers set this up as once a week by default.

For example, Daikin systems are often set to run late on a Friday evening from factory. Other manufacturers do something similar, just at different times.

That weekly cycle isn’t random. It’s a balance.

It gives regular disinfection without constantly forcing the system to run at high temperature.

Does it always need to be once a week?

For most homes, weekly is a sensible default.

But it’s not a hard rule for every situation.

The MCS makes it clear that systems must include a way of controlling bacterial growth, but also that the approach can be influenced by a risk assessment.

In a normal domestic property where:

• Hot water is used daily

• The system is well designed

• There are no unusual pipe layouts

…the actual risk is relatively low.

That’s why in some cases, systems may run:

• Weekly

• Every couple of weeks

• Or adjusted based on usage

But this isn’t something to guess at.

Weekly is used because it is a safe, simple, and broadly appropriate setting for most homes.

What increases the risk?

This is the part that often gets missed.

Legionella risk isn’t just about temperature. It’s about conditions.

Risk increases where:

• Water is left sitting unused

• There are rarely used taps or showers

• The system has dead legs or poor pipework

• The property is unoccupied for long periods

The HSE specifically advises that infrequently used outlets should be flushed through regularly to prevent stagnation.

So a busy household using hot water every day is very different to a property that sits empty for weeks at a time.

How long should a disinfection cycle run?

It’s not just about reaching 60°C.

It needs to stay there long enough to actually be effective.

Most systems are set up to:

• Raise the cylinder temperature above 60°C

• Hold it there for a period of time

Exactly how long depends on the system, but the principle is simple.

It’s a combination of temperature and time, not just hitting a number briefly.

Factory settings are generally designed to handle this correctly, which is why they’re usually a good starting point.

Where people get this wrong

There are two common mistakes.

The first is running the cylinder at 60°C all the time “just to be safe”.

That’s unnecessary in most homes and just increases running costs.

The second is disabling the disinfection cycle completely without understanding the implications.

That’s not something I’d recommend unless there’s a very clear and informed reason for doing it.

R290 systems and the high temperature misunderstanding

Newer R290 heat pumps have added another layer to this.

These systems can reach higher temperatures without needing an immersion heater, which is a genuine improvement.

But it’s also led to a common misunderstanding.

Because the system can run at 60°C, people assume it should.

It shouldn’t.

It’s still a heat pump, and the same rules apply. Higher temperatures reduce efficiency and increase heat loss from the cylinder.

What R290 gives you is flexibility. It allows you to reach higher temperatures when needed, including disinfection, without relying on additional electric heating.

It doesn’t change the overall strategy.

Run lower temperatures day-to-day. Go higher when needed.

Timed hot water vs constant reheating

This is one of the biggest things that gets overlooked, and it has a direct impact on running costs.

A lot of systems are left on what looks like the “safe” option, where hot water is available all day and the system just keeps topping the cylinder up whenever the temperature drops slightly.

On paper, that sounds sensible. In reality, it usually means the heat pump is constantly cycling.

What happens is this. The cylinder cools slightly through normal heat loss or use, the sensor picks it up, and the system fires back up to reheat it. That might happen multiple times throughout the day.

Each time it does, it’s often operating at higher temperatures, which is where the heat pump is least efficient.

So instead of one controlled, efficient run, you end up with lots of smaller, less efficient ones.

A timed approach changes that completely.

Instead of reacting all day, you tell the system exactly when to heat the cylinder. It runs properly, brings the cylinder up to temperature, and then stops.

From there, you’re relying on the insulation of the cylinder to hold that heat.

Most modern cylinders will comfortably retain usable hot water for 8 to 12 hours, often longer depending on size and insulation. That means for a lot of households, one or two well-planned heating periods are more than enough.

It’s not about restricting hot water. It’s about controlling when the system does the work.

Why constant hot water can actually affect your heating

This is something that tends to show up more in winter, and it’s easy to overlook.

A lot of systems are deliberately set to keep hot water on constantly. It’s a choice people make. The thinking behind it is usually that it’s more efficient to let the system tick over and keep the cylinder topped up, in the same way low and steady heating works well for the house.

You’ll even hear this advice from time to time.

The problem is, hot water doesn’t behave in quite the same way as space heating.

Most heat pumps can’t do heating and hot water at the same time. It’s one or the other.

So every time the system switches over to reheat the cylinder, it stops heating the house.

If that happens occasionally, it’s not an issue. But if it’s happening regularly, especially in colder weather when the system is already working hard, it can start to affect how well the house heats.

You can end up in a situation where the system is running, but the property never quite reaches temperature.

I’ve been to cases where others have diagnosed systems as undersized, suspected circulation issues, or even potential refrigerant problems, simply because the heating performance isn’t where it should be.

But when you actually step back and look at the system as a whole, the cause can be much simpler.

The hot water has been set to constant reheat.

So the system keeps switching away from heating, topping up the cylinder, then trying to recover the heating again. It ends up chasing its tail.

The winter icing and defrost side of it

There’s another knock-on effect as well.

When the system is repeatedly switching into hot water mode, it tends to run at higher temperatures and under higher load.

In colder or freezing weather, that can lead to the outdoor unit icing up more quickly.

More icing means:

• More frequent defrost cycles

• More interruptions to heating

• Reduced overall efficiency

So you end up with a system that:

• Heats less effectively

• Stops more often

• Uses more energy

All from a setup that, on the face of it, seems like a sensible idea.

