There have been a lot of changes in the boiler world over the last number of years, much of which has been a drive towards greater energy efficiency. Condensing boilers are now becoming a common option; even the entry level is what used to be called “high efficiency”. You may think that one boiler is much the same as another. They all look roughly the same on the outside but there are a wide range of products on the market and the model chosen ultimately determines the efficiency of the heating system.

Boiler Selection Must Be Done in the Context of the Design of the Overall Heating System

Enda Gilroy October 22, 2012 , , , , ,

Boiler basics
Most heating boilers don’t boil, they generally produce Low Temperature Hot Water (LTHW) at 80°C. Only steam boilers actually boil the water and they are mainly used in very large industrial sites. The boilers at residential places are generally low heat producers which in today’s world must have the best boiler cover to help you out during unpredictable circumstances. As every big industrial boiler comes along with an insurance cover from the manufacturer’s side which is essential, now its time we protect our homes from boiler mishaps too. Some large multi-building sites operate on medium (MTHW) or even high (HTHW) temperature hot water allowing the designer to minimise the diameter of distribution pipework and hence capital costs. Some heating systems (e.g. underfloor) operate as low as 40°C and these are ideal for condensing boilers.

Space heating boilers are most commonly fuelled (fired) by natural gas but oil is still widely used and, with the drive to reduce carbon emission, biomass boilers are gaining in popularity. EU Directives Directive 92/42/EEC of 21.5.92, which comes under the SAVE programme concerning the promotion of energy efficiency in the Community, determines the efficiency requirements applicable to new hot-water boilers fired by liquid or gaseous fuel with a rated output of no less than 4 kW and no more than 400kW.

New boilers, within the size range 4kW to 400kW sold in the European Union, must operate at, or above, the specified minimum percentages efficiencies as per the Directive while running at full load or part load conditions.  New heating appliances, i.e. boiler combustion chambers (bodies) and burners, are normally marketed as separate items, and must meet the relevant efficiency requirements, when they are assembled together to form a complete boiler. Table 1 indicates the minimum requirements to comply with the Directive.

Table 1

Seasonal and part load efficiencies
The key factors determining the seasonal performance of a boiler is the efficiency of the plant at part load, and the load that the plant experiences in response to the seasonally varying building heating demand. Part-load efficiency refers to the ability of a system to handle part-load energy use and it should be taken into consideration when specifying an mini HVAC system. And it is beneficial to you in the long run if you were to consult an HVAC expert and finding more about KCS, a HVAC solutions company, to provide get the most efficacious of results. Here is how you can start installing your own mini split system. Systems generally operate at their peak efficiency when they are working at their maximum capacity and most systems are sized to meet heating conditions that occur only 1% to 2.5% of the time. Because of this, systems are often oversized, rarely operate at full load, and thus do not operate efficiently.

Designing for part load
Proper sizing of a HVAC system can maximise partload efficiency. Selecting the appropriately-sized system requires an understanding of the peak heating load and the system’s load profile.
(1) Determine how often the HVAC system will be running under part-load conditions;
(2) If that will be a frequent occurrence, look for a system that will be efficient for those partload conditions;
(3) Beyond right-sizing equipment, there are system components and modular components that can be selected to improve efficiency. A few examples of these components that can operate efficiently at part-load include variable speed drive controls for pump motors; variable capacity boiler plants, and temperature reset controls for hot water.

In buildings with highly variable loads, which is common in commercial buildings, multiple, modular boilers are an option. Modular systems are more efficient as they permit each boiler to operate around maximum rated load most of the time and reduce standby losses. Other options include condensing boilers, and modulating boilers that can run at partial capacity rather than cycling on and off.

Some engineers design systems with multiple boilers – one can be sized for 75% to 80% of the design load, while another is sized for the part load (30% to 40% of the full load). Operators can then select a unit based on the energy efficiency performance and the heating needs. Careful sequence control is fundamental to this approach.

Benefits include greater energy efficiency, reduced running costs, improved load matching, built-in standby capacity, flexibility in maintenance and allowing the most efficient boilers to take the base load. Overall energy savings of 5-10% are typical.

Gross and net calorific values
There is often confusion about the presentation and use of gross and net calorific values data for heating equipment. In simple terms, the calorific value (CV) is the amount of heat released when a specific amount (weight or volume) of fuel is completely burnt in oxygen. Most commonly used fuels (oil and gas) contain hydrogen and when burnt this hydrogen is converted to water vapour that, when fully cooled, is converted to liquid water.

During the process of converting water vapour to its liquid state a certain amount of heat is released. Condensing boilers recover some of the heat in the water vapour so it is possible to achieve efficiencies greater than 100% net efficiency. This is known as the latent heat of condensation. The possibility exists for the measurement of calorific value to include or to exclude the latent heat of condensation/evaporation, thus there are two values of calorific value for a fuel. The higher value, including the latent heat, is the “gross” CV and the lower value is the “net” CV. See Figure 1.

