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  • Low-temperature domestic heating systems

Low-temperature domestic heating systems

Calculation tool

Introduction

The efficiency of condensing boilers and heat pumps is higher when they supply heat at lower temperature. SAP calculations allow for this in the case of a low-temperature heating system.

A low-temperature heating system is defined as one in which the hot water leaving the heat generator is always at a temperature not exceeding 45°C or 35°C, even on the ‘design day’ (a day with cold weather conditions chosen for calculating the maximum heat losses from the dwelling). The definition does not include heating systems in which the water temperature is lower only some of the time, such as those with weather compensation or load compensation controls, nor does it include underfloor systems in which a thermostatic mixing valve is used to blend water at a high temperature with cooler water before entering the underfloor heating system1.

Low-temperature heating requires a different system design, mainly to ensure that the heat emitters (radiators, fan-assisted radiators or convectors, or underfloor heating pipes) can deliver the same amount of heat at the lower temperature as a traditional radiator system would have done at normal temperature (over 55°C). The emitters in each room must be sized correctly to ensure they are capable of achieving that. Suitable controls must also be installed to ensure the design temperature of water leaving the heat generator is not exceeded while the system is providing space heating, and the system commissioned for low temperature operation. Provided that space heating and water heating are not carried out simultaneously, separate control arrangements can apply to the water heating.

Low temperature emitters apply to SAP calculations when a suitable commissioning certificate that confirms compliance with all aspects of the design, installation and commissioning requirements for low temperature operation has been signed by a suitably qualified individual and supplied to the SAP assessor.

At present the only design guidance recognised by SAP is BRE Trust Report FB 59, Design of low-temperature domestic heating systems2. It includes an example design, installation and commissioning certificate. Other guidance that may become available in future could also be recognised if the same conditions and restrictions are observed.

Condensing boilers

Condensing boilers operate at higher efficiency with lower flow and return temperatures. Default boiler space heating efficiency data held in Table 4b of the SAP 2012 specification and individual boiler records held in the Product Characteristics Database (PCDB) are based on the design flow temperature of the water in the heat distribution system being 55°C or higher.

Where the heating system has been designed to operate at a lower temperature the space heating efficiency of a condensing boiler is increased by the applicable efficiency adjustment given in Table 4c of the SAP 2012 specification. These adjustments apply to all heat emitter types when the design flow temperature as stated on the commissioning certificate (rounded to the nearest whole number) is less than or equal to 45°C or 35°C.

Heat pumps

Heat pumps operate at higher efficiency with lower flow temperatures. The space heating efficiency data in Section 9.2.7 and Table 4a of the SAP 2012 specification provide values for 35°C and 55°C, and Product Characteristics Database (PCDB) records for heat pumps provide values for 35°C, 45°C and 55°C.

The default is a flow temperature of 55°C. If the heating system has been designed to operate at a lower temperature the data for lower flow temperatures are applied where the design flow water temperature as stated on the commissioning certificate (rounded to the nearest whole number) is less than or equal to 45°C or 35°C.

 

Procedure

The supporting calculation tool (available below) implements the design procedure defined in the BRE Trust Report FB 59, Design of low-temperature domestic heating systems2, enabling the specification of design flow temperatures and sizing of heat emitters. The calculation tool is only suitable for determining emitter heat output, not for determining the required power output of a boiler or heat pump. It is based on a room by room heat loss calculation and requires details of dimensions and construction materials or U-values for each heated room.

The supporting calculation tool was developed by the Building Research Establishment Ltd. as one possible implementation of principles defined in the BRE Trust Report FB59 and can only be used in conjunction with this guidance. Alternative calculation tools that implement BRE Trust Report FB59 principles or any alternative guidance that observes the same conditions and restrictions could also be used, subject to approval by the Building Research Establishment. Such alternatives must output a suitable commissioning certificate that confirms compliance with all aspects of the design, installation and commissioning requirements for low temperature operation. Alternative approved calculation tools will be listed on this webpage if/when they become available.

In order to use this tool, it is critical that the following parameters are satisfied:

• If the heat generator also provides hot water service, controls must be installed that ensure it does not supply hot water services and space heating service simultaneously

• The design included a heat loss calculation for every room of the dwelling that is heated by the installation

• The low temperature heating system design must include all emitters in the heating system

• The heat output of any existing radiators that are retained for the low temperature heating system design are assessed in accordance with ‘Heat Emitter Supplement to the Domestic Heat Design Guide’, available at:  http://www.microgenerationcertification.org/images/Supplementary%20tables%20of%20heat%20emitter%20outputs.pdf

• Controls to ensure continued low-temperature operation (limiting flow temperature at the heat generator) have been installed and commissioned in accordance with the design and cannot be overridden by the householder

• The heat emitter system does not use a thermostatic mixing valve to blend water at a high temperature with cooler water before entering the system1

 

1 - Underfloor systems may still have a mixing valve but only as a protection device

2 - Available from www.brebookshop.com

 

Disclaimer

The Building Research Establishment Ltd. has developed a calculation tool that implements the design procedure defined in the BRE Trust Report FB 59, Design of low-temperature domestic heating systems.

The Building Research Establishment Ltd. makes no representation or warranty that the content of the calculation tool made available on this website is suitable for any use or that it constitutes accurate data and/or advice.

 

Download tool(s) here:

BRE - Calculation tool for design of low temperature domestic heating systems - V1.2