Hot Water in HEM: Technical Guide to Domestic Hot Water Modelling

The Home Energy Model (HEM) calculates domestic hot water demand and system performance using a fundamentally different approach from SAP. Rather than spreading a monthly hot water volume across the year, HEM models individual tapping events (every shower, bath and other draw-off), each with a flow rate, duration and delivery temperature. Combined with stratified cylinder modelling that tracks temperature layers within the storage tank, and detailed pipework loss calculations based on pipe length, diameter and insulation, HEM produces a half-hourly profile of hot water energy demand that reflects how real households actually use hot water.

SAP vs HEM: Hot Water Modelling Compared

The table below summarises the key differences between how SAP and HEM approach domestic hot water calculations. Every row represents an area where HEM introduces greater physical realism, and correspondingly greater data requirements.

For a broader comparison covering all calculation modules, see SAP vs HEM. For a non-technical introduction to how HEM replaces SAP, see What is HEM?

Event-Based Demand: HEM-TP-09 Methodology

The single most important change in HEM's hot water methodology is the shift from monthly volumes to individual tapping events. SAP 10.2 builds its daily hot water volume from three parts: showers (each outlet's flow rate multiplied by an assumed six-minute shower), baths (73 litres each), and other uses (9.8 litres per occupant plus 14), with the number of occupants derived from floor area. Monthly factors then vary the total across the year, with no consideration of when in the day the water is drawn.

HEM abandons this entirely. Instead, the model constructs a half-hourly tapping profile from a schedule of discrete draw-off events. Each event is defined by:

• Tapping point: a shower (mixer or electric), a bath, or a generic other tapping point
• Flow rate (litres per minute), taken from the shower outlet the event is allocated to
• Duration: the length of each draw-off event
• Delivery temperature: the temperature at which water is delivered to the user, accounting for mixing with cold water at the tap
• Timing: when during the day each event occurs, following a standardised daily schedule

SAP already scales shower demand by the flow rate of each shower outlet. What HEM adds is timing: the same draw-offs land in particular half-hours, and are met at whatever temperature the cylinder holds and whatever the heat source can deliver at that moment.

Individual Outlet Specifications

HEM asks for the showers and baths installed in the dwelling. Bath and shower events are then allocated in rotation between the outlets present. For each one the model needs:

• Tapping point type: mixer shower, instantaneous electric shower, bath, or a generic other tapping point
• Flow rate or rating: the flow rate in litres per minute for a mixer shower, the rated power in kW for an instantaneous electric shower, and the volume or fill rate for a bath. Other tapping points run at a standard 12 l/min in the FHS assessment
• Connection details: which hot water source serves each tapping point, since every outlet other than an instantaneous electric shower has to be assigned to one

Standardised Tapping Schedules

Although each outlet is individually specified, HEM does not ask the assessor to define when occupants use hot water. Instead, the model applies a standardised daily tapping schedule that distributes draw-off events across the day in a pattern representative of typical UK household behaviour. The schedule varies by the number of assumed occupants (itself derived from the number of bedrooms) and includes morning and evening peaks with lower daytime and overnight usage.

The total hot water demand is therefore determined by the combination of the standardised schedule and the actual outlet characteristics, meaning that the assessor's data on outlet flow rates directly scales the energy demand, while the usage pattern remains consistent for regulatory purposes. This is analogous to how HEM uses standardised occupancy schedules for space heating rather than asking about actual occupant behaviour.

Stratified Cylinder Modelling: HEM-TP-11

In SAP, a hot water cylinder is treated as a single well-mixed volume at a uniform temperature. This is a significant simplification; in reality, hot water cylinders exhibit thermal stratification, where water at the top of the cylinder is hotter than water at the bottom. This layering effect is central to how cylinders actually perform, and HEM models it explicitly.

Temperature Layer Model

HEM-TP-11 follows Method A of BS EN 15316-5:2017 and divides the cylinder into 24 temperature layers. At each half-hourly timestep, the model tracks the temperature of each layer and calculates:

• Heat input from the heating system: whether from an immersion heater, heat pump, boiler coil, or solar thermal coil. The position of each heat source within the cylinder determines which layers receive energy directly.
• Heat drawn off by tapping events: hot water is drawn from the top layer (the hottest), and cold mains water enters at the bottom. This preserves the stratified temperature profile, which is physically realistic: drawing a bath does not cool the entire cylinder uniformly.
• Mixing between layers: where a layer ends up hotter than the layer above it, the two are mixed and their temperatures averaged. The model loops over the layers until each one is no hotter than the layer above, which is what keeps the profile physically ordered.
• Standby heat losses: the cylinder's declared daily loss, apportioned across the layers according to the temperatures they have reached. Because upper layers are hotter, they lose more heat, a subtlety that SAP's uniform temperature model cannot capture.

