Hydronic Heating and Cooling

Water-based heating and cooling, flow and temperature difference, pipe sizing, emitters, expansion vessels and pumps.

On this page

  1. Why water
  2. Flow, temperature difference and heat
  3. Sizing the pipes
  4. Emitters and flow temperature
  5. Expansion vessels and safety
  6. Pumps and balancing
  7. Low-temperature systems and heat pumps
  8. FAQ

Why water

Water carries nearly 3 500 times as much heat per unit volume as air, so a hydronic (water-based) system moves the same heat through pipes a fraction of the size of the ducts. It is used for radiators, underfloor heating, fan coils, chilled beams, boilers and heat pumps, and the same equations serve heating and cooling.

Flow, temperature difference and heat

Q = ṁ cp ΔT = ρ V̇ cp ΔT

Worked example: 20 kW of radiators

  • Heating at 20 kW with a supply of 70 °C and a return of 50 °C (ΔT = 20 K): ṁ = 20 ÷ (4.186 × 20) = 0.239 kg/s.
  • At 60 °C mean (density 983.2 kg/m³) that is 0.243 L/s or 0.87 m³/h.
  • A smaller ΔT needs more flow: with ΔT = 10 K the flow doubles to 0.478 kg/s (2.0 times). Larger pipes and pumps, but better condensing-boiler and heat-pump performance (see below).

Sizing the pipes

Pipe is sized by velocity and by friction. Typical design: 0.5 to 1.2 m/s in small pipes (up to DN50), up to 2 to 3 m/s in mains, and a friction loss of 100 to 300 Pa/m (above that it is noisy, below it the pipe is oversized and costly). The pressure drop comes from the Darcy–Weisbach equation (see Pipe Friction and the Moody Diagram).

Worked example: the pipe for 20 kW

  • For a velocity of 0.8 m/s the inside diameter = √(4 V̇ ÷ (π v)) = 19.7 mm, so a DN20 pipe (inside diameter 21.6 mm) fits.
  • In DN20: v = 0.66 m/s, Re = 30 154, f = 0.0282 (copper, 0.045 mm), so 283 Pa/m: just under the 300 Pa/m ceiling, so DN25 (about 90 Pa/m) is the quieter choice.
  • A 30 m run each way (60 m) therefore loses 17.0 kPa in the pipe; with fittings and valves the circuit needs about 1 to 2 times that.

Emitters and flow temperature

A radiator’s output depends on the temperature difference between its water and the room, approximately as the 1.3th power of the log-mean difference: Q ∝ ΔTlm1.3. So a radiator sized for 70/50 °C gives much less when the water is cooler.

Worked example: running the same radiators at lower temperature

  • At 70/50 °C with a room at 20 °C: ΔTlm = (50 − 30) ÷ ln(50 ÷ 30) = 39.2 K (the design output).
  • At 55/45 °C: ΔTlm = 29.7 K, so output = (29.7 ÷ 39.2)1.3 = 70 percent. To deliver the same heat the emitters must be 1.43 times larger.
  • Underfloor heating at 35/30 °C works with a floor at about 25 to 29 °C: its large area gives the output with a small difference, which is why it suits low flow temperatures (the same radiator would give only 22 percent).

Radiators rely on convection and radiation. Fan coil units boost the convection with a fan and so give more output at a low water temperature, and are also used for cooling (they have a drain pan).

Expansion vessels and safety

Water expands when it is heated: from 20 °C (998.2 kg/m³) to 80 °C (971.8 kg/m³) it grows by 2.7 percent. A sealed system must take up that volume in an expansion vessel (a diaphragm and a gas cushion) or the pressure will rise until the safety valve lifts.

Worked example: the expansion vessel for a 200 L system

  • Expansion volume = 200 × 0.0272 = 5.4 L.
  • Vessel size = Ve ÷ (1 − P₁ ÷ P₂), with the fill (pre-charge) pressure P₁ = 2.0 bar absolute and the maximum working pressure P₂ = 3.0 bar absolute: 5.4 ÷ (1 − 0.67) = 16 L. Choose the next size up, 18 or 25 L.
  • The relief valve must be set above the working pressure and discharge safely. Always use vented or sealed systems built to the code (EN 12828, ASME) with a safety valve, a pressure gauge, and a way to refill and vent air.

Pumps and balancing

The circulator pump has to overcome the pipe, valve, emitter and boiler resistance at the design flow (the system curve, a pressure drop varying with the square of the flow, see Pumps, Curves and NPSH). For the example, 0.243 L/s at 3 m of head needs a hydraulic power of 7.0 W; with a small-pump efficiency of 35 percent the electrical input is about 20 W, small but running 5 000 hours a year. A variable-speed (EC) pump that follows demand cuts it greatly, by the same cube law as fans. Balancing valves or pressure-independent control valves make sure each emitter gets its design flow so that the far radiators are not starved and the return stays cool.

Low-temperature systems and heat pumps

Condensing boilers need a return below about 55 °C to condense (see Combustion and Boiler Efficiency), and heat pumps are far more efficient with a low flow temperature (see Refrigeration and Heat Pumps). With a source at 0 °C and a machine at half of the Carnot value, the COP is about 3.0 for a 55 °C flow and about 4.4 for a 35 °C flow: a large difference. That is why heat-pump installations favor oversized emitters, underfloor heating and a low-temperature weather-compensated curve. For frost protection add glycol: a 30 percent mixture has a lower specific heat (about 3.75 kJ/kg·K) and a higher viscosity, so it needs about 12 percent more mass flow and more pump power for the same heat.

Related

FAQ

Why is my radiator hot at the top and cold at the bottom?

Usually the circuit is unbalanced or the flow is low; the radiator is running at a larger ΔT than designed. It can also mean sludge: flush and add an inhibitor.

What is a good flow temperature for a heat pump?

As low as comfort allows: 35 to 45 °C with underfloor or big radiators, 50 to 55 °C with standard ones, and weather-compensated so it falls when it is mild.

What does a ΔT of 20 K mean for the pump?

Half the flow of a 10 K design, so smaller pipes and a smaller pump, but the emitters must be hotter on average.

Do I need to vent a heating system?

Yes. Trapped air blocks flow and corrodes; use automatic air vents at high points and a deaerator in larger systems.

References

  • ASHRAE, Handbook – HVAC Systems and Equipment, Chapters on hydronic heating and cooling, and Fundamentals, Chapter 22 (pipe design).
  • CIBSE Guide B1 and Domestic Heating Design Guide; EN 442 (radiator output) and EN 12828 (heating systems in buildings).
  • Hydronics Institute and Idronics (Caleffi) design manuals.

Last reviewed: October 2026. Radiator exponents, efficiencies and vessel sizes are typical.