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Pressurisation Units and Automatic System Filling for Sealed Heating Systems
Pressurisation Units and Automatic System Filling for Sealed Heating Systems
Sealed central heating systems rely on maintaining a stable static pressure — typically 1.0–1.5 bar cold — to operate safely and efficiently. When that pressure drops below the low-pressure cutout on the boiler, the system shuts down. In commercial and larger residential installations, manual re-filling via a temporary filling loop is impractical. Pressurisation units provide automatic, continuous pressure maintenance, removing the need for manual top-up and eliminating the compliance risk of permanent filling loops.
This guide covers how pressurisation units work, how to select and size them, current regulations on permanent filling connections, and fault diagnosis for the most common site problems.
Why Sealed Systems Lose Pressure
A healthy sealed system should hold its static fill pressure indefinitely between services. In practice, pressure loss occurs for several reasons:
- Micro-leaks: Pin-hole corrosion, weeping gland packing on valves, and slow seepage at compression joints. Each individual leak may be undetectable without pressure testing but collectively cause gradual loss.
- Waterlogged expansion vessel: When the vessel diaphragm fails or the pre-charge pressure is wrong, the vessel fills with water. The system then over-pressurises on heat-up and relieves through the PRV — pressure drops on cool-down. Repeated cycling causes loss. Article #29 covers expansion vessel diagnosis in full.
- Air ingress: Air dissolved in fill water comes out of solution as the system heats. Automatic air vents (AAVs) and manual radiator bleeding remove this air, but each event results in a small pressure drop. Article #126 covers AAVs and drain valves.
- Degassing during initial system fill: New systems commonly lose 0.2–0.5 bar in the first few weeks as dissolved gases escape.
In a residential system, the engineer or homeowner can top up via a filling loop once or twice a year. In a commercial plant room with a dozen circuits, or a block of flats with a communal boiler, this becomes a maintenance liability. A pressurisation unit automates the top-up.
How Pressurisation Units Work
A pressurisation unit is a compact assembly, typically wall-mounted, that connects to the mains cold water supply and injects water into the heating system whenever system pressure falls below a set point. The core components are:
- Backflow prevention valve: A Type CA or CB anti-siphon check valve or RPZ valve (depending on fluid category — see below). This prevents contaminated heating water from siphoning back into the mains supply.
- Pressure-reducing valve (PRV): Reduces mains pressure to the required static fill pressure, typically adjustable between 0.5–4.0 bar.
- Solenoid valve: Opens automatically when the system pressure sensor detects a drop below the set point. Closes when pressure is restored.
- Pressure sensor/transducer: Monitors system pressure continuously (or periodically in duty-cycling units) and signals the solenoid valve.
- Control unit: Configures set points, low-pressure alarm, high-pressure alarm, and auxiliary contacts for BMS integration.
- Flow meter (on some units): Monitors cumulative water make-up. Excessive make-up triggers an alert, indicating a system leak that needs investigation.
- Pressure gauge: Allows visual check of system and incoming mains pressure.
When system pressure drops — due to any of the reasons above — the solenoid opens, the PRV-reduced mains water flows into the system, and the pressure rises back to the set point. The solenoid closes. The whole sequence is automatic and typically takes a few seconds to a few minutes depending on system volume and the magnitude of the loss.
Fluid Category and Backflow Prevention
Heating system water is Fluid Category 3 (slight health hazard — corrosion inhibitor, antifreeze, or system debris). The Water Regulations Advisory Scheme (WRAS) and the Water Fittings Regulations 1999 require that the connection between the mains supply and a Fluid Category 3 circuit includes at minimum a double check valve assembly (Type BA backflow preventer). Many pressurisation units include this as standard.
If the system contains glycol (antifreeze) — which upgrades the fluid to Category 4 — a reduced pressure zone (RPZ) valve is required. RPZ valves are testable, require annual inspection by a competent person, and must be registered with the water undertaker in many areas.
A permanent filling loop — a simple flexible hose with two isolating valves connecting mains to system — is only acceptable as a temporary connection for initial filling and must be physically disconnected and removed after use. Leaving it permanently connected is a notifiable water fitting that requires approval from the water undertaker. The sealed system pressure guide (Article #45) covers filling loops in detail.
Types of Pressurisation Unit
1. Inline / Trickle Fill Units
The simplest type. A pressure-reducing valve and backflow preventer allow a trickle of water in when pressure drops below the PRV set point. No electrical components. Suitable for small residential sealed systems where the boiler already has adequate overpressure protection. Often called a "filling link" when installed alongside a standard filling loop.
