Four-Pipe Systems

The classic four-pipe boiler chiller system uses chillers for cooling and boilers for heating, each on its own circuit. This central boiler and chiller system is well proven, easy to size and simple to operate.

Air-to-water heat pumps produce hot water efficiently from electricity, replacing or supplementing boilers. They suit the moderate heating temperatures used by FCUs and reheat coils.

A heat recovery chiller captures the heat it removes from the chilled water circuit and delivers it to the hot water circuit. Cooling and heating come from the same electricity, which gives very high combined efficiency whenever the loads overlap.

Multifunction units produce chilled water and hot water at the same time. They recover heat between the two circuits and use outdoor air only for any imbalance, which makes them ideal for four-pipe central plant HVAC with year-round overlapping loads.

Constant speed pumps and three-way valves on each coil keep flow steady in both circuits. This is simple, but pumping energy stays high at part load.

A four-pipe variable flow system uses two-way valves or PICVs with VFD pumps on both circuits, so each circuit's flow follows its own load. This is the efficient choice for most modern buildings.

A dual-coil four-pipe fan coil unit has separate heating and cooling coils, each with its own valve. This arrangement is known as a four-pipe independent load system. The heating coil usually sits downstream of the cooling coil, so it can also provide reheat for dehumidification.

A single coil connects to both circuits through a six-way valve, which selects either chilled or hot water. This is a four-pipe common load system. It is compact and cost-effective, but it cannot cool and reheat at the same time, and valve sealing between circuits is critical.

AHUs serving lobbies, halls, operation theatres and cleanrooms carry separate cooling and heating coils. Each coil is fed from its own circuit and controlled independently.

In humidity-critical spaces, the cooling coil overcools the air to remove moisture, and a hot water reheat coil then brings it back to the required temperature. A four-pipe system provides that reheat from the hot water circuit, without electric heaters.

Each coil in a dual-coil unit has its own two-way control valve. The controller sequences them so only one opens at a time, except in dehumidification mode. Every four-pipe fan coil control valve should close tightly, to prevent energy loss through the idle coil.

A six-way valve connects a single coil to either the chilled water or the hot water circuit through one actuator. Its signal range covers a heating sequence, a closed middle position and a cooling sequence. Tight internal sealing prevents mixing between circuits.

In a PICV four-pipe system, each coil's flow is controlled and balanced automatically, whatever the pressure changes in either circuit. A PICV for four-pipe fan coil duty is fitted on both the cooling and heating coils, making commissioning faster and flows more stable.

In systems without PICVs, manual balancing valves set design flows in each circuit, branch and riser. The chilled water and hot water circuits are balanced separately.

Air-cooled or water-cooled chillers supply the chilled water circuit. They are sized for peak cooling, with redundancy for critical facilities.

Gas, LPG, diesel or electric boilers and hot water generators supply the hot water circuit. They are reliable and responsive, and well suited to peak heating in colder regions.

Heat pumps typically deliver several units of heat for every unit of electricity at moderate hot water temperatures. They are an efficient, low-emission heating source for four-pipe systems.

A heat recovery chiller produces chilled water and useful hot water at the same time. It is sized for the overlap between cooling and heating loads, and delivers very high combined efficiency.

Heat rejected by chiller condensers, process equipment or air compressors can be captured for reheat and domestic hot water preheat. Recovered heat is often the cheapest heat available on site.

Each shaft carries four risers: chilled water supply and return, and hot water supply and return. Floor branches connect every terminal unit to both circuits.

Direct return uses less pipe but needs balancing or PICVs to prevent near units taking too much flow. Reverse return equalizes path lengths for natural balance, at the cost of extra pipe. The choice can differ between the two circuits.

On large floor plates, chilled and hot water mains run horizontally on each floor from central risers, serving FCUs and AHUs across that level.

Four-pipe HVAC piping, with insulation on every line, needs careful coordination with ducts, sprinklers, electrical trays and structure. Early 3D coordination prevents clashes, protects ceiling heights and keeps valves accessible.

Each circuit has its own chilled water pumping system and hot water pumping system, with standby pumps. Hot water flows are usually much smaller, so hot water pumps are smaller too.

VFD pumps on each circuit, controlled from differential pressure at each circuit's index point, let flows follow load independently. This four-pipe hydronic pumping system cuts pumping energy significantly at part load.

Larger plants can use constant primary pumps at the chillers and boilers, with variable secondary pumps serving the distribution. Decouplers or hydraulic separators keep the loops independent.

Where chillers or heat pumps allow variable flow, one set of VFD pumps per circuit serves both plant and distribution. Minimum flow is protected by a bypass.

In India's humid climate, especially during the monsoon, cooling coils must overcool the air to remove moisture. The four-pipe system's hot water circuit then reheats it, so rooms stay dry without feeling cold.

