MCS published an updated version of MIS 3005-D: The Heat Pump Design Standard on 2 December 20241. The certification body said the revision clarifies the circumstances under which a hybrid heat pump heating system would be compliant with MCS requirements, following a consultation in February 20241.
The update introduces definitions of a "hybrid heat pump system" and a "supplementary heat source"1. It removes the requirement that a heat pump should be selected that will provide at least 100% of the calculated heat load when installing a hybrid heat pump system1. In its place, a new procedure for hybrid installations sets two requirements: the heat pump must be selected to meet a minimum of 55% of the calculated heat load of the building at a 55°C flow temperature, at design conditions, and the controls' philosophy must be capable of prioritising the heat pump1.
MCS said feedback from industry suggested there was a lack of clarity regarding installing hybrid heat pump heating systems under MCS, and a lack of guidance on sizing these systems, which was felt to be restricting the installation of such systems1. The changes were agreed in conjunction with the MCS Heat Pump Working Group1.
"The changes were agreed in conjunction with the MCS Heat Pump Working Group."
The standard sits alongside the wider set of MCS installation standards covering MIS 3001, MIS 3002 and the rest, and forms part of the heat pump standards, testing and certification framework. The sizing procedure it sets out relates to heat pump sizing and heat loss calculations.
| Before | After |
|---|---|
| Heat pump selected to provide at least 100% of calculated heat load in a hybrid system | Heat pump selected to meet a minimum of 55% of calculated heat load at 55°C flow temperature, at design conditions |
| No definition of hybrid heat pump system or supplementary heat source | Definitions introduced |
| No stated controls requirement for prioritising the heat pump | Controls' philosophy must be capable of prioritising the heat pump |
Why it matters for households
A hybrid heat pump system pairs a heat pump with a boiler as a supplementary heat source. Under the previous requirement, a hybrid installation needed a heat pump sized to carry the whole calculated heat load, which in practice pushed households towards larger units. The revised standard allows a smaller heat pump, covering at least 55% of the load at a 55°C flow temperature, with the boiler making up the remainder1.
For a home's energy independence, the split matters: the larger the share of heating delivered by the heat pump rather than the boiler, the less gas the property draws. The new controls requirement, that the philosophy be capable of prioritising the heat pump, bears directly on that share, because it determines which source runs first. The standard does not itself set a running-cost outcome, and no figures for expected bill changes appear in the announcement1.
Whether a hybrid system qualifies for grant support is a separate question from MCS compliance, and the announcement does not address Boiler Upgrade Scheme eligibility for hybrid heat pumps1. The wider policy context sits under regulation and policy.
What happens next
MCS said the update is expected to increase the number of heat pumps being installed to support reaching government targets to install 600,000 heat pumps a year by 20281. The standard is available through the MCS Standards and Tools Library, and queries are directed to meetings@mcscertified.com1. No implementation date or transition period for the revised standard has been reported1.