Limitation: No alarm, no monitoring, no make-up tracking. Not suitable for commercial or multi-zone systems.
2. Electronic Pressurisation Units (EPUs)
The standard for commercial installations. Includes pressure transducer, solenoid valve, digital display, and configurable alarms. Most units provide:
- Adjustable set point pressure (fill pressure)
- Low-pressure alarm (system needs investigation)
- High-pressure alarm
- Make-up volume/time logging to detect leaks
- Volt-free contacts for BMS integration
- Mains supply isolation valve
Common manufacturers: Flamco (Fillcontrol), Altecnic (Caleffi), Grundfos, Pneumatex, Reflex (Fill Control Compact), Spirotech.
3. Pump-Assisted Pressurisation Units
Used where mains pressure is too low to fill the system against system pressure, or where system pressure requirements exceed what a PRV alone can deliver (e.g., high-rise buildings, pressures above 3.0 bar). A small centrifugal pump boosts the incoming mains pressure. Often found in duty-standby configurations with automatic changeover.
4. Pressurisation and Deaeration Units
Combined units that both maintain pressure and remove dissolved oxygen and air from the system water. Spirotech SpiroFill and Flamco Flamcovent Clean series offer this. Particularly valuable for larger commercial systems where ongoing degassing is important for corrosion prevention.
Sizing a Pressurisation Unit
For most residential and light commercial applications, a standard single-solenoid EPU (e.g., Flamco Fillcontrol Compact, Altecnic Statefill 2) is adequate. Key parameters:
Set Pressure
The cold static fill pressure is calculated as:
P_fill = P_static_head + 0.3 bar safety margin
Where P_static_head = height of highest point above the pressurisation unit × 0.1 bar/m. For a three-storey building with the unit in the basement (10m height difference), P_static_head = 1.0 bar, so P_fill ≈ 1.3 bar.
Typical fill pressures: 1.0–1.5 bar for residential, 1.5–3.0 bar for commercial.
Make-Up Flow Rate
For a tight system, the pressurisation unit make-up rate only needs to match incidental leakage — 1–2 litres per hour is generally sufficient. However, if the system is being commissioned and is degassing heavily, a higher flow rate reduces nuisance top-ups. Most units offer 15–30 litres/hour; some larger units 60+ litres/hour.
Duty/Standby Configuration
For critical commercial applications (hospitals, hotels, continuous process heating), a duty/standby pair with automatic changeover is standard. Both units are connected to the system; if the duty unit fails or triggers a fault, the standby cuts in. Some manufacturers (Flamco, Grundfos) offer pre-assembled duty/standby pairs on a common manifold.
Mains Pressure Requirements
The unit needs incoming mains static pressure at least 0.5 bar above the required fill pressure (to allow the PRV to regulate). Check available mains pressure at the installation point — in some older buildings or where mains supply runs long distances, available pressure may be lower than expected. A pump-assisted unit is required where mains pressure is marginal.
Installation Requirements
Location
The pressurisation unit is typically installed in the plant room, adjacent to the expansion vessel and in close proximity to the boiler or heat exchanger. It connects to:
- Mains supply: Via isolating valve, filter/strainer, and backflow prevention device
- System: Typically to the return pipework (lower pressure side), with a non-return valve to prevent heating water entering the unit when the solenoid is closed
- Electrical supply: Usually 230V, 50Hz, typically <5W. Some units are battery-backed
- BMS (optional): Volt-free alarm contacts
Water Treatment
Fresh make-up water introduces dissolved oxygen and limescale-forming calcium bicarbonate. Each litre of mains water added contributes to scaling on heat exchanger surfaces and corrosion of steel components. Where make-up volume is significant, consider in-line water softening or a dosing pot on the make-up line. In-line filters and dirt separators (Article #130) protect the pressurisation unit from system debris flowing back during pressure sensing.
System Protection Requirements
A pressurisation unit does not replace the safety devices on a sealed system. The following remain mandatory:
- Pressure relief valve (set at maximum working pressure — typically 3 bar for residential, higher for commercial)
- Expansion vessel of correct size and pre-charge pressure
- Temperature/pressure gauge
- High-limit thermostat on the boiler/heat source
Commissioning and Setting Up
- Check expansion vessel pre-charge: Before filling, verify the vessel nitrogen pre-charge equals the required cold fill pressure. An undercharged vessel will cause the pressurisation unit to run frequently as it struggles to maintain pressure against a waterlogged vessel. See Article #29.
