Air-to-air vs air-to-water heat pumps: which do you need?
An air-to-air heat pump blows warm air through indoor fan units and cannot make domestic hot water; an air-to-water heat pump heats water for radiators, underfloor circuits and a cylinder...
Knowledge Hub
Common Questions, Straight Answers
Sizing, costs, payback, installation, incentives — the questions installers and homeowners actually ask, answered directly.
151 entries
Heat Pumps
Are R290 (propane) heat pumps safe?
R290 (propane) heat pumps are safe when installed to manufacturer clearance rules. A typical outdoor monoblock holds about 1 kg of refrigerant — a fraction of a barbecue cylinder — sealed...
Buffer tank vs volumiser: what is the difference?
A volumiser is an insulated vessel plumbed in series on the return that adds water volume without mixing flow and return; a buffer tank adds volume and hydraulic separation, letting zoned...
Can a heat pump cool my house in summer?
A reversible heat pump can cool, and most air-to-water models sold in Europe contain the four-way valve needed to do it. Through underfloor circuits the flow temperature must stay above the...
Can a heat pump heat a swimming pool efficiently?
Yes — pool heating is the heat pump's best-case application. Because pool water needs only 26–30 °C, the temperature lift from outdoor air is tiny and dedicated pool heat pumps reach COP...
Can a heat pump replace a boiler in an older, uninsulated house?
Yes. In the UK's Electrification of Heat trial, 742 heat pumps were installed across every house type and age band tested, and seasonal performance showed no significant difference between...
Can a heat pump work with microbore pipework?
Usually yes with 10 mm copper, rarely with 8 mm. A heat pump moves three to four times the water a boiler does at the same output, because it runs a 5 °C temperature difference instead of...
Can I run a heat pump alongside my existing boiler (bivalent)?
Yes — a heat pump and an existing boiler can share one heating system in a bivalent (hybrid) arrangement: the heat pump carries the base load while the boiler tops up on the coldest days. A...
Can solar thermal and heat pumps work off-grid?
Yes, with the right split of duties. Solar thermal works off-grid almost perfectly — a thermosiphon collector needs no electricity at all. A heat pump runs well off-grid in mild, sunny...
COP, SCOP and SPF: what is the difference?
COP is a heat pump's instantaneous efficiency at one test point — heat out divided by electricity in; COP 4.0 means 4 kWh of heat per 1 kWh of electricity. SCOP is the seasonal average...
Do buffer tanks reduce heat pump efficiency?
Not when it is correctly piped. A well-integrated buffer costs only 1–3% in standing losses — often less than the short-cycling and defrost losses it prevents on zoned, low-volume and...
Do heat pumps need annual servicing?
Annual servicing is not legally required for a typical domestic heat pump in Europe — hermetically sealed units holding under 10 tonnes CO₂-equivalent of refrigerant are exempt from...
Do heat pumps work below freezing?
Yes. Air source heat pumps heat reliably below freezing: standard units work to −15 °C and EVI (enhanced vapour injection) models to −25 °C, still delivering a COP of about 2 even in deep...
Do I need a buffer tank with a heat pump?
Most heat pump installations benefit from a buffer tank, and systems with zoned circuits, low water volume or cold-climate defrost cycles effectively require one. Only open systems without...
Do I need an electrical upgrade for a heat pump?
Most homes do not. A 5–12 kW domestic heat pump draws only 7–21 A while running, so an existing 60–100 A single-phase supply normally copes. Upgrades are needed when the property's total...
Do I need bigger radiators for a heat pump?
Usually only in some rooms. Radiators are rated at ΔT 50 K (75/65 °C into a 20 °C room), and at a heat pump flow of 45 °C a panel delivers about 35% of that rating — but many existing...
Do I need planning permission for a heat pump?
In most cases no. An outdoor air source heat pump is permitted development across most of Europe provided it meets size, siting and noise conditions. In England a detached house may install...
Ground source vs air source heat pump: which is better?
Ground source heat pumps run about 10–20% more efficiently than air source and hold that efficiency all winter, because ground below 1.5 m stays at 8–12 °C year-round. UK field monitoring...
Heat pump vs gas boiler: which is cheaper to run in 2026?
