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 not. The heat pump coil must be far larger than a boiler coil — 3 m² or more in a 250–300 L store — because it delivers at 50–55 °C, not 75 °C.
Why does the order of the coils matter so much?
Because both machines are temperature-sensitive, in opposite directions. Solar collectors are most efficient when the fluid returning to the roof is cold, so the solar coil belongs at the very bottom of the store, in the coldest water. An air-to-water heat pump loses COP as it is asked for hotter water, so its coil belongs higher up, where it only has to finish the job on a store the sun has already preheated.
Reverse the order and the system quietly destroys itself: the heat pump warms the bottom of the tank to 50 °C, the collector return climbs with it, and solar yield falls by a third or more. Correct layering preserves stratification — a cold bottom, a hot top, and two heat sources that never fight over the same litres.
What are the workable architectures?
| Architecture | How it works | Best for |
|---|---|---|
| Twin-coil DHW cylinder | Solar coil bottom, heat pump coil top, one store | Retrofit and new build, 250–400 L |
| Two tanks in series | Solar pre-heat store feeds the heat pump cylinder | Large solar arrays, existing heat pump cylinder worth keeping |
| Hygienic combination tank | Solar and heat pump charge one buffer volume; DHW made instantaneously in a coil or fresh-water station | Combined heating and hot water, legionella-sensitive sites |
| Buffer plus separate DHW cylinder | Solar charges the DHW store; heat pump uses the buffer for heating | Systems needing hydraulic separation for zoning or defrost volume |
The twin-coil cylinder is the default for domestic hot water. The combination tank earns its place when solar is also expected to contribute to space heating, because a single stratified volume can serve both circuits.
Why does the heat pump coil have to be so much bigger?
A gas boiler hands over heat at 70–80 °C, so a 1.0–1.5 m² coil is enough to transfer its output. A heat pump running at 50–55 °C has less than half the temperature difference to work with, and every degree it is forced to add costs efficiency. That is why heat pump cylinders carry 3 m² or more of coil surface in a 250–300 L body — typically 2.5–3× a boiler coil. The coil surface area question covers the arithmetic in detail.
The solar coil is a different animal. It needs roughly 0.15–0.2 m² per m² of collector aperture and can be smaller in absolute terms, because the collector loop tolerates a bigger temperature difference than a heat pump ever should.
How should the controls be sequenced?
Solar priority, always — the marginal kWh from the roof is free, while the heat pump's is not.
- Let solar charge without restriction up to a store limit of 70–80 °C, behind a thermostatic mixing valve.
- Set the heat pump's DHW target low, typically 48–50 °C, and give it a time window rather than continuous demand. Every degree above 50 °C costs roughly 2–2.5% of COP.
- Delay the heat pump's hot water call into the afternoon in summer, so the sun gets first attempt at the store. Many controllers do this with a simple solar-yield or time-of-day condition.
- Keep the legionella cycle, but let solar satisfy it where it can — a store that reaches 65 °C on solar alone in July does not need an electric pasteurisation cycle that week. See what temperature kills legionella with a heat pump.
- Place sensors deliberately. The heat pump's cylinder sensor belongs at the top third, the solar store sensor at the bottom, near its own coil.
Does solar thermal still pay next to a heat pump?
Yes. A heat pump making hot water runs at a DHW COP of roughly 2.5–3.0, so each solar kWh saves about 0.33–0.4 kWh of electricity. At the EU-average household price of about €0.29/kWh (Eurostat, second half of 2025) that is worth €0.10–0.12 per solar kWh, against roughly €0.14 when the same solar kWh displaced gas at €0.123/kWh through a condensing boiler — so solar keeps roughly 70–85% of its value next to a heat pump. In high-price markets such as Germany and Ireland (€0.39–0.40/kWh) each solar kWh displaces €0.13–0.16 of electricity, as much as it ever saved against gas. A 4 m² array yielding 450–600 kWh/m² saves roughly 1,800–2,400 kWh of heat pump heat a year, and that saving rises with every electricity price increase.
Where it pays best:
- High, year-round hot water demand — large families, guesthouses, small hotels, where the store is emptied daily and the sun always has somewhere to put heat.
- Summer shutdown. A solar array sized for June to September lets the heat pump stop entirely for months, which also removes its worst-COP duty of the year.
- Expensive electricity or weak grid connections, common across southern Europe and Turkey, where displaced kWh are worth more.
- Existing collectors. If solar thermal is already on the roof, integrating it with a new heat pump is far cheaper than replacing it.
Solimpeks builds both sides of this system — Wunder collectors and Solimpeks heat pumps with COP up to 4.9 at A7/W35 — and the honest engineering answer is that the tank, not the brand of either machine, decides whether the combination performs.
Frequently asked questions
Can solar thermal and a heat pump share the same coil?
No. They need different flow temperatures, different coil surfaces and different sensor positions, and sharing one coil means one source is always fighting the other. Use a twin-coil cylinder, or two tanks in series.
Should solar or the heat pump have priority?
Solar, in every case. Solar heat is free at the margin, while every heat pump kilowatt-hour costs electricity. Practically this means a generous solar store limit and a heat pump hot water window shifted into the afternoon or evening.
Can solar thermal feed the heat pump's source side instead of the tank?
That is a different architecture, used with ground-source and brine heat pumps: unglazed or hybrid PVT collectors raise the source temperature or regenerate the ground loop. Standard glazed collectors are usually better used to make hot water directly.
How large should the cylinder be for both?
250–400 L for a family of four: enough volume for a heat pump to run long, efficient charging cycles and enough for 40–60 litres per m² of collector. Smaller stores force both sources into short cycles and push the collectors into stagnation in summer.