Clear definitions of solar thermal, PVT and heat pump terminology — written by the engineers who build the technology.
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ηs, the seasonal space heating energy efficiency, is the EU Ecodesign metric expressing a heater's useful output as a share of the primary energy it consumes. For heat pumps it equals the SCOP divided by 2.5 (the EU electricity conversion factor) minus small corrections. Heat pumps must reach at least 110% — low-temperature units 125% — while condensing gas boilers manage about 92–94%.
Read more →The absorber is the heat-generating core of a solar thermal collector: a copper or aluminium sheet, typically 0.2–0.5 mm thick, whose selective coating converts up to 95% of incident sunlight into heat and passes it to the fluid in tubes welded to its back. Absorber and coating quality largely determine a collector's certified optical efficiency.
Read more →Air separators and air vents remove free and dissolved air from closed heating and solar circuits. Fill water carries roughly 20 millilitres of dissolved air per litre, which leaves solution as the fluid heats and collects at high points — causing corrosion, noise, pump cavitation and stalled circulation. Vents release gathered air at high points; microbubble separators strip it continuously from the flow.
Read more →An air-to-water heat pump extracts heat from outdoor air and transfers it to a water-based heating system — radiators, underfloor circuits and a domestic hot water cylinder. One kWh of electricity typically delivers 3–5 kWh of heat (COP 3.0–5.0). It is Europe's dominant heat pump type for hydronic heating, part of a market that reached 2.88 million units sold in 2025.
Read more →Aperture area is the opening through which solar radiation enters a collector — for a flat plate, the visible glass area inside the frame, typically 90–93% of gross area. Since the 2013 revision of ISO 9806, collector performance and Solar Keymark datasheets are referenced to gross area instead, so always check which area a quoted efficiency figure uses before comparing collectors.
Read more →A bivalent heating system pairs a heat pump with a second heat generator — an existing boiler, an electric element or solar thermal — that assists or takes over below the bivalent point, typically set between −7 and −2 °C in Central Europe. Because deep cold is rare, a heat pump sized for only 70–80% of the design load still delivers 90–98% of the annual heat.
Read more →A brine/water heat pump extracts heat from a closed antifreeze loop — the brine — connected to ground collectors, boreholes or solar absorbers, and delivers it to a hydronic heating system. Its standard rating point is B0/W35: brine entering at 0 °C, heating water leaving at 35 °C, where quality units achieve COP values above 4.
Read more →A buffer tank is an insulated water storage vessel installed between a heat generator and the heating circuit. It stores technical (non-potable) heating water, decouples heat production from demand, prevents heat pump short-cycling, and provides the water volume needed for defrost cycles. Typical sizing for heat pumps is 20–50 litres per kW of heating capacity.
Read more →A cascade heat pump system connects multiple heat pumps — commonly two to eight units — piped in parallel and staged by a master controller so capacity follows the load. A cascade of six 30 kW units covers a 180 kW peak yet can turn down below 10 kW with one unit modulating, giving large buildings redundancy and part-load efficiency a single big machine cannot match.
Read more →A check valve (non-return valve) lets fluid flow in one direction only and closes automatically against reverse flow. In solar circuits its key job is stopping night-time reverse thermosiphoning, in which warm tank water circulates backwards to the cold collector and radiates stored heat to the sky. Solar gravity brakes open at roughly 20 mbar of forward pressure.
Read more →A coil heat exchanger is a spiral tube — smooth steel, enamelled or corrugated stainless — immersed inside a storage tank to transfer heat between two water circuits without mixing them. Solar practice sizes the coil at roughly 0.2–0.3 m² of exchanger surface per m² of collector area; heat pump cylinders need around three times the coil surface of an equivalent boiler cylinder.
Read more →The collector efficiency curve states how a solar collector's efficiency falls as it runs hotter than its surroundings: η = η0 − a1·(Tm−Ta)/G − a2·(Tm−Ta)²/G. η0 is the optical efficiency (0.75–0.82 for good flat plates), a1 and a2 are heat loss coefficients, and G is the irradiance, referenced to 1,000 W/m². All three values come from ISO 9806 testing and appear on every Solar Keymark datasheet.
