Lowering System Temperatures Can Substantially Increase Beneficial Application of Heat Pumps in Existing Building Stock

Improving building shell -insulation and airtightness is the way to go for a further reduction of the heat demand and a simultaneous increase of the seasonal efficiency of hydronic space heaters in the existing building stock of the EU27. With additional replacement of the existing emitters by low temperature emitters (LT-emitters), the seasonal efficiency of inverter-controlled full electric heat pumps can be further increased by another 30% to over 80%.

Keywords: low temperature heating, LT-emitters, heat pumps, existing building stock, energy saving.

Space heating is one of the largest energy consuming services in the existing building stock of the EU27. In 2020, 101 million central hydronic space-heaters were in use, consuming around 1 510 TWh in the same year[1]. There is a large saving potential, and application of heat pumps in the existing building stock is one of the means. But heat pumps generally require lower temperature regimes than those used for fuel boilers. Although some heat pump technologies are capable of producing higher system temperatures, further reduction of the system temperatures is very much preferred in order to achieve better seasonal efficiencies for space heating.

In existing buildings with fuel boilers, emitter-systems are dimensioned for temperature regimes of - depending on the age of the building - either 90/70°C, 80/60°C, 75/65°C or for the newest buildings 55/45°C. A temperature regime of 90/70 means that the envisaged supply temperature at the design point (i.e. −10°C for the average climate) is 90°C and the return temperature 70°C. The associated system design temperature is (90 + 70)/2 = 80°C and the associated system design DT = 20°C.

Only for floor heating system, lower temperature regimes are applied, but they are generally achieved by employing a mixing valve; not by lowering the temperatures coming from the boiler.

For heat pump systems, considerably lower temperatures regimes and preferably also lower DTs are required to maximise their seasonal efficiencies and fully exploit the energy saving potential of heat pumps. Only in newly designed dwellings with high insulation and airtightness, really low temperature regimes of around 35/30°C can be applied and combined with floor/wall heating systems.

For the existing building stock, it is essential to lower the required system temperatures and DTs wherever possible, to fully capitalise on the saving potential of heat pumps.

Heat Load to Emitter Capacity Ratio

The required system temperature (the temperature needed to comfortably heat the house) is determined by the ratio between Heat Load and Emitter Capacity:

HL/EC-ratio

The smaller this HL/EC-ratio, the lower the required system temperature. There are two options to reduce this ratio and consequently lower the system temperature:

1)    Reduce the heat load of the room/dwelling, by improving insulation and airtightness

2)    Increase emitter capacity in the rooms

Reduction of the heat load

A reduction of the heat load of the room or dwelling is the preferred option, because the benefits are twofold: 1) the actual heat demand is reduced by the improved insulation and airtightness and 2) the seasonal efficiency of the heat pump is increased because the lowered HL/EC-ratio reduces the required supply temperature. Lower supply temperatures improve the COP of the heat pump.

Improving insulation of existing dwellings and buildings is therefore the way to go, but unfortunately not in all cases an economically viable option. Filling cavity walls with insulation materials and replacing glass areas by multiple walled insulating glass are generally financially attractive measures. But when cavity walls are lacking, options for heat load reduction can become quite expensive and possibly financially unfeasible. In such cases, increasing the emitters capacity is the only remaining option.

Increasing the emitter capacity

The principle here is, that the existing radiator capacity is increased to a capacity that allows a lower system temperature (preferably £ 40°C) at an outdoor temperature of e.g. −10°C.

An example

Let’s assume a dwelling constructed in the sixties, with a heat load of 11 kW at −10°C for the whole house and a heat load of 5.5 kW for the living room where also the room thermostat is located. For this living room, two 2.0 m long type 21 steel panel radiators with a height of 50cm are needed to cover the 5.5 kW heat loss with the temperature regime of 90/70°C (system design temperature = 80°C) when combined with a flow of 120 l/h per radiator. Over the years, renovations to the building shell of this house have led to a heat load reduction of for instance 25%, bringing back the heat loss in the living room to around 4.1 kW. The same two steel panel type 21 radiators can now compensate for this 4.1 kW heat loss at a temperature regime of 76/61°C (system design temperature = 68.5°C) using the same flowrate. The room thermostat senses that the required room temperature setpoint is achieved at this lower system design temperature and prevents the boiler from further increasing the supply temperature.

When these two emitters are replaced by already existing same size LT emitters (e.g. Radson Ulow-E2 or Stelrad Compact Vento), the temperature regime will be further reduced to 59/44°C (system design temperature =51.5°C).

Radson Ulow-E2 Type-22

Stelrad Compact Vento

This option of further increasing the emitter capacity by replacing one or two of the existing emitters and consequently lowering the system temperatures, is not yet fully recognised in the replacement market. This is probably also caused by the fact that not much data is available as regards the achievable energy savings versus the related costs. Calculations show that this option in many cases is a viable efficiency measure, and sometimes even more economical than improving building shell insulation and glazing. The two new LT-emitters in the example above will cost around €2000, - and reduce the system design temperature with 17°C (from 68.5 to 51.5°C). The seasonal efficiency of a 10-kW inverter-controlled heat pump installed in this house will increase from 125% when combined with the original emitters, to 170% with the selected LT-emitters. The annual savings on energy costs will be around € 800, - resulting in a payback period of around 2.5 years.

