Hybrid heating: heat pump alongside the old boiler

Not everyone wants to or can throw out their old boiler straight away. A hybrid heating system is a middle ground: a heat pump takes over for most of the year, and a second heat generator kicks in when it gets very cold outside. This sounds like the best of both worlds. Whether it really suits you depends on your house, your plans, and a subsidy question that many only notice too late.

What a hybrid heating system is

A hybrid heating system combines a heat pump with a conventional heat source, such as a gas, oil, pellet, or wood boiler. Both are connected to the same heating system and are controlled by a common control system.

This article is about the most common case: the heat pump is newly added, and the existing boiler remains in the cellar as support.

The bivalence point

The central concept in hybrid heating is the bivalence point. It refers to the outdoor temperature below which the output of the heat pump alone is no longer sufficient. According to the energy provider eww, most manufacturers set this point at around minus 5 °C. In individual cases, it is determined during planning to suit the house.

Below this point, there are two operating modes:

Bivalent-parallel: The heat pump and boiler work together. The heat pump continues to deliver what it can, and the boiler covers the rest.
Bivalent-alternative: Below the bivalence point, the boiler takes over alone and the heat pump switches off.

The parallel operating mode uses the heat pump for longer. The alternative operating mode can make sense if the house requires high flow temperatures on very cold days, which the heat pump struggles to achieve.

When a hybrid heating system can make sense

Your house needs a flow temperature of more than 55 °C on the coldest days. klimaaktiv mentions 55 °C as a threshold: if all rooms get warm with this on a cold winter's day, the house is suitable for a heat pump without further measures. If your house is above this, the boiler can cover the few peak days.
A major renovation is planned but not yet implemented. In this case, the heat pump can already be designed for the state after the renovation, and the boiler bridges the time until then.
The boiler is still young and in good condition. Many people find it hard to dispose of it straight away.

When it is rather not suitable

Your house already passes the 55-degree test today. In that case, there is no need for a second heat generator. A pure heat pump is simpler, and you save on maintenance and fixed costs for the boiler.
You want a public subsidy for the switch. More on this in a moment.
The boiler is at the end of its life anyway. An old boiler that is about to fail is not a reliable backup.

The eww also points out that a hybrid heating system does not suit every building and is not the most efficient solution everywhere.

The catch with the public subsidy

Many public subsidy programmes for replacing heating systems require the old fossil fuel system to be decommissioned. One example is the federal public subsidy „Kesseltausch“. It is running again in a relaunch, eligible from 7 October 2026, registration from 2 November 2026, and the decommissioning rules remain the same. According to the info sheet for the relaunch, when replacing fossil fuel heating systems, all remaining heating systems powered by fossil fuels such as oil, gas, coal or coke must be taken out of service, decommissioned and properly disposed of. Oil tanks that cannot be disposed of must in any case be drained, cleaned and sealed, according to the info sheet.

For a hybrid solution with the old gas or oil boiler, this means: you will generally not receive this public subsidy. Individual conditions apply to the federal state subsidies (Landesförderungen), which change constantly. Therefore, before making a decision, check what applies in your federal state in the Subsidy Manager and look at the subsidy overview.

Calculate honestly: if the public subsidy for a pure heat pump is high, it may be economically better to switch completely straight away and invest the money in larger radiators or insulation measures instead. You can run through the comparison of running costs using the cost comparison tool.

What you need to pay attention to during planning

Do not plan the heat pump to be too small, but not too large either. klimaaktiv warns that heating systems are frequently oversized. The basis is a heating load calculation according to the standard. A first orientation is provided by the heating load calculator.
The control system is the heart of the installation. Clarify which device decides when the boiler kicks in, and whether you can adjust the bivalence point yourself.
Flow temperature as low as possible. According to klimaaktiv, every degree less reduces the electricity consumption of the heat pump by about 2 to 2.5 per cent per year. Hydraulic balancing and individual larger radiators help the heat pump to manage as many days as possible on its own. Read more about this in the article Vorlauftemperatur (Flow Temperature).
Consider domestic hot water. Who prepares the hot water, the heat pump or the boiler? In summer, the boiler should ideally not run at all.

Checklist questions for the quote

At what outdoor temperature is the planned bivalence point, and how was it determined?
Parallel or alternative: which operating mode is planned and why?
What proportion of the annual heating energy is the heat pump expected to cover?
Is the existing boiler technically suitable to work together with the heat pump?
Which public subsidy is possible with this solution, and which is not?
What would I need to do later to completely decommission the boiler?

How you evaluate quotes overall is explained in the guide Checking quotes.

Your next step

Whether hybrid or a pure heat pump: an installer should be able to explain both options to you. Via our request for quotes, you will receive quotes from a maximum of three checked companies. Let them know how old your boiler is and whether you want to apply for a public subsidy.