Read more →Compressor modulation is an inverter heat pump's ability to vary output continuously between a minimum and maximum — typically 30–100% of rated capacity, a turndown ratio of about 3:1. Instead of stopping and starting, the compressor slows to match the heat load, which lengthens run times, prevents short cycling and lifts seasonal efficiency.
Read more →COP (Coefficient of Performance) is the ratio of useful heat a heat pump delivers to the electricity it consumes at one fixed test condition. A COP of 4.0 means 1 kWh of electricity produces 4 kWh of heat. Modern air-to-water heat pumps typically achieve a COP between 3.0 and 5.0.
Read more →A dead leg is a length of pipework where water stagnates because nothing regularly flows through it — a capped-off branch, a run to a rarely used outlet, or redundant pipe left after alterations. Stagnant water sitting in the 20–45 °C band is prime territory for legionella growth, which is why codes such as German DVGW W551 require circulation once a run holds more than 3 litres.
Read more →A defrost cycle is the routine by which an air-source heat pump melts frost from its outdoor coil, usually by reversing the refrigerant circuit for 2–10 minutes and drawing heat from the heating water. Frost forms mainly between +5 and -7 °C in humid weather. Defrosting typically consumes 5–10% of winter heat output, and manufacturers require a minimum system volume — often 10–20 litres per kW — to supply it.
Read more →Delta-T (ΔT) is the temperature difference between two points in a hydronic system, most often between flow and return. Together with flow rate it defines transferred power: kW ≈ flow (m³/h) × ΔT (K) × 1.16. Heat pump circuits are designed around a ΔT of 5–8 K; conventional boiler circuits around 15–20 K; solar collector loops typically 10–15 K.
Read more →A DHW heat pump is a compact air-source heat pump factory-integrated with its own storage tank — typically 200–300 litres — dedicated solely to domestic hot water. Drawing heat from ambient or exhaust air, it delivers around 2.5–3.5 kWh of hot water heat per kWh of electricity under EN 16147 testing, roughly a third of the running cost of an electric immersion element.
Read more →Domestic hot water (DHW) is the potable heated water drawn at taps and showers — legally and technically separate from the closed heating-circuit water. A European household member uses roughly 35–45 litres at 45 °C per day, about 1.5–2 kWh, and water heating represents around 15% of household energy use (Eurostat). DHW is stored at 60 °C or protected by scheduled anti-legionella cycles.
Read more →A drain-back system is a solar thermal design in which the heat-transfer fluid — usually plain water — drains by gravity into a reservoir whenever the circulation pump stops, leaving collectors and exposed pipes empty. Freeze and stagnation protection come from geometry rather than chemistry, so no glycol is needed. Every pipe must slope toward the reservoir by at least 2% (20 mm per metre).
Read more →Enamel coating is a glass layer fused onto the steel interior of a hot water tank at roughly 850 °C, forming a chemically neutral barrier between drinking water and the steel wall. Applied to DIN 4753-3 and paired with a magnesium anode that guards its microscopic pores, enamelling is the most widely used corrosion protection for potable water cylinders in Europe.
Read more →The ErP energy label is the EU efficiency label for heating products, created by Regulations 811/2013 and 812/2013 under the Ecodesign framework. Space heaters are rated on a scale reaching A+++, water heaters up to A+, and hot water storage tanks from A+ to F based on standing loss in watts. Every heater and cylinder sold in the EU must carry one.
Read more →The EU F-Gas Regulation (2024/573), in force since 11 March 2024, phases down fluorinated refrigerants by cutting the HFC quota to 42.9 Mt CO2e for 2025–2026 — roughly 48% below the 2023 level — on a path to a full HFC phase-out by 2050. The key heat pump equipment ban starts 1 January 2027 for monoblocks up to 50 kW and small air-to-water splits using refrigerants with GWP ≥150.
