Section 2: Gas, oil and biomass-fired boilers

  1. Section 2

    Section 2

    2.1 Introduction

    This section provides guidance on specifying gas, oil and biomass-fired space heating systems for new and existing buildings to meet relevant energy efficiency requirements in the Building Regulations. It covers relevant boiler types, and describes measures, such as additional controls, that can be used to gain heating efficiency credits to improve the heat generator seasonal efficiency.

    2.2 Scope of guidance

    The guidance applies to wet central heating systems using commercial boilers fired by:
    • natural gas
    • liquid petroleum gas (LPG)
    • oil, and
    • biomass.

    The guidance in this section does not cover:
    • steam boilers (as these are used primarily for processes rather than provision of space heating), or
    • electric boilers (for which see Section 7).

    2.3 Key terms

    The terminology used to describe efficiencies for boiler systems is detailed below.

    In this section the heat generator is a boiler.

    Biomass means all material of biological origin, excluding material embedded in geological formations and transformed to fossil fuel.

    Boiler efficiency means the energy delivered by the water as it leaves the boiler (or boilers in multi-boiler installations) to supply the heat emitters, divided by the energy (based on gross calorific value) in the fuel delivered to the boiler, expressed as a percentage. It is an expression of the boiler’s performance and excludes energy used by boiler auxiliary controls, pumps, boiler room ventilation fans, mechanical flue extraction fans and fan dilution systems. The boiler efficiency is measured according to the standards that are used to demonstrate compliance with the Boiler Efficiency Directive11.

    Effective boiler seasonal efficiency is the boiler seasonal efficiency (as calculated by Equation 2 below for individual boilers, or by Equation 3.1 for multiple boilers), plus any applicable heating efficiency credits.

    Economiser means a device, including a secondary heat exchanger fitted on or near to a boiler, which provides additional heat transfer capacity. For the purposes of this guide, any boiler which will be supplied with an economiser should have the economiser fitted when the boiler efficiency is tested according to the standards that are used to demonstrate compliance with the Boiler Efficiency Directive. The effect of this on the boiler efficiency at 30% and 100% of the boiler output may be taken into account in the values used for the calculation of the boiler seasonal efficiency using Equations 2 or 3.1, or the three-step method and Equations 3.2 and 3.3, as appropriate.

    Condensing boiler means a boiler that offers a higher energy efficiency by recovering heat from the flue gases. This is achieved by increasing the heat exchanger surface area, which recovers extra sensible heat whenever the boiler fires. The boiler becomes even more efficient when system water temperatures are low because the larger heat exchanger area promotes condensation, allowing much of the latent heat to be recaptured. Standing losses (when the boiler is not firing) are low, and part load performance is very good. In multiple-boiler systems, condensing boilers can be used as the lead boiler.

    Standard boiler means, in the context of this document, a non-condensing boiler.
    Zone control means independent control of rooms or areas within buildings that need to be heated to different temperatures at different times. Where several rooms or areas of a building behave in a similar manner, they can be grouped together as a ‘zone’ and put on the same circuit and controller.

    Sequence control enables two or more heating boilers to be switched on or off in sequence when the heating load changes. This maximises the efficiency of the boilers, so reducing fuel consumption, and reduces wear and tear on the boilers.

    Direct acting weather compensation is a type of control that enables a heat generator to work at its optimum efficiency. The control allows the boiler to vary its operating flow temperature to suit the external temperature conditions and the temperatures inside the building. Weather compensation relies on communication between an external sensor and one inside the boiler. The boiler’s water flow temperature is varied accordingly, so that energy is not wasted by the boiler turning on and off.

    Weather compensation via a mixing valve is similar to direct acting weather compensation, except that the outside temperature is used to control the temperature of water supplied to the heat emitters by mixing the boiler flow and return rather than by altering the boiler temperature.

    Optimum start is a control system or algorithm which starts plant operation at the latest time possible to achieve specified conditions at the start of the occupancy period.

    Optimiser is a control system employing an optimum start algorithm.

    Optimum stop is a control system or algorithm which stops plant operation at the earliest possible time such that internal conditions will not deteriorate beyond preset limits by the end of the occupancy period.

    Two-stage burner control is a type of control that offers two distinct boiler firing rates.

    Multi-stage burner control is a type of control that offers more than two distinct firing rates, but without continuous adjustment between firing rates.
    Modulating burner control is a type of control that provides a continuously variable firing rate, which is altered to match the boiler load over the whole turndown ratio.

    Decentralisation means the replacement of centralised boiler plant and its associated distribution pipework with several smaller, more accurately sized boiler plants, installed within or adjacent to the buildings or systems they serve.

    This eliminates long pipe runs between buildings or through unheated areas, so reducing heat losses.

    Building management system (BMS) means a building-wide network which allows communication with and control of items of HVAC plant (and other building systems) from a single control centre, which may be local or remote. More advanced (‘full’) building management systems offer a wide range of functions, including sequential control, zone control, weather compensation, frost protection and night set-back, as well as monitoring and targeting.

