Open vs Closed Loop Heating Systems: Complete Engineering Comparison

Quick Verdict

The choice between open and closed loop heating is largely determined by modern building practices and equipment requirements.

Bottom Line: Closed loop (sealed) systems are the standard for all new construction due to compatibility with condensing boilers, superior corrosion protection, and higher efficiency. Open loop (vented) systems remain functional in existing buildings but should be converted when replacing boilers with condensing units. The cost difference ($100-300 for expansion vessel vs header tank) is negligible compared to efficiency benefits.

For existing open systems that are working well, conversion purely for modernization isn't necessary—maintain and monitor as normal. Conversion becomes mandatory when installing condensing boilers, heat pumps, or aluminum-cored components.

At-a-Glance Comparison Table

System Configuration: The Fundamental Difference

Understanding how each system manages thermal expansion and air removal is essential.

Open Loop (Vented) System

An open vented system has three key components:

1. Feed and Expansion (F&E) Tank: Open tank in roof space, typically 18-25 liters
2. Cold Feed Pipe: Supplies makeup water from tank to system
3. Open Vent Pipe: Rises from boiler flow to terminate above tank, preventing pressurization

Water expands when heated, rising up the vent pipe into the header tank. As it cools, water returns via gravity. The open tank is exposed to atmosphere, allowing:

• Air to enter water (oxygen causing corrosion)
• Water to evaporate (requiring periodic top-up)
• Pressure relief (cannot over-pressurize)

Maximum system pressure equals static head (height difference) plus atmospheric pressure—typically only 0.5-1.5 bar at the boiler.

Closed Loop (Sealed) System

A sealed system has different key components:

1. Expansion Vessel: Pre-charged pressure vessel with rubber diaphragm
2. Pressure Relief Valve (PRV): Typically set at 3 bar, discharges if exceeded
3. Filling Loop: Connects mains water to system for pressurization
4. Pressure Gauge: Indicates system pressure (typically 1.0-2.5 bar)

When water heats and expands, it compresses the gas in the expansion vessel. As it cools, the vessel pushes water back into the system. The sealed configuration:

• Prevents oxygen ingress (no corrosion mechanism)
• Maintains consistent pressure (better pump performance)
• Allows operation above 100°C (pressurized boiling point elevation)

Verdict: Configuration

Winner: Closed Loop — Sealed systems provide superior pressure control, prevent corrosion-causing oxygen ingress, and enable modern high-efficiency boiler operation. Open systems' simplicity doesn't compensate for their inherent limitations.

Corrosion: The Long-Term Factor

Corrosion is the primary reason closed systems dominate modern installations.

Open System Corrosion

Open systems allow continuous oxygen dissolution into water:

1. Oxygen enters through header tank surface contact
2. Circulating water distributes oxygen throughout system
3. Steel corrodes via oxidation: $4\text{Fe} + 3\text{O}_2 + 6\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3$
4. Black sludge (magnetite) accumulates in system

Corrosion rates in open systems:

• Mild steel radiators: 0.1-0.3 mm/year wall thickness loss
• Steel pipes: Similar rates, eventually causing leaks
• Cast iron: More resistant but still affected over decades

Symptoms: black water when draining, cold spots from sludge accumulation, radiator failures, pump damage from abrasive particles.

Closed System Corrosion

Properly maintained closed systems have near-zero corrosion:

1. Initial oxygen consumed during first heating cycles
2. No replacement oxygen enters sealed system
3. Corrosion inhibitor protects metal surfaces
4. System remains clean indefinitely with proper maintenance

Requirements for corrosion-free operation:

• Initial fill with softened/treated water
• Appropriate inhibitor concentration (check annually)
• No air ingress (check AAV, joints, pump seals)
• Periodic testing of inhibitor levels

Verdict: Corrosion

Winner: Closed Loop — Sealed systems with proper inhibitor essentially eliminate corrosion. Open systems have continuous oxygen ingress causing ongoing degradation. The difference in system lifespan is measured in decades.

Pressure and Temperature: Operating Limits

System pressure affects operating capability and component selection.

Open System Limits

Maximum pressure at any point equals: $P = P_{atm} + \rho g h$

Where $h$ is height below the header tank surface.

For a tank 10m above the boiler: $P = 1.0 + (1000 \times 9.81 \times 10) / 100000 = 1.0 + 0.98 = 1.98 \text{ bar (absolute)}$

At upper radiators (near tank level), pressure approaches atmospheric—marginally adequate for circulation.

Temperature limit: Water boils at 100°C at atmospheric pressure. Upper system points at near-atmospheric pressure limit temperature to ~85-95°C to prevent steam formation.

Closed System Capability

System pressure is controllable via:

• Expansion vessel sizing and pre-charge
• Filling loop pressure setting
• PRV setting (maximum limit)

Typical design:

• Cold fill pressure: 1.0-1.5 bar
• Operating pressure: 1.5-2.5 bar
• PRV setting: 3.0 bar

At 2.5 bar absolute, water boiling point is approximately 127°C—well above typical heating temperatures. This allows:

• Condensing boilers operating at optimal conditions
• Higher temperature differentials for smaller components
• Multi-story buildings without zone separation

Verdict: Pressure/Temperature

Winner: Closed Loop — Sealed systems offer controllable pressure independent of building geometry and enable operation above atmospheric boiling point. Open systems are limited by static head and must stay below ~95°C at upper points.

Efficiency: The Bottom Line

System configuration affects both operational efficiency and ongoing losses.