Read more →An evacuated tube collector is a solar thermal collector built from parallel glass tubes, each enclosing its absorber in a vacuum that practically eliminates convective and conductive heat loss. With heat loss coefficients around a1 ≈ 1–2 W/m²K versus 3–4 for flat plates, it holds its efficiency at large temperature differences — at the price of higher cost and stagnation temperatures up to 300 °C.
Read more →An EVI (Enhanced Vapour Injection) compressor adds a second refrigerant injection port fed by an economizer heat exchanger, cooling the compression process mid-stroke. This lets a heat pump hold heating capacity and safe discharge temperatures under extreme conditions — sustaining operation down to -25 °C outdoor air and flow temperatures of 60 °C or more without handing the load to an electric backup heater.
Read more →An expansion vessel is a closed steel tank whose gas-charged diaphragm absorbs the volume growth of heating water as it warms — about 4% between 10 °C and 100 °C — keeping system pressure stable. In solar circuits the vessel must additionally swallow the entire collector fluid content displaced by steam during stagnation, which makes solar vessels two to three times larger than heating vessels of similar system volume.
Read more →A flat-plate collector is the standard European solar thermal collector: a glazed, insulated flat casing containing a selectively coated full-format absorber. Quality units certify optical efficiencies of 0.75–0.82 with heat loss coefficients of 3–4 W/m²K, deliver water at 60–90 °C, and routinely operate for 25 years and more.
Read more →Flow temperature is the temperature of the heating water leaving the heat generator toward the emitters; return temperature is what comes back. Underfloor circuits run at 30–40 °C, modern radiator systems at 45–55 °C, legacy radiators at 70 °C or more. Every 1 °C of flow temperature reduction improves heat pump efficiency by roughly 2–3%, making it the single strongest efficiency lever.
Read more →A forced-circulation solar system uses an electric pump, commanded by a differential temperature controller, to move heat-transfer fluid between the collectors and a storage tank that can stand anywhere in the building. The pump typically starts when the collector runs 5–8 K hotter than the tank. It is the standard solar water heating configuration in central and northern Europe.
Read more →Freeze protection is the set of measures that stops the fluid in a solar thermal circuit from freezing and bursting collectors or pipework. The three established methods are a propylene glycol antifreeze mixture — 40 % by volume protects to roughly −21 °C — a drain-back design that empties the collectors when the pump stops, and a controller frost cycle that circulates warm store water on cold nights.
Read more →GWP (Global Warming Potential) measures how much warming one kilogram of a gas causes over 100 years compared with one kilogram of CO2, which scores 1. Refrigerant examples: R410A about 2,088, R32 about 675, R290 propane roughly 0.02. The EU F-Gas Regulation 2024/573 uses GWP thresholds of 150, 750 and 2,500 to ban refrigerants from new equipment and from servicing.
Read more →A heat dump is a heat-rejection circuit — typically a radiator, fan coil, pool loop or buried pipe coil — that a solar thermal controller uses to discharge surplus collector heat once the storage tank is fully charged. By keeping the circuit below stagnation conditions, where flat-plate collectors can reach 160–200 °C, it protects the glycol, seals and expansion vessel from repeated thermal stress.
Read more →A heat load calculation determines the maximum heating power, in kW, a building needs at its design outdoor temperature — computed room by room to EN 12831 from transmission and ventilation losses. Typical results range from 10 W/m² for a passive house through 30–50 W/m² for recent builds to 120 W/m² and more for uninsulated older stock. It is the basis for sizing every heat generator.
Read more →Heat Pump Keymark is the European CEN Keymark certification for heat pumps, proving that performance was tested by an independent accredited laboratory to EN 14511, EN 14825, EN 16147 and EN 15879. Launched in 2015, it verifies the COP, SCOP and ηs figures on datasheets and feeds the eligibility lists behind national subsidy schemes.