Standby Losses and Cylinder Properties

Both methods start from the same test figure. SAP characterises cylinder performance using a single daily standing loss taken from manufacturer test data (BS 1566 or BS EN 12897). HEM takes that same declared kWh/day figure as an input and applies it layer by layer at each timestep, against the temperature each layer has actually reached. Because the upper layers are hotter, they lose more, and the loss changes through the day as the cylinder heats and cools.

HEM therefore resolves when the losses occur rather than recalculating how large they are. Insulation quality reaches the calculation only through the declared daily loss, so changing the tank volume on its own does not change the modelled standby losses.

Pipework Losses

Pipework losses represent the energy wasted as hot water travels from the heat source (cylinder or combi boiler) to the point of use (tap, shower, or bath). SAP handles this with a fixed distribution loss of 0.15 times the hot water energy content. HEM replaces that with a per-section calculation, set out in HEM-TP-10, that works from the physical characteristics of each run of pipe.

Data Requirements

For each pipework section, the core engine requires:

• Pipe lengths: the length of each section of hot water distribution pipework, in metres. In the FHS assessment these are not measured on site: the wrapper calculates total pipework length from the building's length, width, internal height and number of storeys, following BS EN 15316-3:2017
• Pipe diameters: the internal diameter of each pipe section, which determines the volume of water that cools between draw-off events
• Insulation specification: the type, thickness, and thermal conductivity of pipe insulation (or the absence of insulation)
• Pipe location: whether the pipework runs through heated or unheated spaces, which affects the rate of heat loss and whether losses contribute as useful internal gains

At each timestep, HEM calculates the heat lost from each pipe section based on the temperature difference between the water in the pipe and the surrounding air, the pipe's surface area, and the insulation resistance. Between draw-off events, the water in the pipes cools, and the next draw-off must first displace this cooled water before delivering hot water to the user. This “dead leg” effect is a significant source of wasted energy in real homes that SAP largely ignores.

Impact on System Design

Because HEM calculates pipework losses from first principles, system design decisions that SAP overlooked now directly affect the calculated energy performance:

• Short pipe runs are rewarded: locating the cylinder or combi boiler close to the main points of use reduces both heat loss and dead leg waste
• Proper insulation matters: insulated pipes lose less heat between draws and keep the standing water warmer for longer
• Pipe diameter selection affects dead leg volumes; oversized pipes hold more water that cools between uses
• Secondary circulation loops (common in larger dwellings) are not modelled at present, so neither their pump energy nor their continuous heat losses reach the result

Combi Boiler vs Cylinder-Based Systems

HEM treats combi boilers and cylinder-based systems as fundamentally different hot water delivery mechanisms, each with its own calculation path.

Cylinder-Based Systems

For systems with a hot water storage cylinder (whether heated by a boiler, heat pump, immersion heater, or any combination), HEM uses the full stratified cylinder model described above. Energy flows into the cylinder from the heating system according to a thermostat-controlled schedule, and energy flows out through tapping events, standby losses, and pipework losses. The half-hourly timestep captures the dynamic interaction between the heating system's recovery capacity and the household's draw-off pattern.

Combi Boiler Systems

Combi boilers heat water instantaneously on demand, with no storage tank. In HEM, each tapping event triggers a calculation of the energy required to raise cold mains water from its current temperature to the required delivery temperature. The cold feed temperature varies through the year: the FHS assessment uses monthly UK average values, with separate figures for a mains feed and a header tank, and assumes no regional variation. The boiler's efficiency at the specific operating conditions, including part-load performance and any condensing behaviour, is applied to determine the fuel consumed.

On top of that, HEM adds a combi loss: the standing penalty a combi carries for keeping itself ready to deliver hot water. It is derived from the boiler's BS EN 13203-2 tapping profile test results, using the medium profile with the small or large profile where those are also submitted. Where no additional test results exist, a combi loss of 600 kWh per year is assumed.

The combi loss is an annual figure spread across the timesteps rather than a per-event firing penalty. HEM-TP-14 records relating it to the number of events, or the volume of water in the test tapping profile, as a possible future improvement.

Heat Pump Water Heating: Two Test Routes

When a heat pump serves the hot water system, HEM-TP-12 splits the calculation in two. A heat pump that also provides space heating, the normal case in a new home, is calculated from its EN 14825 test data with the cylinder as the sink. A heat pump that heats water only is calculated from EN 16147 test data instead.

COP for Water Heating

A heat pump's coefficient of performance (COP) when heating water is typically lower than its COP for space heating, because the target temperature is higher. Space heating may require a flow temperature of 35–45°C (particularly with underfloor heating), while the FHS assessment holds the cylinder setpoint at 60°C for legionella control. The larger temperature lift reduces the heat pump's thermodynamic efficiency.

On the EN 14825 route, HEM models this by calculating the heat pump's COP at each half-hourly timestep based on the source temperature (outdoor air for an ASHP, or a ground source temperature derived from outdoor air for a GSHP) and the required sink temperature for the cylinder. As the cylinder temperature rises during a heating cycle, the sink temperature increases and the COP falls. HEM captures this progressive degradation rather than applying a single average figure. Hot-water-only units are the exception: the EN 16147 route yields one seasonal performance factor for the year.

