Understanding Bivalent Heating Systems
Convention 6.15
Charlotte Lancaster
Last Update 3 months ago
A bivalent heating system is when a zone uses two (or more) different heat sources to provide space heating. These sources can work in sequence or combination depending on system design and external conditions.
Common configurations include:
A heat pump operating as the main source with a boiler for backup in colder conditions
A biomass boiler as the primary source with an oil or gas boiler for peak loads
Electric room heaters as well as a heat pump
These arrangements are designed to maximise efficiency, reduce carbon emissions, and ensure reliable heating under all conditions.
EPC Convention 6.15
Convention 6.15 addresses how bivalent systems should be treated in SBEM when producing EPCs. The convention exists because, without clear guidance, assessors could model these systems differently, leading to inconsistent results.
What does the Convention 6.15 state?
Identify the Main System
The primary system is the one intended to provide cooling as the software will only account for cooling from the main system.If neither system provides cooling, then the main system will be the generator which provides the greater proportion of heating load.This should be determined by reviewing the evidence of the heating load design. If this design does not exist then the following 4 options will need to be followed:Option 1: One central (multi-zone) system and one or more local (single-zone) systems
Load split rule: 80% is assigned to the central system and 20% to the local system(s).
If there are multiple local systems, the 20% is divided equally between them.
Example:
A large office building has:
A gas-fired central boiler with radiators serving most of the building (multi-zone).
A local electric heater serving one meeting room (single-zone).
In this case:
80% of the load is assigned to the boiler system.
20% is assigned to the local electric heater.
If there were two local electric heaters instead of one, each would take 10% of the load.
Option 2: Two central (multi-zone) systems only
Load split rule: 50% is assigned to each central system.
Example:
A university library has:
A biomass boiler serving all zones.
A gas-fired boiler also serving all zones, providing backup.
In this case:
50% of the load is assigned to the biomass boiler.
50% of the load is assigned to the gas boiler.
Load split rule: 40% is assigned to each central system and 20% to the local system(s).
If more than one local system is present, the 20% is divided equally.
Example:
A retail building has:
A VRF system with heat recovery serving most of the building (multi-zone).
A gas boiler with fan coils also serving the building (multi-zone).
A local split system air-conditioner serving a small IT room (single-zone).
In this case:
40% of the load is assigned to the VRF system.
40% to the gas boiler system.
20% to the local split system AC.
Option 4: No central (multi-zone) system and two or more local (single-zone) systems
Load split rule: Load is equally divided between the local systems.
Example:
A small retail unit has:
Two wall-mounted split system AC units, each serving different shop zones.
Both are classed as local systems (single-zone).
In this case:
50% of the load is assigned to each split AC unit.
If there were three split units instead of two, each would take 33.3% of the load.
For three local systems:
Key Note
For the purpose of this convention:
Central (multi-zone) system = serves multiple spaces from a common plant (e.g., boilers, chillers, VRF with multiple terminals).
Local (single-zone) system = serves only one space or is a split/multi-split unit.
Top tip: In practical submissions, explicitly state that you applied Convention 6.15, and which part, when dealing with a hybrid system. This shows you are aware of conventions and are applying them appropriately.
Summary
Bivalent heating systems are becoming increasingly common as the UK transitions to low-carbon energy. For NDEAs at Level 4, the ability to correctly interpret and model such systems is vital. Commercial EPC Convention 6.15 provides the framework: identify the primary and secondary systems, input them correctly into SBEM, and apply conservative assumptions where detailed data is lacking. By following this approach, you can produce consistent, reliable EPCs that meet professional standards.