Read more →Hydraulic separation decouples the heat generator's circuit from the emitter circuits so that each runs its own flow rate and pump without interference. It is achieved with a buffer tank, a low-loss header or a plate heat exchanger. Heat pumps, which need constant flow sized for a 5–8 K temperature spread, rely on it whenever zone valves or multiple circuits vary the secondary flow.
Read more →A hygienic tank is a combination store that heats drinking water instantaneously as it flows through a corrugated stainless-steel coil immersed in a large volume of non-potable heating water. Because no potable water is stored, the 20–45 °C band in which legionella bacteria multiply contains no standing risk volume. One vessel serves heating buffer, solar storage and hot water duties at once.
Read more →An immersion heater is an electric resistance element screwed directly into a hot water cylinder, typically rated 2–3 kW in domestic sizes. Convection means it heats only the water above its own level, so its mounting position decides how much of the tank it serves. At 3 kW, reheating 200 litres by 50 °C takes about 3.9 hours and 11.6 kWh of electricity.
Read more →An inverter compressor varies its rotational speed through a variable-frequency drive, so a heat pump modulates output continuously instead of switching on and off at full power. Typical modulation range is 25–100% of rated capacity. Modulation cuts start-stop cycling, softens starting currents, and is the main reason modern air-to-water heat pumps reach SCOP values of 3.5–5.0.
Read more →ISO 9806 is the single global test standard for solar thermal collectors, defining thermal performance measurement plus durability and safety tests for glazed, unglazed, air and PVT collectors. One accredited ISO 9806 test campaign generates the data behind both Solar Keymark certification in Europe and SRCC certification in North America — one test, many markets. The current edition is ISO 9806:2017.
Read more →The JAZ (Jahresarbeitszahl), internationally the seasonal performance factor (SPF), is the ratio of heat a heat pump delivers to the electricity it consumes over a full year of real operation. A JAZ of 3.0 means 1 kWh of electricity produced 3 kWh of heat. Air-to-water systems typically achieve 3.0–4.0; Germany's BEG subsidy requires a calculated JAZ of at least 3.0.
Read more →Legionella are waterborne bacteria that multiply in stored water between 20 and 45 °C and are killed within minutes above 60 °C. Hot water systems control them by storing at 60 °C, distributing at 55 °C or higher, or by eliminating stored potable volume entirely. An anti-legionella cycle periodically heats the whole tank to 60–70 °C to disinfect it.
Read more →A low loss header is a short, oversized vertical vessel with four connections that hydraulically separates the heat generator circuit from the heating circuits while storing almost no water — typically under 20 litres, against 100 litres or more for a buffer tank. Because the pressure drop across it is near zero, the generator pump and each circuit pump operate independently without interfering with one another.
Read more →A magnesium anode is a sacrificial rod fitted inside enamelled hot water tanks that corrodes deliberately so the steel tank does not. Together with the enamel coating it forms the corrosion protection system required by DIN 4753; the anode is a wear part that should be inspected roughly every two years and replaced when consumed.
Read more →MCS (Microgeneration Certification Scheme) is the UK's quality-assurance scheme for small-scale renewables, certifying both products and installers of heat pumps, solar thermal, and solar PV up to 45 kW thermal or 50 kW electrical. It is the gateway to UK funding: the Boiler Upgrade Scheme grant — £7,500 toward an air or ground source heat pump — is paid only for MCS-certified installations.
Read more →A monoblock heat pump contains the complete refrigerant circuit in a single outdoor unit; only insulated water pipes cross the wall into the building. No refrigerant work is done on site, so the sealed circuit needs no F-gas certified installer. Under the EU F-Gas Regulation, new monoblock units up to 12 kW placed on the EU market from 1 January 2027 must use refrigerants with a GWP below 150 — which is why R290 propane models, such as Solimpeks' Varm Boost Series, are becoming the standard.
