HTM04-01 Specification for
Hot Water Calorifiers and Buffer Vessels

McDonald Water Storage specialise in manufacturing commercial water heaters with an extensive range of calorifiers and buffer tanks, to any size and capacity, and to suit vented and unvented installations.

When cylinders are specified for NHS installations, the Health Technical Memorandum 04-01, a Department of Health document outlines the requirements for Safe Water within Healthcare Premises. Part of this document outlines the specification of all hot water storage vessels used on NHS premises, with the primary requirement that they are to be manufactured from copper!

Our expert commercial water heater specialists can advise on the HTM04-01 requirements, in addition to the optimum design for the installation.

As part of our market leading calorifier and buffer vessel solution, you can enjoy the following benefits:

  • 2 week delivery period on most calorifier projects – meaning you get the cylinder when you need it
  • Manufactured to meet BS853 and follow an approved and assessed quality plan to ISO 9001 – meets all the required regulations and legislation for the strictest specifications
  • Deliver a superior level of quality, reliability and durability which is simply unrivalled in the industry – providing peace of mind
  • Manufactured from copper, which kills 99.9% of all bacteria such as legionella and Ecoli, minimising any risk to public healthn
  1. Calorifiers and buffer vessels (Cylinders) to be manufactured from copper to BS853-2, BS EN 12897 (Unvented) and meeting the design requirements of HTM04-01.
  2. All cylinders to be convex bottomed and fitted with a pre-plumbed drain connection at the lowest point, which extends beyond an integral raised base to accommodate the convex bottom and drain connection.
  3. Drain pipework to be connected to a three-port T-type ball valve (avoiding dead leg) drain point and a preplumbed destratification circuit, including pump with integral timer and quarter turn full bore pump isolation ball valves.
  4. All cylinders to incorporate a fully insulated and removeable inspection hatchway, with a minimum 250mm and maximum 450mm diameter clear opening, (size dependant on the diameter of the cylinder) and PTFE gasket.
  5. All cylinders to be fully insulated with either factory applied, non-removeable high density foam insulation (preferred) and outer metal casing, or where access restrictions dictate, a fully removeable clip together metal casing incorporating a 50 to 100mm foil-backed rockwool insulation jacket, (dependant on cylinder size and standing heat losses).
  6. Indirect calorifiers to incorporate finned copper primary heat exchanger coils, (to maximise heat transfer for use with low carbon primary heat sources such as heat pumps), pressure tested to a minimum of 5 bar, rated to achieve a 30-minute recovery time from 10 to 60, with a maximum pressure drop of 20kPa.
  7. Cylinders to include a minimum of two commercial grade single/three-phase (on site configurable), immersion heaters, each incorporating a working and high limit thermostat, rated to raise the cylinder contents to greater than 70 centigrade, (working in conjunction with the primary coil and BMS controlled), for pasteurisation and where resilience of supply is critical, (e.g. Tier 1 sites) rated to raise the cylinder contents from 10 to 60 centigrade in a maximum of 3 hours without input from the primary coil.
  8. Buffer vessels heated via a packaged plate heat exchanger (PHE) to incorporate a pre-plumbed priming circuit pump and flow regulator connected to the drain connection and secondary side of the PHE and include quarter turn pump isolation valves and a PHE insulation jacket.
  9. External dimensions, capacity and orientation of the cylinder connection points to be specified for each cylinder to optimise installation, minimise extraneous pipe work and to ensure clearances are provided above
    and around each cylinder, allowing safe access to all connections and components to facilitate monitoring, maintenance, and repair activities.
  10. Where unvented cylinders are used, these are to be supplied with flow-through expansion vessels, designed to be mounted vertically only with the inlet at the bottom and of a type which do not use EPDM bladders. Each vessel must also incorporate a means of flushing / purging its contents.

All cylinders to include the relevant ancillary components, (refer to drawing below), including:

  • Separate control and high limit thermostats and corresponding connection points in the cylinder.
  • 2 No. 100mm face diameter temperature gauges and corresponding high and low level connection points in the cylinder.
  • 1 No. 100mm face diameter pressure gauge and corresponding connection point in the cylinder.
  • Low level (below primary coil) hot water return connection.
  • Non-return valve pre-fitted into the cold-water feed connection.
  • Pre-fitted anti-vacuum valve.
  • Destratification pump circuit and quarter turn, full bore pump isolation valves.
  • Three port valved destratification circuit / drain point connection (avoiding drain dead leg).

Additional requirements for unvented cylinders:

  • Pressure reducing valve.
  • Pre-fitted combined temperature and pressure sensing safety valve.


Primary heating coils, PHE and back up immersion heater kW ratings are dependent on the cylinder volume and system demand. Manufacturers should be consulted to advise for each application.

  • Bespoke Calorifier tank solution to suit Vented and Unvented Installations
  • Manufactured from Copper enhancing heat recovery process
  • Superior non-corrosive and anti-bacterial qualities to kill bacteria such as Legionella and Ecoli
  • All units can be manufactured to suit renewable energy, such as Solar or Heat Pumps
  • All tanks and commercial water heaters manufactured to BS853
  • Delivery in approximately 2 weeks from order
  • Galvanised Cladded Units and White cased units available

Have you ever wondered what is involved in manufacturing a commercial copper cylinder?

Check out our time lapse video


The growth in demand for Copper calorifiers is largely thanks to coppers’ enhanced thermal conductivity, as well as its excellent non-corrosive and anti-bacterial qualities. These are proven to be more effective at eradicating germs such as MRSA, E-coli and Legionella.

For further information on the benefits of copper, see our brochure The Ultimate Supermetal or visit our Copper Facts page.

Sizing and technical advice are readily available to ensure tank is supplied suitable for the capacity and pressure required. Design sketches are also available for individual projects.

We have an enviable position in the market for our ability to design, manufacture and supply calorifier tanks thanks to our in-house expertise and our advanced machining capabilities.

We also carry a wide range of sheets, spinnings and other materials which helps us to provide a rapid delivery – often within one week.

Whether its sizing, multi-fuel applications or connections, our team are only too happy to help. This enables you to get exactly what your project requires without compromising on any aspect.

To discuss your requirements, simply contact us on 01592 611123.

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