Heating system balancing: optimise your system for consistent comfort

Two-pipe system

The two-pipe system is the most common in France. Each radiator is fed by a flow pipe and a return pipe. Balancing is done by setting the Kv of the balancing valves (e.g. Caleffi, IMI TA STAD, Danfoss AB-QM) so that flow is distributed in proportion to each radiator's output. The design delta T is typically 10 to 20 °C depending on the temperature regime. Radiators close to the plant room must be throttled more (lower Kv) to push water towards the index radiators further away.

One-pipe system

Older and less efficient, the one-pipe system passes water from one radiator to the next in series. The temperature drops progressively, so the last radiators have to be oversized. Balancing is trickier: the bypass valve on each radiator is adjusted to control the proportion of flow passing through the emitter. This type of system is common in apartment buildings from the 1960s to 1980s.

Principle

Low-temperature underfloor heating runs on water at 25 to 40 °C, circulating through loops embedded in the screed. Balancing is carried out at the manifold by setting the flow rate of each loop. Loop length, pipe diameter and the floor area to be heated determine the design flow rate of each circuit.

Specific considerations

The delta T is small (5 to 10 °C), which makes measurement more delicate. An accurate flow meter is essential. Modern manifolds (Rehau, Uponor, Giacomini) have built-in sight-glass flow meters that make adjustment easier. Beware of the screed's high thermal mass: the effect of any adjustment only shows after several hours.

Operation

Fan coil units (FCUs) combine a water coil and a fan. They are used for both heating and cooling. Hydronic balancing is carried out as for radiators, but fan speed and airflow must also be taken into account. Motorised 2-port or 3-port valves add an extra layer of control.

Points to watch

The design flow rate depends on the operating mode (heating or cooling). In heating mode, the delta T is 10 to 15 °C; in cooling mode, 5 to 7 °C. Balancing must be checked in both modes. Clogged filters reduce airflow and distort performance, so clean them first.

Architecture

A communal boiler plant serves several buildings or stairwells via a primary circuit. Balancing takes place at two levels: the primary (between the plant room and the substations) and the secondary (between each substation and the heat emitters). The main balancing valves (DN 50 to DN 150) are set first, then you work down to the terminal valves.

Challenges

Balancing a communal heating system directly affects how heating costs are shared between co-owners. An unbalanced system causes overconsumption in some buildings and comfort complaints in others. French law now requires individual heat cost allocation, which makes balancing even more critical for fair billing.

Air-to-water heat pump

Air-to-water heat pumps are increasingly replacing boilers in both new-build and refurbishment. They operate at low temperatures (35-45 °C), which calls for more precise balancing than a conventional boiler. The delta T is small (5 to 8 °C), so flow rates must be high to compensate. The heat pump's built-in circulator has a specific pump curve that must be respected.

Ground source heat pump

A ground source heat pump extracts energy from the ground via vertical boreholes or horizontal collectors. On the ground loop side, the flow in each borehole must be balanced to optimise heat exchange. On the emitter side, balancing follows the same rules as for an air-to-water heat pump. The key point is that the heat pump's COP (coefficient of performance) is directly linked to the quality of balancing: poor balancing lowers the COP and increases electricity consumption.

Review the system and drawings

Before any work on site, the engineer studies the heating system drawings: hydraulic schematic, pipe sizes, emitter layout and balancing valve locations. They identify the main branches, sub-branches and fittings (bends, reducers, tees). This preliminary review explains how the system is laid out and highlights likely problem areas. Calculating the friction and fitting pressure losses on each branch then determines the valve settings required.

Calculate design flow rates for each emitter

Each emitter (radiator, underfloor heating loop, fan coil unit) has a design output specified by the design engineer. The design flow rate is calculated with the formula Q = P / (Cp × delta T), where P is the output in watts, Cp the specific heat capacity of water (4,185 J/kg·K) and delta T the difference between supply and return temperatures. For example, a 1,000 W radiator with a delta T of 10 °C needs a flow rate of 86 litres per hour. These calculations are recorded in a balancing schedule that serves as the reference for the valve settings.

Check the circulator pump

The circulator pump is the heart of the hydronic system. Its flow rate and head must match the system's requirements. An undersized pump cannot adequately supply the most remote emitters; an oversized pump creates excessive velocities and noise. Modern variable-speed (A-rated) circulators adapt automatically, but their operating curve still needs checking. The duty point (where the pump curve meets the system resistance curve) must lie within the best-efficiency zone.

Fully vent the system

Before taking any measurements, the entire system must be vented to remove trapped air. Air in the pipework distorts flow measurements and prevents proper circulation. Venting starts at the system's high points and works progressively downwards. Automatic air vents must be checked and, where necessary, operated manually. A properly vented system should no longer gurgle or show cold spots at the top of radiators.