Read more →Peak draw is the largest simultaneous hot water demand a system must serve — the busiest half hour of showers, baths and taps running together. It, not daily consumption, dictates cylinder size and coil power. As a working rule, allow roughly 50 litres of stored hot water per simultaneous shower; a home whose three bathrooms are used at once must store for that overlap, not for the daily average.
Read more →Propylene glycol is the non-toxic antifreeze mixed with water to protect closed solar thermal circuits from freezing. A 40% glycol mixture protects to about -21 °C and a 50% mixture to about -33 °C. Solar-grade fluids add corrosion inhibitors and must be checked every 2–3 years, because repeated stagnation above roughly 170 °C degrades glycol into corrosive acids.
Read more →A PVT (photovoltaic-thermal) hybrid collector generates electricity and hot water from the same panel area: photovoltaic cells produce power while a heat exchanger behind them captures the waste heat. Because the cells are actively cooled, a PVT panel produces up to 15% more electricity than a comparable PV panel and delivers a combined efficiency of up to 80%.
Read more →R290 is refrigerant-grade propane, a natural refrigerant with a GWP of about 0.02 (IPCC AR6) that sits outside the EU's HFC phase-down. It delivers 70 °C flow temperatures and is the default choice for new monoblock heat pumps, because EU rules ban GWP ≥150 refrigerants in new units up to 12 kW from 1 January 2027. As an A3 flammable gas it requires charge limits and trained installers.
Read more →R32 (difluoromethane) is a mildly flammable A2L refrigerant with a GWP of 675 — about one third of the 2,088 of R410A, which it replaced across air-conditioners and heat pumps during the 2010s and 2020s. It remains widely used worldwide and permitted in many equipment categories; under the latest EU F-Gas Regulation rules, new monoblocks and air-to-water splits up to 12 kW placed on the EU market from 1 January 2027 must use refrigerants below GWP 150. Installed R32 units keep operating and can be serviced for life.
Read more →Recovery rate is the speed at which a hot water cylinder reheats after a draw-off, set by the heat input in kW rather than by tank volume. The physics is fixed: water needs 1.16 Wh per litre per degree, so a 15 kW boiler coil reheats 200 litres by 45 °C in about 42 minutes, while a 3 kW immersion element needs around 3.5 hours.
Read more →A refrigerant is the working fluid of a heat pump: it absorbs heat by evaporating at low pressure outdoors and releases it by condensing at high pressure indoors. Refrigerants are identified by R-numbers (R290, R32, R410A), a safety class from A1 to A3, and a GWP value. The EU is phasing down high-GWP HFCs — the 2025–2026 quota of 42.9 Mt CO2e sits about 48% below the 2023 level.
Read more →SCOP (Seasonal Coefficient of Performance) is a heat pump's average efficiency over an entire heating season, measured under EN 14825. Unlike COP, which describes one fixed condition, SCOP weights performance across mild and cold weather. A good SCOP for an air-to-water heat pump is 3.5 or higher; European subsidy schemes typically require at least 3.0.
Read more →A secondary return is a pipe that carries domestic hot water from the far end of the distribution pipework back to the cylinder, with a small circulation pump keeping the loop warm. Hot water then arrives at distant taps in a few seconds instead of after tens of litres of wasted draw-off. The comfort costs energy: the loop adds continuous pipe losses and disturbs tank stratification, so control strategy matters.
Read more →A selective coating is the optical surface applied to a solar absorber that absorbs 94–95% of incoming sunlight while re-emitting only 4–5% of it as thermal infrared radiation. By suppressing radiant losses it keeps the collector efficient as it runs hotter, and it is the single biggest difference between a modern collector and a black-painted one.
Read more →SG-Ready (Smart Grid Ready) is a standardised heat pump control interface defined by the German Heat Pump Association (BWP) in 2012. Two switching contacts encode four operating states — from a temporary block to forced operation — letting utilities and home energy managers shift heat pump operation toward cheap or surplus electricity. Germany's BEG subsidy requires grid-responsive capability of this kind.
