Clear definitions of the terms you will meet when specifying heat pumps, solar collectors and storage tanks. Each term has its own page you can link to.
The absorber is the metal plate inside a solar collector that captures sunlight and passes the heat to the fluid in the tubes bonded to it. A selective coating, usually applied by vacuum sputtering, absorbs about 95 % of sunlight while emitting only around 5 % as thermal radiation, so the absorber stays efficient at high temperatures. Absorbers are made of copper or aluminium sheet with copper tubes in harp or meander form.
An air-to-water heat pump extracts heat from outdoor air and transfers it to water circulating in radiators, underfloor heating or a hot water tank. It is the most common heat pump type for homes and small commercial buildings because it needs no boreholes or ground loops. Modern inverter units operate down to about −20 °C to −25 °C, with reduced output at very low temperatures.
Aperture area is the transparent opening of a solar collector through which sunlight enters, as distinct from gross area (the outer dimensions) and absorber area (the plate itself). Efficiency coefficients and outputs must always state which reference area they use; Solar Keymark datasheets based on ISO 9806 now use gross area. Storage sizing rules such as 50–70 L per m² normally refer to aperture area.
A buffer tank is an insulated vessel that stores heating water (not drinking water) between a heat source and the heating circuits. It gives a heat pump the volume it needs for defrost and minimum run times, provides hydraulic separation from zoned circuits and can collect heat from solar or other sources. Typical sizing is 15–25 L per kW of heat pump output for decoupling, or 50 L per kW and more for storage.
A combi tank, or tank-in-tank cylinder, is a storage unit in which a smaller drinking-water tank sits inside a larger buffer volume of heating water. The buffer is charged by a heat pump, boiler or solar coil and heats the inner tank through its wall, combining DHW and buffer storage in one footprint. It suits compact single-family systems, especially with solar support.
COP is the ratio of heat output to electrical input of a heat pump at one defined operating condition, measured in Europe under EN 14511. It is quoted with the test point, for example COP 4.8 at A7/W35 (7 °C air, 35 °C water outlet). A higher COP means less electricity per unit of heat, but figures are only comparable at the same test point.
Domestic hot water (DHW) is the potable hot water used for washing, showering and kitchens, as opposed to heating water that circulates in radiators or underfloor systems. Typical demand is 30–50 L per person per day at 60 °C. DHW is stored in enamel or stainless-steel cylinders or produced instantaneously by hygienic tanks and fresh-water stations.
A drainback system is a pumped solar thermal system in which the collector fluid drains by gravity into a reservoir whenever the pump stops, leaving the collectors and exposed pipes empty. This protects against freezing and stagnation without antifreeze and simply halts collection when the tank is full, which is valuable in hot climates. It requires continuously sloping pipework and a pump able to refill the collectors on start-up.
Enamel coating is a glass-like layer fused onto the inside of a carbon-steel hot water tank at around 850 °C to protect the steel from corrosion by drinking water. Standards such as DIN 4753-3 define its quality, and because microscopic pores are unavoidable, enamel tanks are always fitted with a magnesium or impressed-current anode. Enamel is the cost-effective standard for DHW cylinders and inner tanks of combi units.
The ErP energy label is the EU efficiency label for energy-related products such as heat pumps, boilers and hot water tanks, introduced under the Ecodesign and Energy Labelling framework. For heat pumps it shows the seasonal space heating efficiency class from A+++ to D at 35 °C and 55 °C, sound power levels and rated output for three climate zones. The European Commission has proposed rescaling the label to a simple A–G scale.
An expansion vessel is a closed tank with a gas-filled membrane that absorbs the volume increase of water or glycol as it heats up, keeping the system pressure within safe limits. Solar expansion vessels are sized to hold the entire collector fluid volume as well, so that steam formation during stagnation cannot lift the safety valve. Heating and DHW systems each need their own vessel matched to volume, temperature and pressure.