What this means in practice

This is why constant hot water is rarely the best option.

It’s not that it doesn’t work. It’s that it doesn’t work efficiently, and in colder conditions it can start to affect overall system performance.

A properly timed setup avoids this.

The cylinder is heated when it needs to be, and the system is then free to focus on heating the house without constantly switching between modes.

That’s when everything starts behaving properly.

Using tariffs properly (and where people get it wrong)

Tariffs can make a big difference, but they’re often applied in a very simplistic way.

A lot of advice out there just says:

“Heat your water overnight when it’s cheap”

That can be right, but it’s not always.

If you’re on a tariff with significantly cheaper overnight rates, like many EV tariffs, then yes, storing hot water during that period can reduce costs.

But you need to understand what’s happening in the background.

Heat pumps don’t operate in a vacuum. Their efficiency is heavily influenced by outdoor temperature.

At night, particularly in winter, the outside air is colder. That means the heat pump has to work harder to produce the same hot water temperature, and its efficiency drops.

So now you’ve got two competing factors:

• Cheaper electricity

• Lower system efficiency

In many cases, the cheaper electricity still wins, but not always.

If the price difference is small, or the system is really struggling in cold conditions, you might be better off heating water during the day when the air temperature is higher and the heat pump is more efficient.

This is why copying someone else’s setup doesn’t always work.

The best approach depends on:

• Your tariff

• Your system

• Your usage

Real-world scenarios (this is where it actually matters)

Take a typical family home.

Four people, showers in the morning, maybe baths in the evening, 200–250 litre cylinder, and a cheap overnight tariff.

A well-set system might heat the cylinder overnight to around 48°C, using cheaper electricity, then allow a second shorter heating period later in the day if needed.

The disinfection cycle can also be scheduled overnight, because that’s one of the most energy-intensive parts of the process.

Now compare that to a smaller household.

One or two people, low hot water usage, standard flat tariff.

In that case, there’s no real benefit to heating water overnight. You’re better off running a single heating period during the day, when the heat pump is operating more efficiently.

Same technology, completely different strategy.

That’s why there isn’t a one-size-fits-all answer.

How much hot water do you actually use?

There’s a lot of talk about litres per person, but in reality, it’s easier to understand this through behaviour.

Some households use a lot of hot water without realising it. Long showers, multiple people back-to-back, baths, all add up quickly.

Others barely touch it.

Government-backed studies have put average domestic hot water usage somewhere around 90 to 120 litres per household per day, but averages don’t tell you much about your own home.

What matters is how your system behaves.

If you’re running out of hot water, you either need:

• A higher storage temperature

• Better timing

• Or more storage capacity

If you’ve always got plenty left, you’re probably storing more heat than you actually need.

Cylinder size and why bigger isn’t always better

It’s easy to assume a bigger cylinder is always the safer option.

More storage means more hot water available, which sounds like a win.

But there’s a trade-off.

A larger cylinder holds more energy, but it also loses more heat over time. That’s because you’ve got a greater volume of hot water sitting there, constantly losing heat to the surrounding space.

Smaller cylinders lose less heat overall, but they need to be managed more carefully to avoid running out.

The right answer isn’t “biggest possible”.

It’s:

big enough to meet demand, without carrying unnecessary heat loss

Standing heat loss (the bit no one sees)

This is one of the quiet contributors to higher running costs.

Even modern, well-insulated cylinders lose heat continuously.

Typical figures for a decent cylinder might be somewhere around 1 to 1.5 kWh per day in standing losses. That’s just from sitting there, doing nothing.

Now factor in temperature.

The higher the stored temperature, the greater the temperature difference between the cylinder and the room around it. That increases the rate of heat loss.

So a cylinder stored at 60°C will lose heat faster than one stored at 48°C.

That lost heat has to be replaced, which means more energy used over time.

It’s basic physics, but it has a real impact on bills.

Seasonal changes and system behaviour

Hot water demand itself doesn’t change massively through the year, but system performance does.

In winter, the heat pump is already under more load from heating, and outdoor temperatures are lower. Producing hot water becomes more demanding, and timing becomes more important.

This is where poor setups start to show. Systems running constant reheat or poorly timed cycles end up working harder than they need to.

In summer, the heat pump has an easier job. Outdoor temperatures are higher, there’s no heating demand, and hot water production is generally more efficient.

That often means you can reduce the number of heating cycles or slightly lower the target temperature without affecting comfort.

It doesn’t need constant adjustment, but understanding the difference helps.

The biggest issue I see

It’s not usually a fault.

It’s that the system is never revisited after installation.

It gets commissioned, left on default settings, and that’s it.

Because it works, no one questions it.

But default settings are just a starting point. They’re not tailored to the property, the usage, or the tariff.

And over time, that’s what leads to higher running costs.

A lot of the time when someone says their heat pump is expensive, the system itself isn’t the problem.

It just hasn’t been properly set up for how the house actually uses it.

A sensible starting point

For most homes, this is a good baseline:

• Store hot water at 45–50°C

• Use timed heating periods rather than constant reheating

• Run a weekly disinfection cycle above 60°C

• Adjust based on actual usage

• Factor in tariff where relevant

From there, it’s about refining it.

Final thought

The best hot water setup isn’t about running the system as hot as possible or keeping it topped up all day.

It’s about understanding what the system is doing and controlling it properly.

Enough hot water to meet demand, without the system working harder than it needs to.

Get that right, and everything settles down. The system runs more smoothly, and the costs usually follow.