    2.4 Determining boiler seasonal efficiency

    Single-boiler systems and multiple-boiler systems with identical boilers
    For boilers the relevant heat generator seasonal efficiency is the boiler seasonal efficiency. The boiler seasonal efficiency is a ‘weighted’ average of the efficiencies of the boiler at 15%, 30% and 100% of the boiler output (the efficiency at 15% being taken to be the same as that at 30%). This is usually quoted by the boiler manufacturer. Note that the efficiencies based on net calorific value should be converted to efficiencies based on gross calorific value using the appropriate conversion factor in SAP 2012 Table E4.

    The boiler efficiencies, measured at 100% load and at 30% load, are used in Equation 2 to calculate the boiler seasonal efficiency. The weighting factors in Equation 2 reflect typical seasonal operating conditions for a boiler.

    ****Equation 2****

    Equation 2 applies to:

    • single-boiler systems where the boiler output is  400 kW and the boiler will operate on a low temperature system
    • multiple-boiler systems where all individual boilers have identical efficiencies and where the output of each boiler is  400 kW operating on low temperature systems.

    For boilers with an output  400 kW, the manufacturer’s declared efficiencies should be used.

    Multiple-boiler systems with non-identical boilers replacing existing systems

    Where more than one boiler is installed on the same heating system and the efficiencies of the boilers are not all identical, Equation 3.1 should be used to calculate the overall boiler seasonal efficiency. All boilers should be included in the calculation, even when some are identical.

    The boiler seasonal efficiency for multiple-boiler systems with non-identical boilers is:

    ****Equation 3.1****

    Where:

    OBSE is the gross overall boiler seasonal efficiency, being an average weighted by boiler output of the individual seasonal boiler efficiencies

    BSE is the gross boiler seasonal efficiency of each individual boiler calculated
    using Equation 2 R is the rated output in kW of each individual boiler (at 80/60°C).

    Multiple-boiler systems in new buildings

    In the case of multiple boilers in new buildings, the more accurate three-step method described below should be used to calculate the overall seasonal boiler efficiency. These steps can readily be programmed into a spreadsheet to automate the calculation.

    Step 1

    Determine the load on each boiler for each of the three system part-load conditions of 15%, 30% and 100%. For example, if the total system output is made up of three equally sized boilers, at 15% of system output the lead boiler will be operating at 45% of its rated output, with the other two boilers switched off.

    Step 2

    Determine the efficiency of each boiler for the above operating conditions. In the above example, the efficiency of the boiler operating at 45% can be determined by linear interpolation between its efficiencies at 30% and 100% of rated output. Where it is necessary to determine the efficiency of an individual boiler at 15% of rated output, this should be taken as the same as the efficiency at 30% of rated output. (Note that the efficiency at 15% of rated output will only be needed if a single boiler meets the full design output.)

    Step 3

    Calculate the overall operating efficiency at each of the system part load conditions using:

    ****Equation 3.2****

    Calculate the overall boiler seasonal efficiency as the weighted average of the efficiencies at the three load conditions using:

    ****Equation 3.3****

    Table 2 is a worksheet for following through these calculations (using manufacturer data for boiler efficiency at 100% and 30% output). Table 3 shows a completed example calculation using this worksheet, for the case where a system with a rated output of 625 kW is served by three boilers, each rated at 250 kW. The first two boilers are condensing boilers, while the third is a standard boiler. Because the installation is oversized (750 kW compared to 625 kW), at full system output the final boiler is only operating at 50% output (125/250).

    The notes at the foot of the table illustrate how the various values are calculated.

    ****Table 2 Worksheet for calculating the overall boiler seasonal efficiency of a multiple-boiler system using the alternative three-step method****

    ****Table 3 Example calculation of the overall boiler seasonal efficiency of a multiple-boiler system in a new building****

    2.5 Boilers in new buildings

    Background

    New buildings should be provided with high efficiency condensing or non-condensing boilers that meet the recommended minimum standards for heat generator seasonal efficiency in this guide.

    Commercial heating systems are inherently more complicated than domestic systems with a wider range of temperatures and heat emitters. The selection of condensing or non-condensing boilers will be determined by application and physical constraints.

    Note: Water quality can have a major impact on system efficiency. It is important that designers take appropriate measures to ensure that the system water is of good quality.

    Condensing boilers will meet projected efficiencies only when they operate with a system return temperature between 30°C and 40°C for 80% of the annual operating hours. With a return temperature of 55°C and above, condensing boilers will not produce condensate and will have similar efficiencies to non-condensing high efficiency boilers. Some systems are suitable for weather compensation, which allows return temperatures to fall into the condensing range for some periods of the heating season, and they may be best served by a mixture of condensing and non-condensing boilers.