The F-gas Regulation is EU law — currently Regulation (EU) 2024/573, in force since 11 March 2024 — that phases down fluorinated greenhouse gases through a shrinking quota and bans them in specific products. For heat pumps it prohibits refrigerants with a GWP of 150 or more in monoblocks up to 50 kW and small split air-to-water units from 2027, and all F-gases in monoblocks up to 12 kW from 2032. HFC consumption in the EU is to reach zero by 2050.
A flat-plate solar collector is a glazed, insulated panel containing a selectively coated absorber with fluid channels that converts sunlight into heat for hot water, heating support or pool heating. Good models reach peak efficiencies of 75–82 % and typically operate at 40–80 °C. It is the most widely installed solar thermal collector type and the basis of most thermosiphon and pumped solar water heaters.
Glycol, usually non-toxic propylene glycol mixed with water at 30–45 %, is the antifreeze heat-transfer fluid used in closed solar collector loops and outdoor heat pump pipework in climates with frost. Solar-grade mixtures include inhibitors and must withstand stagnation temperatures around 200 °C without degrading. The fluid should be checked every few years for pH and freezing point and replaced when it has aged.
GWP is a measure of how much heat a gas traps in the atmosphere over a set period (usually 100 years) compared with the same mass of CO2, which has a GWP of 1. Refrigerants range from about 3 for propane (R290) to 675 for R32 and around 2,088 for R410A. EU F-gas rules use GWP thresholds of 150 and 750 to decide which refrigerants may be used in new equipment.
A heat exchanger coil is a spiral of smooth or corrugated tube inside a storage tank through which one fluid flows to transfer heat to the water surrounding it, keeping the two circuits separate. Solar tanks use a low coil for the collector loop, and heat pump cylinders need a large coil — around 0.25–0.3 m² per kW — because their low flow temperature reduces the temperature difference available for heat transfer.
A heat pump is an electrically driven machine that moves heat from a cooler source (air, ground or water) to a warmer place, such as a building's heating system, using a refrigerant cycle. Because it transfers existing heat rather than generating it by combustion, it typically delivers 3–5 kWh of heat per kWh of electricity. Heat pumps are classified by source and sink, for example air-to-water or brine-to-water.
Hydraulic separation is the decoupling of a heat generator's circuit from the distribution circuits so that each pump sees only its own resistance and flow rates can differ. It is achieved with a buffer tank or a low-loss header and is important for heat pumps, which need a guaranteed minimum flow and volume even when thermostatic valves or zone controls close off parts of the system.
A hygienic tank is a storage tank filled with heating water in which drinking water is heated instantaneously as it flows through a long corrugated stainless-steel coil, so almost no potable water is stored. A fresh-water station achieves the same with an external plate heat exchanger and pump. Both minimise Legionella risk and allow lower storage temperatures, which suits heat pumps and solar systems.
An inverter compressor is a variable-speed compressor whose motor is driven by a frequency inverter, allowing a heat pump to modulate output — typically from about 20 % to 100 % — instead of switching on and off. Matching output to demand improves seasonal efficiency, reduces cycling wear, lowers noise at part load and avoids high starting currents. Nearly all modern air-to-water and pool heat pumps use inverter compressors.
Legionella protection is the set of design and operating measures that stop Legionella bacteria multiplying in stored or circulating hot water, where they thrive between 25 °C and 45 °C. The main measures are keeping stored drinking water at 60 °C and circulation returns at 55 °C or above, running a periodic anti-Legionella heat-up cycle (many heat pumps do this weekly), or avoiding storage altogether with hygienic tanks and fresh-water stations. The EU Drinking Water Directive 2020/2184 requires risk-based Legionella monitoring in buildings.
A magnesium anode is a sacrificial rod fitted inside an enamel-lined hot water tank that corrodes in place of the steel at any pores or welds in the enamel, extending the tank's life. It should be inspected every one to two years and replaced when it is largely consumed. Impressed-current (titanium) anodes are a maintenance-free alternative powered by a small electronic unit.
