HEATING. EXPLAINED.
Heating systems: understand how the whole system works
A practical, engineering-led guide to radiators, underfloor heating, pipework, flow rates, circulation pumps, balancing and controls. Better performance begins with understanding how the components work together.
A heating system is more than a heat source
The boiler or heat pump supplies heat, but comfort and efficiency also depend on how that heat moves through the building. Heat emitters, water temperatures, flow rates, pipework, pumps, valves and controls must all suit the building’s calculated demand.
Good design starts with room-by-room heat loss. From that, an engineer can assess each emitter, establish suitable design temperatures and determine the hydraulic requirements.
Heat emitters: matching output to the room
Radiators
Panel and column radiators transfer heat to rooms. Output depends on mean water temperature, room temperature, radiator construction and size.
Underfloor heating
A large heated floor area can provide room heat at comparatively low water temperatures, subject to floor construction, covering and heat loss.
Fan-assisted emitters
Fan convectors and similar emitters can provide useful output at lower water temperatures, but require consideration of noise, power and maintenance.
An emitter must deliver at least the required room heat output at the intended design conditions. Manufacturer ratings are often quoted at a reference temperature difference; the actual output at lower water temperatures can be substantially less.
Radiator sizing and temperature difference
For a radiator, the mean water temperature is approximately the average of flow and return temperatures. The radiator temperature difference (ΔT) is this mean water temperature minus the room air temperature.
Worked example: at 55°C flow, 45°C return and 20°C room temperature, mean water temperature is 50°C and radiator ΔT is 30 K.
A commonly used approximate correction for a radiator rated at ΔT50 is: output at ΔT30 ≈ rated output × (30 ÷ 50)1.3. This gives about 51% of its ΔT50 rating. The exponent is indicative; use the manufacturer’s published correction factors and data for design.
A radiator rated at 2.0 kW at ΔT50 might therefore provide roughly 1.03 kW at ΔT30 under this simplified estimate. Do not size an entire system from nominal dimensions alone.
Underfloor heating: design considerations
- Heat output: calculate the room load and check achievable floor output at acceptable floor surface temperatures.
- Floor construction: screed depth, timber floors, insulation and floor coverings affect response and output.
- Pipe spacing and circuit length: select to meet output and pressure-loss requirements.
- Controls: coordinate zone operation, mixing arrangements and weather compensation with the heat source.
- Commissioning: fill, purge, balance and document individual circuits.
Underfloor heating is not automatically required for a heat pump; correctly sized radiators or other emitters may also be suitable.
Water flow rate: the core heating calculation
For water-based heating, the approximate relationship is heat output (kW) = 4.18 × flow (litres/second) × water temperature drop (K). Rearranging gives flow (L/s) = heat output (kW) ÷ [4.18 × ΔT (K)].
Worked example: a circuit delivering 10 kW at a 5 K flow-to-return drop requires about 0.478 L/s, equivalent to 28.7 L/min or 1.72 m³/h.
For the same 10 kW at a 10 K drop, the flow is about 14.4 L/min. The chosen temperature drop changes the required water flow and must match the heat source and emitter design.
These are approximate water calculations; glycol mixtures, operating temperature and fluid properties can alter the result.
Pipe sizing: flow, velocity and pressure loss
Pipe diameter cannot be selected reliably from boiler or heat-pump output alone. The designer must consider required flow, internal pipe diameter, material, circuit length, fittings, valves and available pump head.
The relationship flow = cross-sectional area × velocity allows a preliminary velocity check. In practice, pressure loss through the complete circuit is also essential.
Small-bore and microbore systems deserve particular attention during low-temperature conversions: an existing branch may carry enough flow for a high-temperature radiator but be restrictive when greater flow is required.
Use actual internal diameters and manufacturer pressure-loss data. Nominal 10 mm or 15 mm tube sizes do not describe every product’s internal bore.
Circulation pumps and hydraulic design
- Calculate design flow through the heat source and distribution circuits.
- Estimate the resistance of the most demanding circuit, including fittings, control valves and equipment.
- Select a pump that delivers the required flow at the necessary differential pressure, without excessive noise or energy use.
- Check minimum-flow requirements, variable-speed operation and interactions with thermostatic radiator valves.
- Assess whether hydraulic separation, a buffer or a volumiser is necessary for the specific heat source and controls; these are not universal requirements.
Pump selection should be based on a duty point, not simply on the size of the previous boiler or pump.
Hydraulic balancing: delivering heat where it is needed
Balancing adjusts flow distribution so that each circuit and emitter receives the intended share of water. Without it, nearby radiators may receive excessive flow while more distant rooms remain cold.
- Verify the system is clean, properly filled and free of trapped air.
- Check radiator valves, strainers, pump settings and design flow requirements.
- Measure or infer flows using appropriate valves, instruments and temperature readings.
- Adjust circuits progressively and confirm room performance under stable operating conditions.
- Record commissioning settings for future maintenance.
Balancing cannot compensate for an undersized radiator, blocked pipe, incorrectly selected pump or fundamentally unsuitable design.
Controls and weather compensation
Controls should help the heating system meet comfort requirements with as little unnecessary energy use as practicable. Important elements include time scheduling, room temperature control, suitable zoning, domestic hot-water priority and weather compensation.
Weather compensation adjusts heating-water temperature according to outdoor conditions. When correctly commissioned, it can help heat pumps operate at lower flow temperatures during milder weather.
Avoid unnecessary short cycling and control arrangements that regularly prevent the heat source from achieving its required flow. The correct approach depends on the equipment and system layout.
System efficiency and common faults
- High flow temperatures: may increase heat-pump electricity use; investigate emitter sizing, heat loss and control settings.
- Cold radiators: may indicate air, poor balancing, stuck valves, restricted flow or inadequate pump duty.
- Noise: may be associated with excessive velocity, valve differential pressure, trapped air or pump operation.
- Frequent cycling: may relate to load, controls, system water volume or hydraulic configuration.
- High energy bills: require assessment of heat demand, tariffs, operating schedules and actual seasonal performance.
A systematic diagnosis uses measurements rather than changing several settings at once.
Heating upgrade checklist
- Obtain a room-by-room heat-loss calculation.
- Confirm the intended heat source and design flow/return temperatures.
- Verify radiator or underfloor output in every room.
- Calculate design circuit flows and pipework pressure losses.
- Select and check circulation pump duty and hydraulic arrangement.
- Specify controls, weather compensation and hot-water strategy.
- Plan cleaning, water treatment, testing, balancing and commissioning.
- Record as-built settings, operating guidance and maintenance requirements.
Frequently asked questions
Can existing radiators be used with a heat pump?
Often, yes, but their output must be checked at the proposed lower water temperatures. Some radiators may need upgrading.
Does microbore pipework always need replacing?
No. Its suitability depends on actual pipe lengths, internal diameters, required flows and pressure losses. A hydraulic assessment is needed before deciding.
What is the difference between system ΔT and radiator ΔT?
System ΔT usually means the water temperature drop between flow and return. Radiator ΔT often means the difference between mean radiator water temperature and room air temperature. They are different quantities.
Will balancing reduce my energy bills?
Correct balancing can improve heat distribution and support efficient operation, but savings depend on the initial fault, heat source and controls. It should not be presented as a guaranteed percentage saving.
Build a better heating system from the fundamentals
Understand the building load, match the emitters, establish the flows and commission the controls. These steps matter whether you are retaining a boiler or planning a heat pump.
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