Set the Kv of the balancing valves

The Kv (flow coefficient) of a balancing valve determines the flow through the valve for a given pressure drop. It is set by turning the valve's presetting handwheel. Each manufacturer (Caleffi, IMI TA, Danfoss, Oventrop) provides charts or selection software giving the number of turns for the required Kv. Start with the valves furthest from the plant room (the least throttled), then work progressively back towards the plant room, throttling more as you go. This sequence is essential for a stable balance.

Measure supply/return temperatures on each loop

Using contact thermometers or temperature probes clamped to the pipework, the supply and return temperatures of each emitter or loop are measured. These readings confirm whether the actual flow rate matches the calculated one. If the delta T is too high, the flow is too low; if the delta T is too low, the flow is too high. An infrared thermometer can supplement the readings for quick checks, but contact probes are more accurate for fine-tuning.

Check the delta T

The delta T (the temperature difference between supply and return) is the main indicator of balancing quality. Under design conditions, the delta T should be consistent across all emitters. For a high-temperature radiator system, the design delta T is 15 to 20 °C; for a low-temperature system (underfloor heating, heat pump), it is 5 to 10 °C. A deviation of more than 2 °C between measured and design delta T indicates an imbalance that must be corrected. Balancing is iterative: adjusting one valve can affect the others, hence the need for several verification passes.

Write up the balancing report

The balancing report is the formal document certifying that balancing has been carried out correctly. It contains: project details, date of the works, measurement conditions (outdoor temperature, operating regime), a table of design and measured flow rates for each emitter, valve settings (number of turns), measured supply/return temperatures and the resulting delta T. It is handed to the client and forms part of the DOE (the French as-built file, equivalent to O&M manuals). It serves as the reference for future maintenance and rebalancing.

Context

A six-storey building (ground + 5) with 40 apartments, served by a communal gas condensing boiler plant. The two-pipe system feeds 100 steel panel radiators. Handover is due in 3 weeks and the client requires an RE2020-compliant balancing report.

Problem

Radiators on the top floors are lukewarm while those on the ground floor are scalding. The measured delta T ranges from 5 °C (ground floor) to 25 °C (5th floor), against a design delta T of 15 °C. The circulator pump is set to maximum speed, causing noise in the risers.

Solution

After checking the drawings, the engineer finds that the balancing valves at the base of the risers have never been set (all fully open). They calculate the design flow rate for each riser, then for each radiator. The riser valves (IMI TA STAD DN25) are set first, starting with the most remote riser. The radiator valves are then preset using the manufacturer's charts. The circulator pump is switched back to variable speed (proportional pressure mode). After balancing, the delta T is a consistent 15 °C ± 2 °C across the whole system, and the pump uses 40% less electricity.

Result

Compliant balancing report, handover signed off, and estimated energy savings of 22% compared with an unbalanced system.

Context

A 3,000 m² office building with fan coil units. The 15-year-old plant room has just been refurbished: the oil-fired boiler has been replaced with a 120 kW air-to-water heat pump and the circulator pump has been changed. The existing two-pipe distribution is retained.

Problem

Since the replacement, occupants on the 2nd floor have been complaining of cold. The fan coil units run at full speed but cannot heat the space. The supply temperature has dropped from 80 °C (old boiler) to 45 °C (heat pump), so a much higher flow rate is needed for the same output, yet the balancing valves are still set for the old high-temperature regime.

Solution

The engineer recalculates all design flow rates for the new temperature regime (45/38 °C, a delta T of 7 °C instead of 20 °C). The required flow rates almost triple. The balancing valves are opened up and reset to pass the new flows. Some small-bore valves are replaced because their maximum Kv cannot deliver the required flow. The new circulator is set to constant pressure mode with a 40 kPa set point. After rebalancing, temperatures are even across all offices.

Result

Comfort restored in every office, heat pump COP up by 0.5 thanks to better delta T control, and overall energy consumption down 35% compared with the old oil-fired installation.

Context

A 180 m² detached house with low-temperature underfloor heating supplied by a ground source heat pump on vertical boreholes (2 × 100 m). The manifold serves 8 underfloor heating loops of different lengths (60 to 120 m). The owner complains of cold areas in the living room and master bedroom.

Problem

The longest loops (living room 120 m, bedroom 100 m) are starved of flow because their pressure drop is much higher than that of the short loops (bathroom 60 m, WC 65 m). Flow naturally favours the shortest loops, which offer the least resistance. The measured delta T ranges from 3 °C (short loops, excess flow) to 12 °C (long loops, insufficient flow).

Solution

At the manifold, the engineer throttles the short loops by partially closing their fine-adjustment valves and opens up the long loops. Flow on each loop is checked using the manifold's built-in flow meters (Rehau HKV-D). The temperature regime is set to 32/27 °C (a 5 °C delta T) to optimise the ground source heat pump's COP. On the ground loop side, the flows in both boreholes are checked and balanced to ensure even heat extraction from the ground.

Result

Even floor temperatures in every room (± 0.5 °C), the heat pump's seasonal COP up from 3.8 to 4.3, electricity consumption down 12%, and the cold areas reported by the owner eliminated.