We've blogged before about the cost effectiveness of heat pumps: Are heat pumps cost effective? More on the cost effectiveness of heat pumps Heat pumps are more cost effective to run than gas or oil boilers
We're back with a series of 3 update posts...
The cost of running a heat pump is dependent on many factors, but 3 of significance are:
The flow temperature from the heat pump to provide space heating
How much the electricity is costing to power the heat pump
Weather compensation
We’re looking at each one in turn over the 3 posts as there is quite a lot to discuss!
Heat pump flow temperature
So why does the flow temperature affect how much it costs to run a heat pump?
The energy output of a heat pump compared with the energy input is defined as the ‘Coefficient of Performance’ (CoP), and annual average of CoP is the ‘Seasonal Coefficient of Performance’ (SCoP). For example, a heat pump with a SCoP of 2.7 will deliver 2.7kW of heat for 1kW of electricity used. The following chart details the SCoP for one of the new Samsung R290 units. I have chosen this unit as we have just fitted one at Artimus Towers and so I can provide information from personal experience.

This chart shows that, as the flow temperature increases, the SCoP decreases. For example, with a flow temperature of 35°C, the heat pump has a SCoP of 4.74, which means, on average over a year, the heat pump delivers 4.74kW of heating for every 1kW used. Now with a flow temperature of 65°C, the heat pump has a SCoP of 2.84, so delivers 2.84kW of heat for every 1kW used. A significant difference.
So, between the two values discussed, there is an increase in running costs of just under 67%. You may now be wondering why I don’t just run the heat pump at the lower value and gain the benefit? The heating system needs to deliver sufficient heat to meet the design outside temperature, which is between -2°C and -4°C for the UK. The design of the heating system will require a certain flow temperature to meet that need.
The amount of heat delivered by the heating system emitters (radiators etc.) depends primarily on 3 factors:
The flow temperature of the water. The rate at which heat transfers is dependent on the temperature gradient between the hot part and the cooler part. The greater the temperature difference between hot and cold, the faster the rate of heat transference. So, the hotter the water compared with the room temperature, the more heat will be transferred in a given time.
The surface area of the emitter. The greater the surface area of the emitter, the more heat will be transferred in a given time.Â
The flow rate of the water. The water flowing in the emitter needs to carry sufficient heat to meet the demand. With a faster flow rate, more heat is carried.

Flow temperature
An older property with gas or oil heating is likely to have the emitters sized for a flow temperature of between 65°C and 75°C, so reducing the flow temperature to 35°C will not heat the house in the winter. However, many houses have oversized radiators from build, so it may be possible to reduce the flow temperature and improve efficiency. An alternative is to increase the size of the radiators. If this is an option, we generally design in K3 radiators to maximise the heat output.
Building regulations require that new heating systems be designed with a maximum flow temperature of 55°C. However, for existing systems this is an advisory target that can be exceeded if required. With current pricing of electricity, oil and gas, this is a good maximum design point as the Samsung should be cheaper to run at this temperature. In the second post in this series, we’ll discuss how you can reduce this cost further.
Surface area of the emitter
The following is only intended as a guide to the flow temperature that may be needed, and detailed heat loss calculations are required to confirm the heating system design:
Cold house in winter, single panel radiators that are too hot to keep a hand on. A heat pump may not be suitable without increasing the size of the radiators and ideally improving the insulation.
Cold house in winter in some rooms but warm in others. A heat pump is likely to be suitable with a flow temperature between 50°C and 65°C, but some radiators will need upgrading.
Warm house in the winter and it is possible to keep a hand on the radiator. A heat pump is likely to be suitable with a flow temperature between 50°C and 65°C. We have completed a similar install on 10mm microbore to the radiators with a good outcome.
Warm house in the winter and it is possible to keep a hand on the radiator but there is a requirement to reduce the flow temperature as far as possible. Depending on the heating pipe sizing, it may be possible to reduce the flow temperature to between 45°C and 55°C.
Underfloor heating throughout. It may be possible to have a flow temperature of between 35°C and 45°C.
The type of radiator can make a significant difference to the required flow temperature. Here's an example taken from a supplier’s website: a type 11 single panel radiator, 600mm by 600mm, has a stated output of 570W with a Delta T of 50°C. The Delta T is the difference between the average water temperature in the radiator and the room temperature. A type 33 radiator with triple panels of the same size has an output of 1550W for a Delta T of 50°C. This is nearly 3 times the output. This means it may be possible to upgrade the existing radiators and drop the flow temperature. We have just completed an install of a heat pump with upgrade of all radiators and will hopefully post about this in the near future.
An upgrade on the conventional radiator is the fan-assisted radiator. This type of radiator has a heat exchanger similar in construction to a car radiator and so has a much larger surface area than a conventional radiator. It has a fan to push the air through the heat exchanger and so requires a power supply and is designed to work at lower temperatures. We will be replacing the conventional radiators at Artimus Towers over the coming year, so we can let you know how we get on. Another advantage is that they can provide cooling as long as the heat pump can operate in cooling mode. Have a look at the following link: https://www.olimpiasplendid.com/fan-coils-fan-radiators
Underfloor heating provides by far the greatest surface area and will enable the flow temperature to be minimised.
Flow rate of the water
The water flowing in the emitter needs to carry sufficient heat to meet the demand. With a faster flow rate, the more heat is carried but the flow rate is limited by the size of the heating pipework.
Summary
In the second post in this series, you will see that keeping the flow temperature at or below 55°C will mean that the heat pump is currently cheaper to run than gas or oil. By making use of a dedicated tariff such as Cosy Octopus, there is also the potential to significantly reduce heating costs by making good use of battery storage.
The third post will detail how temperature compensation works with a heat pump and how it can significantly save on running costs if set up correctly.
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