Evidently, the existing emitter capacity can also be increased by installing bigger or more radiators. Replacing the two type-21 steel panel emitters by two almost 90-kilograms weighing same size type-33 steel panel radiators, will also result in lower system design temperatures. But generally, people prefer replacement by a similar sized or possibly smaller and lighter emitter. This is where the LT-emitter comes in.

LT-emitters are emitters that emit considerably more heat at low temperatures compared to their same-sized convectional counterparts.

LT-emitters

Several companies already manufacture and sell their version of the so called ‘LT-emitter’. These products re-use the already manufactured heat-exchangers/emitters and add multiple axial low-noise fans to boost the heat output at low temperatures.

Tables 1-2 give a concise representation of the heat output at a system temperature of 40°C for some 2-meter-long conventional steel panel emitters and their similar sized LT-counterparts. The data are based on EN 442 test results and related characteristic equations, provided by the manufacturers. The tests were done with the fans switched on, producing a sound power level LwA of maximum 38 dB(A). Table 1 lists the data for emitters with a maximum distance from front to wall between 10 and 15 cm, and Table 2 with this maximum distance being between 15 –20 cm. The bold text and figures in the tables represent the LT-emitters. They illustrate that the similar sized LT-counterparts of conventional emitters, can deliver 40 to almost 70% more heat at low temperature.

Table 1. Heat output at system temperature of 40°C (= mean emitter temperature) and ambient temperature of 20°C. Emitter dimensions: l = 200 cm, h = 50 cm, front-to-wall distance between 10–15 cm.

Table 2. Heat output at system temperature of 40°C (= mean emitter temperature) and ambient temperature of 20°C. Emitter dimensions: l = 200 cm, h = 50 cm, front-to-wall distance between 15–20 cm.

However, Tables 1-2 also reveal that the EN 442-tests result in higher flow rates for LT-emitters, up to 484 l/h for a single emitter. Real installations in existing dwellings however generally cannot achieve such flowrates because the pipework used to connect the emitters to the hydronic system is the limiting factor. A 15 mm diameter pipe can handle a flow of around 200 l/h without noticeable flow-generated noise. At higher flowrates, flow resistance and noise become limiting factors. Furthermore, if an existing emitter has a flowrate of for instance 130 l/h, this flowrate does not automatically change when this emitter is exchanged by an LT-emitter or a bigger emitter. These differences in test flowrates and real-life flowrate will also result in differences between measured and real-life heat-output. These effects will differ per type of heat emitters.

It is therefore recommended to adjust the related standards (EN 442, EN 16430) and apply test flowrates that are typical for pipe-dimensions predominantly used in existing installations and/or for the inlet opening and flow path area of the emitter under test.

Saving potential

The saving potential of this additional option of replacing existing emitters by similar sized LT-emitters on top of economically feasible building shell renovation is huge. According to the impact assessment report accompanying the Energy Performance of Buildings Directive, around 55% of all existing dwellings hold Energy Label class E, F or G, having an average primary energy consumption for space heating of around 162 kWh/m² per year[2]. For an average sized dwelling with exactly this primary energy consumption for space heating, calculations were made to examine how big the energy saving can be when system temperatures are further reduced. These energy saving calculations were performed with a purpose-built calculation model. See Table 3.

JAGA Strada Hybrid

Table 3. Calculation of achievable energy savings of a further reduction of system temperatures for an EU-average dwelling with a heated surface of 94 m² and a design heat load after already implemented building shell improvements 9 024 · 75% = 6 768 watts.

The calculated savings of around 40% with existing average-performing LT-emitters and average flowrates, are already respectable. These figures include the electricity consumption of the auxiliary fans mounted on the LT-emitters (for this living room around 90 kWh per year).

With completely new LT-emitters, specifically designed to maximize the heat transfer at low temperatures and with optimized flowrates and water content for the emitter system, it is expected that in combination with heat pumps, savings can be achieved of over 80%, compared to this baseline primary energy consumption of 162 kWh/m² per year.

All in all, the energy saving potential of LT-emitters in the existing building stock is enormous and all actors (manufacturers, system designers, installers, legislative authorities, EPC-assessors, consumers etc.) are invited to move into action and help realising this potential.

Further explanation of the purpose-built calculation model

Standards

EN 14825:2022 Air conditioners, liquid chilling packages and heat pumps, with electrically driven compressors, for space heating and cooling, commercial and process cooling - Testing and rating at part load conditions and calculation of seasonal performance.

EN 15316-4-2: 2017 Energy performance of buildings - Method for calculation of system energy requirements and system efficiencies - Part 4-2: Space heating generation systems, heat pump systems, Module M3-8-2, M8-8-2. This standard is currently under revision, it is expected that a new prEN version will be published in summer 2025, a related updated excel file will become available at the EPB Center website: https://epb.center/support/overview-epb-standards/m3/.

EN 442-2:2014 Radiators and convectors - Part 2: Test methods and rating.

EN 16430-2:2014 Fan assisted radiators, convectors and trench convectors - Part 2: Test method and rating for thermal output.

References