Duo Valve for Hot Water Systems: Complete Buyer’s Guide

A duo valve is a common plumbing valve used on many storage hot water system installations. It can combine isolation and non-return functions in one valve body, helping control the cold water supply into the hot water system. If you are replacing a hot water system, the existing duo valve should be checked before assuming it can be reused. At Hot Water Outlet, you can compare supply-only hot water systems and compatible valves, pipework and fittings, including options for electric hot water systems, heat pump hot water systems, gas hot water systems and instant gas hot water systems.

Quick Answer

A duo valve is commonly used on the cold water inlet side of storage hot water systems. It can provide isolation and non-return function, depending on the valve type and installation setup.

A duo valve may be relevant for:

• Electric storage hot water systems

• Heat pump hot water systems

• Gas storage hot water systems

• Some solar hot water systems

• Replacement storage tanks

• Valve kit upgrades

• Hot water system changeovers

The exact valve setup depends on the hot water system type, water pressure, pipework layout, temperature control, drainage and compliance requirements.

A licensed plumber should confirm whether a duo valve is required, whether the old valve can be reused and what additional valves may be needed.

Supply-only pricing for a hot water system usually does not include duo valves, valve kits, installation, pipework changes, plumbing labour, drainage work, old unit removal, disposal or compliance upgrades unless specifically shown at checkout.

Duo Valve Buyer Checklist

What Is a Duo Valve?

A duo valve is a plumbing valve commonly used on hot water system cold water inlet pipework.

Depending on the specific valve type, it can combine two functions:

• Isolation

• Non-return

The isolation function allows the water supply to the hot water system to be turned off.

The non-return function helps prevent water from flowing backwards through the valve.

This makes a duo valve useful in many storage hot water setups, but it does not replace every other valve required on a hot water system.

Why Duo Valves Matter

A duo valve can help with:

• Isolating the hot water system for servicing

• Isolating the hot water system for replacement

• Preventing reverse flow

• Supporting correct cold water inlet valve setup

• Making future maintenance easier

• Forming part of a compliant valve arrangement

• Reducing the need for separate isolation and non-return valves in some setups

If the duo valve is faulty, seized, leaking or missing, it can create problems during replacement or future servicing.

Duo Valve vs Isolation Valve vs Non-Return Valve

A duo valve can combine functions that may otherwise be handled separately.

A duo valve does not control water temperature. It also does not reduce incoming mains pressure unless it is part of a wider valve arrangement that includes pressure control.

Where Is a Duo Valve Installed?

A duo valve is commonly installed on the cold water supply line feeding a storage hot water system.

The exact location depends on:

• System type

• Pipework layout

• Existing valve setup

• Water supply location

• Access for servicing

• Space around the hot water system

• Manufacturer requirements

• Local compliance requirements

It should be accessible so the water supply can be isolated when needed.

A duo valve hidden behind walls, blocked by the tank or installed where it cannot be reached may create issues during future servicing.

What Hot Water Systems Use Duo Valves?

Duo valves are mainly associated with storage hot water systems.

They may be used with:

• Electric hot water systems

• Heat pump hot water systems

• Gas hot water systems

• Solar storage systems

• Replacement storage tanks

• Domestic hot water systems

• Some rental property replacements

• Some apartment hot water systems

They are not usually the main valve people think of for instant gas hot water systems, although instant systems still need their own isolation valves, gas valves, fittings and pressure checks.

Duo Valve for Electric Hot Water Systems

Electric hot water systems commonly use storage tanks, so a duo valve may form part of the cold water inlet arrangement.

When replacing electric storage, check:

• Existing duo valve condition

• Whether it turns off fully

• Whether it leaks

• Whether it is accessible

• Pipe size

• Flow direction

• Water pressure

• Pressure limiting valve requirements

• Tempering valve requirements

• Expansion control requirements

• Relief valve drain setup

A new electric tank does not automatically mean the old duo valve is suitable.

Duo Valve for Heat Pump Hot Water Systems

Heat pump hot water systems also store hot water, so they may need a similar valve arrangement to electric storage systems.

