The relationship between flow, temperature difference and transferred heat output in a water-based system, with a worked example.
Introduction
In a water-based heating or cooling system the energy is carried by the water. How much power is transferred depends on how much water flows and by how many degrees it is cooled (or heated) along the way.
For water near room temperature the formula reduces to a handy rule of thumb: P[kW] ≈ q[l/s] · 4.19 · ΔT. A ΔT of 20 K means, for example, 70/50 °C flow/return.
Worked example
A flow of 0.2 l/s with ΔT = 20 K gives P = 0.2 · 4.19 · 20 ≈ 16.8 kW. If you want to move more power through the same pipe, you must either increase the flow (= higher velocity) or increase ΔT.
Assumptions
- 01c and ρ for water vary slightly with temperature — the common values near room temperature are used here.
- 02Applies to pure water. For glycol mixtures and other fluids, the relevant material constants must be entered.
Mass flow and volume flow at different temperatures
Formlen bygger på massestrømmen ṁ [kg/s], men på pladsen måler og indregulerer man som regel volumenstrømmen q [l/s eller m³/h]. De to hænger sammen via massefylden: ṁ = q · ρ. For koldt vand er ρ ≈ 1000 kg/m³ (1 kg/l), men varmt vand fylder mere: ved 70 °C er ρ ≈ 978 kg/m³, ved 90 °C ≈ 965 kg/m³. Ved varmeanlæg er forskellen på nogle få procent og forsvinder ofte i huskereglen P[kW] ≈ q[l/s] · 4,19 · ΔT, men ved præcis indregulering og ved varmt brugsvand skal du bruge den aktuelle massefylde.
Radiator dimensioning and EN 442
A radiator's rated output is not a fixed number — it applies at specific temperatures. All European radiators are measured according to DS/EN 442 at 75 °C supply, 65 °C return and 20 °C room, which gives a logarithmic mean temperature difference of 50 K. If the system runs colder (e.g. combined with a heat pump), the radiator emits less.
| System (forward / return / room) | ΔT (log.) | Share of the EN 442-effect |
|---|---|---|
| 75 / 65 / 20 (EN 442-reference) | 50 K | 100 % |
| 70 / 40 / 20 | ca. 33 K | ca. 58 % |
| 60 / 40 / 20 | ca. 29 K | ca. 49 % |
| 55 / 45 / 20 | ca. 30 K | ca. 51 % |
| 45 / 40 / 20 (varmepumpe) | ca. 22 K | ca. 35 % |
| 40 / 35 / 20 (varmepumpe) | ca. 17 K | ca. 25 % |
Andelene er beregnet med n = 1,3 og logaritmisk ΔT efter EN 442. Pointen er markant: ved lavtemperaturdrift til en varmepumpe leverer en given radiator kun en tredjedel til en fjerdedel af sin katalogeffekt — derfor kræver varmepumpekonvertering ofte større radiatorflader eller gulvvarme. (Kilde: DS/EN 442; REHVA Journal — vejledende beregning.)
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