Read more →Short cycling is a heat pump compressor starting and stopping in rapid succession — run times under about 10 minutes, or more than four to six compressor starts per hour — instead of long, steady runs. It is caused mainly by oversizing and too little system water volume, and it raises electricity use while accelerating compressor wear.
Read more →A solar controller, or differential thermostat, switches the solar circuit pump by comparing collector and tank temperatures: it starts the pump when the collector runs typically 6–8 K hotter than the tank bottom and stops it when the difference falls below 3–4 K. Modern controllers add pump speed control, tank temperature limits, stagnation routines and heat metering.
Read more →Solar district heating (SDH) feeds heat from large ground-mounted collector fields into district heating networks. Plants range from about 1 MWth to the 110 MWth Silkeborg field in Denmark, whose 156,694 m² of flat-plate collectors supply around 20% of the annual heat demand of 21,000 connected customers. Combined with seasonal pit storage, solar fractions of 40–50% are achievable.
Read more →Solar fraction is the percentage of a heat demand met by solar energy: f = solar heat delivered ÷ total heat demand. Well-sized domestic hot water systems reach 50–65% in central Europe and 70–90% in Mediterranean climates, while solar combi systems typically cover 20–30% of combined space heating and hot water. It is the primary design target when sizing a solar thermal system.
Read more →Solar irradiation is the solar energy arriving on one square metre over a given period, measured in kWh/m². Annual global horizontal irradiation ranges from roughly 950–1,200 kWh/m² in Germany and the UK to about 1,500 kWh/m² in Türkiye and 1,700–1,900 kWh/m² in southern Spain. It is the resource figure every solar yield and payback calculation starts from.
Read more →Solar Keymark is the leading European certification for solar thermal products, issued under the CEN Keymark scheme. It proves a collector or system was tested to EN 12975/EN ISO 9806 by an accredited laboratory and that production is inspected annually. Most European subsidy schemes — including Germany's BEG — accept only Solar Keymark-certified collectors.
Read more →Solar process heat — SHIP, Solar Heat for Industrial Processes — is the use of solar thermal collectors to supply industrial heat for washing, drying, pasteurisation and boiler feed-water preheating. At least 1,315 SHIP plants totalling roughly 1,071 MWth operated worldwide at the end of 2024, and about 30% of industrial heat demand sits below 150 °C — the range standard collectors serve best.
Read more →A solar pump station is the pre-assembled hydraulic centre of a pressurised solar thermal system: circulation pump, 6 bar safety valve, pressure gauge, flow meter, check valves and fill-and-drain ports in one insulated housing. It circulates heat-transfer fluid between collectors and storage tank at 10–50 litres per square metre of collector per hour, depending on the flow concept.
Read more →A solar-assisted heat pump (SAHP) is any system in which solar energy improves a heat pump's performance — feeding its evaporator as the heat source (PVT or unglazed collectors), working alongside it as a parallel generator, or powering its compressor with PV electricity. Because compressor efficiency rises roughly 2–3 % for every kelvin of warmer source temperature, solar-boosted sources translate directly into higher seasonal performance.
Read more →A split heat pump divides the system into an outdoor unit holding the evaporator, fan and compressor, and an indoor hydrobox holding the condenser and hydraulics, connected by refrigerant lines. The water circuit stays entirely indoors, so it cannot freeze. Because refrigerant joints are made on site, an F-gas certified installer is required, and new air-to-water splits up to 12 kW must use sub-150-GWP refrigerants from 1 January 2027.
Read more →SRCC OG-100 and OG-300 are the North American certification programs for solar heating products, operated by ICC-SRCC. OG-100 certifies individual collectors from ISO 9806 test data; OG-300 certifies complete residential solar water heating systems and adds annual energy ratings for around 200 North American locations. US model building codes and incentive programs reference both.
Read more →Stagnation temperature is the maximum temperature a solar collector reaches when circulation stops while full sunshine continues — for a quality flat-plate collector typically 160–200 °C, for evacuated tubes up to 300 °C. Every component of the solar circuit must be specified to survive repeated stagnation events.