At 2026 EU-average energy prices a heat pump is cheaper to run than a gas boiler: with electricity at €0.29/kWh and gas at €0.12/kWh (Eurostat, H2 2025), a SCOP 3.5 heat pump delivers heat...
Heat pump with underfloor heating vs radiators: what changes?
Underfloor heating lets a heat pump run at about 35 °C instead of 45–55 °C, which is worth roughly 20–40% on running cost. Radiators work perfectly well too when sized for 40–50 °C — which...
How do I judge whether a heat pump design and installation is good?
Judge a heat pump installation on six checkable items: a room-by-room heat loss calculation, a stated design flow temperature of 50 °C or below, pipework sized for the flow rate, a stated...
How do I set the weather compensation curve on a heat pump?
Set the curve from two points: your design outdoor temperature paired with the design flow temperature from the heat loss calculation, and mild weather around 15 °C paired with a flow of...
How does an air source heat pump work?
An air source heat pump extracts heat from outdoor air and moves it into heating water using a vapour-compression refrigeration cycle: evaporation, compression, condensation and expansion...
How long does a heat pump last, and what fails first?
A well-installed air source heat pump lasts 15–20 years, and the compressor is what usually decides the end of life. Circulation pumps, expansion vessels and sensors are 8–12 year...
How long should a heat pump take to reheat a hot water cylinder?
At 3 kW of delivered coil power, a heat pump reheats a fully cold 200-litre cylinder from 10 °C to 50 °C in roughly 3 hours; at 8 kW, about 70 minutes. Energy in kWh = litres × temperature...
How loud is a heat pump — and what are the EU noise limits?
A modern air source heat pump emits 50–65 dB(A) sound power, heard as roughly 35–48 dB(A) at 3 metres — between a quiet library and light rainfall. EU Ecodesign Regulation 813/2013 caps...
Is my heat pump oversized — and why does it matter?
A heat pump is oversized when its minimum modulated output still exceeds the building's heat demand for most of the season, forcing short-cycling instead of steady running. Telltale signs...
Is storing hot water at 48-50 °C safe (legionella)?
Storing hot water at 48–50 °C is defensible only in a low-risk system: a single-family home whose cylinder turns over daily, has no dead legs, and runs a regular anti-legionella cycle...
Low loss header vs buffer tank: which do I need?
A low loss header provides hydraulic separation with almost no stored volume, while a buffer tank provides separation plus 10–50 litres of water per kW of heat pump capacity. Choose a...
Monoblock vs split heat pump: which should you choose?
Choose a monoblock heat pump when you want the entire refrigerant circuit factory-sealed outdoors, water-only connections into the house and R290 refrigerant — the configuration the market...
Should a buffer tank be piped 2-port or 4-port?
Pipe a buffer tank 2-port — in series with the circuit, normally in the heating return — whenever the hydraulics allow: it adds full water volume with almost no temperature distortion...
Should a heat pump run continuously at low temperature?
In a reasonably insulated home, a heat pump runs cheapest when it runs continuously at the lowest flow temperature that keeps rooms comfortable — every 1 °C cut in flow temperature improves...
What are the disadvantages of an air source heat pump?
An air source heat pump's six commonly cited disadvantages each have a standard answer: the higher upfront cost is largely offset by 2026 grants and by running costs about 35–40% below gas...
What flow temperature should I run my heat pump at?
Run the lowest flow temperature that still holds every room at target on the coldest design day: about 35 °C for underfloor heating, 40–45 °C for upgraded radiators, and no more than 50 °C...
What is a high temperature heat pump, and do I need one?
A high temperature heat pump delivers 65–80 °C flow water instead of the 35–55 °C a standard unit is built for, so it can replace a boiler without changing radiators. Run permanently at a...
What is a solar-assisted heat pump (SAHP)?
A solar-assisted heat pump (SAHP) combines solar collectors with a heat pump so solar energy either raises the heat pump's source temperature or powers its compressor. In the most common...
What size heat pump do I need for my home?
Size a heat pump to the building's calculated heat load at design outdoor temperature, not to the old boiler. Typical specific loads are 30–50 W/m² for well-insulated homes and 80–120 W/m²...
What temperature kills legionella in a heat pump system?
Legionella is killed by temperature and time together: the bacteria die off in about 32 minutes at 60 °C, in roughly 2 minutes at 65 °C and almost instantly at 70 °C. A heat pump cylinder...
