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Last updated: 15th April, 2024


Combi Boiler Valve Positions Explained

Combi Boiler Valve Positions Explained

Key takeaways

  • Combi boilers efficiently supply both heating and hot water, negating the need for separate storage tanks.
  • Valve positions control crucial aspects of boiler function, thereby ensuring efficient operation and system safety.
  • Proper knowledge and maintenance of valve positions contribute to the energy efficiency and longevity of the boiler.

With the advancements in boiler technology by 2024, understanding the various valve positions and their functions is essential for any homeowner or installer looking to optimise their heating system's performance and efficiency.

A combi boiler, or combination boiler, is at the heart of many UK homes, providing central heating and hot water from a single compact unit. As an efficient space-saving option, these boilers are designed to be cost-effective and convenient, eliminating the need for separate hot water cylinders and cold water storage tanks often found with traditional systems.

With the advancements in boiler technology by 2024, understanding the various valve positions and their functions is essential for any homeowner or installer looking to optimise their heating system's performance and efficiency.

Valve positions in combi boilers play a pivotal role in regulating the flow of water and gas within the system. Knowledge of these valves ensures proper installation and operation – a key aspect of maintaining energy efficiency and prolonging the lifespan of the unit.

From safety valves that prevent overpressure situations to diverter valves that control whether the heated water is channelled to radiators or taps, each type of valve has a distinct function critical for the correct operation of the heating system.

To ensure the reliability and effectiveness of a combi boiler, users must familiarise themselves with these components, facilitated by comprehensive guidelines available in the latest resources.

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Understanding Combi Boilers-

Combi boilers, an abbreviation for 'combination boilers', have revolutionised the way homes in the UK are heated. They're esteemed for their proficiency in providing heating and hot water on demand without the footprint of additional storage.

Basics of Combi Boilers

Combi boilers are a type of heating system unique in providing both central heating and hot water directly from one compact unit. They draw cold water straight from the mains and heat it instantaneously when a tap is turned on. This eliminates the need for bulky hot water storage cylinders or tanks, saving precious space in homes.

Advantages of Using Combi Boilers

Efficiency: Combi boilers are lauded for their energy efficiency. By heating water directly from the mains on a need-only basis, it avoids energy losses associated with storing hot water.Maintenance: With fewer parts and no hot water storage tank, maintenance can be simplified compared to conventional systems, potentially reducing costs and inconvenience resulting from repairs.

How Combi Boilers Differ From Other Types

Combi boilers differ distinctly from traditional systems, which typically consist of a cold water tank, often housed in the loft, and a separate hot water cylinder. The compactness of a combi boiler allows for significant space savings, and the absence of a cold water tank reduces the risk of pipework freezing in the winter. Their ability to provide hot water on demand also means no wait time for a storage cylinder to refill and reheat, enhancing the users’ convenience and comfort.

What are Boiler Valves?

Boiler valves are crucial components in heating systems that manage the flow of water and steam within boilers. These devices ensure efficient operation, safety, and maintenance capabilities.

Where to Find Boiler Valves?

Boiler valves are usually located on the pipes near the boiler itself. Homeowners typically find these valves on either the water inlet or outlet pipes, which control the flow of water into and out of the boiler. In combi boilers, which provide both heating and hot water directly from the mains without the need for tanks, the valves play a pivotal role in the immediate heating of water.

Valve Positions and Their Functions

Accurate valve positioning is essential for the optimal operation of a combi boiler. Each valve performs a distinct function that contributes to system efficiency and safety.

Understanding Valve Mechanics

Valves within a combi boiler regulate the flow of water and ensure the proper operation of the system. The valve positions are crucial, as they dictate the water supply path, affecting the flow rate and overall system performance.

Diverter Valve Operation

The diverter valve plays a pivotal role in directing the hot water to either the heating system or the taps. When a user initiates hot water demand, the diverter valve's position adjusts, temporarily ceasing water flow to the heating circuit and redirecting it to the taps. Routine maintenance of the diverter valve ensures consistent operation and extends the lifetime of the boiler.

  • Closed Position: Heating system operates normally.

  • Open Position: Hot water is directed to taps/shower.

Pressure Relief Valve Importance

The pressure relief valve is a safety feature designed to maintain the pressure within a safe limit. Should the system pressure exceed the designated threshold, this valve's opening will alleviate excess pressure, thereby preventing potential damage to the boiler and the associated pipework.

  • Normal Operation: Valve remains closed, maintaining system pressure.

  • Excess Pressure: Valve opens to release water and reduce pressure.

Installation Guidelines

This section addresses the critical elements of installing a combi boiler effectively, ensuring compliance with regulations and achieving an efficient heating system.

