This article is the second in a series on peak demand in electrified heating systems. The first – Peak Demand and the Shift to Storage-Led Design – made the case for storage-led energy centre design. This one goes further back: to the decisions that determine how much peak demand a storage system has to manage in the first place.
The Decisions That Set the Scale
Peak demand is not a fixed property of a building. It is the accumulated output of decisions made long before plant is specified, determined by choices made in separate conversations, without reference to their effect on the energy centre. The size of the energy centre, and the electrical connection it requires, is set by peak demand. Manage it at briefing stage, and everything downstream becomes smaller, cheaper, and more grid-compatible. Fail to manage it, and storage-led design becomes an expensive retrofit to a problem of a scale that good design should have avoided.
Before Plant Is Specified
Plant is sized for the coldest day of the year: typically -5°C, building unoccupied, ventilation at maximum flow rate, warming up to 21°C from cold. This scenario may never occur, yet designers must account for it, and the electrical connection must be capable of serving it. Fabric performance is no longer the primary variable – Part L, BREEAM, and Passivhaus have made adequate insulation effectively standard. What remains is a set of design decisions that directly determine how much of that worst-case condition reaches the plant.
Demand at Source
The more significant variable, and the one most sensitive to budget pressure, is ventilation strategy. Natural ventilation offers no heat recovery; infiltration and fresh air loads pass directly to the plant. Hybrid ventilation, a stepping stone adopted in many cases as a budgetary compromise rather than a considered long-term strategy, captures a maximum of around 40% of heat loss. Mechanical ventilation with heat recovery (MVHR) typically captures 70–90% before it becomes a load on the system. The choice between them does not change the building. It changes the size of everything downstream.
Domestic hot water compounds the problem. Stored DHW requires periodic high-temperature cycling for Legionella prevention, driving high return temperatures back to the plant and degrading heat pump performance at precisely the wrong moment. Decoupling DHW from the primary system using heat interface units (HIUs) eliminates stored potable water entirely and returns low temperature water to the energy centre, improving COP.
Distribution temperature is the final variable. High ΔT systems, operating across 30–40 K, reduce flow rates, maximise thermal storage density, and reduce the electrical connection the plant requires. The effect is cumulative: each decision prior to plant selection reduces what the next has to manage, and high ΔT distribution amplifies the benefit of every decision that preceded it.
The Refurbishment Trap
In refurbishment projects, a further failure mode compounds all of these. When gas boilers are replaced with heat pumps on a like-for-like basis, plant is sized to match the installed capacity of its predecessor rather than the actual energy demand of the building. Replacing it like-for-like does not electrify a building’s heat demand; it electrifies its historical oversizing. The result is an energy centre larger, and an electrical connection more expensive, than the building has ever required.
The Scale of the Difference
The cumulative effect of these decisions is not marginal. Applied in full, they can reduce peak heat demand by up to 75% compared to an unmanaged baseline. That is the difference between an energy centre sized for a problem that rarely occurs and one sized for the building as it actually operates, with a corresponding reduction in installed capacity, electrical connection size, and capital cost.
Lean System Design
These are not independent decisions. They are expressions of a single organising principle: Lean System Design (LSD). The goal of LSD is to minimise installed capacity, reduce the electrical connection the plant requires, and create the conditions for storage-led operation. Demand reduction is not a separate objective – it is the mechanism by which the energy centre becomes smaller, cheaper, and more compatible with the grid it sits on.
What Lean System Design produces in practice – and what it costs when the sequence is ignored – is the subject of the next article.
