Guidance for EPC assessors and retrofit professionals.
Open chimneys represent one of the largest sources of uncontrolled ventilation heat loss in UK housing. Chimella provides a recognised method of controlling this airflow within SAP assessments while still allowing fireplaces to be used when required.
Chimella is a chimney airflow control measure recognised for modelling within SAP 10.3 and RdSAP10 assessments as a blocked chimney.. When installed and verified, Chimella reduces ventilation heat loss associated with open chimneys.
Within SAP 10.2 / RdSAP10, an open chimney is modelled with a ventilation airflow rate of 80 m³/hr as part of the dwelling ventilation heat loss calculation.
When a chimney is treated as blocked, airflow is modelled at 20 m³/hr. Installation of Chimella enables this reduction when installed and verified.
Reducing chimney airflow from 80 m³/hr to 20 m³/hr represents up to a 75% reduction in modelled chimney airflow and can typically contribute around 2–5 SAP points uplift per chimney within an EPC calculation.
Chimella installations can be verified through the Chimella Pro SAP Compliance System, which provides an installation reference tag and QR access to supporting technical documentation.
Chimella meets the SAP requirements for a chimney blocking device capable of reducing ventilation heat loss when permanence of installation can be verified.
Recognition and guidance for SAP assessors to model Chimella (The Chimney Umbrella) as a chimney blocking device
Scope
This technical note provides guidance for DEAs to model the Chimella chimney blocking device in SAP10.2 and RdSAP10. It does not apply to earlier versions of SAP and RdSAP.
Treatment of chimneys in SAP10.2 and RdSAP10
The air flow through chimneys is modelled in SAP as part of the ventilation heat loss calculation for a dwelling. SAP defines an open chimney as a vertical duct for combustion gases of diameter 200mm or more, or a rectangular duct of equivalent area 125,000 mm2. In SAP 10.2 and RdSAP10 the flow rate is taken to be 80 m3/hr.
A chimney can be decommissioned with the fireplace filled in and the chimney stack capped or removed completely. Such a chimney may require ventilation to manage moisture through the provision of air bricks. Where a decommissioned chimney is provided with a ventilation area less than 30,000mm2 it is treated as a blocked chimney and is modelled with a flow rate of 20 m3/hr.
Further guidance on identifying and modelling chimneys in SAP is provided in Convention 9.01 of the RdSAP10 Conventions document[1].
Technology
A chimney blocking device can be inserted into an open chimney flue to reduce draughts and ventilation heat loss. According to RdSAP10 Convention 9.01, such devices are generally assumed to be a temporary measure and so any chimneys where these devices are used are to be modelled as open, i.e. assigned an air flow rate of 80 m3/hr.
However, Convention 9.01 also notes that if a chimney blocking device is supported by evidence (i.e. recognition though the SAP Appendix Q process) that it:
then a chimney where such a device is installed can be treated as blocked.
The Chimella (The Chimney Umbrella) device has demonstrated that it meets these requirements. Therefore, where it is installed, the associated chimney is counted as blocked and is assigned a flow of 20 m3/hr in a SAP assessment.
This is available at https://bregroup.com/documents/d/bre-group/rdsap_conventions_v12_15_may_2025_final
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