Indirect unvented cylinders – Unvented indirect hot water cylinders

Indirect unvented cylinders, or indirect unvented hot water cylinders, take their supply water directly from the mains water for the domestic hot water requirements. Because they can supply hot water at mains water pressure, indirect unvented hot water cylinders are great for properties with multiple bathrooms, as they have the flow capacity to run multiple showers, baths or taps with a reliable hot water supply at a consistent pressure.

Mains pressure supply has many benefits, the most important being a better flow rate compared to a gravity-feed system hot water cylinder heating products. Vented cylinders rely on gravity and an open vent, usually a feed cold water storage tank and expansion header tank in the loft space, while unvented cylinders work under mains water pressure.

These cylinders are also known as an indirect pressurised hot water cylinder, an unvented indirect hot water cylinder or an unvented indirect cylinder.

Indirect unvented hot water cylinders require an external source of heating, normally a gas boiler, heat pump, solar thermal system, or oil boiler, as a heat source, for example. This warm water then circulates via a stainless steel heat exchanger coil in the water cylinder, heating the surrounding domestic water without mixing with it. Thus, the term is an “indirect” water heating system. This heat source is often used to supply the central heating system as well.

 

A direct unvented hot water cylinder, on the other hand, uses electric immersion heaters as a heat source, which are installed in the water tank / cylinder to heat the hot water directly, again at mains water supply pressure. In contrast, an indirect cylinder has a heating internal coil within the tank, powered by a boiler or another heating source.

Thus, the term “main’s pressure direct unvented cylinders” or “direct unvented cylinder”

We do an extensive range of this type of direct unvented cylinder on this dedicated webpage – direct unvented cylinder

Benefits of Indirect Unvented Hot Water Cylinders

So, what are the benefits of unvented hot water cylinders?

In one line – it’s a mains pressure domestic water hot system, with all the benefits of higher pressure and flow rate for your hot water heating

No Cold Water Storage Tank Required

As the water is fed directly, properties with limited space will not have to give up valuable storage space in the attic, which you require with gravity-fed systems. The eradication of this cold-water feed tank also eliminates the possibility of it freezing during prolonged cold spells.

Mains Pressure Hot Water and Higher Flow Rates

This advantage arises because water is supplied from the cold-water mains rather than from a water tank. Higher pressure is especially significant when there are many bathrooms that may be running warm water from multiple showers or taps at the same time.

Quieter Hot Water System

This advantage is due to the feed tank not being filled with cold water, as none is required in an unvented system.

Reduced Risk of Water Contamination

It benefits from reduced danger of contamination, with fewer opportunities for foreign contaminants to enter the hot water cylinder and, consequently, cause harm or damage to the domestic water heating system. Such contamination is a real possibility with vented cylinders in a property.

Thermal Expansion and the Internal Air Gap

Unvented hot water cylinders require an internal air bubble or expansion vessel to be installed to work correctly, as any stored water in the hot water cylinder expands when it heats up. This heated water volume expands and needs to be absorbed so that the pressure does not become excessive, and a vessel or air bubble takes this expansion.

Expansion vessels are installed externally to the unvented cylinder, whilst an internal air bubble is obviously internal to the unvented cylinder. As the name suggests, these systems are used by the Heatrae Sadia Megaflo or Sadia Megaflo Eco units.

Self-Sustaining Air Gap and Venturi System

Our innovating and unique solution to this problem is a self-sustaining air gap system for our unvented cylinders.

Details on how the system works on our unvented cylinders

The mixing of liquid with air, or, more precisely, the induction of air bubbles into the liquid, is what distinguishes the patented self-sustaining system technology, with the system now capable of maintaining the internal air gap when hot water is drawn off further upstream during hot water demand, i.e., when taking a shower or bath or using the hot taps in the kitchen or sinks.

There is no requirement for expansion vessel refilling or regular servicing since it continually replaces the internal expansion mechanism in this high-quality indirect stainless steel cylinder range.

