What Temperature Should Radiators Run at With a Heat Pump?

What Temperature Should Radiators Run at With a Heat Pump? (UK Guide)

If you've recently had a heat pump installed or you're in the process of planning one the question of what temperature your radiators should actually run at comes up early, and it matters more than most people realise.

It's one of the most significant practical differences between a heat pump and a traditional gas boiler. And it's one of the main reasons why some heat pump systems run efficiently with low electricity bills, while others end up costing far more to run than the homeowner expected.

The short answer is that heat pump radiators run at lower temperatures than boiler-fed systems. But the more important answer the one that actually determines whether your system performs well is that the correct temperature should come directly from the system design, not from a general guideline you've found online or a setting someone suggested.

This guide explains why radiator temperature matters so much, what it should typically be in a well-designed UK system, and what it means if your system is running hotter or colder than it should be.

Why Heat Pump Radiator Temperatures Are Lower Than a Boiler

With a traditional gas boiler, flow temperatures of 65–75°C are standard. Radiators in most UK homes were sized with those temperatures in mind, which is why they could be relatively compact and still heat a room effectively.

A heat pump works on a completely different principle. Rather than burning fuel to generate heat, it extracts heat energy from the outside air and transfers it into the heating system. The efficiency of that process measured as COP, or Coefficient of Performance is directly linked to the temperature difference between the outside air and the water leaving the heat pump.

The smaller that temperature gap, the less work the compressor has to do, and the more efficiently the system runs.

At a flow temperature of 35°C, a modern air source heat pump might achieve a COP of 4.0 or higher meaning it produces four units of heat for every unit of electricity it consumes. Push that flow temperature up to 55°C and the same heat pump might only achieve a COP of 2.5 or lower. The house gets the same heat, but you're paying significantly more electricity to produce it.

That relationship between flow temperature and efficiency is why getting radiator temperatures right isn't just a technical detail it's the difference between a heat pump that saves money and one that doesn't.

What Temperature Should Heat Pump Radiators Run at in the UK?

For most UK homes, heat pump systems are designed to operate somewhere between 35°C and 50°C flow temperature, depending on the property and the time of year.

That said, quoting a range doesn't actually answer the question for your specific system. The correct flow temperature for any given installation comes from three things working together:

The heat loss of the property. Every building loses heat at a different rate depending on its construction, insulation, glazing, and air tightness. A proper room-by-room heat loss calculation done to BS EN 12831 standards tells you exactly how much heat each room needs to stay at the desired temperature when it's coldest outside.

The radiator output at that temperature. Radiators deliver less heat as the water temperature drops. A radiator that outputs 1,000W at 75°C might only output 400–450W at 45°C. Once you know the heat demand of each room, you can work out whether the existing radiators can meet it at a lower temperature, or whether they need to be upgraded.

The heat pump's design flow temperature. This is the temperature the system was specified to run at under design conditions typically the coldest outdoor temperature expected for the location. In most parts of England, that design point is around -3°C.

When all three of those are calculated and matched correctly, the system has a specific design flow temperature often somewhere between 40°C and 55°C for older UK properties, and as low as 35°C for well-insulated modern homes. That design figure is what the system should be set to, not a number chosen through trial and error.

What Happens When Radiator Temperatures Are Too High?

If a heat pump is regularly running at flow temperatures of 55°C or above, it will still heat the house but efficiency takes a significant hit, and running costs climb.

In most cases, high flow temperatures aren't a deliberate choice. They're a compensation mechanism. The system is running hotter because it has to, usually because of one of the following:

Radiators that are too small to deliver sufficient heat at lower temperatures. This is extremely common in older UK properties where radiators were originally sized for a boiler running at 70–75°C. Without upgrading them, the only way to get enough heat into the room is to push the flow temperature up.

A heat loss calculation that underestimated the property's actual heat demand. If the heat loss figure was too optimistic perhaps assuming insulation improvements that haven't been made, or using standardised U-values rather than measured ones the system will be working harder than its design suggests it should.

Poor hydraulic balance across the system. If some radiators are getting significantly more flow than others, the system can't deliver heat evenly, and the flow temperature may be increased to compensate.