EN 16147 Tapping Cycles

EN 16147 tests a heat pump water heater against standardised tapping cycles: a sequence of draw-offs at defined times, volumes and temperatures across 24 hours. For a hot-water-only heat pump, HEM requires test data for load profile M; without it, no seasonal performance factor can be derived. Where profile L data is also submitted, HEM interpolates or extrapolates between the two according to the daily hot water requirement of the dwelling being assessed.

The measured figures that feed the calculation are the COP over the cycle, the electrical input over 24 hours, the standby power, and the declared daily vessel heat loss. This creates a direct link between the dwelling's modelled daily hot water volume and the test data used to characterise the unit.

Solar Thermal Contribution

HEM models solar thermal panels as an additional heat source that feeds into the hot water cylinder. At each half-hourly timestep, the model calculates the useful solar energy collected based on:

• Solar irradiance: direct and diffuse radiation on the collector surface, derived from the weather file and the collector's orientation and tilt (using the same solar calculation methodology as solar gains through the building fabric)
• Collector characteristics: peak collector efficiency, first-order and second-order heat loss coefficients, the incidence angle modifier, and the module reference area
• Temperature difference: the difference between the collector temperature and the temperature of the cylinder layer to which the solar coil is connected. As the cylinder heats up, the useful output decreases because the temperature lift is reduced.
• Pump and controller behaviour: the solar pump only runs when the collector temperature exceeds the cylinder temperature by a defined margin, preventing reverse circulation

SAP's approach to solar thermal is considerably simpler: it uses a monthly calculation that estimates the total useful output based on the collector area, orientation, and regional solar radiation data, without accounting for the dynamic interaction between the collector and the cylinder temperature at different times of day.

HEM's half-hourly approach captures important real-world effects: solar thermal output peaks around midday, but hot water demand peaks in the morning and evening. A well-sized cylinder can store midday solar energy for evening use, but an undersized cylinder reaches temperature early in the day and rejects further solar input. HEM models these interactions explicitly.

Waste Water Heat Recovery

Waste water heat recovery (WWHRS) systems capture heat from used shower water to pre-heat the incoming cold mains supply. HEM models WWHRS by calculating the temperature rise of the cold water feed based on:

• The shower flow rate and temperature (from the individual outlet specification)
• The WWHRS heat exchanger efficiency (from manufacturer test data, typically 30–60% for instantaneous systems)
• The system configuration: whether the pre-heated water feeds both the shower and the water heating system (System A), the shower only (System B), or the water heating system only (System C)

Because HEM models individual tapping events, it can calculate the WWHRS benefit specifically for shower events rather than spreading it across all hot water use. This is physically more accurate than SAP's approach, which applies a simplified efficiency factor to the total hot water demand. The result is that HEM can properly credit WWHRS installations, particularly on dwellings where showers represent a large proportion of total hot water use.

Data Input Requirements: What Assessors Must Provide

The shift to event-based hot water modelling places a considerably greater data collection burden on assessors. The table below summarises the additional information HEM requires compared to SAP:

For assessors accustomed to SAP's relatively straightforward hot water inputs, this represents a fundamental change in working practice. The increased data requirements contribute significantly to the overall increase in assessment time: some respondents to the FHS consultation reported HEM assessments taking around three times as long as the equivalent SAP assessment. Our SAP Assessor Transition Guide provides practical strategies for managing this transition, including recommended approaches for systematic data collection and the use of project data rooms.

Integration with the Whole-Building Model

Hot water calculations do not exist in isolation within HEM. The domestic hot water module interacts with several other calculation modules at each half-hourly timestep:

• Internal heat gains: cylinder standby losses and pipework losses within the heated envelope contribute as internal heat gains to the relevant zone, reducing the space heating demand. This interaction means that “wasted” hot water energy is partially recovered as useful heating; an effect that is more significant in winter (when heating is needed) than in summer (when it contributes to overheating risk).
• Heating system demand: the energy required to heat the cylinder or serve combi tapping events is added to the total demand on the heating system at each timestep. For heat pumps, this affects the COP calculation because hot water demand may coincide with space heating demand, requiring the system to operate at higher output.
• Electricity demand: immersion heaters, electric showers, and solar thermal pumps contribute to the dwelling's half-hourly electricity demand profile, which interacts with PV generation and battery storage modelling.
• Overheating assessment: in summer, hot water system losses contribute to internal heat gains that can increase overheating risk, particularly in well-insulated, airtight dwellings.

This interconnected approach is a fundamental advantage of HEM's half-hourly simulation over SAP's monthly method. In SAP, interactions between building systems are handled through simplified correction factors that approximate the average effect over a month. In HEM, the interactions are calculated explicitly at each timestep, capturing the dynamic interplay between hot water use, space heating, and electricity demand that characterises real building performance.

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