Where should the heat pump outdoor unit go?
Put the outdoor unit where air moves freely: manufacturers typically require 300 mm behind, 150–300 mm at the sides, 500 mm above and 1–2 m clear in front of the fan. Raise it 200–300 mm...
Why does my heat pump defrost so often?
Defrosting every 40–90 minutes is normal between about −5 °C and +7 °C when the air is damp, and a cycle should finish in 3–10 minutes. Defrosting every 20–30 minutes, or for longer than 12...
Why does my heat pump keep switching on and off?
A heat pump short-cycles when it makes more heat than the system can absorb. Two to three compressor starts per hour in mild weather is normal; six or more means something is wrong. The...
Why is my heat pump COP so low?
A well-designed air-to-water system returns a seasonal COP of 3.2–4.0; UK monitoring of 428 air source installations found a median of 2.78, and the best monitored homes exceed 4.4. A...
Why is my heat pump electricity bill higher than I was told?
Heat pump bills overrun for four measurable — and usually fixable — reasons: the quote used a laboratory SCOP instead of a field SPF, the flow temperature is running above design, the...
Incentives & Costs
Are PVT hybrid panels eligible for grants in 2026?
Yes, in several markets. Germany funds PVT both as a solar thermal system and as a heat source for a heat pump, at 30% base and up to 70% with bonuses. France removed hybrid panels from...
At what COP does a heat pump beat a gas boiler on running cost?
A heat pump beats a gas boiler on running cost when its COP exceeds the electricity-to-gas price ratio multiplied by the boiler's efficiency. With a 90%-efficient boiler that means a...
Do heat pumps and solar systems add value to a property?
Yes. Research published in Nature Energy found heat pumps add a $10,400–17,000 price premium in nearly half of US states, and UK analysis links an EPC move from band D to C with roughly 3%...
Do new builds and self-builds qualify for heating grants?
Usually not. Almost every European heating grant funds replacement of an existing fossil system, so new builds are excluded by design: Ireland requires the home to be built and occupied...
How do I compare two heat pump quotes fairly?
Compare the design, not the total. Line up four numbers from each quote: calculated heat loss in kW to EN 12831-1, design flow temperature, cylinder coil area in m², and expected SCOP at...
How do you choose a good heat pump installer?
Choose on design method, not on price. Insist on a room-by-room heat loss calculation to EN 12831-1, a stated design flow temperature of 45 °C or lower where the building allows, a named...
How does the UK's £9,000 Boiler Upgrade Scheme work? (2026)
The Boiler Upgrade Scheme pays a £7,500 grant towards an air-source or ground-source heat pump in England and Wales, raised to £9,000 for homes currently heated by oil or LPG from 21 July...
How much do PVT panels cost compared with standard PV?
A PVT panel costs more than a commodity PV module of the same electrical rating, and the hydraulic installation adds to the invoice. In return it delivers 3–4 times the total energy per...
How much does a heat pump cost to install in 2026?
A whole-house air-to-water heat pump typically costs €10,000–18,000 installed in Western Europe in 2026, or £8,000–15,000 in the UK, before grants. Germany is the outlier at €27,000–40,000...
How much does a solar water heating system cost in 2026?
A domestic solar water heating system costs €4,000–8,000 installed in central and northern Europe in 2026, around £4,000–6,000 in the UK, and €1,500–2,500 for a Mediterranean thermosiphon...
How much does certification add to the cost of an installation?
Installer certification typically adds £500–1,500 to a domestic job in the UK — a figure that reflects £1,500–3,000 of initial accreditation, £500–1,500 of annual audits, roughly £30 per...
Is a hybrid solar + heat pump system worth the investment?
A hybrid solar + heat pump system is worth the investment for households with steady hot water demand: solar covers 50–70% of annual water heating, the heat pump's seasonal efficiency rises...
Solar thermal or a heat pump: which is better for hot water only?
For hot water only, solar thermal wins where sunshine and roof space allow: collectors convert 60–70% of sunlight into heat and typically cover 50–70% of annual demand at zero fuel cost. A...
What actually drives the price of a heat pump installation?