The tell-tale sign is a system that runs at higher temperatures and still doesn't feel entirely comfortable, or where electricity bills are noticeably higher than the installer's estimate suggested they would be.

What Happens When Radiator Temperatures Are Too Low?

Running at low flow temperatures only works when the system has been properly designed to do so. If the temperature is set lower than what the heat demand of the property actually requires, the problems are different but equally frustrating.

Common signs that the flow temperature is too low for the system design include:

The house takes a long time to reach the set temperature, particularly on colder days.

Individual rooms feel slightly underheated even when the heat pump has been running for several hours.

The system runs almost continuously without ever getting the house fully up to temperature.

It's worth noting that heat pumps running for long periods isn't automatically a problem they're designed to do exactly that. The concern is when continuous running still doesn't achieve the desired indoor temperature, which suggests the flow temperature is insufficient for the actual heat demand.

Why Radiator Size Has More Impact Than the Temperature Setting

The temperature displayed on your controller is a consequence of your system design, not really a variable you should be adjusting manually on a regular basis.

The component that actually determines whether your radiators can work at low temperatures is their physical size. A large radiator has more surface area to release heat into the room, which means it can deliver the same amount of warmth at a lower water temperature. A small radiator has less surface area and needs hotter water to produce the same output.

This is why radiator upgrades are one of the most common and most impactful changes made when retrofitting a heat pump into an older UK property. It's not about making the system more powerful — it's about giving the heat pump the ability to run at the lower temperatures where it operates most efficiently, rather than forcing it to run hot to compensate for radiators that are too small.

If you haven't had a proper radiator output check as part of your system design, it's one of the first things worth looking at if performance isn't what you expected.

How Weather Compensation Changes the Picture

In practice, your heat pump won't run at the same flow temperature all year round and it shouldn't. This is where weather compensation comes in.

Weather compensation automatically adjusts the flow temperature based on how cold it is outside. On a mild autumn day at 12°C, the system might only need to run at 35–38°C. On a cold January day at -1°C, it might run closer to its design temperature of 48–50°C.

The practical result is that across a full heating season, the system spends most of its time running at lower temperatures than the design maximum which means it's running at higher efficiency for most of the year.

When weather compensation is set up correctly, you shouldn't need to manually adjust the flow temperature at all. The system increases and decreases it automatically in response to outdoor conditions.

If you find yourself regularly increasing the flow temperature manually to keep the house warm, that's a signal that either the weather compensation curve isn't set correctly for your property, or there's an underlying design issue that settings adjustments can't fully resolve.

For a more detailed look at how to set and adjust weather compensation, see our guide: How to Set Weather Compensation on a Heat Pump (UK Guide).

Signs Your System Isn't Running at the Right Temperature

You don't need to understand the technical data in detail to recognise when something isn't right. The most common warning signs are:

Your electricity bills are noticeably higher than your installer estimated, and haven't improved since the system was commissioned.

You find yourself manually increasing the flow temperature on the controller regularly just to maintain comfort.

The house feels warm in some rooms but cold in others, despite radiators being open throughout.

The system runs constantly but the house still doesn't quite reach the target temperature in cold weather.

The heat pump is frequently cycling on and off rather than running in long, steady periods.

Any of these suggest the system's flow temperature and the design behind it is worth reviewing properly.

Getting the Temperature Right Before Installation

The easiest time to get radiator temperatures right is before the system is installed.

A proper pre-installation design process including accurate heat loss calculations and radiator output checks at the intended flow temperature removes the guesswork entirely. You know before the heat pump is connected what flow temperature the system is designed to run at, which radiators (if any) need upgrading, and what to expect from running costs.

That foundation means the system can be commissioned at the correct settings from day one, rather than spending weeks or months adjusting temperatures trying to find a comfortable balance.

Already Installed and Not Running Efficiently?

If your system is already in place and you're not confident it's running at the right temperature, our Full Performance Review looks at exactly this flow temperatures, radiator performance, system design, and control setup. You'll receive a clear written report explaining what your system is actually doing, what the correct temperatures should be, and what adjustments are likely to make the biggest difference.

If you're still planning, our Pre-Installation Design & Heat Loss Review ensures the system is designed to run at the right temperature from the start — so you're not discovering problems after the installation is done.