A monoblock heat pump is an air-to-water heat pump whose entire refrigerant circuit is sealed inside a single outdoor unit, so only water pipes enter the building. It needs no refrigerant work on site and is the standard format for propane (R290) units, since no flammable refrigerant is brought indoors. Split heat pumps, by contrast, place the condenser indoors and connect the two parts with refrigerant lines.
A pool heat pump is an air-to-water heat pump designed to heat swimming pool water, using a corrosion-resistant titanium heat exchanger and operating at low temperature lifts of 26–30 °C. Rated at 26 °C air / 26 °C water, good units reach COPs of 5–7, and full-inverter models are quieter and 20–40 % more efficient over a season than on/off types. Sizing rules of thumb start at about 0.2 kW per m³ of pool volume for a covered outdoor pool in a temperate climate.
A PV-T hybrid panel is a photovoltaic module with a fluid-cooled absorber on its back that produces electricity and low-temperature heat from the same roof area. Cooling the cells recovers some electrical output, while the captured heat serves hot water preheating, pools or a heat pump source. Unglazed PV-T works best near ambient temperature; glazed versions deliver hotter water at the cost of electrical yield.
R290 is refrigerant-grade propane, a natural hydrocarbon refrigerant with a GWP of about 3 and no ozone depletion potential. Its high critical temperature (96.7 °C) lets heat pumps deliver flow temperatures of 70–75 °C, ideal for radiator retrofits and hot water. It is classed A3 (highly flammable), so units use small charges, spark-free components and are usually built as outdoor monoblocks.
R32 (difluoromethane) is a single-component HFC refrigerant with a GWP of 675, widely used in residential heat pumps and air conditioners since the 2010s as a lower-impact successor to R410A. It is classed A2L (mildly flammable) and works well up to flow temperatures of about 60–65 °C. Under EU F-gas Regulation 2024/573 it may no longer be used in new monoblock heat pumps up to 50 kW or small split air-to-water units placed on the market from 1 January 2027.
A refrigerant is the working fluid in a heat pump or air conditioner that absorbs heat when it evaporates at low pressure and releases it when it condenses at high pressure. Its properties set the achievable flow temperature, efficiency, safety class and climate impact. Common heat pump refrigerants include R32 (an HFC) and R290 (propane, a natural refrigerant).
SCOP is the average efficiency of a heat pump over a whole heating season, calculated under EN 14825 from part-load tests at several outdoor temperatures and including defrost and standby consumption. It is stated for a climate zone (average, warmer or colder) and for a flow temperature of 35 °C or 55 °C. SCOP is the basis of the EU energy label and a better predictor of running costs than a single COP.
Solar fraction is the share of a building's hot water or heating demand that is covered by solar energy over a year, expressed as a percentage. Well-sized solar water heaters reach 50–70 % in Central Europe and 70–90 % in Southern Europe, the Middle East and Africa. Higher fractions require more collector area and storage, so systems are usually optimised for cost per kWh rather than the maximum fraction.
Solar Keymark is the voluntary European certification mark for solar thermal collectors and factory-made systems, owned by CEN and based on testing to ISO 9806 / EN 12975 and EN 12976 at accredited laboratories plus annual factory inspections. Its public database lists every valid certificate with efficiency coefficients and annual output figures. Most European subsidy schemes and many international tenders require it, and the scheme now covers PV-T collectors.
Stratification is the natural layering of water in a storage tank, with the hottest water at the top and the coldest at the bottom, caused by differences in density. Good stratification means hot water is available at the top while the heat source or solar collector still sees cold return water at the bottom, which raises system efficiency. Diffusers, tall tank shapes and low flow velocities help preserve it.
A thermosiphon system is a solar water heater in which the storage tank is mounted above the collector so that heated water rises naturally into the tank and cooler water sinks back, without any pump or controller. Typical residential units combine 1.5–4 m² of flat-plate collector with a 150–300 L tank, in direct or indirect (heat-exchanger jacket) versions. They dominate residential solar water heating in the Mediterranean, Middle East and Africa.
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