What are the true benefits of separating heating and hot water systems, and how does the use of direct or indirect fired units compare? Kevin Potter has the answers
When it comes to designing heating and hot water systems we may all take a different approach depending on our own philosophies; the application; type of building; environmental aspirations; and ultimately the client's objectives. With such a wide choice of systems and products available for the efficient generation of domestic hot water (DHW) it is important to understand what are the true benefits of separating the heating and hot water systems, and the comparison between using direct or indirect fired units.

Containing heat exchanger coils
Direct fired units for hot water generation have an integral gas or oil burner that directly heats the water stored in its tank by supplying hot gases through one or more of its heat exchanger coils to heat the surrounding stored water. Indirect fired units such as calorifiers have no integral burner, but contain one or more heat exchanger coils that are filled with hot liquids (water or solar fluid) that have already been heated 'indirectly' by one or more external heat source, such as a boiler or solar collectors in a solar circuit.

There are significant benefits to be gained with using a correctly sized, self-contained, direct fired DHW system that operates independently from the main building heating system, particularly if the application is one that can take advantage of being able to switch off the heating boilers during the summer months.

Formerly indirect fired systems would have a dedicated hot water boiler for heating a calorifier, but with modern highly efficient boilers the same boiler can now be used for the heating circuit and the DHW system - provided it is sized to meet the maximum heating and hot water conditions. However during the summer months when the space heating should not be required, the boiler will still need to be operating to provide the heat for the hot water system.

Direct fired water heaters are solely dedicated to the job of hot water generation providing a faster heat up and response time compared with an indirect system which is in fact 'heating water to heat water'. This increases the chance of heat losses in the boiler, and associated pipework between the boiler and calorifier, before the water even reaches the hot water distribution system pipework.

Water heaters can often be installed close to the point of use, reducing the energy required to supply water over a long pipe run and minimising distribution heat losses. This also helps lessen the length of deadlegs in the system and supports compliance with regulations such as the Health and Safety Executive L8 - Approved Code of Practice & Guidance for The Control of Legionella Bacteria in Water Systems which states that the distribution pipework should be adequately designed to enable the water to reach all outlets at 50 deg C within 1 minute of turning on the tap.

Calorifiers are simpler to install
Calorifiers used for DHW generation do not require flues or have a gas supply directly connected to them. This makes them simpler to install, particularly where the heat source may be located remote from the calorifier itself, and no changes are required to the building fabric to install flue runs.

With rising energy bills, and pressure to reduce carbon emissions, as well as comply with forthcoming regulations, the efficiencies of both direct and indirect fired hot water systems have never been under such scrutiny as they are now. Manufacturers are continually investing in new and improved products to meet this demand and provide energy efficient, compliant solutions.

This has caused some debate in the industry about the fairness of the efficiency criteria in the Part L Building Regulations document, as there are some discrepancies between the method of calculating efficiencies for an indirect and direct fired hot water system.

For direct fired water heaters, Part L takes the gross seasonal efficiency of the appliance which is measured using the efficiency during one heat up cycle from hot to cold. On the other hand, for an indirect fired hot water system it allows the gross seasonal efficiency of the associated boiler to be used.

This is where the problem occurs. The gross seasonal efficiency of a boiler is calculated using part load and full load operating efficiency; however an indirect fired system will generally operate at 80/60, temperatures at which a boiler cannot condense consequently not actually operating at part load (which requires a return temperature of 30 deg C).

This means that what is recorded on paper is not a true reflection of what will actually be happening in practice; skewing the efficiencies which may ultimately affect the customer's choice of hot water plant.

For example using the Building Regulations Part L document, a direct fired condensing water heater with a gross thermal efficiency of 96 per cent may compete against a calorifier where the boiler's gross seasonal efficiency is used, which for a modern condensing boiler can also easily be 96 per cent.

However, when producing hot water with return boiler temperatures likely to be 60 deg C or higher, the boiler is more likely to be operating at a gross efficiency in the region of 88 per cent.

If you also take into account the additional electrical load requirements of the boiler primary pump and calorifier loading pump then the energy requirements and true efficiencies of an indirect fired solution differ greatly to the proposition on paper.

Even more reason to thoroughly review all the options available for your heating and hot water plant.

// The author is business development manager - hot water solutions at
Hamworthy Heating //