Figure 1

The two approaches are simply different scales for measuring the same. For product comparisons and sizing boilers, ensure that all the information is based on either gross calorific value or net calorific value — don’t mix the two. Heat output shown on manufacturers’ literature might be based on either gross or net and this can make a significant difference when specifying equipment.

Burner/boiler technology
The objective of a burner is to achieve combustion with the correct mix of fuel and air so that all the fuel is burnt efficiently. There are various types of burner, brief details being as follows:

Atmospheric burners – gas is injected through the burner which entrains the air necessary for combustion. This is the most basic and least efficient approach, and one that the market is moving away from;

Pressure jet burners – a fan forces air into the burner, the fuel (gas or oil) is then mixed in at  the burner nozzle and fired into a combustion chamber. Usually used on larger boilers;

Pre-mixed burners – gas/air is mixed before combustion in a mixing chamber, then forced through a burner and the flame sits on the burner. The main advantage of the pre-mix method is that the combustion air can be controlled very closely to achieve the correct ratio of air and gas mixture at all times. This has the effect of improving combustion efficiency.

High-efficiency boilers – These boilers generally have low water content (and/or low thermal mass) with even greater heat exchange surface and insulation. They achieve around 85% at full load falling slightly to around 80% at 30% part load. The higher part load efficiencies make them particularly suitable for applications with a wide range of loads.

Condensing boilers – These boilers use an additional heat exchanger to extract extra heat by condensing water vapour from the products of combustion. They operate at a minimum efficiency of around 85%, even when not condensing and can achieve efficiencies in the range 85/95% depending upon the system return water temperature. Condensation begins to occur at return water temperatures below 55°C and the lower the return the more efficient the boiler. In underfloor heating systems that operate at 30-40°C they can achieve seasonal efficiencies over 90%

However, the more common approach for standard radiator systems is direct weather compensation to achieve around 88%. Constant temperature 80°C flow systems for fan coil units or air handling units are less appropriate for condensing boilers as payback periods will be less attractive. Condensing boilers provide typical energy savings of 10/20% when replacing existing older plant, resulting in paybacks of between two and five years depending on the installation.

Boiler arrangement and system integration
The first step in achieving an energy efficient heating system is to minimise the demand for heat. The structure and fabric composition of the building will influence the heating strategy and can be designed to minimise heating energy consumption. Before designing a heating system it is essential to ask: ‘Have the demands been minimised?’

There are a few useful rules to follow when designing energy efficient heating systems.
Designers should:
• Select fuels that promote high efficiency, low emissions and minimise running costs;
• Segregate hot water services generation wherever possible;
• Locate plant to minimise distribution system losses;
• Insulate pipework, valves, storage vessels etc effectively;
• Choose efficient primary plant, such as condensing boilers;
• Consider energy recovery, e.g. from air exhaust streams;
• Distribute heat effectively by avoiding excessive pipe lengths and system resistance;
• Use effective controls through good zoning,effective time control and variable flow control where possible;
• Consider de-centralised heating and hot water services generation plant on large sites to reduce standing     losses and improve load matching.

But:
• Avoid over-designing the heating system itself as oversizing can lead to a significant drop in efficiency;
• Ensure that the base load is provided by the most efficient plant;
• Always consider the part load efficiency of the overall system since much of the year will be spent operating at part load; ensure that large central systems do not operate to meet relatively small loads.

Using multiple boilers (modules) in one installation can improve energy efficiency by enabling a good match between boiler output and system demand. During maximum demand on the system in midwinter, all the boilers will be firing. During periods of lower demand (e.g. spring/ autumn) only a proportion of the boilers will be required to supply heat. As the load increases,individual modules are progressively switched on.

The smaller modules spend more time operating at full load compared to a single large boiler, hence, improved seasonal efficiencies. Although this is less pronounced in modern boilers, the principle remains the same, unless the plant has higher efficiency at part load (e.g. condensing boilers).

Figure 2

Figure 2 shows a simple heating system that can be used as a basic building block, which when it includes the following features, can often help to reach a simple energy efficient heating system:
– A pumped boiler primary circuit;
– A common primary circuit pump set (larger boilers);
– A reverse return primary circuit;
– Decouple primary and secondary circuits via a common header;
– Ensure correct set points for boiler sequence controller;
– Set boiler thermostats higher than the boiler sequence controls and ensure that adequate system pressure is available.

Condensing boilers can be more expensive than the standard boiler. To keep capital cost to a minimum while still retaining high efficiencies, it is sensible to mix and match condensing and non-condensing boilers. Other than very low temperature systems, combinations of condensing and non-condensing boilers are normally more cost effective than all condensing boilers. Specifying the lead boiler(s) as condensing, with high efficiency to top-up, optimises capital cost while still keeping overall plant efficiency high. It is common to find that 50/75% condensing plant provides the most economic approach.