Operation theatres, ICUs, cleanrooms and laboratories need tight temperature and humidity bands. Four-pipe AHUs with cooling and reheat coils, humidity sensors and well-tuned controls hold these conditions steadily.

Reheat energy can be expensive when it comes from boilers or electric heaters. Recovering chiller condenser heat for reheat turns heat that would otherwise be rejected into useful energy, cutting running costs substantially.

Two circuits mean more pipe, valves, insulation, pumps and controls, so capital cost is higher than for two-pipe systems.

Every terminal unit has four pipe connections and two valves, or a six-way valve. Installation quantities rise accordingly.

Four insulated pipes need more ceiling depth and shaft area, which must be planned early in the building design.

Two circuits, more connections and more controls demand careful coordination, workmanship and commissioning.

Poor controls, small deadbands or leaking valves can make a unit heat and cool at the same time without any dehumidification benefit. Good control design and maintenance prevent this.

More valves, actuators, coils and pumps mean more items to inspect and service. A structured AMC keeps this under control.

Buildings with mixed facades, deep floor plates or diverse occupancies benefit from heating and cooling at the same time.

Pharma plants, laboratories, museums and archives need reheat-based humidity control, which a four-pipe system delivers directly.

Guests and tenants in premium properties expect individual control and year-round comfort, and four-pipe systems deliver both.

Operation theatres, ICUs and patient rooms need stable temperatures, reheat and individual control. Four-pipe is often the right choice for these areas.

In North India and hill regions, winters bring genuine heating demand while internal zones may still need cooling. Four-pipe systems handle both without changeover.

Cooling, heating and reheat loads are calculated zone by zone and hour by hour. Knowing when the loads overlap is essential for sizing heat recovery equipment.

Flows for both circuits use the same formula:

Flow (m³/h) = Load (kW) ÷ (1.163 × ΔT in °C)

For example, a hotel floor with 45 TR (158 kW) of cooling at a 5.5 °C delta-T needs about 24.7 m³/h of chilled water. A 60 kW heating load at a 10 °C delta-T needs only about 5.2 m³/h of hot water.

Chilled water commonly runs at around 6.7/12.2 °C, or a wider delta-T to cut flow. Hot water for FCUs and reheat is often designed around 45–60 °C. Lower hot water temperatures improve the efficiency of heat pumps and heat recovery chillers.

Each circuit is sized for its own design flow within velocity and friction limits. Hot water flows are usually much smaller, so hot water pipes are typically several sizes smaller than chilled water pipes, which saves space and cost.

Each circuit has its own index path, and its pump head covers:

• plant losses
• risers, mains and branches
• the index coil, its control valve and balancing device

Hot water expands as it heats: about 1.4% from a 25 °C fill to 60 °C. For a 3,000-litre hot water circuit with a 2 bar fill and a 4.5 bar maximum, the tank needs roughly 90–100 litres. The chilled water circuit has its own, smaller expansion vessel.

We compare the hourly cooling and heating profiles to find where they overlap. The overlap sets the right size for a heat recovery chiller or multifunction unit, while boilers or heat pumps cover the remaining peaks.

MS pipe is standard for mains and risers in both circuits. GI is sometimes used for smaller bores. Copper suits FCU branches in hotels and apartments, where compact, corrosion-resistant connections matter.

Chilled water supports use insulated inserts to prevent thermal bridging. Hot water supports and guides allow the pipe to move with thermal expansion.

Hot water lines expand as they heat up. Expansion loops, offsets or bellows with properly placed anchors and guides absorb this movement without stressing joints or equipment.

Automatic air vents at the high points of both circuits and drain valves at the low points keep circulation quiet and maintenance easy.

Chilled water pipes, valves and fittings need closed-cell insulation with a continuous, sealed vapour barrier to prevent condensation in humid ceiling spaces.

Insulation rated for the hot water temperature cuts heat loss and protects people from hot surfaces. It also stops hot pipes from warming nearby chilled water lines.

Cooling coils produce condensate, which increases in reheat dehumidification mode. Drain pans and lines are sized, sloped, trapped and insulated to carry it away reliably.

Chilled water flows are balanced to design at every cooling coil, branch and riser, starting from the index circuit.

The hot water circuit is balanced separately to design flows at every heating and reheat coil. Smaller flows make accurate valve selection especially important.

With PICVs on both circuits, each coil balances itself at every load. Commissioning is faster, and future changes don't disturb the rest of the system.

Room controllers manage fan speed plus separate heating and cooling valve outputs, or a six-way valve signal. Networked controllers allow central scheduling, setpoint limits and monitoring.

A heating-cooling deadband is a neutral band between the heating and cooling setpoints where neither valve opens. It prevents rapid switching between modes and saves energy. ASHRAE 90.1 generally calls for a deadband of at least about 3 °C (5 °F) in zone controls that manage both heating and cooling, with exceptions for special applications.

