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, plus 60–80 litres of store per m² of collector. It works with underfloor heating at 30–35 °C; high-temperature radiators get very little.
Can solar thermal actually heat a house?
Partly, and the limit is calendar, not technology. In cold and temperate climates the great majority of the annual solar resource arrives outside the main heating season — in Ottawa, roughly 85% of it falls outside mid-November to mid-March, and central European sites are not much kinder. Solar heat is abundant exactly when the house does not need it.
So a solar combisystem — one that serves both domestic hot water and space heating — does its real work in the shoulder months. From March to May and September to November it can carry most of a well-insulated house's heating load on sunny days, letting the boiler or heat pump stand down. In January it contributes preheat, not heat.
How much collector area does space heating need?
Three times a hot water array, near enough. The German rule of thumb is about 1 m² of flat-plate collector per 10 m² of heated floor area, adjusted for how well the building holds heat:
| Building standard | Collector area per m² of floor | 150 m² house |
|---|---|---|
| New build, well insulated | 0.04–0.06 m² | 6–9 m² |
| Energy-renovated | 0.06–0.08 m² | 9–12 m² |
| Uninsulated older house | 0.08–0.12 m² | 12–18 m² |
A typical single-family combisystem therefore uses 10–15 m² of flat-plate collectors (or correspondingly fewer evacuated tubes) with a 600–1,000 L store. Combisystem stores should be sized from collector area and building standard together, because the same 12 m² array behaves very differently on a 1990s house and a new build. Storage runs at 60–80 litres per m² of collector — more than the 40–60 L/m² of a hot water-only system, because the heat has to survive from a sunny afternoon to a cold evening. The collector-area-to-store-volume ratio sets out that arithmetic.
Which emitters can actually use solar heat?
The lower the flow temperature, the more hours a year the collectors can contribute:
| Emitter | Flow temperature | Solar usefulness |
|---|---|---|
| Underfloor heating | 30–35 °C | Excellent — collectors reach this on most bright days, even in winter |
| Oversized radiators | 40–45 °C | Good in the shoulder season |
| Standard radiators | 60–70 °C | Poor — collectors rarely hold this in the heating season |
This is the same physics that makes low flow temperatures matter for heat pumps, and the reason a combisystem and a low-temperature emitter circuit are usually specified together. A house with 70 °C radiators will see its solar contribution restricted almost entirely to hot water.
How is a combisystem plumbed?
Around a single stratified store, not two separate tanks. The usual arrangement is a buffer with an internal hot water tank or a fresh-water station on top, charged by the collectors through a stratification lance or a two-port charging valve so the heat lands at the layer matching its own temperature. The heating return is drawn from the middle of the store and passed back to the boiler or heat pump only if the store cannot cover the load — return-temperature raising, in the trade.
Two details decide performance: sensor placement (the store needs at least three) and a solar controller that can prioritise hot water, then heating support, then dumping surplus. Get those wrong and a 15 m² array performs like a 6 m² one.
Is a combisystem worth it, or should the money go elsewhere?
Be clear-eyed about the marginal return. A hot water-only array delivers 400–500 kWh per m² per year because every kilowatt-hour it makes gets used. Push the array to combisystem size and specific yield falls to roughly 250–350 kWh/m², because the extra collectors mostly produce surplus in summer. You are buying heat at a rising unit price.
That trade-off is worth taking when: the house has low-temperature emitters, the heating season is long and sunny (Alpine and continental climates, high-altitude Turkey), the alternative fuel is expensive oil, LPG or direct electricity, or a subsidy specifically rewards heating support. It is a poor bet on a leaky house with 70 °C radiators, where insulation buys more kilowatt-hours per euro than any collector will.
Frequently asked questions
What percentage of heating can solar thermal cover?
A well-designed combisystem covers 20–35% of total annual heat demand (space heating plus hot water) in central Europe. Of the space heating portion alone, expect 15–25%, concentrated in spring and autumn. Systems claiming much more usually rely on seasonal storage or a very low-energy building.
Can solar thermal work with radiators?
It works well with oversized radiators sized for 40–45 °C flow, and poorly with standard radiators designed for 70 °C. Increasing radiator surface, as you would for a heat pump, is what makes solar heating support viable in a retrofit.
Do I need a bigger tank for a combisystem?
Yes — 600–1,000 L is typical for a single-family house, against 250–300 L for hot water only. Plan 60–80 litres per m² of collector, and make sure the store is stratified rather than a single mixed volume.
Is a combisystem better than adding a heat pump?
They work best together: the heat pump carries the winter, while the solar array covers most of the hot water and much of the spring and autumn load, so the compressor runs less. On its own, a combisystem is the stronger option where the fuel being displaced is expensive, the emitters already run cool, and a subsidy rewards solar heating support.