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 estimating a monthly hot water volume from the dwelling's floor area, HEM models individual tapping events — every bath, shower, basin draw, and kitchen sink use — each with specified flow rates, durations, and delivery temperatures. Combined with stratified cylinder modelling that tracks temperature layers within the storage tank, and detailed pipework loss calculations based on actual pipe lengths 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 aggregate monthly demand to individual tapping events. In SAP, the daily hot water demand in litres is derived from the formula V_d,average = 36 + 25N (where N is the assumed number of occupants, itself derived from floor area). This produces a single daily volume that is distributed across the year using monthly factors — with no consideration of when, where, or how 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:

• Outlet type — bath, shower (mixer or electric), basin hot tap, or kitchen sink hot tap
• Flow rate (litres per minute) — specific to each outlet as installed
• 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

This means HEM can distinguish between a household with a single low-flow shower and one with a high-flow rain shower, a bath, and multiple basin taps. SAP treats both identically because it never asks about individual outlets.

Individual Outlet Specifications

HEM requires the assessor to specify each hot water outlet in the dwelling. This is one of the most significant changes to the data collection process compared to SAP, where hot water outlets are not individually recorded at all. For each outlet, the assessor must provide:

• Outlet type — categorised as bath, mixer shower, electric shower, basin hot tap, or kitchen sink hot tap
• Flow rate — the measured or manufacturer-specified flow rate in litres per minute. Typical values range from approximately 6–8 l/min for a standard basin tap to 10–12 l/min for a mixer shower, and up to 15–20 l/min for baths and high-flow showers
• Connection details — which hot water system and pipework circuit serves each outlet

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 divides the cylinder into a series of discrete temperature layers (sometimes called nodes or slices). 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 — the model accounts for conduction between adjacent layers and turbulent mixing caused by high flow rates or the entry of cold water. Excessive mixing degrades stratification and reduces system efficiency.
• Standby heat losses — losses from each layer to the surrounding environment, calculated from the cylinder's insulation properties and the temperature difference between each layer and the ambient temperature. Because upper layers are hotter, they lose more heat — a subtlety that SAP's uniform temperature model cannot capture.

Standby Losses and Cylinder Properties

SAP characterises cylinder performance using a single daily standing loss value taken from manufacturer test data (typically to BS 1566 or the equivalent). HEM goes further by using the cylinder's physical properties — volume, dimensions, insulation thickness and conductivity — to calculate standby losses from first principles at each timestep. The loss from each temperature layer is proportional to the temperature difference between that layer and the surrounding space, so the model captures how losses change throughout the day as the cylinder heats and cools.

This approach means that HEM properly credits well-insulated cylinders and penalises poorly insulated ones with greater precision than SAP's single daily figure. It also captures the benefit of locating a cylinder within the heated envelope (where its losses contribute to useful space heating) versus in an unheated cupboard or garage.

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). In SAP, pipework losses are handled through simplified defaults based on the system type, with basic adjustments for insulation. HEM replaces this with a detailed calculation that requires specific data about the pipework installation.

Data Requirements

For each pipework circuit, HEM requires:

• Pipe lengths — the actual measured or designed lengths of hot water distribution pipework from the heat source to each outlet, in metres
• 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 modelled with their associated pump energy and continuous heat losses

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 (which varies seasonally, using half-hourly mains temperature data) to the required delivery temperature. The boiler's efficiency at the specific operating conditions — including part-load performance and any condensing behaviour — is applied to determine the fuel consumed.

HEM also accounts for two combi-specific energy penalties:

• Keep-hot facility losses — many combi boilers maintain a small internal store of pre-heated water or keep the primary heat exchanger warm to reduce the delay before hot water reaches the tap. HEM models the continuous energy cost of this feature, which can be significant over a full year.
• Firing cycle losses — each time the combi boiler fires to serve a tapping event, there are start-up and shut-down losses. With HEM's event-based approach, these losses are calculated for each individual draw-off rather than averaged across the month.

Heat Pump Water Heating — EN 16147

When a heat pump is the primary heating source, its interaction with the hot water system presents specific modelling challenges that HEM addresses through the methodology in HEM-TP-12, drawing on test data from EN 16147 (the European standard for heat pump water heaters).

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), but a hot water cylinder must be heated to at least 50–60°C to meet Legionella safety requirements. The larger temperature lift reduces the heat pump's thermodynamic efficiency.

HEM models this by calculating the heat pump's COP at each half-hourly timestep based on the actual source temperature (outdoor air for an ASHP, or ground/borehole temperature 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.

EN 16147 Tapping Cycles

EN 16147 defines standardised tapping cycles (labelled S, M, L, XL, and XXL) that are used to test heat pump water heaters under controlled conditions. These cycles specify a sequence of draw-off events at defined times, volumes, and temperatures throughout a 24-hour period. HEM uses the test results from the appropriate tapping cycle to characterise the heat pump's performance when serving hot water demand, including its energy consumption during standby periods and its recovery rate after large draw-offs.

The choice of tapping cycle depends on the dwelling's hot water demand profile, which HEM derives from the individual outlet specifications and the standardised daily schedule. This creates a direct link between the specific outlets installed in the dwelling and the heat pump performance data used in the calculation.

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 — optical efficiency (eta-zero), first-order heat loss coefficient (a1), and second-order heat loss coefficient (a2), as determined by EN 12975 testing
• 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 the cold supply to the shower only (System A), the cold supply to the entire hot water system (System B), or both (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. Consultation testing showed that the increased data requirements contribute significantly to the overall increase in assessment time — from approximately 20 minutes in SAP to 1 hour 40 minutes in HEM for a standard house type (excluding geometry and U-value calculations). 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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