Boiler strategies for low-carbon HVAC

The theory that will lead to more efficient heating systems served by boilers is often not borne out in practice. Steve Cooper provides food for thought.

If you’re looking to reduce energy consumption and upfront cost in a heating system, the boiler strategy is the place to start. Something as basic as focusing on the system Δt, the difference between flow and return temperature, will have a dramatic effect on the capital cost and energy usage of the system.

You would be surprised, though, how many UK systems incorporating the latest technology are under-performing because the strategies for boiler operation fail to optimise the energy-saving opportunities. The arrival of ultra-efficient condensing and biomass boilers has actually accentuated, rather than reversed, this situation.

The problem is often the operating temperatures. In North America and the UK, low-temperature hot-water heating systems were traditionally designed with a supply temperature of 80°C (180°F) and a return of 70°C (160°F). In Continental Europe 80°C (180°F) supply and 60°C (140°F) was the norm; this doubling of the system ∆t immediately halves the flow required.

Efficiency of Armstrong’s MBS condensing for various loads and system temperatures

For example, a 240 kW system with a 10 K ∆t would require a flow of 5.7 l/s. With a ∆T of 20 K the flow would be halved to 2.85 l/s. This reduction in flow has a dramatic effect on pipe sizing, pump sizing, friction losses, electrical power consumption and lifecycle cost.

Yes, the higher ∆t does reduce the mean temperature from 75 to 70°C, but in an age when building air tightness and thermal insulation is vastly improved, this has little impact on emitter size.

High flow temperatures such as 80/70°C become a real problem though in systems with condensing boilers, which will never reach the high efficiencies stated in manufacturers’ literature unless the system temperature is below the dewpoint of the flue gases. The ideal flow/return temperatures are 50/30°C. The difference in efficiency can be as much as 10%, depending on boiler type and load (see table).

The lower temperatures in the table are ideal for underfloor heating, and can also work successfully with fan coils and air handling units. They also enable standard radiators to be used in place of expensive low-surface-temperature radiators in applications like hospitals, care homes and nurseries. The only thing low flow temperatures are not suitable for is producing domestic hot water, which should be designed as a stand-alone constant-temperature system to avoid problems with Legionella.

Fig. 1: Modern boilers, especially condensing boilers are more efficient at part load, unlike traditional non-condensing boilers, which are inefficient at part load.

The control strategy is also crucial. With today’s integrated systems it is no longer difficult to employ the optimum methodology. Modern condensing boilers are at their most efficient at low load, unlike traditional boilers where the opposite is true (Fig. 1). With integrated heating solutions such as prefabricated systems, control of the entire system (not just the boilers) is already pre-designed to suit the more efficient part-load environment.

Integrating biomass boilers

The greatest challenge often arises in low carbon HVAC systems with a biomass boiler for the base load and gas-fired condensing boilers for peak loads and back-up. The biomass boiler wants to run at 80/60°C whilst the condensing boiler wants to run at 50/30°C. Typically, the biomass camp wins, because of the technical problems of operating below 60°C. Thus the system is designed for, at best, 80/60°C or, at worst, for 80/70°C — even though there are usually variable- or low-temperature circuits requiring far lower temperatures.

Fig. 2: This typical design for a combination of biomass boiler and gas-fired condensing boilers is expensive to install and will cost more to run than a truly integrated system.

Fig. 2 shows a typical design for a system comprising a biomass boiler and gas-fired condensing boilers. This system will work, but will cost more to install and won’t operate at optimum efficiency. A 400 kW system, designed at 80/70°C, would cost around 19% more to install (because of larger pipe and pump sizes) and cost around 13% more to run than a truly integrated low- and zero-carbon system which operates each sub-system at its optimum operating temperatures.

Fig. 3 shows an integrated design. In this solution, the biomass boiler feeds a buffer vessel. On start-up, the hot supply water is diverted via the 3-port valve into the biomass boiler return. Once the return water reaches 60°C, the 3-port valve starts to close the by-pass and allows water through to the buffer vessel and eventually heats it to 80°C. The pump on the constant-temperature (CT) circuit draws water from the buffer vessel through the 3-port valve which is normally closed to the by-pass port.

Fig. 3: This integrated design for a biomass boiler and gas-fired condensing boilers enables both types of boiler to deliver their full potential.

The gas-fired condensing boilers serve the variable-temperature (VT) circuit(s) with LTHW at 65/45°C. A 3-port mixing valve and pump on each VT circuit reduces the system temperatures further as required. Normally, the biomass and condensing boiler systems are separated by the 3-port diverting valve on the common flow header. However, in the event of a failure of either, the 3-port valve will open to allow flow from one to the other.

The Armstrong white paper ‘Integrated low- and zero-carbon heating solutions’ provides more detail on this and other system design strategies. It is available free of charge by e-mailing

Steve Cooper is director for sustainable design with Armstrong Integrated.

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