The heat pump itself is only 30–40% of the installed price. The building supplies the rest: emitter upgrades, a heat pump cylinder, pipework, electrical supply, design and certification...
What is Ireland's SEAI heat pump grant now it rose to €12,500?
Ireland's SEAI heat pump support doubled on 3 February 2026 to a maximum of €12,500 for a house: €6,500 for the heat pump itself, up to €2,000 for heating-system upgrades such as radiators...
What is the payback on commercial solar thermal for hotels and factories?
Commercial solar thermal typically pays back in 4–7 years for hotels and 3–6 years for factories with steady daytime process heat, because both have flat year-round demand. Delivered solar...
What VAT rate applies to a heat pump or solar installation?
Most European countries apply a reduced VAT rate to renewable heating. Great Britain charges 0% on energy-saving materials until 31 March 2027, then 5%; Ireland charges 0% on solar panels...
What warranty should a solar thermal or heat pump system carry?
Expect 5–7 years on a heat pump as standard, extending to 10 years on parts and compressor when the unit is registered and serviced annually. Solar thermal collectors should carry 10 years...
When does a heat pump grant actually get paid?
It depends on who receives the money. In the UK the grant is deducted from the quote before you pay anything and Ofgem reimburses the installer on a weekly payment cycle, usually within...
Which electricity tariff is cheapest for running a heat pump?
A time-of-use tariff designed for heat pumps beats a flat rate in every major market. UK heat pump tariffs price roughly 8 off-peak hours near 14–15p/kWh and save £200–350 a year against...
Which European countries offer heat pump grants in 2026?
In 2026 every major European market subsidises heat pumps: Germany covers 30–80% of costs through the reformed BEG, France pays €3,000–5,000 per air-to-water unit via MaPrimeRénov', Italy's...
Which subsidies support solar thermal in 2026?
Solar thermal is subsidised across most of Europe in 2026: Germany's BEG pays 30% (more with bonuses), France funds solar water heaters through CEE energy-saving premiums after removing...
PVT
Are PVT panels actually better than plain PV?
Yes, whenever the building can use the heat — and most can, through a heat pump source circuit, a hot water cylinder or a pool. The same square metre then yields three to four times more...
Can a PVT panel really reach 80% efficiency?
The 80% figure is real but it is a peak combined number, not an annual one: roughly 15–22% electrical plus 55–60% thermal, measured at low fluid temperature and full sun. Annual combined...
Can one solar panel make electricity and hot water at the same time?
Yes. A PVT (photovoltaic-thermal) hybrid panel produces electricity and hot water simultaneously from the same surface: PV cells on top generate power while an absorber behind them captures...
Can PVT panels be the only heat source for a heat pump?
Yes. An uncovered, back-finned PVT array sized at roughly 2.5 m² per kW of heat pump output can serve as the sole source for a brine/water heat pump, with no borehole and no outdoor fan...
Can PVT panels heat a swimming pool?
Yes — a swimming pool is arguably the best possible load for PVT panels. Pools want large volumes of low-grade heat at 24–30 °C — exactly where an unglazed hybrid collector is most...
Do PVT panels work in winter and at night?
Uncovered PVT panels keep harvesting heat in winter and after dark, because they absorb energy from ambient air and rain as well as from sunlight. Field systems sustain a seasonal...
Do solar panels produce more electricity when cooled?
Yes. Crystalline silicon PV loses 0.3–0.45% of its output for every degree the cells rise above the 25 °C rating point, and rooftop cells routinely reach 60–70 °C in summer — a 14–18%...
How do PVT hybrid panels work?
A PVT hybrid panel stacks photovoltaic cells over a heat absorber in a single module: the cells convert sunlight into electricity while the absorber bonded to their rear captures the heat...
How does a PVT panel work as a heat pump source?
A PVT panel feeds 25–45 °C low-grade heat from the absorber on its rear side into the source circuit of a brine/water heat pump. The elevated source temperature lifts heat pump COP —...
How many PVT panels does a heat pump need as a source?
Size a PVT array at 2.5–4 m² per kW of heat pump heating capacity — about 1.5 to 2 panels per kW, so 12–16 panels for an 8 kW brine/water heat pump. Arrays below 2 m² per kW cannot hold the...
Insulated, uninsulated or finned PVT — which should I choose?
