• Sizing of products
  • Energy saving
  • Hydronic heating
  • Radiators

The important role of radiators in a heat pump system

In the Nordic countries heat pumps have been used for years. The combination of well-insulated buildings and heat pump systems running at temperatures of 55/45 °C is certainly nothing new. Nevertheless, if an old boiler is replaced by a heat pump or a heat pump is already installed but the system temperatures are to be lowered further to, for example, 35/25 °C, it’s important to remember to review all the elements of the heat pump system, including the heat distribution and emitters. For an optimal energy-efficiency, in most cases this will mean replacing the old radiators with new ones.
heat pumps do match with radiators

Complete heat pump system

For a heat pump system to operate efficiently, as for all systems in fact, all components must be fully coordinated with each other. Only then will the system be able to reach its full potential and guarantee optimal efficiency. Therefore, careful planning and design of the complete heat pump system, including heat source, heat pump, heat distribution and heat emitters, is crucial.

In more detail, this entails determining the heat load of the building and individual rooms, selecting suitable hydronic components, checking the design of the existing heat emitters and, if necessary, replacing undersized or unsuitable emitters or adding additional heating surfaces. On top of that, the existing piping system needs to be flushed, the fittings and valves must be checked and possibly replaced and the heating system must be balanced hydronically.

Increased efficiency

When all components are matched to the heat pump as well as the building, the efficiency of a heat pump system automatically increases. It’s when we’re looking for ways to further optimise that efficiency that it becomes clear just how important radiators are within the system.

A first way to increase the efficiency of a heat pump is to further reduce the system temperatures from 55/45 °C to 35/25 °C. This can result in energy savings of about 25%(1). Lower system temperatures will improve the SCOP (Seasonal Coefficient of Performance) value of the heat pump, meaning its annual performance factor within different operating states, which are weighted according to climate zones. The higher the SCOP, the lower the energy consumption. This makes sense, because the smaller the temperature difference between the heat source (albeit air, water or geothermal heat) and the heat transfer medium (e.g. a radiator), the more economically a heat pump functions.

Secondly, hydronic balancing of the heating system is important for increased efficiency. All radiators within the system must be hydronically balanced to ensure that even the radiator farthest from the heat source is still warm. In the past, the heating water circulation pump was often dimensioned larger and/or the flow temperatures were set higher than actually necessary. Both actions cause unnecessary energy losses. Hydronic balancing helps to create an efficient heating system and leads to energy savings of 7 to 11%(2).

A further step towards improving the efficiency of a heat pump system is the use of a dynamic radiator valve insert. Especially in existing buildings, the invisible piping can pose a problem if documents are incomplete or unavailable. Pipe lengths and diameters remain uncertain, leading the design to be based on assumptions and empirical values. A dynamic valve insert solves this problem. When the required water volume is set, taking into account the radiator output and the system temperatures, the valve insert will ensure a constant volume flow. The setting needs to be done only once with the scale key included in the scope of delivery. The scaling in l/h makes the setting quick and uncomplicated. However, a prerequisite for the use of the dynamic valve is system water that is free of contamination.

Furthermore, a modern thermostatic valve head with 1K instead of 2K proportional deviation ensures fewer oscillations and thus guarantees exact compliance with the desired room temperature (setpoint temperature). With the 1K thermostatic head, the room temperature can be controlled more precisely, which in turn can quickly lead to energy savings since every degree less corresponds to approximately 6% savings(3). It should be noted, however, that this advantage is only possible if the pipe network has been designed with a 1K proportional deviation. 

Finally, the efficiency of the radiators is also affected by the state they are in. Old radiators often have corrosion residues caused by the reaction of steel to the combination of moisture and oxygen. This corrosion acts as an insulating layer inside the radiator and consequently inhibit its efficiency. To avoid corrosion, regular bleeding of the radiators is required. 

Low temperature and heat pump radiators

As mentioned, lower flow temperatures help to optimise the SCOP of a heat pump. Flow temperatures in the range of 35 to 50 °C have proven to be compatible with a radiator system linked to a heat pump. However, the temperatures influence the performance of the radiator. Whereas it normally combines convective heat, i.e. it warms up the airflow in its vicinity to gradually heat the room, with radiant heat that is directly felt where it lands, the convective heating share is significantly reduced when the flow temperature drops below 40 °C. Below this temperature fan-assisted radiators or convectors are recommended as they create a forced airflow in the radiator, and this way continue to heat the air with convective heat. 

To allow the heating system to operate with lower flow temperatures, it might be necessary to enlarge the heating surfaces. Since radiant heat is effective even at low temperatures, radiators with sufficiently large radiant surfaces are decisive when it comes to heating in the temperature range below 50 °C. This will also increase the cosy feeling of warmth.

Radiator size

In the past, the discussion has always been about whether radiators would need to become smaller or larger when optimising thermal insulation and changing system temperatures. The fact is that a lower heat load as a result of improvements to the building envelope combined with changes in the system technology allows for more efficient and economical heat generators.

In general, it can be stated that existing radiators should be checked mathematically in each individual case, taking into account the lower system temperature, and replaced if necessary. Even if some radiators are seized sufficiently, their age and condition should be considered. Moreover, hydronic balancing is an important factor as this ensures an even heat distribution in the pipe network and guarantees that every radiator receives the required amount of hot water. 

Sources
(1) According to data gathered within Purmo Group from the MIRAI-SMI 4.0 EH1218DC heat pump.
(2) According to the 2019 research report from the ITG Dresden “Potential Energy Savings and Economic Evaluation of Hydronic Balancing in Technical Building Systems”.
(3) Communicated, among others, by the International Energy Agency (IEA) and the European Commission.