Heat pump in an old building with radiators: how to test your house

Many owners of older houses assume that a heat pump will not work for them: too old, poorly insulated, no underfloor heating. This is often not true. Whether it is suitable depends less on the year of construction than on a single figure: the flow temperature your house needs on the coldest day. And you can test this before you buy anything.

Why radiators are not a dealbreaker

A heat pump operates more economically the cooler the heating water can be. According to klimaaktiv, every degree lower flow temperature reduces the heat pump's electricity consumption by about 2 to 2.5 per cent per year. At the same time, technology has improved: according to klimaaktiv, modern appliances operating at up to 55 °C are just as efficient as appliances operating at 40 °C a few years ago.

In addition, there is an advantage that many old buildings bring with them unnoticed: in the past, radiators were often generously sized. klimaaktiv points out that existing radiators can therefore sometimes continue to be used with a lower flow temperature during renovation, especially if the house has been insulated beforehand.

We explain exactly how the flow temperature works in detail in the article Flow temperature: the figure on which almost everything depends.

The 55-degree test

klimaaktiv suggests a simple rule of thumb: if all living spaces get sufficiently warm on a cold winter's day with a flow temperature of at most 55 °C, the house is suitable for a heat pump without any further measures.

Here is what you do:

1. Wait for a cold week. On mild days, the test is not meaningful.
2. Lower the flow temperature step by step. Using the controller of your existing heating system, in small steps over several days.
3. Go through all the rooms. Not just the living room. Note down which room gets too cold first.
4. Make a note of the value where it gets tight. Take this room and this figure with you to your discussion with the installer.

The figure of 55 °C also has a practical side: the federal public subsidy „Kesseltausch“ requires a maximum flow temperature of the heat distribution system of 55 °C according to the info sheet for the 2026 relaunch (eligible from 7 October, registration from 2 November). In some federal states, additional, sometimes stricter limits apply. You can see what applies to you in the Förder-Manager.

Check and selectively replace radiators

If your test shows that only one or two rooms cannot keep up, this is the most common and at the same time most harmless case. Usually, these are rooms with large window areas, corner locations, or a small radiator under the window.

Checklist questions for each problem room:

• How large is the radiator in relation to the room? A narrow radiator in a large corner room is a typical bottleneck.
• Is it blocked? Furniture, curtains, or covers slow down the heat emission.
• Is the water actually flowing through it properly? Without hydraulic balancing (hydraulischer Abgleich), some radiators do not get enough water.
• Is there space for a larger one? klimaaktiv recommends installing larger radiators where space permits, which emit the necessary heat at a lower temperature.

Where there is no space, there is a second option: according to klimaaktiv, fans can be mounted on existing radiators to increase heat emission at a low flow temperature. klimaaktiv emphasises that even a targeted radiator replacement can lower the flow temperature by a few degrees and permanently save operating costs.

Insulation steps in a sensible order

Not every renovation has to happen before the heat pump. But some steps are so cost-effective in relation to their impact that they belong beforehand. Before sizing the heat pump, klimaaktiv explicitly mentions two measures: insulating the top floor ceiling and hydraulic balancing (hydraulischer Abgleich).

A sensible order for many old buildings:

1. Hydraulic balancing (hydraulischer Abgleich). Distributes the heating water evenly so that the worst room does not dictate the temperature for the whole house.
2. Insulating the top floor ceiling. Often doable with manageable effort and highly effective.
3. Replacing individual radiators. Exactly where your test showed bottlenecks.
4. Windows, cellar ceiling, facade. Larger steps that you plan with an energy consultant. klimaaktiv recommends also having an energy performance certificate (Energieausweis) drawn up for this.

The reason for this order: what you improve before installation reduces the heating load. What you improve afterwards makes the already installed heat pump oversized.

The heating load: no reliable quote without a calculation

According to klimaaktiv, heating systems are frequently oversized in practice. This has a negative impact on the lifespan and efficiency of the heat pump. klimaaktiv advises asking the planner how the heating load is calculated: empirical values or estimates are not enough; the calculation should be carried out in accordance with the applicable standard.

Particularly in old buildings, there is a strong temptation to simply adopt the output of the old boiler. However, old boilers were often significantly larger than necessary. You can get a first assessment with our heating load calculator. It does not replace the calculation by the installer, but it gives you a feel for the scale.

Before you speak to companies, it is also worth taking a look at the suitability test.

Checklist for the discussion with the installer

• I know which rooms get cold first at a low flow temperature.
• I know whether hydraulic balancing (hydraulischer Abgleich) has been carried out.
• I will ask for a heating load calculation according to the standard, room by room.
• I will ask what flow temperature the system is being designed for, as a specific figure.
• I will ask which radiators should be replaced or supplemented with fans, and why.
• I will clarify whether the planned design meets the subsidy conditions of my federal state.

How to go through a quote point by point afterwards is explained in the guide Checking the quote.

Your next step

If your house passes the 55-degree test or only a few rooms need retrofitting, you are a typical candidate for a heat pump in an existing building. Via our request for quotes, you will receive quotes from at most three verified installers from your region. Take your test results with you to the discussion: they are more valuable than any estimate.