Because there are no internal moving components in the cylinder, and once the venturi valve is installed, no more parts are required, this automated replenishment of the air gap saves time and money. This pinnacle of fit-and-forget technology will provide landlords, developers, self-builders and heating installers with confidence that they do not have to inspect for wear and tear or be called out to repair problems on a regular basis.

This means no external expansion vessel; thus, less space is required – great for limited space or access locations

Because there is no external expansion vessel, the cylinder may be installed in more compact locations, providing builders and heating installers considerably more freedom in where to install the unvented cylinder, whether for a new build or restoration project.

 

Download the installation details here for the self-sustaining unvented system

Flexiheat UK Installation 3S Air Gap Device information for unvented hot water cylinders

Heat pump indirect cylinders

If you require an indirect hot water cylinder for use with a heat pump, as lower-temperature heat pumps require a larger coil surface area, then you require a larger heating coil than these standard indirect hot water cylinders – we have a range of heat pump cylinders, which are listed on this webpage – hot water cylinders for heat pumps

Unvented Cylinder Safety Controls and G3 Requirements

Unvented hot-water storage systems must have at least two independent safety devices, in addition to the normal thermostat, and must include a non-self-resetting energy cut-out and a temperature or combined temperature-and-pressure relief valve as per the standard BS EN 12897:2016+A1:2020.

The installation of an unvented hot-water cylinder must comply with the relevant requirements of Approved Document G of the Building Regulations, including Requirement G3 for hot-water supply and systems.

Cylinder Thermostat and Overheat Protection

Each unit comes with a cylinder thermostat for normal temperature control of the water and a high-limit thermostat with a non-self-resetting thermal cut-out for overheat protection.

Temperature and Pressure Relief Valve

Each cylinder is factory fitted with a temperature and pressure relief valve. This valve is pre-calibrated to open at 7 bar or 90°C and provide final safety protection for the unvented hot water system, as required by Building Regulations Part G. A suitable discharge route for the relief valve has to be provided, i.e., T&P relief valve to the tundish, to discharge pipework and finally to a safe discharge location.

Cold Water Inlet Control Group

Prior to the hot water tank, the supplied combined cold water inlet control group (which consists of a pressure-reducing valve and an expansion relief valve) can be installed anywhere on the cold water mains. If it is more convenient, it can be positioned near where the water mains supply enters the property.

Water Pressure and Flow Rate Requirements

The minimal criteria for satisfactory operation of our cylinder are 1.5 bars of pressure and a flow rate of 20 litres per minute. The unvented hot water cylinder will continue to function below this level, but two or more hot water outlets will not be able to be used at the same time.

Which Heat Sources Can Be Used with an Indirect Unvented Cylinder?

Gas, Oil and Electric Boilers

Any type of boiler can feed these unvented indirect cylinders, be it gas boilers, oil boilers, or electric boilers powering the internal coil of the domestic hot water cylinder.

Solid Fuel and Wood-Fired Boilers

Unvented cylinders that are indirect are not suitable for solid fuel boilers or wood-fired boilers. For solid fuel heat sources you must use a vented hot water cylinder, which is listed on this webpage –indirect vented hot water cylinders 

Heat pumps

As mentioned above, due to the lower temperatures associated with heat pumps, you require an indirect heat pump hot water cylinder that has a large coil heat exchanger surface area.

What Size Indirect Unvented Cylinder Do I Need?

The Hot Water Association of the United Kingdom provides a solid rule of thumb that might help you determine the size of your water cylinder.

Hot Water Demand per Person Method

In a typical home, 30 to 50 litres of warm water per person per day is required.

A low-consumption household will require 20–30 litres of warm water per person per day.

In a high-consumption residence, 50–70 litres of warm water per person per day is recommended.

Sizing for Baths and Showers

Please bear in mind the following –

A standard UK bath requires 60 litres at 60°C, with the other 40 litres being cold – to get the mixed water temperature of 40°C.

A standard UK showerhead is around 8 litres of water per minute – it will use around 5 litres of 60°C water. A drench-type shower can use an awful lot more!