Condensing boilers should always be the first choice for ‘lead’ gas boilers in multiple installations. In mixed boiler systems, the additional hydraulic resistance of condensing boilers must be considered when designing boiler circuits and suitable regulating valves used to ensure balanced flows. Always select the most efficient plant. Typical seasonal efficiencies of boiler plant are shown in Table 2.

Table 2

Biomass boilers
Biomass usually refers to the use of logs, wood chip or wood pellets that are converted to heat in purpose-designed boilers. The carbon that is released during their combustion is equivalent to the amount that was absorbed during growth, and so the fuel itself is not only renewable, but also almost carbon-neutral. However, there are some carbon emissions associated with processing the wood into fuel and with its transportation.

There are many forms of biomass. This article will briefly cover wood chips and pellets for use in boilers in commercial developments. The application of wood-fired boilers to building developments, where there is a significant space heating or domestic hot water demand, offers the possibility of considerable reductions in carbon dioxide emissions, generally greater than any other currently available on-site renewable technology.

The design of wood-burning boiler installations is very different, particularly in relation to:
• physical size
• fuel handling and storage
• fuel properties and availability
• emissions and flueing requirements
• operating characteristics
• sizing of plant
• capital costs

Pellet boilers range from domestic models through to units rated at several hundred kilowatts. The level of automation associated with the boiler increases as the boiler capacity rating increases. Wood pellets are relatively easy to handle and have a much higher calorific value than wood chips, and the fuel handling systems required are much simpler.

Wood chip is a cheaper fuel but is difficult to handle and has a lower calorific value. It therefore requires a large fuel store and sophisticated fuel transport system. It is generally true that a wood chip installation can burn pellets but a pellet installation cannot burn wood chip.

Wood pellet
Pellet burning installations generally take advantage of the fact that boilers are fully automatic just like oil and gas boilers. Pellet boilers use advanced microprocessors to control the amount of fuel and air being supplied to the
combustion chamber. This ensures extremely high efficiencies (up to 90%) and ultra-low emissions.

Pellets can also be burned in boilers designed to burn wood chips, which makes the technical upper limit for the application of pellet burning the same as wood chip burning. It would be economic considerations of fuel cost that would indicate a switch to wood chip burning at larger capacities.

Wood chip
Wood chip boilers are generally restricted to stepped moving grate burners which can cope with the handling characteristics and higher ash content of wood chip fuel. There are two types of stoking mechanism, which are applied differently dependant on fuel type used and the boiler capacity; the larger the boiler and the wetter the fuel the greater mechanical intervention required to stoke the boiler.

Boiler and system controls
It is very difficult to separate boilers from the heating system as the two interact to form an overall system efficiency. Equally important is the dynamic nature of heating systems with heat demand changing almost constantly. The most efficient systems have efficient boilers, good heat distribution systems and good controls. The key requirement is to provide heat only when and where it is needed and at the right temperature while minimising boiler cycling.

Use optimum start/stop for time control and weather compensation for temperature control, trimmed by motorised valves or TRVs for zone control. Good sequence control is fundamental to achieving an energy-efficient multiple boiler installation. In particular, careful location of the sensor in a representative part of a constant flow primary circuit is essential for stable control.

All boilers have a boiler control thermostat and a high limit thermostat for safety purposes. In multiple boiler installations these should be set much higher than the sequence controls so that they allow the sequence control to act without interference.

Figure 3

Weather compensation controls reduce the flow temperature in variable temperature circuits as the external temperature increases, see Figure 3. The most common version requires a three-port motorised valve to control water temperature. Weather compensation can provide low return water temperatures in milder weather causing condensing boilers to operate at higher efficiencies.

Optimum start controls are weather dependent time-switches that vary the start-up time in the morning to achieve the building temperature by the start of occupancy. Heat-up times are reduced during milder weather, thus saving around 5/10% of heating energy. Optimum stop controls turn the heating system off early without compromising comfort in milder weather. Figure 4 shows the operation of optimum start controls and the potential energy savings compared with a time-switch.

Figure 4

It is essential to select the most efficient plant and ensure that plant and equipment are not oversized. Plant that is too large will operate further down the part load curve and hence at lower efficiencies unless it is condensing. Where possible, segregate domestic hot water from space heating in order to avoid poor summertime efficiencies. Plant sized to meet space heating and hot water will effectively be far too large for small summer hot water demands and this could reduce seasonal efficiency significantly.

Even a well-designed system can perform badly with poor controls. Conversely, you can’t fix a poor heating design by just adding controls. The boilers, heating distribution and controls have to be seen as one overall system. Regularly carrying out good boiler maintenance is essential to ensure continual high efficiencies. This includes cleaning and setting up the burner, cleaning the heat exchanger to ensure good heat transfer, and setting the boiler controls correctly. ■

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References
• Building control systems CIBSE Guide H
• Energy Efficiency in Buildings – CIBSE Guide F

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BSNews July/August 2019

About the Author:

Enda Gilroy