Control sequences ensure the heating and cooling valves at a unit are never open together, except in a deliberate dehumidification-with-reheat mode. Tight-closing valves prevent hidden energy waste.

When humidity rises above setpoint, the cooling valve opens further to remove moisture, and the reheat valve holds the room or supply air temperature. Limits and priorities keep comfort and energy use balanced.

Pumps prove flow before chillers, boilers or heat pumps start. Heat recovery equipment runs first whenever cooling and heating loads overlap, with boilers or heat pumps topping up.

Plant equipment, pumps, valves and room controllers integrate with the BMS over BACnet, Modbus or hardwired points. Operators get live status, alarms, trends and energy data for both circuits.

Pump power for each circuit is estimated as:

Pump power (kW) = Flow (m³/h) × Head (m) ÷ (367 × pump-motor efficiency)

For example:

• Chilled water circuit: 150 m³/h at 30 m with 72% efficiency draws about 17 kW.
• Hot water circuit: 30 m³/h at 25 m with 60% efficiency draws about 3.4 kW.

VFDs on both circuits reduce these figures sharply at part load.

Thermal energy is load multiplied by hours, and the energy you pay for depends on how that heat is made. For example, 100 kW of reheat over 4,000 hours is 4,00,000 kWh of heat. That heat costs very different amounts depending on its source:

• Electric heaters: 4,00,000 kWh of electricity.
• Heat pump at a COP of 3: about 1,33,000 kWh.
• Recovered condenser heat: very little extra energy.

A heat recovery chiller delivers cooling and heating from the same input. At a cooling COP of 3.5, 100 kW of cooling uses about 29 kW of electricity and delivers about 129 kW of useful heat. That is a combined efficiency of around 8, far above any boiler or standalone heat pump.

• Add heat recovery chillers or multifunction units where loads overlap
• Convert both circuits to variable flow with PICVs and VFD pumps
• Widen deadbands and prevent unintended simultaneous heating and cooling
• Lower hot water temperatures to suit heat pumps and heat recovery
• Replace leaking valves, and upgrade FCUs to EC fan motors
• Repair insulation on both circuits

Both circuits are pressure tested separately before insulation, at the specified test pressures, to confirm every joint and connection.

Each circuit is flushed through bypasses to remove debris before water reaches coils and valves, then filled with treated water.

Chilled water and hot water flows are balanced to design at every terminal, or PICV presets are verified. Pump duties are confirmed against design.

We stroke every valve and verify the six-way valve sequences. We also confirm deadbands and check that heating and cooling never operate together except in dehumidification mode.

Critical areas are tested under operating conditions. This confirms that temperature and humidity hold within the specified bands through load changes and plant sequencing.

The usual causes are overlapping setpoints, a missing deadband, faulty sequences or valves that don't close. Controller reconfiguration and valve repairs stop the waste.

Worn seals in six-way valves can let chilled and hot water mix, wasting energy and disturbing temperatures. Leak-through testing and valve replacement fix it.

Poor balancing, blocked strainers, faulty valves or undersized branches can leave some zones under-served. Rebalancing, cleaning and PICV upgrades restore even performance.

Trapped air causes noise and poor circulation, especially in hot water lines where air comes out of solution as the water heats. Venting and working air separators resolve it.

Damaged chilled water insulation or broken vapour barriers cause dripping. Hot pipes run too close to chilled lines can also make things worse. Repairing insulation and correcting routing prevents both.

Pressure swings and relief valve discharges usually point to an undersized or waterlogged expansion tank, or incorrect pre-charge. Checking and resizing the tank restores stable pressure.

Adding a hot water circuit and heating coils converts a two-pipe building to four-pipe. Where only some zones need it, a targeted conversion keeps cost and disruption down.

Adding heat recovery equipment to an existing four-pipe plant captures condenser heat for heating and reheat. This is often the fastest-payback upgrade available.

Air-to-water heat pumps can replace or supplement boilers, cutting fuel use and emissions. They are especially effective when hot water temperatures are moderate.

PICVs and VFD pumps on both circuits reduce pumping energy and improve control and balancing throughout the building.

New four-pipe FCUs with EC fan motors improve comfort, reduce fan energy and run more quietly.

Modern networked controllers with proper deadbands, humidity logic and BMS integration eliminate unintended simultaneous heating and cooling. They also make performance visible.

Is your four-pipe system using more energy than it should? Ask Compress India for a performance and heat recovery assessment.

• Building size, height and number of terminal units
• Heating source: boilers, heat pumps, heat recovery chillers or multifunction units
• Chiller plant capacity and redundancy
• Pipe materials, sizes and lengths for both circuits
• Dual-coil FCUs or single-coil FCUs with six-way valves
• Control valves: two-way, six-way or PICV
• Pumps and VFDs for both circuits
• Insulation for chilled and hot water lines
• Controls, humidity sensors and BMS integration
• Ceiling and shaft coordination, and site access
• Testing, balancing, commissioning and AMC