Open System Efficiency Losses

1. Standing losses from header tank:

   • Tank in cold roof space loses heat continuously
   • Estimated 100-300 watts during heating season
   • Over 6 months: 500-1,500 kWh annual loss

2. Evaporation losses:

   • Heated water evaporates from open tank
   • Requires makeup water addition (hard water = scale)
   • Energy lost in evaporated water heat content

3. Lower temperature operation:

   • Cannot achieve optimal condensing temperatures
   • Boiler efficiency reduced (85-88% vs 92-95%)

4. Circulation limitations:

   • Low pressure at upper floors limits flow rates
   • May require larger pipes or stronger pumps

Total efficiency impact: 5-15% higher fuel consumption vs equivalent sealed system.

Closed System Efficiency Advantages

1. No standing tank losses:

   • Expansion vessel is small and insulated by system water
   • Negligible thermal loss

2. No evaporation:

   • Sealed system retains all water
   • No makeup water required (stable inhibitor concentration)

3. Optimal boiler operation:

   • Condensing boilers achieve full efficiency
   • Precise temperature control

4. Better circulation:

   • Consistent pressure throughout
   • Smaller pipes and lower pump power acceptable

Verdict: Efficiency

Winner: Closed Loop — Sealed systems eliminate standing losses, enable condensing boilers, and provide better circulation. The 5-15% efficiency advantage typically pays for conversion within 2-5 years.

Application-Specific Recommendations

When to Keep Open Loop

Maintain existing open systems when:

• System is working well with no corrosion problems
• Traditional cast iron boiler not being replaced
• Gravity circulation (no pump) desired for simplicity
• DIY maintenance preference (simpler troubleshooting)
• Budget doesn't allow conversion plus any necessary upgrades

Typical Applications:

• Older residential properties with adequate existing systems
• Listed/heritage buildings with restrictions on modification
• Very simple installations with one or two radiators
• Properties where maximum simplicity is valued over efficiency

When to Choose Closed Loop

Use closed loop systems for:

• All new construction (standard practice, often code required)
• Installing condensing boiler (mandatory for warranty)
• Adding underfloor heating (requires sealed operation)
• Heat pump installation (always sealed)
• Multi-story buildings over 10m height
• Systems with aluminum components (oxygen-sensitive)
• Converting from open system when replacing boiler

Typical Applications:

• New residential and commercial construction
• Boiler replacement projects
• Energy efficiency retrofits
• Multi-story buildings
• Systems requiring antifreeze (glycol evaporates in open)

Conversion Considerations

Converting from open to closed is common when replacing boilers.

Conversion Process

1. Drain system and remove header tank
2. Cap cold feed and vent pipe connections at boiler
3. Install expansion vessel sized per EN 12828
4. Add filling loop with double check valve
5. Install pressure relief valve (typically 3 bar)
6. Add pressure gauge and automatic air vent
7. Flush system thoroughly (remove old sludge)
8. Fill and pressurize with inhibitor-treated water
9. Commission and verify operation

Conversion Cost

If boiler replacement is concurrent, many installers include conversion in the price.

Maintenance Comparison

Open System Maintenance

Regular Tasks:

• Check header tank water level (monthly)
• Inspect for overflow or debris in tank (annually)
• Clean tank if contaminated (every 5-10 years)
• Check ball valve operation (annually)
• Inspect vent pipe for blockage (annually)

Common Problems:

• Overflow from failed ball valve
• Contamination from tank debris
• Freezing in unheated roof spaces
• Evaporation requiring top-up
• Corrosion requiring system flush

Closed System Maintenance

Regular Tasks:

• Check pressure gauge (monthly when cold)
• Verify expansion vessel charge (annually)
• Test PRV operation (annually)
• Check inhibitor concentration (annually)
• Bleed any accumulated air (as needed)

Common Problems:

• Pressure loss (leak or vessel failure)
• Over-pressurization (vessel waterlogged)
• PRV dripping (vessel undercharged or oversized)
• Loss of inhibitor protection

Verdict: Maintenance

Winner: Tie — Both systems require regular attention. Open systems have simpler troubleshooting but more degradation issues. Closed systems have pressure-related complexity but fewer corrosion problems. The maintenance skill requirement is similar.

Common Mistakes to Avoid

Related Tools

Use these calculators to design your heating system:

• Expansion Tank Calculator - Size expansion vessels for sealed systems
• Circulation Pump Calculator - Size pumps for system requirements
• Heat Loss Calculator - Determine system heat demand

Key Takeaways

• Corrosion: Open systems allow oxygen ingress causing ongoing corrosion; closed systems prevent this
• Efficiency: Closed systems eliminate tank losses and enable condensing boilers—5-15% efficiency improvement
• When to choose open: Existing working systems, simple gravity installations, maximum simplicity priority
• When to choose closed: New construction, condensing boilers, heat pumps, multi-story buildings
• Conversion: Common and cost-effective ($400-1,000) when replacing boilers

Further Reading

• Understanding Expansion Tanks - Detailed expansion vessel sizing guide
• Understanding Circulation Pumps - Pump selection and sizing
• Boiler vs Heat Pump - Heat source selection guide

References & Standards

• EN 12828: Heating systems in buildings — Design for water-based heating systems
• BS 7593: Code of practice for the preparation, commissioning and maintenance of domestic central heating
• EN 13831: Closed expansion vessels with built-in diaphragm for installation in water systems
• CIBSE Guide B1: Heating systems
• Building Regulations Part L: Conservation of fuel and power

Disclaimer: This comparison provides general technical guidance based on international standards. Actual system requirements depend on specific building characteristics and local regulations. Always consult qualified engineers for system design and installation.