Choose by working temperature. Finned uncovered PVT is for heat pump source duty below about 25 °C, plain uncovered PVT for pool heating and cylinder pre-heat at 25–45 °C, and insulated or...
What are the disadvantages of PVT hybrid panels?
PVT panels cost more per square metre than a plain PV module (roughly 1.5–2.5 times the module price), add a small glycol circuit with pump, vessel and periodic checks, deliver their heat...
What are the temperature limits of a PVT panel?
A PVT panel operates between roughly −25 °C (glycol-protected source operation) and a stagnation temperature that is far milder than a pure thermal collector: an uncovered PVT panel...
What do "WISC" and "unglazed" mean on a Solar Keymark certificate?
WISC stands for Wind and Infrared Sensitive Collector — the ISO 9806 category for collectors without a glazing cover, which includes most PVT hybrid panels. Because no glass cover shields...
What maintenance do PVT panels need, and how long do they last?
A PVT panel lasts 25 years or more — the laminate is built like a standard PV laminate, for which 25–30 year performance warranties are common, and the hydraulic circuit is what needs...
What water temperature does a PVT panel actually produce?
An uncovered (WISC) PVT panel delivers water at 10–40 °C, typically only 3–8 K above its inlet on a single pass, which is why it feeds a heat pump source circuit rather than a hot water...
Which heat pump water heaters are compatible with PVT panels?
A heat pump water heater accepts PVT panels only if it has a liquid source circuit — a brine/water unit whose source inlet is approved from roughly −15 °C to +25 °C. Air-source models have...
Solar
How many solar collectors do I need for a family home?
For domestic hot water, plan roughly 1–1.5 m² of flat-plate collector area per person in southern climates and 1.5–2.5 m² per person in central/northern Europe, with 40–60 litres of storage...
PVT panels vs separate PV and solar thermal: which is better?
PVT hybrid panels win when roof area is limited and both electricity and hot water are needed: one collector field delivers up to 80% combined efficiency and the water cooling lifts PV...
Solar Thermal
Can I combine solar thermal with a combi boiler?
Yes, using a solar pre-heat store and a thermostatic blending valve. Combi boilers not approved for pre-heated water typically accept a cold inlet no higher than 25–28 °C, so the valve caps...
Can I retrofit solar thermal to my existing hot water system?
Yes, in most homes — but the cylinder decides the job, not the roof. Solar needs its own heat exchanger low in the store, so a single-coil cylinder is normally swapped for a twin-coil solar...
Can solar thermal collectors heat a swimming pool?
Yes — a pool is the easiest thing solar thermal can heat. Because the water needs only 26–28 °C, unglazed absorber mats run near their optical maximum, and the sizing rule is collector area...
Can solar thermal contribute to space heating and radiators?
Yes. A solar combisystem covers 20–35% of a house's total heat demand, but it needs about three times the collector area of a hot water system — roughly 1 m² per 10 m² of heated floor area...
Can solar thermal panels work with an unvented cylinder?
Yes — an unvented cylinder is a good match for solar thermal, provided it is a solar-rated twin-coil model with the solar coil at the bottom. The installation keeps its full safety chain...
Direct vs indirect solar water heating: what is the difference?
A direct (open-loop) system pumps the drinking water itself through the collectors. An indirect (closed-loop) system circulates a propylene glycol mixture through the collectors and hands...
Do I need a heat dump on a solar hot water system?
Most correctly sized domestic solar hot water systems do not need a heat dump: certified collectors are designed to survive stagnation, and a right-sized array saturates the tank without...
Do solar thermal panels need planning permission?
In most of Europe, no. Roof-mounted solar collectors on a house are permitted development in England when they project no more than 200 mm from the roof slope and stay below the ridge, and...
Do solar water heaters really work? The honest disadvantages
Yes — a correctly sized solar water heater covers 50–65% of a household's annual hot water in central Europe and 70–90% around the Mediterranean. Replacing electric or LPG water heating in...
Does collector orientation and tilt really matter?
Less than most people fear. A collector facing due south at 30–50° is the annual optimum, but anything within 45° of south loses only 5–8% of yield, and a due-east or due-west roof still...
Does solar water heating work in winter and on cloudy days?