When buying or replacing a heat pump, check:

• Existing duo valve condition

• Cold water inlet position

• Pipework layout

• Water pressure

• Pressure limiting valve requirements

• Tempering valve requirements

• Relief valve drainage

• Condensate drainage

• Existing base or slab

• Access around the unit

Heat pumps also need airflow, noise placement, electrical supply and condensate drainage. The duo valve is only one part of the overall setup.

Heat pump rebates, STCs and incentives can change and depend on product, installer, property, purchase type and program rules. Do not assume a rebate applies without checking current eligibility.

Duo Valve for Gas Storage Hot Water Systems

Gas hot water systems that store hot water may also use a duo valve on the cold water inlet.

When replacing gas storage, check:

• Natural gas or LPG

• Existing duo valve condition

• Cold water inlet pipework

• Gas isolation valve

• Water pressure

• Relief valve drainage

• Tempering valve requirements

• Flue or ventilation requirements if applicable

• Tank size

• Installation location

Natural gas and LPG are not interchangeable. Gas fitting work must be completed by a licensed gas fitter.

Duo Valve for Solar Hot Water Systems

Solar hot water systems can include storage tanks, boosters, collectors and extra pipework.

A duo valve may be part of the cold water inlet arrangement on the storage tank, but solar systems can have extra valve and drainage requirements.

When replacing or modifying solar hot water, check:

• Existing tank valve setup

• Duo valve condition

• Solar pipework

• Booster type

• Tempering valve requirements

• Pressure limiting valve requirements

• Expansion control requirements

• Roof collector setup

• Drainage

• Whether the system is being changed to heat pump, electric or gas

Solar hot water replacement may involve extra work such as roof collector removal, solar pipework isolation, booster disconnection, crane work or roof repairs.

Do Instant Gas Systems Need Duo Valves?

Instant gas hot water systems do not store hot water in the same way as storage tanks, so their valve setup is different.

Instant gas systems may require:

• Cold water isolation valve

• Gas isolation valve

• Connection fittings

• Pressure control where required

• Controller wiring where applicable

• Power point where required

• Flue kit for approved internal/flued models

• Correct gas pipe sizing

• Correct water pressure and flow

A duo valve is not usually the key valve buyers are looking for with instant gas, but a licensed plumber or gas fitter should confirm the correct valve arrangement.

External gas hot water units are outdoor/external only unless the product is specifically designed and approved for internal or flued installation.

What Does the Isolation Function Do?

The isolation part of a duo valve allows the cold water supply to the hot water system to be shut off.

This is useful for:

• Hot water system replacement

• Valve replacement

• Leak control

• Servicing

• Emergency isolation

• Future maintenance

If the isolation function does not work, the water may need to be turned off elsewhere at the property, which can make servicing more difficult.

What Does the Non-Return Function Do?

The non-return part helps prevent reverse flow through the valve.

This matters because hot water systems and pressure control setups can be affected by reverse flow.

A non-return function may help:

• Support correct water direction

• Prevent backflow through the cold inlet

• Support valve arrangement

• Protect system operation

• Help meet installation requirements

The exact need for non-return protection depends on the plumbing setup and local requirements.

Can an Old Duo Valve Be Reused?

Sometimes, but only if it is suitable, accessible, compliant and in good working condition.

An old duo valve may not be suitable if it is:

• Leaking

• Seized

• Corroded

• Difficult to access

• Installed in the wrong location

• Installed the wrong way around

• Not shutting off properly

• Not compatible with the new system

• Not compliant with the replacement setup

• Showing signs of wear or age

If a valve does not turn freely or does not isolate properly, it may need replacement.

Signs a Duo Valve May Need Replacing

A duo valve may need attention if:

• It leaks from the body or connection

• It will not turn

• It turns but does not isolate water

• It is corroded

• It is installed in an inaccessible location

• It is damaged

• It looks old and worn

• It is the wrong size

• It is not suitable for the replacement system

• The plumber cannot safely reuse it

A faulty duo valve can delay replacement or make future servicing harder.

Is a Duo Valve Included With a Hot Water System?

Not always.

Some hot water systems may include certain valves or fittings, but many supply-only product prices do not include a full valve kit.

