• For use in Domestic Properties

• For use with solid fuel appliances

•Can be specified for a gravity system






Indirect Vented Cylinders are heated by a boiler of some sort which heats water within a coil of the cylinder. They are also fed from a cold water feed and expansion tank usually situated in the loft space within the property. These cylinders are generally classed as low pressure cylinders due to the low static pressures better known as working head. The working head is determined by the distance between the feed tank’s water level, and the draw off point on top of the cylinder.


 The working head of a cylinder determines which of the three grades of copper it should be manufactured from;

• Grade 1 is the heaviest or thickest grade and is specified for a working head of 25 metres.

• Grade 2  which is specified for a working head of 15 metres

• Grade 3 which is specified for a working head of 10 metres.


Grade 3 cylinders are usually specified and are more than adequate for general domestic properties. Although you may also come across Grade 4 or Medium Duty cylinders, both have now been superseded by the Grade 3 cylinder since 2002.

Indirect Coil-

By definition, Indirect cylinders are fitted with a spiral heating coil designed to work in conjunction with domestic gas /oil boilers, heat pump as well as solid fuel heating appliances. Heat is transferred to the potable water contained within the cylinder via a spiral coil that acts as the heat exchanger keeping the potable bathing water and the contaminated system water separate.

Other compatible heat inputs can be in the form of solid fuel appliances such as wood burners or multi fuel stoves. As most of these systems rely on natural thermo syphoning to achieve the circulation of heated water through the coil (as opposed to a circulation pump), the spiral coil needs to be 28mm and have a continuous fall to minimise the risk of any air traps or air pockets from forming within the coil.


Our polyurethane foam insulation can be applied up to a thickness of 75mm deep and is the most environmentally friendly type available. Rather than the more common blowing agents that are used in spray foam applications , such as CFC, HCFC and HFC .Our spray system uses water as a blowing agent. The code for sustainable homes states that materials must have an ozone depletion potential of zero and a global warming potential of less than five. As a result of the water used in our spray system , our insulation has an ozone depletion potential of zero and a global warming potential of one.

British Standard-

Manufactured in accordance with British Standard, BS 1566-1 : 2002 and in compliance with Part L of the Building Regulations, these cylinders combine a high performance heating coil with low heat loss, factory applied insulation, to provide a highly efficient and effective way to store and provide domestic hot water.


See our Online Designer or contact us if you have any specific requirements.



When ordering a particular cylinder its worth taking into consideration upgrade options or requirements as listed below, a marginal cost now can save a considerable amount over the life of the cylinder.


Material grade is important to consider especially in areas of hard or aggressive water quality, by designing a cylinder from thicker material can prolong the life of the cylinder.


There are a wide variety of connection sizes available and its worth taking the time to refer to our Connection Size List before ordering a direct replacement cylinder. If you are unsure then the simplest option is to specify the cylinder with 22mm or 28mm compression connections, this saves buying extra plumbing fittings and any worry over ordering the incorrect size connections.

Connection layout-

We understand how difficult it can be to install a cylinder with pipes that are inaccessible, by specifying connections in the most convenient places will not only save money on materials but make the the install easier.

Additional connections-

Specifying additional connection such as a shower or a secondary return connection can save not only the trouble of fitting an Essex flange but also cost as well. A Surrey or Warrick flange while easy to install are considerably more expensive than having an extra connection added when specifying a cylinder.

Quick recovery coil-

For situations that need faster than usual heat-up times or where a large boiler is installed, these cylinders can be supplied with a larger than usual coil, designated as a “Quick Recovery” coil. There is also a benefit if you have a space and volume restriction due to cupboard size, by having a quick recovery coil fitted and timing the boiler to come on when there are peak hot water demands, this will replace the volume used whilst bathing or showering. These units not only improve on heat-up times but also reduce boiler cycling times which in turn will maximize the boiler’s efficiency, this can make a significant saving over the year.

Gravity Coil-

For cylinders that are intended for use with a solid fuel appliance, a 28mm gravity coil is required with connection sizes of 1" or above. It is important to specify that the cylinder is intended for gravity use when ordering, we will then carry out a test to ensure the coil is compatible for gravity use.

Header Tanks-

Due to the uncontrolled heat input and high temperatures that a solid fuel appliance can emit over prolonged periods of time, it is recommended that a metal feed and expansion tank is installed as opposed to the more common plastic types. For more information on our copper and stainless steel header tanks, please see our Header tank page.


