Important
Different manufacturers use completely different scales. A "curve 5" on NIBE is not the same as "curve 5" on IVT. Use the correct table for your brand.
NIBE
Parameters: NIBE FAQ · Values: Byggahus.se, Installer handbook (PDF)
Parameters
- Curve (Menu 1.9.1) – Curve slope, scale 0–15
- Offset (Menu 1.1) – Parallel shift, scale -10 to +10 (each step ≈ 2.5°C)
Note
NIBE does not provide fixed recommended values. Official manuals use regional tables based on DUT (design outdoor temperature). The values below are approximate guidelines from installers and forums.
Approximate values
| System | Curve | Offset |
|---|---|---|
| Underfloor heating in concrete slab | 4–5 | 0 to -1 |
| Underfloor heating + shunted radiators | 8–9 | varies |
| Radiators (low-temp) | 8–10 | 0 to -2 |
Example from installer handbook (Region 10, Markaryd)
- Underfloor heating in concrete joists: Curve 5
- Low-temperature radiator system: Curve 10
IVT
Source: IVT Energispararen · IVT 690 Manual (PDF)
Parameters
- Curve/Curve slope – Scale 0–10 (decimal values like 2.1, 3.8 supported, factory setting 4)
- Fine-tuning heating c. – Parallel shift (10 "clicks" ≈ 0.6°C)
- Base point – Adjustment for mild weather (around 0°C)
- End point – Adjustment for cold weather
Recommended values Official source
| System | Curve slope |
|---|---|
| Underfloor heating in concrete slab | 1–2 |
| Underfloor heating on wooden joists | 3–4 |
| Radiators | 4–6.5 |
| Abnormally high (small radiators) | 7–10 |
Quote from IVT manual
"Curve slope 1-2: Normal setting for underfloor heating in concrete slab. Curve slope 4-6.5: Normal setting for radiators."
Thermia
Source: Byggahus.se, Värmepumpsforum
Parameters
Thermia uses a different system where CURVE specifies the supply temperature in °C at 0°C outdoors – not an abstract slope number:
- CURVE – Supply temp at 0°C outdoors (factory setting 40°C)
- CURVE +5 – Adjustment at +5°C outdoors (default 0)
- CURVE 0 – Adjustment at 0°C outdoors (default 0)
- CURVE -5 – Adjustment at -5°C outdoors (default 0)
- ROOM – Parallel shift (each 1°C changes supply by 3°C)
- MIN TARGET – Minimum supply temp (default 22°C)
- MAX TARGET – Maximum supply temp (default 55–70°C)
Recommended values
| System | CURVE (°C at 0°C outdoors) | MAX TARGET |
|---|---|---|
| Underfloor heating | 28–35 | 35–40 |
| Radiators (well-insulated) | 35–40 | 50–55 |
| Radiators (older house) | 40–50 | 55–70 |
Example from Thermia technician
Curve 40°C, Min 22°C, Max 70°C, +5: 0, 0: 0, -5: 0, Room: 20°C
How to adjust
- Generally too cold/warm → Change ROOM
- Cold only in freezing weather → Increase CURVE or CURVE -5
- Cold around 0°C → Increase CURVE 0
CTC
Source: CTC EcoHeat 400 Manual (PDF) · CTC Glossary
Parameters
CTC specifies the curve as supply temperature at -15°C outdoors:
- Curve slope/Slope – Supply temp at -15°C outdoors, scale 20–70°C
- Curve adjustment – Offset in °C
- Underfloor heating limit – Max temp for underfloor heating
Recommended values Official source
| System | Slope | Description |
|---|---|---|
| Underfloor heating only | 35 | Low-temperature system |
| Well-insulated house | 40 | Low-temperature system |
| Normal house | 50 | Factory setting |
| Older house, small radiators | 60 | High-temperature system |
Quote from CTC manual
"Curve slope 50 means that the value set is the outgoing temperature to the radiators at -15°C outdoor temperature."
Viessmann
Source: Viessmann Sweden
Parameters
Viessmann uses German terminology even in other markets:
- Neigung (Slope) – Curve steepness, scale 0.2–1.6+ (factory setting 1.4)
- Niveau (Level) – Parallel shift from 0 (factory setting 0 = 20°C room temperature)
Recommended values Official source
| System | Neigung (Slope) |
|---|---|
| Underfloor heating (well-insulated) | 0.3–0.5 |
| Radiators (well-insulated) | 1.0–1.2 |
| Older house with radiators | 1.4–1.6 |
Adjustment guide from Viessmann
- Generally too cold → Increase Niveau
- Cold in freezing weather but OK otherwise → Increase Neigung
- Cold in mild weather, OK in freezing weather → Increase Niveau, decrease Neigung
Comparison table
Overview of all brands' scales:
| Brand | Slope parameter | Scale | Underfloor heating | Radiators |
|---|---|---|---|---|
| NIBE | Curve | 0–15 | 4–5 | 8–10 |
| IVT | Curve | 0–10 | 1–4 | 4–6.5 |
| Thermia | CURVE | °C at 0°C outdoors | 28–35°C | 40–50°C |
| CTC | Slope | 20–70°C | 35 | 50 |
| Viessmann | Neigung | 0.2–1.6 | 0.3–0.5 | 1.0–1.2 |
Energy savings
Lowering the supply temperature results in energy savings, especially for heat pumps:
| Action | Approximate savings |
|---|---|
| 1°C lower supply temperature | 2–3% energy |
| 5°C lower supply temperature | 10–15% energy |
Why does lower temperature save energy?
Heat pumps operate more efficiently with a smaller temperature difference between the heat source and supply. COP (coefficient of performance) increases significantly at lower supply temperatures.