Before ordering, check:

• Product inclusions

• Whether a duo valve is included

• Whether a valve kit is separate

• Whether a pressure limiting valve is needed

• Whether a tempering valve is needed

• Whether an expansion control valve is needed

• Whether relief drain materials are included

• Whether your plumber is supplying parts

• Whether valves, pipework and fittings need to be ordered separately

Do not assume the hot water system comes with every valve needed for installation.

Duo Valve vs Pressure Limiting Valve

A duo valve and a pressure limiting valve do different jobs.

A duo valve can help isolate the water supply and prevent reverse flow.

A pressure limiting valve reduces incoming water pressure.

If the property has high water pressure, a duo valve alone does not solve that issue. A pressure limiting valve may also be needed.

Duo Valve vs Tempering Valve

A duo valve does not control outlet temperature.

A tempering valve mixes hot and cold water to deliver safer controlled hot water temperatures to selected fixtures.

A hot water system may need both:

• Duo valve on the cold water inlet

• Tempering valve on the hot water outlet or mixed water line, depending on setup

Do not confuse these two valves. They do completely different jobs.

Duo Valve vs Expansion Control Valve

An expansion control valve helps manage pressure caused by water expanding as it heats.

A duo valve does not perform the same role.

Some storage systems may need:

• Duo valve

• Pressure limiting valve

• Expansion control valve

• Tempering valve

• Relief valve and drain line

The exact setup depends on the system and property.

Duo Valve and Water Pressure

Water pressure can affect the wider valve setup.

If incoming pressure is too high, the system may need a pressure limiting valve in addition to other parts.

High water pressure may contribute to:

• Relief valve discharge

• Valve wear

• Leaks

• Water hammer

• System stress

• Reduced component life

A licensed plumber should check water pressure before deciding which valve arrangement is suitable.

Duo Valve and Hot Water System Replacement

When replacing a hot water system, the duo valve should be checked along with the rest of the valve setup.

A proper replacement check may include:

• Duo valve

• Tempering valve

• Pressure limiting valve

• Expansion control valve

• Relief valve

• Isolation valves

• Non-return valves

• Drainage

• Pipework condition

• Safe tray if indoors

• Gas isolation if gas

• Electrical requirements if electric or heat pump

Replacing the tank without checking the valves can lead to leaks, poor performance or compliance problems.

Duo Valve for Like-for-Like Replacements

Even a like-for-like hot water replacement may need valve changes.

Examples:

• Electric storage to electric storage

• Heat pump to heat pump

• Gas storage to gas storage

• Solar tank to storage tank

The existing duo valve may not be reusable if it is leaking, seized, corroded or not suitable for the new system.

Like-for-like does not always mean all old valves stay.

Duo Valve When Changing System Type

Changing system type can affect the valve arrangement.

Examples include:

• Electric storage to heat pump

• Gas storage to electric storage

• Gas to heat pump

• Solar to heat pump

• Storage to instant gas

• Instant gas to storage

Changing system type may require:

• New cold water inlet arrangement

• New isolation valve setup

• New tempering valve

• New pressure limiting valve

• New expansion control valve

• New relief drain pipework

• Condensate drainage for heat pumps

• Gas isolation or capping

• Electrical circuit changes

• Pipework alterations

Before changing system type, check the full installed cost, not just the supply-only system price.

Buying a Duo Valve Online

Before buying a duo valve online, confirm:

• Hot water system type

• Storage or instant system

• Pipe size

• Connection size

• Thread type

• Flow direction

• Pressure rating

• Existing valve setup

• Whether other valves are needed

• Whether the plumber is supplying parts

• Whether the valve suits the application

Do not buy a duo valve based only on appearance. It must suit the system and pipework.

What Else Might You Need With a Duo Valve?

A duo valve may be only one part of the full valve kit.

Depending on the system, you may also need:

• Tempering valve

• Pressure limiting valve

• Expansion control valve

• Isolation valve

• Non-return valve

• Pressure and temperature relief valve

• Relief drain pipework

• Safe tray components

• Connection fittings

• Pipework materials

• Heat pump condensate drainage parts

• Gas isolation valve for gas systems

You can compare compatible valves, pipework and fittings online.

Common Mistakes With Duo Valves

Assuming the Old Valve Still Works

A valve can look fine but be seized, leaking or unable to isolate properly.

Confusing a Duo Valve With a Tempering Valve

A duo valve does not control hot water temperature.