Yes. Solar collectors harvest diffuse as well as direct radiation, so they preheat water even under overcast skies. In central Europe a winter month delivers 10–25% of the July yield...
Flat-plate vs evacuated tube collectors: which is better?
Flat-plate collectors are the better choice for domestic hot water in sunny and temperate climates: higher optical efficiency (0.75–0.82), greater robustness and 20–40% lower cost per m²...
How do I combine solar thermal and a heat pump on one cylinder?
Use a twin-coil cylinder with the solar coil in the bottom third and the heat pump coil above it, and give solar priority in the controls. The heat pump then only lifts what the sun could...
How do I get air out of a solar thermal circuit?
Purge a solar loop with a filling station, never with the system pump: circulate the glycol mixture for 20–30 minutes at 3–4 bar so the velocity carries bubbles to the air separator, then...
How do I keep a solar thermal system from freezing?
Three proven methods protect a solar thermal system from frost: an indirect circuit filled with propylene glycol antifreeze (a 40% mix protects to roughly -21 °C, 50% to roughly -32 °C), a...
How do I size the expansion vessel on a solar thermal loop?
Size a solar expansion vessel to swallow the entire collector content as steam, not just the fluid's thermal expansion. Add the collector volume, the expansion volume and a 3 litre reserve...
How do I stop my solar thermal system overheating in summer?
Stop solar thermal overheating by giving the surplus heat somewhere to go and letting the collector field stagnate safely. Raise the tank limit to 75–80 °C behind a mixing valve, enable the...
How does a drain-back solar system work, and when should I choose it?
A drain-back solar system empties its heat transfer fluid out of the collectors by gravity into an indoor reservoir every time the pump stops, so the collectors stand dry during frost and...
How does a thermosiphon solar water heater work without a pump?
A thermosiphon solar water heater circulates on density difference alone: fluid heated in the collector becomes lighter and rises into a tank mounted above it, while cooler fluid sinks back...
How long do solar thermal collectors last?
A quality flat-plate solar collector lasts 25–30 years, and often longer. The collector itself has no moving parts; system lifetime is set by the consumables around it: glycol fluid (5–10...
How much heat does one square metre of solar collector produce per year?
A well-designed domestic system delivers 350–500 kWh per m² of collector per year in central Europe and 500–700 kWh/m² around the Mediterranean, measured as heat into the tank. Large...
How much of my hot water will solar cover in summer vs winter?
In central and northern Europe a correctly sized solar water heating system covers around 90% of hot water demand in high summer but only around 25% in mid-winter, averaging 50–70% across...
How much roof space does solar water heating need?
A domestic solar water heating system needs 4–6 m² of unshaded roof for a family of four — typically two flat plate collectors or 20–30 evacuated tubes, with a 250–300 litre cylinder. Plan...
How often should solar glycol be replaced, and how?
Solar glycol should be tested every 2 years and replaced every 5–8 years, or immediately if its pH falls below 7. Two instruments decide it: a refractometer for frost protection and a pH...
Is solar thermal obsolete now that PV is cheap?
No. Solar thermal still delivers 400–500 kWh of heat per m² a year — two to three times what the same roof yields as PV electricity — and 544 GWth of collectors supply 443 TWh of heat...
Solar thermal vs PV + diverter vs PV + heat pump: which should you buy?
Solar thermal delivers the most heat per square metre — 400–500 kWh/m² a year against 150–200 kWh/m² from PV — so it wins wherever roof area is limited or hot water demand is high. A hot...
What antifreeze should a solar thermal loop use?
Use an inhibited, high-temperature-stabilised propylene glycol made specifically for solar thermal systems, mixed with demineralised water at 30–50% concentration for frost protection...
What happens to solar collectors in hail, frost and storms?
Certified solar collectors survive hail, frost and storms because ISO 9806 makes them prove it: the impact-resistance test fires 25 mm ice balls at about 23 m/s, and mechanical load tests...
What is a solar thermal system and how does it work?
A solar thermal system converts sunlight directly into heat — usually for domestic hot water — using roof-mounted collectors, a circulation circuit and an insulated storage tank. Collectors...
What is the difference between active and passive solar water heating?
Active solar water heaters use an electric pump and controller to move fluid between collector and storage tank; passive systems move it by natural convection alone, with the tank mounted...