Gas Safe Engineers

When installing a combi boiler, using a Gas Safe registered engineer is imperative. Gas Safe registration is a legal requirement in the UK for anyone carrying out work on gas appliances. These professionals are equipped with the necessary skills and knowledge to ensure a safe and correct installation.

Steps for Installing a Combini Boiler

The process starts with removing the old boiler, followed by preparing the necessary connections – water, gas, and electrical. After this preparation, the new combi boiler is mounted and connected. The system is then filled with water, and the heating engineer will purge any air from the radiators. Finally, a series of safety and performance checks are conducted before commissioning the boiler.

  1. Remove the old boiler

  2. Prepare connections

  3. Mount the new combi boiler

  4. Fill and purge the system

  5. Conduct safety and performance checks

Selecting the Right Location

Choosing the appropriate location for a combi boiler involves considering factors such as proximity to the gas line and water pipes, as well as ventilation and flue options. Ideally, the location should also allow for easy access for future maintenance.

  • Proximity: Close to gas and water connections

  • Ventilation: Sufficient air supply

  • Flue options: Correct positioning of the flue for safe exhaust

  • Maintenance: Convenient access for servicing and repairs

Understanding Installation Costs

The cost of installing a combi boiler varies, with labour representing a significant portion of the expense. Factors influencing the total cost include boiler size, location, and complexity of installation. Prices typically range from £1,500 to £2,700, with additional expenses for relocating the boiler or upgrading existing pipework.

  • Boiler size: Bigger boilers may increase the cost.

  • Location: Relocating a boiler can add to the price.

  • Complexity: Complexity of the installation can affect labour time.

  • Additional upgrades: Extra work like pipework changes adds to the cost.

The installation cost often reflects the time and skill required by the heating engineer, underlining the value of professional service for a safely installed and efficient heating system.

System Requirements and Compatibility

When integrating a combi boiler into a home's central heating system, certain prerequisites ensure optimal performance. Water pressure, pipe compatibility, and proper ventilation make up the trifecta of considerations for a smooth installation and operation.

Water Pressure Considerations

The combi boiler's efficacy is contingent on adequate mains water pressure. The boiler requires a minimum pressure to operate correctly, typically around 1.2 bar. If the existing water pressure is insufficient, additional measures, such as installing a pump, might be necessary.

  • Minimum Required Pressure: 1.2 bar

  • Optimal Operating Range: 1.5-2.0 bar

Homes with lower mains water pressure may encounter challenges in maintaining an even supply of hot water to multiple outlets.

Pipes and Connections

Combi boilers are connected to the central heating system through an intricate network of pipes. Compatibility with existing pipes and connections is paramount:

  • Gas Supply Pipe: Must support the combi boiler's requirements.

  • Hot Water Pipes: Should be insulated to preserve heat.

  • Condensate Pipe: Needs to be resistant to acidic water and have a fall towards the drain.

Selecting the correct diameter and material for the pipes is crucial for efficient operation and avoiding leaks or damage.

Flue and Ventilation Requirements

A safe installation abides by strict regulations for the flue and ventilation to expel exhaust gases:

  • Flue Position: Must adhere to the UK's building regulations, avoiding windows and property boundaries.

  • Ventilation: Needs to be sufficient to ensure an ample supply of oxygen for combustion.

By ensuring the above requirements are met, a combi boiler can be successfully incorporated into the central heating system, providing efficient heating and hot water.

Operating Your Combi Boiler

In managing a Combi Boiler, one balances temperature settings for comfort, adjusts water pressure for optimal performance, and applies energy-saving measures to cut costs. These facets work in tandem to sustain an efficient and effective home heating and hot water system.

Setting Optimal Temperatures

To maximise efficiency whilst ensuring a cosy home, it's essential to regulate the boiler's temperature. For central heating, experts recommend setting the boiler temperature to about 65°C; this allows for a warm home without excessive energy consumption. Hot water should be set at around 55°C to prevent scalding while still killing bacteria.

Central Heating:

Hot Water:

  • Ideal Temperature: 55°C

  • Balance: Safety and hygiene

Adjusting Water Pressure

The water pressure within your combi boiler should typically stand between 1 and 2 bars; visible on the boiler's pressure gauge. Should it drop below 1 bar, the boiler may cease functioning correctly. To remedy this, one can often find a filling loop beneath the boiler to reintroduce water into the system and restore the pressure to its optimal level.

Pressure Gauge Readings:

  • Normal Pressure: 1 - 2 bars

  • Low Pressure: Below 1 bar (requires action)

Energy Saving Tips

Employing energy-saving practices not only alleviates environmental strain but also reduces household utility bills. An annual boiler service ensures your system's energy-efficient operation. Additionally, insulating pipes minimises heat loss, and installing thermostatic radiator valves grants individual control over the temperature in different rooms.

