MCS published a four-week consultation on 12 February 2024 on changes to MIS 3005-D, the heat pump design standard, intended to clarify when a hybrid heat pump heating system would be compliant with MCS requirements1. Responses are due by 10.00am on Monday 11 March 20241.
Under the current standard, "a heat pump should be selected that will provide at least 100% of the calculated heat loss"1. Because the word "should" prescribes a requirement intended to be complied with unless reasonable justification can be given, hybrid heating systems are allowable for MCS Certification only where there is reasonable justification as to why the heat pump cannot be sized to cover at least 100% of the calculated heat loss1. MCS states that what counts as reasonable justification can be subjective and open to interpretation, and that the standard currently gives no minimum sizing requirement for the portion of heating to be delivered by the heat pump, which it says risks poorly performing systems being installed1.
The consultation proposes three changes1:
| Proposed change |
|---|
| Introduction of a definition of a hybrid heat pump system |
| Removal of the requirement that a heat pump should be selected to provide at least 100% of the calculated heat loss when installing a hybrid heat pump system |
| Requirement for a minimum 55% peak power output contribution from the heat pump at a 55°C flow temperature and at design conditions when installing a hybrid heat pump system |
It also seeks feedback on whether the heat pump should always be prioritised in a hybrid heat pump system, and whether there should be a set minimum of heat supplied from the heat pump1.
"Industry feedback suggests that this lack of clarity regarding 'reasonable justification' and lack of guidance on sizing hybrid heat pump systems is restricting the installation of such systems"
MCS says hybrid systems can act as a stepping stone to 100% heat pump solutions, and that the proposed updates are expected to increase the number of heat pumps installed to support reaching the government target to install 600,000 heat pumps a year by 20281. Responses go to consultations@mcscertified.com using the form provided1.
Why it matters for households
MIS 3005-D governs how a heat pump system is designed before it is installed, so its wording shapes what an installer can propose and what an MCS certificate can cover. The 100% heat loss rule ties into heat pump sizing and heat loss calculations: a home that cannot reasonably be served by a heat pump sized to its full calculated heat loss currently needs a justification before a hybrid heat pump can be certified. The proposal would replace that judgement call with a fixed threshold, a heat pump contributing at least 55% of peak power output at a 55°C flow temperature at design conditions1. For a household, the practical effect is that a retained boiler would carry a defined share of the heating rather than an undefined one, and the heat pump's share of the work would be set in the standard rather than left to interpretation. The consultation does not state how the 55% figure was derived, and no assessment of running costs, emissions or Boiler Upgrade Scheme eligibility for hybrids appears in the published material1. MCS has not reported how many responses it has received or when a decision will be made.
What happens next
Comments must be submitted by 10.00am on Monday 11 March 2024, by email to consultations@mcscertified.com using the response form1. MCS has not reported a date for publishing the outcome or for any revised standard taking effect1. The changes sit within the wider set of MCS installation standards and the heat pump standards and certification framework, and feed into UK heat pump policy and targets.
Sources1 cited
- Consultation: MIS 3005-D: The Heat Pump Design Standard - MCS, mcscertified.com