What is the difference between solar PV and solar thermal?
Solar PV converts sunlight into electricity at around 20% efficiency; solar thermal converts it into heat at 60–70%. That means a thermal collector harvests roughly three times more energy...
What is the payback period of a solar water heater? (2026)
A solar water heater typically pays for itself in 3–5 years where it replaces electric water heating in sunny markets such as Türkiye, Greece, Cyprus and southern Spain, in 4–6 years...
What maintenance does a solar thermal system need?
A solar thermal system needs an annual owner inspection and a professional service every 2–3 years. The service visit tests the glycol's frost protection and pH, checks system pressure and...
What settings should I use on a solar thermal controller?
Start with a switch-on difference of 6–8 K, a switch-off difference of 3–4 K, a store limit of 60 °C for daily use (75–80 °C in summer behind a mixing valve) and a collector emergency stop...
Why does my solar thermal system keep losing pressure?
In most cases it is the expansion vessel, not a leak. A burst diaphragm or a pre-charge that has drifted below the loop's operating pressure lets the 6 bar safety valve blow off fluid every...
Storage Tanks
Can I fit a twin coil cylinder now and connect solar later?
Yes, and it is the cheapest way to buy into solar. Within one manufacturer's range a twin coil cylinder costs roughly 10–20% more than the single coil version, while replacing the tank...
Can I replace a hot water tank anode rod myself?
Yes on most tanks. Isolate the power and the cold feed, release the pressure, drain 10–20 litres, undo the anode with a 27 mm socket and fit the new one with PTFE tape on the thread. Allow...
Can I reuse my existing hot water cylinder with a heat pump?
Usually not. A boiler cylinder carries a 0.5–0.8 m² coil designed for 70–80 °C flow, while a heat pump runs at 45–55 °C and needs 2.5–3.0 m² to reheat the tank in a sensible time. Keeping...
Does a hot water recirculation loop waste energy, and how should it be controlled?
A hot water recirculation loop trades energy for convenience: an uninsulated loop run continuously loses roughly 10–15 W per metre of pipe, which on a 20-metre loop can exceed what the...
Enamel-coated vs stainless steel hot water tanks: which lasts longer?
Both enamel-coated and stainless steel tanks last 15–25 years when matched to the local water. Enamel-coated steel tolerates hard, chlorinated and variable water but depends on a...
How do I descale a hot water cylinder in a hard water area?
Descale the heating surfaces and the sludge at the base, not the tank walls. Withdraw the immersion element and soak it in citric or sulphamic acid, then flush the bottom of the tank...
How do I know if my hot water cylinder is the wrong size?
A cylinder is undersized when the last shower runs cold more than about once a week or the tank cannot recover between morning and evening peaks; it is oversized when the water is still hot...
How is a solar ready hot water tank different from a standard tank?
A solar ready tank has a large coil low in the vessel, a sensor pocket at the base, and its backup heater in the top third so the lower two-thirds stay cold for the collectors. A standard...
How long does a hot water tank stay hot without reheating?
A well-insulated modern cylinder stays usefully hot for a full day or more without reheating, cooling by only 3–5 °C in 24 hours. That is 0.8–1.0 kWh of standing loss for a 200 litre tank...
How many litres of buffer per kW of heat pump? Sizing rules
Size a heat pump buffer tank at 20–50 litres per kW of heating capacity for radiator and zoned systems, 12–20 L/kW for open underfloor circuits, and 30 L/kW or more where defrost cycles are...
How much heat does a hot water cylinder lose per day?
A modern insulated cylinder loses 1–5 kWh per day. Multiply the declared standing loss in watts by 24 and divide by 1,000: a 50 L tank at 54 W loses about 1.3 kWh per day, a 245 L tank at...
How much tank volume do I need per m² of solar collector?
Allow 50–70 litres of storage per m² of collector aperture for domestic hot water, with 60 L/m² as the planning figure across most of Europe. A 4 m² flat plate array therefore pairs with a...
How much usable hot water does a cylinder actually give?
A 200 litre cylinder stored at 60 °C delivers about 300 litres of usable water at 40 °C, because every stored litre is blended with roughly 0.67 litres of cold at the tap. That is the V40...
How often should the magnesium anode be checked and replaced?
