Optimising boiler efficiency

As the power-house of any low or zero carbon heating/hot water system, the boiler is the make or break system component, and the latest generation of condensing or biomass boilers ensure that buildings have the greatest possible chance of delivering energy and cost savings.

Whilst these technologies are inherently energy efficient, to harness the full potential, it is also critically important to consider system design temperatures, control philosophy, hydraulic system design and system integration.

So, having specified a low or zero carbon boiler, how do you ensure that the capabilities of the technology are being optimised?

Condensing boilers

Let’s start with a very common pitfall. We estimate that the majority of condensing boilers installed in the UK will never actually operate in condense mode because so many system designs have them operating at 80°/70°C.

Gas fired condensing boilers are designed to be ultra efficient by extracting latent heat from the flue gases. This is done by dropping the temperature of the flue gases below the dew point, which for natural gas is around 54°C. So to operate in condensing mode, a return temperature of 54?C or below is required. More effective maximum system temperatures would therefore be 70?C supply and 50?C return or, to condense continuously, 50° supply and 30° return.

The difference in efficiency can be as much as 10% depending upon boiler type and load. The reasons for designing to 80°/70°C are largely historic, based on concerns regarding over-pressurisation and corrosion. Plant was also sized for ‘design day’ conditions that are rarely met in temperate climates.

In a world where condensation of the flue gases was a real problem but a high surface temperature was not (and also where energy was relatively cheap) then the old order of 80°/70°C was fine. But when we’re specifying latest generation gas fired condensing boilers, where reducing energy usage and carbon emissions are the key priorities, and boilers are operating in a part load environment, it’s time to think again.

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

Optimum control strategy

The accepted control strategy for multiple boiler installations is to stage each boiler up to full load and then sequentially bring in each of the other boilers until the load is satisfied.

That works well for traditional boilers, because we’re switching them in and out at their peak efficiency point. But, for condensing boilers it’s far from perfect. Condensing boilers are at their most efficient at part load.

This is because the heat exchanger surface remains constant but the fuel/air mixture is variable. On low loads there is a greater surface area available to extract heat energy from the reduced gas/air mix. So if we sequence condensing boilers at peak load we are actually switching them at their least efficient point.

A more effective strategy is to run all multiple condensing boilers simultaneously at their lowest possible load to meet the building demand. This is called demand based control as opposed to the more widely used capacity based control. Remember, space heating is a part load application. We size the plant for design day conditions but these conditions are rarely met in temperate climates. So it makes sense to try to match boiler load to the building demand rather than to the capacity of the boilers.

As well as making sense from an efficiency viewpoint, demand based control gives us other benefits. A boiler is like any other machine in that it likes to run. Constant stop/starting of any machine accelerates wear and tear. In addition, when a boiler is restarted, it must go through a pre-purge sequence that ensures any un-burnt fuel and products of combustion are purged from the system. To achieve this, the fan blows cold air through the heat exchanger and up out through the flue, losing heat on the way. This wastes small amounts of energy, but the costs mount up over the years.

Balancing boiler demands

Let’s move on to a more complex problem. How do you optimise the energy efficiency of a system incorporating a biomass boiler (installed to take the base load) alongside gas condensing boilers (for peak loads and standby back-up)?

Here, the biomass boiler wants to run at 80°/60°C whilst the latter 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, at best, for 80°/60°C or, at worst, for 80°/70°C even though there are usually variable or low temperature circuits which require far less.

This system shown in Figure 1 is a typical biomass/gas condensing boiler design which will work, but will cost more to install and won’t operate at optimum efficiency. A 400kW 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.

There is a better solution however which is an integrated design as shown in Figure 2.

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 to the by-pass and allows water through to the buffer vessel and eventually heats it to 80°C. The pump on the CT circuit draws water from the buffer vessel through the 3-port valve which is normally closed to the by-pass port.

The gas fired condensing boilers serve the VT circuit(s) with LTHW at 65°/45°. 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.

In conclusion, having decided to invest in low and/or zero carbon boiler technologies, the stage is already set for highly energy efficient performance of the building. However, the vital next step of optimisation will be the deciding factor.