Kirk Technical Solutions Ltd
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Technical Articles from KTS Ltd

Friday 22nd April 2016

District Heating & SAP 2016

A consultation is expected later this year on proposed changes to the UK government's standard assessment procedure for the energy performance of buildings.  This will include proposals on how SAP 2016 should assess "community" or district heating.  It is important that SAP provides an accurate method of assessing the energy and carbon impact of community heating, particularly so that the technology can be reliably compared to alternatives such as local gas condensing boilers.

At KTS Ltd we find there are some issues with the current system which need to be addressed.  There is a significant difference between "good" and "bad" community heating in terms of efficiency and carbon so it is important that SAP treats community heating fairly and doesn't lump the good in with the bad.

While we understand that SAP is a simplified method, some systems are complex and their assessment should not be simplified beyond the stage at which the assessment becomes meaningless.

Two aspects of SAP 2012 have been identified which are a source of inaccuracy.

Boiler Efficiency

  • The calculation of SEDBUK seasonal efficiency involves using a simplified heat load based on assumptions which are invalid when applied to community heating.

Network Efficiency

  • The method of characterising the distribution losses from a district heating network between the energy centre and the dwelling does not allow a valid comparison with a local boiler.

  • The assumption that network losses apply to each dwelling on the network equally is invalid.

Calculation of Boiler Seasonal Efficiency

SAP 2012 uses SEDBUK efficiency for boilers, which is based on an assumed load duration curve typical of a domestic dwelling.   For central boilers, SEDBUK relies upon assumptions which are invalid (i.e. the load duration curve), and for domestic boilers it is no longer necessary to simplify the calculation of seasonal efficiency, as load duration curves can now be reliably obtained for a given building through dynamic thermal simulation using approved software.

Why make an assumtion when the real data is available?

On a well-designed community heating scheme, metered data should be available from the energy centre which would allow calculation of actual boiler seasonal efficiency and this is always preferable to using simplifications based on invalid assumptions.  We would also like to see no “default value” option available to the assessor in place of actual data as this data should be available on any well designed community heating scheme as it is a key factor in the financial viability of such a project.

Calculation of Network Efficiency

Network Heat Loss Factor

Characterising the standing loss from a heat network as a factor (“distribution loss factor”) as per SAP 2012 rather than as a kWh value does not allow a valid comparison with alternative heating technologies.

% vs Factor

In SAP 2012, a boiler efficiency figure as a percentage e.g. for a gas boiler in a dwelling, is used for dwellings with a local boiler.  For dwellings connected to community heating, a central boiler efficiency is combined with a standing loss factor (“distribution loss factor”).  The standing loss is not independent of the total dwelling demand and will be affected by:

  • Total network demand

  • heat density of demand

  • heat demand variance due to weather

  • the effect of weather compensated primary flow temperature on network losses

  • Heat demand influenced by occupant behaviour

Since one is dependent only on boiler selection and the other partly dependent on demand, we cannot adequately compare boiler efficiency with network efficiency because the distribution loss is a standing loss, not a product efficiency. Standing losses should instead be characterised as an absolute kWh value.

Dwelling Specific Loss vs Network Loss

It is invalid to assume that distribution losses apply to each dwelling equally on a network.  The approach in SAP 2012 gives a flat rate for the network heat loss factor across the network which does not take account of what percentage of standing loss relates to the dwelling being assessed, e.g. some dwellings closer to each other or the energy centre will have a lower standing loss resulting from their connection to the network in comparison to more distant buildings, or buildings with lower heat demand.

Why make an assumption when real data is available?

The standing loss for the dwelling (e.g. external heat interface unit losses and branch pipe connection losses) must be measured for any asssessment to be valid.

The standing losses for the network (e.g. mains pipework and energy centre losses) should be allocated to each dwelling on the network based on a ratio of the dwelling demand to the network demand.

We cannot allocate network distribution losses to each dwelling equally because this does not reflect the energy and carbon impact of connecting that

In order to accurately calculate standing losses for a particular dwelling, the loss for a dwelling in kWh we could use the following equation:

Total Dwelling Loss (kWh) = HIU Loss + Branch Loss + F*Mains Network Loss + F*Energy Centre Loss

Where F is the ratio of estimated dwelling heat demand to estimated total heat demand of the network.

 

The method should:

  • take into account the impact of heat density and quality of HIU and pipework.
  • characterise the heat loss as a standing loss in kWh, allowing a valid comparison with alternatives. 
  • The network loss would also be specific to the dwelling being assessed, in order to allow a valid comparison between the energy and carbon impact of the community heating scheme vs a local boiler for the dwelling being assessed.

Conclusion

The following changes to SAP 2012 are recommended in order to assess the energy and carbon impact of community heating schemes:

  • Boiler seasonal efficiency should be obtained by direct measurement for all existing community heating boilers,

  • Heat loads/load duration curves and boiler seasonal efficiency should be obtained by dynamic simulation for proposed community heating boilers and for local boilers in a dwelling

  • Distribution losses should be characterised as kWh and applied specifically to the dwelling being assessed

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