- Set PRV to required fill pressure: Turn the PRV adjustment screw with the mains valve partially open and system drain valves closed. Observe the pressure gauge. Set to the calculated static fill pressure.
- Fill the system: Open the mains isolating valve. The solenoid opens (on EPUs, with power on) and fills the system. Monitor pressure rise. Close system vents as filling progresses.
- Bleed and vent: Bleed all radiators and air vents. Allow pressure to restabilise; the unit will top-up automatically.
- Set alarms on EPU: Configure low-pressure alarm (typically set 0.2 bar below fill pressure) and high-pressure alarm (typically 0.5 bar below PRV setting).
- Log commissioning values: Record fill pressure, alarm settings, and expansion vessel pre-charge on commissioning sheet. Include unit serial number, mains connection details, and fluid category assessment.
Fault Diagnosis
Frequent Make-Up (Unit Running Constantly)
The system is losing water faster than the unit can keep up, or the expansion vessel is waterlogged.
- Check expansion vessel — if it discharges water when Schrader valve is depressed, the diaphragm has failed. Replace or recharge the vessel.
- Check PRV for weeping — if PRV is opening and discharging, system is over-pressurising on heat-up (waterlogged vessel).
- If vessel is healthy, the system has a genuine leak. Pressure-test pipework and inspect all joints, gland packings, and radiator valves.
- Check the make-up volume log on the EPU — if it records litres per hour, this helps quantify the leak rate and justify investigation.
Low Pressure Alarm, Unit Not Making Up
- Check mains supply is on and pressure adequate at the unit inlet.
- Check mains isolating valve is open.
- Check solenoid valve — if power is on and set point is not reached, solenoid should be open. Test by listening for water flow. If solenoid is energised but not flowing, check for debris blocking the orifice.
- Check backflow preventer — can seize closed with scale or debris. Remove and inspect/clean.
Unit Running but No Pressure Rise
- System has a large leak — locate and repair first.
- Make-up connection is isolated downstream — check valves between unit and system.
- Non-return valve on system connection is stuck closed.
High Pressure Alarm
- PRV set point too high, or PRV has failed open — isolate mains supply. Reset or replace PRV.
- Solenoid valve stuck open — isolate mains supply, inspect solenoid coil and valve seat.
- Expansion vessel waterlogged — pressure rising unchecked on heat-up.
Regulations and Compliance
- Water Fittings Regulations 1999: Govern backflow prevention requirements. A permanent connection to the system must be notified to the water undertaker if not a Type BA double check or higher protection device for Category 3 fluid. For Category 4 (glycol), RPZ valve required.
- BS EN 12828: Design standard for hot water heating systems — covers pressurisation and expansion requirements for sealed systems.
- BSRIA BG 29: Water treatment for closed heating and cooling systems — sets maximum make-up water volume thresholds (typically 5% of system volume per year) to limit oxygen ingress.
- Part L Building Regulations: New and replacement boiler installations must be sealed systems with adequate pressurisation. Vented systems are now rare in new build.
- Manufacturer warranty: Most boiler manufacturers require pressurisation units on sealed systems above a certain output (commonly 30–50 kW) to maintain warranty. Check manufacturer commissioning requirements.
Key Products and Brands
- Flamco Fillcontrol Compact: Wall-mounted EPU with digital display, two set-point pressure settings, volt-free alarm contacts. The most widely specified unit in UK commercial heating.
- Altecnic Statefill 2: Dual-circuit pressurisation unit allowing two independent circuits from one mains connection. Useful for primary/secondary split systems.
- Reflex Fill Control Compact: Integrated with the Reflex expansion vessel range. Can be factory-coupled to Reflex vessels for supply as a pressurisation set.
- Grundfos Autofill: Electronic unit with LCD display and advanced BMS integration. Supports remote monitoring via Grundfos GO app.
- Caleffi Hydronic Solutions: Caleffi 5534 and 5535 series automatic fill valves — compact units for smaller commercial and large residential applications.
Related Guides
- Article #29 — Expansion Vessels: Heating vs Potable, Sizing, Diagnosis
- Article #45 — Sealed System Pressure, Filling Loops, and PRVs
- Article #130 — Y-Strainers, Dirt Separators, and In-Line Filters
- Article #136 — Water Pressure Problems in UK Plumbing
- Article #138 — Magnetic Filters for Central Heating Systems
- Article #126 — Drain Valves, Manual Air Vents, and Automatic Air Eliminators
- Article #102 — Combi vs System vs Regular Boilers
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