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.
What is a bivalent heating system?
A bivalent system covers a building's heat demand with two generators instead of one: a heat pump carries the base load, and a second source — usually an existing gas or oil boiler, a biomass boiler or solar thermal — covers the coldest peaks. The outdoor temperature at which the second generator joins in is called the bivalent point.
A close relative is the mono-energetic system: heat pump plus built-in electric heating element. Two heat generators, but one energy carrier — electricity — which keeps the installation simple while the element typically contributes only a few percent of the annual heat.
Where should the bivalent point sit?
Between −7 and −2 °C for most Central European buildings — low enough that the heat pump carries the season, high enough that the unit is not oversized for 95% of its operating hours. The load-duration curve does the heavy lifting: peak-cold hours are rare, so capacity and energy diverge sharply.
| Heat pump share of design load | Share of annual heat delivered (typical) |
|---|---|
| 100% (monovalent) | 100% |
| 80% | 97–99% |
| 70% | 93–96% |
| 60% | 87–92% |
| 50% | 78–85% |
The practical consequence: sizing the heat pump from an honest heat load calculation at 70–80% of the design load buys almost the entire annual energy share at markedly lower investment — and avoids the short-cycling an oversized compressor suffers in mild weather.
Bivalent-parallel, bivalent-alternative or mono-energetic?
Bivalent-parallel means both generators run together below the bivalent point; the heat pump keeps producing as long as it can contribute efficiently. It suits systems whose flow temperature stays within heat pump range, roughly up to 55 °C.
Bivalent-alternative means the boiler takes over entirely below the switch point — the classic retrofit answer for legacy radiator systems designed for 70 °C and above, where compressor efficiency would collapse.
Mono-energetic relies on the electric element for the last few percent; simple and cheap to build, but every element kilowatt-hour is bought at direct-electric cost, so the element should remain a rarely used reserve.
In all three layouts, a buffer tank usually acts as the hydraulic hub that decouples the two generators from each other and from the heating circuits.
Are bivalent systems still subsidised in the EU?
Yes — with an important asymmetry. Under the EPBD, stand-alone fossil-fuel boilers have been excluded from public purchase incentives since 1 January 2025, but hybrid systems combining a boiler with a heat pump or solar thermal may still receive support for the renewable share. Germany's BEG funds the heat pump component of a hybrid, while Austria's 2026 heating-replacement programme has already exhausted its budget — check the current official rules of each programme, as rates and budgets changed repeatedly during 2026. The regulatory background is covered in the EPBD guide.
When is bivalent the right design?
Three recurring cases. First, radiator retrofits: keep the functioning boiler for the coldest weeks instead of replacing every radiator at once. Second, security of supply for buildings that cannot tolerate outage risk. Third, staged decarbonisation: the boiler's runtime shrinks to a few hundred hours a year, and from 2028 the EU ETS2 carbon price makes each of those fossil hours progressively more expensive.
The counter-case is modern cold-climate equipment: EVI compressor technology, as used in the Solimpeks Varm Up Series operating down to −30 °C, keeps capacity available in deep frost and turns many former bivalent designs into straightforward monovalent ones.
Frequently asked questions
Can I keep my gas boiler when installing a heat pump?
Yes — that is precisely a bivalent (hybrid) system. The heat pump carries the base load, the boiler covers peak cold, and correctly configured controls make the heat pump the priority generator. It is a common staged path away from fossil heating in radiator buildings.
What is a mono-energetic heat pump system?
A heat pump backed only by its electric heating element. Both run on electricity, so no second fuel is needed; the element typically supplies just a few percent of annual heat if the bivalent point is chosen sensibly, but careless sizing can silently push expensive direct-electric operation upward.
Does a bivalent system need a buffer tank?
Usually yes. Two generators with different flow rates and temperatures need hydraulic decoupling, and the buffer provides it while adding defrost volume for the heat pump. Typical heat pump buffer sizing of 20–50 litres per kW applies.
Is a hybrid heat pump cheaper to run than a boiler alone?
In most European price environments yes, because the heat pump covers 90%+ of annual heat at a seasonal efficiency of 3 or more. The economics improve further from 2028, when the ETS2 carbon price raises gas and oil costs while the heat pump share is unaffected.