Confusing a Duo Valve With a Pressure Limiting Valve

A duo valve does not reduce mains pressure.

Installing the Valve the Wrong Way Around

Flow direction matters. A licensed plumber should install the valve correctly.

Forgetting Other Valves

A hot water system may also need tempering, pressure limiting, expansion control and relief drainage.

Buying the Wrong Size

The valve must suit the pipe size and connection type.

Ignoring Water Pressure

High water pressure may require pressure control in addition to a duo valve.

Comparing Supply-Only Price to Installed Cost

Supply-only pricing usually does not include valves, fittings, pipework, labour or compliance upgrades.

Not Using Licensed Trades

Plumbing, gas fitting and electrical work must be completed by appropriately licensed tradespeople.

Buyer Checklist Before Ordering a Duo Valve

Before ordering, confirm:

• Hot water system type

• Storage or instant system

• Electric, gas, heat pump or solar setup

• Pipe size

• Connection size

• Flow direction

• Existing valve condition

• Water pressure

• Whether a pressure limiting valve is required

• Whether a tempering valve is required

• Whether an expansion control valve is required

• Whether relief drainage is suitable

• Whether a full valve kit is needed

• Whether the valve is accessible

• Whether the plumber is supplying parts

• Licensed plumber requirements

Supply-Only vs Installed Cost

Hot Water Outlet sells supply-only hot water systems, valves and accessories online. Supply-only pricing is useful for comparing product costs, but it is not the same as total installed cost.

The final installed cost may depend on:

• Licensed plumbing labour

• Valve installation

• Pipework changes

• Drainage work

• Existing valve removal

• Access

• Water pressure issues

• Additional fittings

• Safe tray work

• Hot water system replacement

• Electrical work if the system is being replaced

• Gas fitting if gas is involved

• Compliance upgrades

Where appropriate, Hot Water Outlet may help organise quotes from reputable installers.

FAQ: Duo Valves for Hot Water Systems

What does a duo valve do on a hot water system?

A duo valve can combine isolation and non-return functions on the cold water supply to a hot water system.

Is a duo valve the same as an isolation valve?

No. An isolation valve turns water on or off. A duo valve may include isolation plus non-return function, depending on the valve type.

Is a duo valve the same as a tempering valve?

No. A tempering valve controls delivered hot water temperature by mixing hot and cold water. A duo valve does not control temperature.

Is a duo valve the same as a pressure limiting valve?

No. A pressure limiting valve reduces incoming water pressure. A duo valve does not reduce pressure unless it is part of a separate pressure control arrangement.

Do electric hot water systems need a duo valve?

Many electric storage systems may use a duo valve as part of the cold water inlet setup. A licensed plumber should confirm the correct valve arrangement.

Do heat pump hot water systems need a duo valve?

Heat pump storage systems may use a duo valve, but they may also need pressure limiting, tempering, expansion control, relief drainage and condensate drainage.

Do gas hot water systems need a duo valve?

Gas storage systems may use a duo valve on the cold water inlet. Instant gas systems usually have different valve requirements.

Can I reuse my old duo valve?

Sometimes, but only if it is suitable, compliant, accessible and in good working condition. Old valves may be seized, leaking or unsuitable.

Is a duo valve included with a hot water system?

Not always. Supply-only hot water systems may not include a full valve kit unless specifically listed.

Can I install a duo valve myself?

No. Plumbing work must be completed by appropriately licensed tradespeople.

Can Hot Water Outlet install a duo valve?

Hot Water Outlet sells supply-only hot water systems, valves and accessories online. Plumbing work must be completed by appropriately licensed tradespeople. Where appropriate, Hot Water Outlet may help organise quotes from reputable installers.

Final Thoughts

A duo valve is a useful part of many storage hot water system installations because it can provide isolation and non-return function on the cold water inlet. When replacing a hot water system, the duo valve should be checked along with the tempering valve, pressure limiting valve, expansion control valve, relief valve, drainage and pipework.

Do not assume the old valve is reusable, and do not confuse a duo valve with a tempering valve or pressure limiting valve.

Browse valves, pipework and fittings, compare hot water systems, electric hot water systems, heat pump hot water systems, gas hot water systems and instant gas hot water systems online at Hot Water Outlet.