This tool sizes a domestic hot water cylinder for a low-temperature or heat-pump system: how much hot water the property needs, how big a cylinder that means at your storage temperature, how long it takes to reheat, and what that reheat costs. This guide explains how to use it, how each figure is worked out, what it assumes, and where it stops.
Approved Document L1 (2021), clause 5.11, requires domestic hot water systems to be sized in accordance with BS EN 12831-3 or the CIPHE Plumbing Engineering Services Design Guide, and that systems are not significantly oversized.
Cylinder sizing follows the CIBSE DHDG 2026 simplified method (Section 4.5.1, Tables 4-3 and 4-4) and matches DHDG Table 4-1 to within a litre. Reheat times use the BS 8558 equation, so they differ from DHDG Equation 4.3.
MCS uses a different calculation: 45 litres per person per day, taking people as bedrooms + 1, or occupants if more (MIS 3005-D 3.5.2). Use that for MCS installations.
| Outlet | Volume at 42°C |
|---|---|
| Standard shower (9 L/min × 7.5 min) | 67.5 L |
| Low-flow shower (6 L/min × 7.5 min) | 45 L |
| Deep bath | 180 L |
| Shallow bath | 100 L |
| Storage temp | f | Useable |
|---|---|---|
| 60°C (recommended) | 0.729 | 73% |
| 55°C | 0.573 | 57% |
| 50°C | 0.417 | 42% |
The tool has a volume override for when the actual cylinder differs from the calculated minimum — space or structural constraints, or a specific product choice.
Undersized: if the override is below the calculated minimum, the tool flags it with reduced usable capacity. Record the decision in the system criteria document, and consider low-flow outlets to reduce demand.
Oversized: extra capacity, but longer reheat and higher standing losses. Make sure the heat generator can reheat the larger volume in an acceptable time.
The demand formula uses bedrooms and bathrooms as a proxy for maximum occupancy: a 3-bed property is sized for 4 people regardless of who lives there now. This is consistent with Part L, which sizes to the building.
Stagnation risk: low-occupancy properties may use far less than the cylinder holds. The DHDG permits using actual consumption data (Section 4.5.2) to produce a smaller cylinder.
Water Fittings Regulations G18.2 recommends storage at not less than 60°C and distribution at not less than 55°C. R290 heat pumps comfortably deliver 65°C flow for DHW reheat.
Scalding protection (DHDG 2026, 4.9.2): fit TMVs at each point of use (Type 2+, NSF certified). Max outlet temps: baths 46°C, showers 41°C, washbasins 41°C, bidets 38°C.
At 60°C, 90% of legionella is killed in 2 minutes; at 50°C the same kill rate takes about 2 hours.
The DHDG 2026 (Section 4.9.1) permits below-60°C storage for privately owned and occupied residential properties where risk is assessed as minimal. A thermal disinfection routine must raise all stored water to ≥60°C, held for ≥1 hour, at least once per week.
R290 units at 65°C flow can perform pasteurisation themselves, and remain more efficient than an immersion heater (1.0). Frequency is set per risk assessment (BS 8580-1).
T from cold: stored − mains (worst case). T after use: stored − 42°C (normal recovery).
P is the lesser of the heat generator output at the DHW flow temperature or the cylinder coil’s heat-transfer capacity. For immersion heaters, P is the element rating.
The cost depends on the thermal energy required, the reheat efficiency (SCOP or COP), and the electricity tariff.
A heat pump at 2.0 uses half the electricity of an immersion heater (1.0) to deliver the same heat; at 3.5 it uses less than a third.
Efficiency for cylinder reheat is lower than for space heating because the heat pump runs at a higher flow temperature (around 55–65°C rather than 35–45°C). Nothing is pre-selected: costs, and the efficiency line on the report, appear only once an option is chosen. Every option has a named source.
| Setting | Value | Source and when to use |
|---|---|---|
| R290 heat pump — typical | 3.5 | Mean published SCOP at 55°C flow of 12 current R290 models from three manufacturers (see below). A realistic general figure for a well-designed R290 system. |
| MCS default — air source | 1.9 | MCS 031 a) Issue 2.0, 2.5(n). The fixed hot water figure in the MCS pre-sale energy estimate (2.2 for ground source). Use when matching the MCS estimate. |
| Part L minimum | 2.0 | Approved Document L Volume 1, 2021 edition incorporating 2023 amendments, 6.37(b): minimum COP for heating domestic hot water. |
| User-defined | 1.0–8.0 | The manufacturer’s figure for the actual heat pump, or a measured figure. Enter the source — it prints on the report. |
| Boiler / other | 50–100% | Enter the boiler’s efficiency and its source. AD L 2021 minimums (ErP): gas 92% (Tables 6.1/6.2), oil regular boiler 91% (Table 6.3). These are seasonal space-heating figures and many installations fall short of them. |
| Immersion | 1.0 | Direct electric — no heat pump benefit. |
Fuel bought by the litre: kerosene and LPG prices are converted to p/kWh on the gross basis that ErP efficiencies use — kerosene 10.28 kWh/litre, LPG 7.41 kWh/litre (DUKES 2025, Table A.1 gross calorific value ÷ Table A.3 litres per tonne). The LPG figure is the UK average propane/butane mix. Gas is billed per kWh, so enter it directly.