Daily heat loss can be in the region of 5c and higher depending on the insulation depth of the cylinder and the store temperature, by adding extra insulation up to 75mm deep will save on daily heat loss over life span of the cylinder


Matching customers' requirements is particularly important when space is at a premium, so the most common customisation for a hot water cylinder, is the “mixing and matching” of the various heights and diameters that we offer. As additional parts of a Bespoke design, the customer is able to specify a number of options including coil sizes (where applicable), connection sizes/positions, insulation thickness, whether it is manufactured from copper or stainless steel (see our Copper vs. Stainless Steel page for a comparison), and the finish of the unit (the metal case being the premium option). We can offer design drawings prior to manufacture to ensure that all connection sizes, positions and functions are to the customer's requirements.







Indirect cylinders are available in a range of heights (from 400mm to 2000mm) and to the specific diameters listed below. Please see our Slimfit products for small diameters (all sizes stated are excluding insulation):

  • 400mm Diameter

    Height Diameter Literage Cold Feed Boiler Flow Boiler Return Secondary Return*
    900 400 96 100 450 100 700
    400 120 100 500 100 800
    400 135 100 650 100 950
    1500 400 170 100 650 100 1200
    1800 400 210 100 1100 100 1450
  • 450mm Diameter

    Height Diameter Literage Cold Feed Boiler Flow Boiler Return Secondary Return*
    900 400 96 100 450 100 700
    400 120 100 500 100 800
    400 135 100 650 100 950
    1500 400 170 100 650 100 1200
    1800 400 210 100 1100 100 1450
  • 500mm Diameter

    Height Diameter Literage Cold Feed Boiler Flow Boiler Return Secondary Return*
    1200 500 204 100 800 100 950
    1500 500 260 100 850 100 1200
    1800 500 320 100 1100 100 1450
    2000 500 360 100 1100 100 1650
  • 600mm Diameter

    Height Diameter Literage Cold Feed Boiler Flow Boiler Return Secondary Return*
    1200 600 290 100 800 100 950
    1500 600 373 100 850 100 1200
    1800 600 456 100 1100 100 1450
    2000 600 512 100 1100 100 1650

All height and diameter measurements are excluding insulation and all connection port heights are approximate.


For smaller diameters, please see the Slimfit Page

Diameter Options 300mm, 350mm, 375mm, 400mm, 450mm, 500mm, 600mm
Height Options Variable height from 400mm to 2000mm
Copper Grading Grade 1 (25mtr Mwh), Grade 2 (15mtr Mwh) and Grade 3 (10mtr Mwh)
Shell Connection Sizes Available 1/2", 3/4", 1", 1 1/4", 1 1/2", 2", 15mm, 22mm, 28mm
Insulation Thickness 25mm to 100mm
Coil Heating Surface Relative to cylinder size (General Guidance - 25 minute recovery)
Coil Working Pressure 3.5 Bar
Coil Connection Sizes 3/4", 1", 1 1/4", 1 1/2", 2", 22mm, 28mm

Capacity Calculator

This calculator is designed to assist the specification of a cylinder by determining the amount of hot water available from a cylinder of a specific size; this can be particularly useful in situations where space for a cylinder is limited.

Height mm
Diameter mm
Literage Litres.

Uninsulated Diameter Calculator

This calculator is designed to assist the specification of a cylinder by determining the size of the cylinder from it's circumference and insulation thickness; this can be particularly useful in situations where a replacement cylinder is required, but due to the insulation it is difficult to determine the size of cylinder required.


Please note - The final figures are approximate and should be rounded to the nearest standard diameter (375,400,450 etc.)


Circumference mm
Average Insulation Thickness mm
Insulated Diameter mm
Uninsulated Diameter mm

Tapping Chart

To assist with determining the correct size connection for replacement units or units to fit your current system, we have created a table of external and internal thread sizes.

BSP Male Female
1/2" 20.7mm 19.2mm
3/8" 16.6mm 15mm
3/4" 26.1mm 24.2mm
1" 32.8mm 30.4mm
1 1/4" 41.6mm 40mm
1 1/2" 47.6mm 44.9mm
2" 59.3mm 56.9mm
1 3/4" N/A 50.8mm
2 1/4" N/A 63mm
15mm Compression 20.7mm 19.2mm
22mm Compression 28.3mm 26.7mm
28mm Compression 34.6mm 32.6mm

Brunel Drive

Northern Road Industrial Estate



NG24 2EG


Tel: 01636 678 437

Fax: 01636 678 964

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