Heat pumps for older houses and radiators
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A heat pump can also heat an older house with radiators. First compare each room's heat loss with its radiators' output at the proposed water temperature.
Neither the age of the house nor its floor area determines the output needed. A high-temperature model does not guarantee low consumption or that the existing radiators will be enough.
See a radiator example at a lower water temperatureWhat to find out about the house and existing heating
The designer needs the condition of the house as it will actually be heated. If you are planning new windows or insulation, ask for an assessment of the house both before and after the work. The old boiler's output need not be the output the new heat pump needs.
Noticing that the house stays warm with cooler water on a mild day is useful information. It does not prove there is enough output in freezing weather. Discuss settings and tests on the existing system with a heating engineer.
Heat loss at the design temperature
Heat loss in W or kW describes the heating output needed under stated conditions. Annual heat demand in kWh is the amount of energy over a period. One does not replace the other.
Ask for a room-by-room calculation that states the outdoor design temperature and the required indoor temperatures. The public ÚNMS catalogue entry for STN EN 12831-1 confirms the standard for calculating design heat load; the catalogue itself is not a design calculation.
What happens to radiator output with cooler water
A radiator gives off less heat at a lower water temperature. The figures for the specific radiator decide this, not just its size or age. The notation 55/45/20 °C means 55 °C flow, 45 °C return and a 20 °C room.
An illustrative room with a 800 W heat loss
The room's heat loss of 800 W is an assumption. It is not a measurement of a house. The radiator is a Purmo Paros PAV211800405, type 21 from the archived 01/2015 catalogue, with no claim about current availability.
| Flow / return / room | Radiator output | Against a 800 W loss |
|---|---|---|
| 75/65/20 °C | 886 Wcatalogue | Enough in this example |
| 65/55/20 °C | 660 Wcalculated | Short by 140 W |
| 55/45/20 °C | 452 Wcatalogue | Short by 348 W |
Product data source: Purmo Paros, 01/2015 catalogue, PDF pages 2 and 5, checked 20 October 2026. The catalogue gives 886 W at 75/65/20 °C, 452 W at 55/45/20 °C and an exponent n = 1.3192.
How the row between catalogue temperatures is calculated
Following the manufacturer's method, first find the difference between the mean water temperature and the room: ΔT = (flow + return) / 2 − room. Output = 886 × (ΔT / 50)1.3192 W. Only the result is rounded; an available catalogue output takes precedence.
The exponent belongs to this specific radiator. For a different radiator, use its own data. The calculation does not check flow rate, pipe connections or hydraulic balancing and is not a complete design to EN 12831.
At 55/45/20 °C this radiator is not enough for the assumed room. That does not mean every radiator in the house must be replaced: each room needs checking separately.
Insulation, emitter changes or bivalent operation
Reduce heat demand
Assessed insulation or new windows can reduce heat loss. The existing radiator may then be enough at a lower water temperature too. Compare measures for the same house and its planned condition, rather than by a general rule for old houses.
Change only the radiators that fall short
If radiators are inadequate only in some rooms, the design can include a larger or additional emitter. For fan-assisted emitters, use their own operating data; do not apply the passive-radiator calculation above to them.
Add a second heat source
Bivalent operation means the heat pump works together with another heat source, such as a boiler or electric backup. The design must set when it switches on and what share of the heat it supplies. Its energy, servicing and any retained gas standing charge belong in the budget.
Is buying a high-temperature heat pump enough?
The maximum water temperature does not tell you what output and efficiency the system has at that temperature in freezing weather. Ask for data for the exact model at the design outdoor temperature and the radiator flow temperature.
This is a technical example of one system, not a recommendation for your house. At the same outdoor temperature, the table shows the difference with warmer water. COP at a single point does not determine the annual bill; the design must cover both output and consumption.
Cost and comparison with your current boiler
The investment includes the equipment, installation, any changes to radiators and pipework, the electrical supply, the cylinder and the second heat source. Dated offers with their scope and VAT are on the heat pump price page. We do not derive the cost of a conversion from the age of the house.
To compare running costs, you need the same amount of useful heat for both systems:
- Current boiler: fuel consumed, its price, seasonal efficiency and standing charges. The energy on the bill is not automatically the heat delivered to the rooms.
- Heat pump: a realistic seasonal factor at the required water temperature, the electricity price and consumption by backup heating and other equipment.
- Same scope: space heating, hot water, required room temperatures, servicing and retained connections.
The annual difference in running costs is not the payback on the whole investment. Upfront costs, repairs, financing and future energy prices also affect that. Without this information, we do not promise savings.
Assess any heat pump subsidy separately, according to the exact equipment and the household's conditions. Having radiators does not by itself determine eligibility.
The principle of assessing the system and comparing under the same conditions: SIEA, choosing a heat pump (Slovak).
Common questions
Do I have to replace all the radiators?
Not automatically. Compare each radiator's output at the proposed water temperature with its room's heat loss. In some rooms the existing radiator may be enough; in others, losses need reducing or the heating surface needs enlarging.
Is it better to insulate first?
If you are planning insulation, have it included in the design before choosing the heat pump. Comparing the condition before and after the work shows the output needed and what the radiators can do. The order of investments depends on the house's condition and your budget.
Is a high-temperature model enough?
Not the maximum temperature alone. You need to know the available output and efficiency in winter, whether the radiators have enough output and the hydraulic design. A higher achievable water temperature is no guarantee of cheap running costs.
Sources
We are an independent information site. We do not sell or install equipment. The room example is explicitly illustrative; it is not a survey or a design for a particular house.
- Purmo Paros, 01/2015 catalogue (PDF) — the exact radiator, outputs and conversion method; an archived example.
- SIEA, regional planning 2022 (PDF, Slovak) — climate references with limited use.
- ÚNMS, STN EN 12831-1 — the standard's catalogue entry, not its full text.
- Panasonic: technical data for the exact system — test output and COP with stated conditions.
- SIEA: choosing a heat pump (Slovak) — designing and comparing the system.