Read more →Standing loss is the rate at which a hot water storage tank loses heat while held at operating temperature, expressed in watts and measured with the store at 65 °C in a 20 °C room. Under EU Regulation 812/2013 it sets the tank's ErP energy label class: a 300-litre class B tank may lose no more than about 70 W — roughly 1.7 kWh per day.
Read more →Stratification is the natural layering of water in a storage tank by temperature: hot water, being less dense, sits at the top while cooler water stays at the bottom. A well-stratified tank delivers usable hot water from the top while feeding its coolest bottom water to the collector or heat pump, raising whole-system efficiency by 10–20% compared to a fully mixed tank.
Read more →A temperature & pressure relief valve (T&P valve) is the last-line safety device on a sealed hot water cylinder, opening automatically if the stored water gets dangerously hot or the vessel over-pressurises. It exists because sealed water can be heated far past 100 °C without boiling — at 3 bar gauge water boils at about 144 °C — and would flash to steam, expanding roughly 1,600-fold, if containment failed.
Read more →A thermosiphon system is a solar water heater that circulates water by natural convection alone: water heated in the collector rises into a tank mounted above it, while cooler tank water sinks back — no pump, no controller, no electricity. It is the simplest and most widespread solar water heating configuration in sunny climates.
Read more →A thermostatic mixing valve blends hot water from the storage tank with cold water to deliver a fixed, safe outlet temperature — typically set between 38 and 50 °C — regardless of how hot the tank is. It lets a system store water at 60 °C or more for legionella control while preventing scalds: at 60 °C, skin suffers serious burns within seconds.
Read more →A tundish is a funnel-shaped fitting with a visible air gap, installed in the discharge pipe from a hot water cylinder's relief valves — in UK practice within 600 mm of the temperature & pressure relief valve. It does two jobs: the air break stops discharged water being siphoned back into the system, and the open window makes any discharge instantly visible. A healthy cylinder shows a dry tundish.
Read more →A twin-coil cylinder is an indirect hot water tank with two separate internal heat exchangers, letting two heat sources — classically solar thermal below and a boiler or heat pump above — heat one store of water. The solar coil sits low to harvest the coldest water; a common sizing rule is at least 0.2 m² of solar coil surface per m² of collector area.
Read more →An unvented cylinder is a sealed hot water storage vessel fed directly from the cold mains, so it delivers hot water at mains pressure — commonly 3–3.5 bar after the pressure-reducing valve — to every tap in the house. Because there is no open vent for heated water to expand into, it relies on an engineered safety chain: expansion provision, relief valves and a visible tundish discharge.
Read more →A vented cylinder is an open hot water storage tank fed by gravity from a cold water header tank, with an open vent pipe instead of sealed safety valves. Delivery pressure comes purely from the height of the header tank — about 0.1 bar per metre of head — so flow is gentler than mains pressure, but the system physically cannot over-pressurise.
Read more →A volumiser is a small insulated vessel — typically 20–100 litres — piped in series into a heat pump's heating circuit to increase total system water volume. Unlike a buffer tank it has only two connections, provides no hydraulic separation and causes no mixing; it simply adds thermal mass so the compressor cycles less often and the defrost cycle has a guaranteed energy reserve.
Read more →Weather compensation is a control strategy that continuously matches the heating flow temperature to the outdoor temperature along a preset heating curve — the colder it gets outside, the warmer the flow. By keeping the flow temperature as low as each moment allows, it exploits the 2–3% efficiency gain per avoided degree, and EU energy labelling credits such controls with up to 5% on the package label.
Read more →A WISC — Wind and Infrared Sensitive Collector — is an unglazed solar collector whose output depends measurably on wind speed and long-wave sky radiation, and which is therefore tested under ISO 9806 with wind- and infrared-dependent coefficients. Lacking a cover, it exchanges heat directly with the air and can even harvest ambient and condensation heat below air temperature, making it the standard collector category for heat pump source duty.
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