Manufacturers rarely publish a hot-water-specific seasonal figure, but all publish SCOP at 55°C flow (BS EN 14825, average climate). 55°C is close to the flow temperature used for cylinder reheat, so it is the closest published equivalent. It is a space-heating figure used as a proxy, and is labelled as such.
| Manufacturer (R290) | Models | SCOP at 55°C |
|---|---|---|
| Grant Aerona 290 (installation instructions DOC 0204, Rev 1.9, August 2026, Table 2-1) | 4, 6.5, 9, 12, 15.5 kW | 3.66, 3.62, 3.69, 3.74, 3.33 |
| Mitsubishi Electric Ecodan R290 (specification October 2023; PUZ-WZ80VAA sheet January 2025) | 5, 6, 8 kW | 3.53, 3.56, 3.45 |
| Samsung EHS Mono R290 Gen 7 (datasheet 19/12/23) | 5, 8, 12, 16 kW | 3.42, 3.41, 3.51, 3.43 |
Cross-check: the single test point COP at A7/W55 (BS EN 14511, 7°C air, 55°C flow) is consistently about 3.0 across manufacturers (Grant Aerona 290 mean 3.08; Vaillant aroTHERM plus 2.93–3.02; Logic Air about 3.0). The seasonal figure is higher than the 7°C test point because the heating season includes many milder days.
When storing below 60°C, the tool shows pasteurisation costs per cycle, weekly, and annually, for the selected heat source and, for heat pumps, an immersion heater for comparison. Frequency is per risk assessment (BS 8580-1); annual costs assume weekly (×52).
Note: costs cover the energy to heat the water only — not standing losses, distribution losses, or pump energy during the hold period.
Works out how much stored hot water a specific outlet draws from the cylinder. With a tariff and COP entered, the cost to reheat that draw-off is shown too — useful for a per-shower or per-bath running-cost conversation with the client.
The reheat time and cost for one draw-off don’t change with storage temperature. Store it hotter and you draw fewer litres, but each litre has a bigger rise back up to temperature; store it cooler and you draw more litres, but each has a smaller rise. Fewer litres with a bigger rise comes to the same energy — and the same reheat time — as more litres with a smaller rise, so both are set by the outlet demand, not by the storage temperature.
Rearranges the reheat equation to find the minimum heat input for a target reheat time.
With DHW priority, the heat generator covers the larger of the two loads. Check the modulation range to avoid short-cycling.
“Your hot water cylinder is sized to hold a full day’s hot water. It may be larger than you’re used to because the system reheats over a longer period.”
“The system reheats overnight or in scheduled blocks. After heavy use it takes about 1–2 hours to recover with a heat pump, compared to 20–30 minutes with a gas boiler.”
“Each shower costs roughly [X]p to reheat with the heat pump — the tool can show you the exact figure.” Use the outlet and cost calculations in Additional Considerations to generate a per-shower or per-bath cost.
“The water is stored at 60°C for hygiene. The mixing valves blend it to a safe temperature — please don’t remove or tamper with them.”
Fill in Report details at the top of the tool — property, address, assessment date, assessed by, company and contact. The assessment date starts as today’s date; change it if the survey was on another day. Your own details are kept when you clear the form, so you only enter them once.
Complete Parts A to C before viewing the report. Check the storage temperature in Part B: it starts at 60°C and sets the cylinder size. Without Part C the report has no reheat times or running costs.
Then tap View report (under the Summary, or in the ⋮ menu) to read it on screen, and Save as PDF to keep or send it; the file name includes the property.
The report shows the outcome and its basis — demand, cylinder volume against the calculated minimum, reheat times, any pasteurisation requirement, and running costs when an efficiency and fuel price are entered — with the sources and a disclaimer. The working (equations, outlet tables, additional considerations) stays in the tool.
CIBSE DHDG 2026, Section 4.5.2 — detailed sizing with demand profiling.
BS EN 12831-3:2017 — the full dynamic computational model.
HWA 002:2022 — cylinder test specification for heat pump use.
CIBSE DHDG 2026, Section 5.7 — heat generator sizing and modulation.
Data privacy. This tool runs entirely on your device. Everything you enter — inputs, calculations and results — stays in this device’s browser storage and is not sent to any server or third party. There is no account, no tracking and no analytics. Nothing is shared unless you choose to share it yourself by producing a report or sending an email.
Disclaimer. This tool sizes domestic hot water cylinders and estimates reheat time and cost to assist competent persons. It is a calculation aid only. It does not constitute engineering certification, a design, or a substitute for professional judgement, and it does not by itself satisfy any MCS, Building Regulations or other regulatory requirement.
Results depend on the information entered and on the published data cited (CIBSE DHDG 2026 Tables 4-3 and 4-4, BS EN 12831-3, BS 8558 and CIPHE guidance). Cylinder capacity, coil heat-transfer rating and heat-generator output must be confirmed against current manufacturer data for the actual products installed. When fitting a heat pump, always consult the heat pump and cylinder manufacturers for the recommended size and specification.
The user is responsible for checking all inputs and outputs, for working within their demonstrated scope of competence, and for compliance with current UK legislation, standards and manufacturers’ instructions — including scalding protection and Legionella risk assessment where storage below 60°C is used.
To the fullest extent permitted by law, the author accepts no liability for any error, omission, or any direct or indirect loss or damage arising from use of this tool or reliance on its output.
Backups. Your work is stored only on this device. Clearing the browser’s site data, uninstalling the app, or losing or resetting the device will delete it, with no means of recovery.
© 2026 Easy Heating Engineer. All rights reserved. May not be reproduced, redistributed, or modified without written permission.
If a calculation looks wrong, the tool misbehaves, or you have an idea to improve it, please get in touch. Use the link below, or the Support & feedback link at the bottom of the tool. Your build version is added to the email automatically.
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