However, it should be emphasised that personal behaviours have a significant role in overall warm water usage. Two identically sized families can consume drastically different quantities of warm water, with one needing twice as much as the other, as people’s usage patterns can differ drastically.

Should you require larger cylinder-type commercial units for high demand hot water requirements, then please look at our webpage – Commercial indirect hot water cylinders

Indirect Unvented Cylinder Range and Specifications

All of our indirect unvented hot water cylinder products have a 22 mm diameter stainless steel high-performance coil & outlet with a 1 x 3 kW immersion heater as a backup direct unit should you lose your external source of heat indirectly feeding the hot water cylinder/store.

Our unvented hot water cylinder range is manufactured from duplex stainless steel, which, for your information, is the best available – the product warranty is 25 years for our unvented cylinders when used in a domestic property and 5 years when used as a commercial indirect unvented cylinder, i.e., in non-domestic hot water heating applications.

Product ErP information / ErP information product / library ErP information – The 80 to 130 litre capacity cylinders are “A” rated for heat retention and energy efficiency; the 150 to 280 litre capacity cylinders are “A” minus or “B” plus; and the 305 to 500 litre unvented indirect cylinders are better than “C” rated heat loss for this hot water unvented cylinder range of products.

All unvented cylinders in this range have a maximum working pressure of 5.5 bar and are tested to 10 bar pressure for the hot water cylinders.

The 170 litre indirect unvented cylinder to 500 litre indirect unvented cylinder are fitted with a DHW return connection to the unvented hot water tank for large hot water heating systems.

We can also supply “Hi-Flow” models with a larger 28 mm inlet/outlet and a 28 mm Venturi valve for high flow rates; these units are available from our 255 litre to 500 litre indirect unvented cylinder models.

Please note that scale build-up on the coil in hard-water areas can reduce the performance of the water heater. Please consider using a scale inhibitor or water softener to prevent this issue in hard water areas. 

Indirect Unvented Cylinder Dimensions and Technical Data

 

Hot water heating times, recovery rates and coil output ratings based of a flow temperature of 85C to the heat exchanger coil.

If you require larger kW outputs than our standard coil units, then we do a range of rapid recovery cylinders with larger kW coil ratings; these are detailed here – high recovery cylinders

Download the Hot water cylinders indirect and direct unvented cylinder brochure from Flexiheat UK

The installation manual is here –

Flexiheat manual for direct & indirect hot water cylinders

We have dedicated webpages for our most popular indirect unvented cylinders –

150 Litre Indirect Unvented Cylinder

210 Litre Indirect Unvented Cylinder

250 Litre Indirect Unvented Cylinder

300 Litre Indirect Unvented Cylinder

400 Litre Indirect Unvented Cylinder

and the largest in this range 

500 Litre Indirect Unvented Cylinder

We supply larger indirect cylinders above 500 litres – please look at this webpage – Commercial indirect hot water cylinders

Immersion Heater and Backup Heating

Our immersion heater is manufactured in Incoloy 825, which offers excellent resistance to hard water.

Horizontal indirect cylinder option

We also provide a range of unvented indirect horizontal cylinders from 200 to 500 litres – which are detailed on this webpage – Indirect horizontal unvented cylinders

Please note these units use a traditional external expansion vessel for the unvented hot water system.

Pre-plumbed vertical indirect hot water cylinders 

We can also do pre-plumbed units as well, from 175 litres to 300 litres.

Indirect Unvented Cylinder Installation

Unvented water cylinders require installation and maintenance by suitably qualified plumbers/heating engineers, as they run at a greater pressure than a vented system and have numerous additional safety precautions incorporated into the hot water unvented cylinders. All heating engineers / plumbers installing the cylinder must be qualified in “G3” unvented cylinder system installs.

Contact us at Flexiheat UK

Should you require any more information or discount prices on the best indirect unvented cylinder / unvented indirect hot water cylinders on the UK market with a warranty of 25 years (hi-flow commercial units are 5 years), as they are duplex stainless steel construction, then give our sales team a call on 01202 822221 or use our contact us email page, as you can direct shop with us. Follow us on Facebook or Twitter online for our latest offers.