Key Energy-Saving Practices:

  • Annual Boiler Service: Maintains efficiency.

  • Insulation: For pipes, mitigates heat loss.

  • Thermostatic Radiator Valves: Allows precise temperature control for each radiator.

Utilising these techniques effectively contributes to a reduction in energy usage without compromising the warmth of showers, the comfort of radiators, and the performance of taps within the living space.

Maintenance and Servicing

Ensuring that a combi boiler operates efficiently requires a commitment to consistent maintenance and servicing. This vigilance not only safeguards the boiler's functionality but can also help in extending its lifespan and honouring its warranty terms.

Regular Servicing Schedule

Annual servicing by a Gas Safe registered engineer is essential to maintain the boiler in proper working order and to fulfil warranty requirements. These routine inspections should include:

  • Checks on the flue and combustion. A flue gas analysis ensures that the boiler burns gas correctly.

  • Inspection of internal components. The heat exchanger, burner, main injector, and spark/sensor probes are among the critical parts examined.

  • System pressure and flow rate checks. Proper flow rates guarantee the system's operational efficiency.

  • Seal and gasket assessments. To prevent gas and water leaks, all seals should be checked and replaced as necessary.

  • Safety device testing. This ensures devices such as the pressure relief valve and thermostat are functioning correctly.

Common Issues and Troubleshooting

Homeowners may encounter several common issues with their combi boilers, such as:

  1. Loss of pressure: Often due to leaks, it can also result from a faulty pressure relief valve.

  2. No heating or hot water: Causes could range from broken diaphragms and airlocks to issues with the thermostat.

  3. Strange banging, whistling, or gurgling noises: These could denote air in the system, low water pressure, or imminent pump failure.

For troubleshooting:

  • Restarting the boiler can sometimes reset the system and resolve temporary glitches.

  • Bleeding radiators releases trapped air, which may restore optimal performance in cases of noise or uneven heating.

  • Checking the pressure gauge and re-pressurising if necessary can solve some pressure-related issues.

Homeowners should familiarise themselves with their boiler's instruction manual for specific troubleshooting guidance and when in doubt, contact a qualified engineer. Early detection and resolution of issues not only enhance the boiler's efficiency but are also cost-effective, mitigating the need for more extensive repairs or even premature replacement.

Choosing the Right Combi Boiler

When selecting a new combi boiler for your home, it is essential to evaluate a range of critical factors and compare the different brands and models available.

Factors to Consider When Buying

Manufacturer Reputation and Reviews: Established manufacturers like Worcester Bosch, Vaillant, and Viessmann have a track record of producing reliable combi boilers. Homeowners should read current reviews to get a sense of performance and the level of after-sales support.

Price and Costs: The initial purchase price is crucial, but it's equally important to consider ongoing running costs. Seek out boilers that balance upfront expenses with long-term energy efficiency to manage bills effectively.

Warranty: Robust warranty offerings signal a manufacturer's confidence in their product. Brands such as Ideal often provide substantial warranties, ensuring peace of mind and potentially reducing future repair costs.

New Combi Boiler Features: Look for modern boilers with features that improve efficiency and control, such as smart thermostat compatibility and condensing technology, which can significantly cut gas bills.

Comparing Boiler Brands and Models

Comparing Worcester Bosch, Vaillant, and Viessmann:


Price Bracket

Energy Efficiency

Warranty Period

Notable Features

Worcester Bosch



Up to 10 years

Smart controls, Quiet operation




Up to 7 years

Eco-friendly, User-friendly interface



Very High

Up to 12 years

Advanced condensing technology

Boiler Models and Specifications: Within each brand, various models target different needs. Comparing specifications such as heating output, hot water flow rate, and dimensions is crucial for tailoring your choice to your home's requirements.

Costs and Installation: The overall cost can vary widely depending on the complexity of the installation. It's recommended to get multiple quotes from certified installers for the most accurate price comparison.

Reading comprehensively is key, while factoring in these aspects into your decision can maximise your investment in a new combi boiler.

Financial Considerations

Investing in a combi boiler entails examining both the initial outlay and the long-term financial implications. This section sheds light on the total ownership costs, potential savings over time, and the hidden expenses that may arise from opting for budget boilers.

Total Cost of Ownership

The Total Cost of Ownership (TCO) for a combi boiler in the UK encompasses the purchase price, installation fees, and ongoing running costs. Prices for the boiler unit itself generally range from £500 to £2,500, with installation costs adding another £500 to £1,500. These figures can fluctuate depending on the complexity of the installation. Additional costs such as annual servicing or unexpected repairs should also be factored into the TCO.