    The efficiency value that should be entered into accredited NCM tools to calculate the carbon dioxide emission rate is the effective heat generator seasonal efficiency. For boilers in new buildings, no heating efficiency credits can be gained and the effective heat generator seasonal efficiency is therefore the same as the heat generator seasonal efficiency.

    Recommended minimum standards

    To meet relevant energy efficiency requirements in the Building Regulations when installing boiler plant in new buildings:

    a. where a single boiler is used to meet the heat demand, its boiler seasonal efficiency (gross calorific value) calculated using Equation 2 should be not less than the value in Table 4

    b. for multiple-boiler systems, the boiler seasonal efficiency of each boiler should be not less than 82% (gross calorific value), as calculated using Equation 2; and the overall boiler seasonal efficiency of the multiple-boiler system, as defined by the three-step method and calculated using Equations 3.2 and 3.3, should be not less than the value in Table 4

    c. the relevant minimum controls package in Table 5 should be adopted.

    ****Table 4 Recommended minimum heat generator seasonal efficiency for boiler  systems in new buildings****

    ****Table 5 Recommended minimum controls package for new boilers and multiple-boiler systems****

    2.6 Boilers in existing buildings

    Background

    Boiler efficiencies have improved markedly over recent years. A modern boiler meeting the minimum requirements of the Boiler Efficiency Directive has a boiler seasonal efficiency of approximately 78.5% (based on gross calorific value).

    This guidance recognises that in many cases using condensing boiler technology in existing buildings would be either technically impractical (due to flueing constraints) or economically unviable. For this reason non-condensing boilers may be used provided that they meet the recommended minimum efficiency standards given in this section.

    Replacement boilers

    To meet relevant energy efficiency requirements in the Building Regulations when installing boiler plant in existing buildings:

    a. the boiler seasonal efficiency of each boiler (in a single-boiler system or a multiple-boiler system with identical boilers) calculated using Equation 2 should be not less than the value in Table 6

    b. for multiple-boiler systems using non-identical boilers, the overall boiler seasonal efficiency calculated using Equation 3.1 should be not less than the value in Table 6

    c. the controls package in Table 7 should be adopted – i.e. zone control, demand control and time control

    d. the effective boiler seasonal efficiency should be not less than the value in Table 6. To meet the standard, it may be necessary to adopt additional measures from Table 8 in order to gain heating efficiency credits (see below).

    ****Table 6 Recommended minimum heat generator seasonal efficiency for boiler systems in existing buildings****

    ****Table 7 Recommended minimum controls package for replacement voilers in exisating buildings****

    2.7 Heating efficiency credits for replacement boilers

    Where the boiler seasonal efficiency is less than the minimum effective boiler seasonal efficiency for that type of boiler, additional measures will need to be adopted to achieve the minimum effective heat generator seasonal efficiency in Table 6.

    Table 8 indicates the measures that may be adopted and the relevant heating efficiency credits that are applicable. It should be noted that the maximum number of heating efficiency credits that can be claimed is 4 percentage points.

    ****Table 8 Heating efficiency credits for measures applicable to boiler replacement in existing buildings****

    Example: Using heating efficiency credits to achieve the minimum effective heat generator seasonal efficiency for a boiler system in an existing building
    An existing boiler is to be replaced with a gas boiler with a boiler seasonal efficiency of 82%, the minimum allowed by Table 6.

    The boiler’s effective boiler seasonal efficiency needs to be at least 84% according to Table 6, which means that 2 percentage points of heating efficiency credits are needed.

    The following approach would achieve this:

    a. restrict boiler oversizing to 15% (after a detailed assessment of load)

    b. fit a room thermostat to control boiler water temperature in relation to heat load

    c. use two equally sized boilers to meet the heat load in place of the existing single boiler

    d. fit TRVs to control the temperature in areas other than where the room thermostat is fitted.

    Table 9 below shows how credits would be awarded in this example.

    ****Table 9 Example to illustrate allocation of heating efficiency credits for a replacement boiler in****

    Effective boiler seasonal efficiency
    = boiler seasonal efficiencymaximum of 4 heating efficiency credits
    = 82486%

    In this example the minimum effective boiler seasonal efficiency of 84% is exceeded by 2 percentage points.

    2.8 Biomass boilers

    Background

    The method in Section 2.4 for calculating the seasonal efficiency of single and multiple boilers fired by gas, LPG and oil is not appropriate for biomass boilers.
    For biomass boilers, requirements and test methods are covered by BS EN 12809:2001+A1:2004 Residential independent boilers fired by solid fuel. Nominal heat output up to 50 kW. Requirements and test methods.

    Recommended minimum standards
    To meet relevant energy efficiency requirements in the Building Regulations:

    a. the efficiency of biomass boilers at their nominal load should be at least:
    i. 65% for independent gravity-fed boilers  20.5 kW
    ii. 75% for independent automatic pellet/woodchip boilers

    b. controls as for gas, LPG and oil boilers in Table 5 should be provided, where technically feasible.