Inspect the magnesium anode every two years and replace it once about 60% of its mass is consumed or its diameter falls below 10–15 mm. In average mains water that means a new anode every...
In a twin coil cylinder, which coil heats which part of the tank?
The lower coil heats the whole tank; the upper coil heats only the water above it, typically the top third or about 60–80 litres of a 200 litre cylinder. Heat rises, so a coil warms...
Is it cheaper to leave the hot water tank on all day or use a timer?
A timer is cheaper. A 200 litre cylinder loses roughly 0.8–2 kWh a day standing, and every one of those kilowatt-hours is paid for again if the tank is held hot around the clock. Heating...
Magnesium, aluminium or zinc anode: which should I fit?
Fit magnesium as the default sacrificial anode: its electrode potential of around −1.6 V gives the strongest protective current in almost any water, including soft, poorly conductive...
Plate heat exchanger or internal coil for domestic hot water?
Use an internal coil for a single household and a plate heat exchanger where hygiene or peak output matters more than simplicity. A coil stores 150–300 litres of drinking water and reheats...
Thermal store or unvented cylinder: which is better for hot water?
An unvented cylinder stores drinking water under mains pressure; a thermal store does the opposite, storing heating water and making hot water instantaneously through a plate heat...
Vented vs unvented hot water cylinder: which is right for me?
An unvented cylinder is sealed and fed straight from the cold mains, so it delivers hot water at 2–3 bar and 20 or more litres a minute. A vented cylinder is gravity-fed from a loft tank...
What are the common problems with unvented cylinders?
Six faults account for almost every unvented cylinder call-out: a lost expansion vessel charge, a failed pressure-reducing valve, a blocked inlet strainer, a scaled immersion element, a...
What coil surface area does a heat pump cylinder need?
A heat pump hot water cylinder needs 2.5–3.0 m² of coil surface area — roughly 1 m² per 100 litres, and three to four times the 0.5–0.8 m² fitted to a boiler cylinder. The reason is...
What is cylinder recovery rate, and does it matter more than volume?
Recovery rate is the speed at which a hot water cylinder reheats, set by the power its coil can transfer — not by tank size. Each kilowatt of coil power heats about 21 litres of water...
What is tank stratification and why does it matter?
Stratification is the natural layering of hot water above cold inside a storage tank — water at 60 °C is about 1.5% less dense than at 20 °C, so it floats. A well-stratified tank delivers...
What is the difference between a direct and an indirect hot water cylinder?
A direct cylinder heats water with an electric immersion heater inside the tank; an indirect cylinder heats it through a coil fed by a boiler, heat pump or solar collector. The difference...
What makes a cylinder "heat pump ready"?
A heat pump ready cylinder carries an indirect coil of 2.0–3.0 m² — three to five times the 0.5–0.8 m² coil in a boiler cylinder — so it can absorb full heat pump output at a 45–55 °C...
What pressure should a cylinder expansion vessel be set to?
Set the expansion vessel pre-charge equal to the cold-water pressure feeding the cylinder — the pressure-reducing-valve setting, typically 3.0–3.5 bar on an unvented system. Check the...
What size cylinder does a multi-bathroom property or small hotel need?
Size a multi-bathroom property to the peak hour, not the daily total: allow roughly 50 L of stored hot water per simultaneous shower and 80–100 L per bathtub. A three-to-four-bathroom house...
What size hot water tank does my household need?
Size a hot water tank at 30–50 litres per person per day: 80–120 L for two people, 150–200 L for a family of four on a boiler, and 200–300 L when a heat pump or solar thermal system charges...
Which hot water cylinder energy class (ErP) should I buy?
Buy class B or C: a cylinder's ErP class is set purely by standing loss, and at 200 L class C allows at most 86 W of continuous heat leakage (about 2.1 kWh per day) versus 61 W for class B...
Why do I need a hot water cylinder if I have a combi boiler?
Because a heat pump cannot make hot water on demand. A combi delivers 10–15 litres a minute by burning 24–35 kW instantly; an air source heat pump produces 5–12 kW at 50 °C, so it must...
Why is my unvented cylinder discharging through the tundish?
A dripping tundish almost always means the expansion vessel has lost its air charge. Water then has nowhere to expand, pressure climbs past the expansion relief valve at around 6 bar, and...