  • Example TCO Calculation:

    • Boiler price: £1,500

    • Installation: £1,000

    • Yearly servicing: £75 for 10 years = £750

    • Total: £3,250 over 10 years

Long-term Savings

Combi boilers are touted for their energy efficiency, which can lead to significant cost reductions in energy bills. A high-efficiency combi boiler could save a household up to £800 annually compared to older, less efficient models. It is essential to assess the boiler's energy rating and potential savings over its lifetime to understand its true value.

Why Budget Boilers May End Up Costing More

Opting for a budget combi boiler can appear as an attractive short-term saving. However, such models may have lower energy efficiency, leading to higher bills over time. Moreover, cheaper units can incur increased maintenance costs or earlier replacement needs, negating any initial savings. It's imperative to strike a balance between a reasonable price and the boiler's quality and efficiency to prevent unforeseen expenditures.

  • Financial Pitfalls of Budget Boilers:

    • Increased energy consumption

    • Frequent repairs

    • Potential early replacement costs

Various finance options are available, from upfront payments to monthly financing plans, enabling homeowners to spread the cost of their investment. It is recommended to scrutinise these options carefully to select the most cost-effective plan tailored to individual financial circumstances.

Quick Radiator Valve Guide (TRVs, Lockshields)

Thermostatic Radiator Valves (TRVs) feature a numbered or symbolic system, indicating the heat output of a radiator. By adjusting a TRV, users control the temperature for individual rooms. TRVs bear settings usually from 1 to 6; setting the valve to 3 often correlates with approximately 20°C.

Lockshield valves, conversely, serve to balance the heating system, securing an even distribution of heat. These are paired with TRVs and are typically fitted with a plastic or metal cap to prevent inadvertent adjustments. They require a specific key or an adjustable spanner to alter their position.

Valve Type



Temperature Control


Individual room temperature control

Numbered settings



Balancing overall heating system

Key/spanner adjust


When installing or adjusting valves:

  1. Turn off the heating system to prevent burns or leaks.

  2. Drain the radiator when replacing a valve to avoid water damage.

  3. After installing TRVs, leave the lockshield valve open and then adjust according to the heating requirements.

By following these steps and understanding the functions of each valve type, one maximises the efficacy of a combi boiler system.

iHeat New Boiler Costs

Boilers can be a daunting purchase for many people as they’re an appliance with a lot of responsibility, providing heat for you and your family is something you want to get right. Boilers aren’t exactly a quickly disposable item either, potentially lasting you a decade.

Effectively, new boiler cost can be split into two segments: the first is the actual boiler itself (unit price), and the second is the cost of the boiler being installed (set up) in your property by an expert engineer. 

Here at iHeat, we want to remove all of this undue stress and make the decision making process of upgrading to a new central heating system, as easy as possible.

Boiler costs can vary depending on a number of factors including their brand, model, fuel, output, warranty, labour and installation type. Typically a new boiler will cost between £1,845 and £3,500, below is a list of average boiler installations offered by iHeat (guide only).

Installation Type

Price (inc VAT)

Combi to combi swap


Get a quote

System to combi conversion


Get a quote

New boiler install


Get a quote

Back boiler to a combi


Get a quote

System to system


Get a quote

Frequently Asked Questions

What position should boiler valves be in?

Boiler valves should be in an open position during normal operation to allow for the flow of water through the system. It's essential for the valves to be correctly positioned to maintain the system's integrity and functionality.

Which valves should be open on a boiler?

The inlet and outlet valves connected to your boiler should be open to enable water circulation. The gas isolation valve must also be open for the boiler to operate. Safety valves, like the pressure relief valve, will open automatically if necessary.

Should both boiler valves be open or closed?

During normal operation, both main boiler valves should be open to allow for proper water circulation. When servicing or repairing, both valves may need to be closed to isolate the boiler from the water supply.

What are the 5 pipes on a combi boiler?

A typical combi boiler has five main pipes: the flow pipe, return pipe, gas supply pipe, cold water inlet, and condensate discharge pipe. These ensure the boiler functions by supplying water, fuel, and disposing of waste by-products.

Which pipe is flow and return on a boiler?

On a combi boiler, the flow pipe transports heated water from the boiler to the radiators, while the return pipe brings cooler water back to be reheated. Correct identification is necessary for maintenance and troubleshooting.

What happens if you mix up flow and return?

Mixing up the flow and return pipes on a boiler can lead to inefficient heating and potential damage to the system. Radiators may not heat up properly, and the boiler could suffer from internal stress due to incorrect water flow.

Should lockshield valve be fully open?

The lockshield valve, found on a radiator, should not be fully open as it is used to balance the heating system. It should be adjusted to manage water flow, ensuring even heat distribution across all radiators in the property.

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Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.