Heat Pump Cylinder Sizing: Unvented DHW Cylinders, Buffer Tanks, Coil Size and Legionella Pasteurisation

Quick Answer: A heat pump DHW cylinder must be sized for the household's daily hot water demand plus the heat pump's recovery characteristics. A typical 3–4 person household requires a 200–250 litre unvented cylinder with a large heat exchanger coil (to allow effective heat transfer at heat pump flow temperatures of 50–55°C). An immersion heater provides the weekly 60°C legionella pasteurisation cycle. Buffer tanks (50–200 litres) are sometimes added to prevent short-cycling and smooth heat pump operation.

Summary

Domestic hot water (DHW) is one of two primary demands on a heat pump (the other being space heating). Heat pump DHW cylinders differ from conventional indirect cylinders in two critical ways: the heat exchanger coil must be large enough to transfer adequate heat at low flow temperatures (50–55°C rather than the 70–80°C of a boiler), and an immersion heater is required for legionella pasteurisation.

Correct cylinder sizing is a required element of MCS 007 design documentation. Undersized cylinders lead to morning DHW recovery failures; incorrectly sized coils lead to poor heat pump efficiency or an inability to heat the cylinder to the required temperature.

Key Facts

• Unvented cylinder — the standard specification for heat pump DHW; operates at mains pressure (typically 2.5–3.5 bar); provides good flow rates throughout the property; requires an annual inspection of the pressure relief valve (G3 requirements)
• Heat pump compatible cylinder — must have a large-area heat exchanger coil (typically 2–3× the coil surface area of a conventional indirect cylinder); allows adequate heat transfer at 50–55°C flow temperature; often described as a "high-efficiency" or "heat pump ready" cylinder
• Cylinder sizing rule — typically 40–50 litres per person for a domestic property; 3-person household = 150–200L; 4-person household = 200–250L; add 50L for households with baths as the primary bathing method
• Recovery rate — the rate at which the heat pump can heat the cylinder from cold to set temperature; a 10kW ASHP can recover a 200L cylinder in approximately 3–4 hours (depending on flow temperature and coil size)
• Immersion heater — an electric immersion heater (2–3kW) is fitted in all heat pump DHW cylinders; used exclusively for the weekly legionella pasteurisation cycle (60°C for 1 hour); not used for normal DHW heating (to preserve heat pump efficiency)
• Legionella (L8) — HSE guidance L8 requires the DHW storage temperature to prevent Legionella proliferation; heat pump cylinders are typically set to 50–55°C for daily use; a weekly pasteurisation to 60°C (using the immersion heater) is required
• Buffer tank — a small storage vessel (50–200L) inserted in the space heating circuit (not DHW circuit); decouples the heat pump's output from the heating circuit demand; prevents short-cycling when all zone thermostats are satisfied; more commonly used where weather compensation is not aggressive and zone control causes frequent heat pump stop/start
• DHW priority — most heat pump controllers have a DHW priority mode; when the DHW cylinder needs heating, the heat pump diverts output from space heating to DHW; prevents simultaneous heating demand from overloading the heat pump
• Thermostatic blending valve (TMV) — required on unvented cylinders where the stored temperature exceeds 50°C; blends stored hot water with cold to deliver a safe outlet temperature (typically set to 45–50°C) at taps; required under Water Regulations/Water Fittings Regulations
• G3 compliance — unvented hot water storage systems must be installed by a G3-qualified operative (or notified to building control); an annual check of the pressure relief valve, expansion vessel, and temperature/pressure relief (T&P) valve is required

Quick Reference Table: Cylinder Sizing by Household

Detailed Guidance

Heat Pump Cylinder Selection

Coil size and heat transfer: A conventional boiler-heated indirect cylinder typically has a coil surface area of 0.8–1.2 m². At boiler flow temperatures of 70–80°C, this is adequate for rapid heat transfer. At heat pump flow temperatures of 50–55°C, the log mean temperature difference (LMTD) across the coil is much smaller — heat transfer is proportional to coil area × LMTD. A coil that is too small cannot transfer enough heat at heat pump temperatures, resulting in:

• The heat pump running at a higher flow temperature than necessary (reducing COP)
• Slow recovery of the cylinder (DHW shortfall in the morning)
• Heat pump short-cycling as it struggles to raise the cylinder temperature

Heat pump compatible cylinders have coil surface areas of 2.0–3.0 m² — typically 2–3× the area of a standard indirect cylinder. This larger coil allows effective heat transfer at 50–55°C flow temperature.

Standing heat loss: Well-insulated cylinders (factory-fitted foam insulation, minimum 50mm) minimise standing heat loss. For heat pumps operating on tight running cost margins, a cylinder with a low daily heat loss figure (≤1.0 kWh/day for a 200L cylinder) is preferable.

Common heat pump cylinders:

• Gledhill Torrent Eco, Telford Tempest, Megaflo Eco HP, Joule Cyclone, Sunamp heat batteries
• Each has specific coil area, flow rate requirements, and clearance dimensions — check compatibility with the heat pump model

Buffer Tank Application

When a buffer tank is NOT required: For most correctly designed heat pump systems with weather compensation and either:

• Underfloor heating (single zone, continuous operation)
• Radiators in multiple rooms with no individual zone thermostats (or with a thermostatic bypass valve)

... a buffer tank is not required. The heating circuit provides sufficient water volume to prevent short-cycling.

When a buffer tank IS required:

• Where zone thermostats close down large portions of the heating circuit simultaneously (e.g., a property with separate room thermostats on most radiators and no bypass valve)
• Where the primary circuit volume is very low (a compact system with short pipe runs and small emitters)
• Where the heat pump's minimum run time cannot be satisfied by the heating circuit alone

Buffer tank sizing: A buffer tank is typically 50–150 litres for a domestic ASHP installation. The buffer adds thermal mass to the circuit, allowing the heat pump to run for longer continuous periods before stopping.

Hydraulic separation: Some buffer tanks are installed as hydraulic separators (low-loss headers) between the heat pump primary circuit and the heating distribution circuit. This allows the heat pump and heating circuit pump to operate at different flow rates without hydraulic interference.

Legionella Pasteurisation

Why heat pump DHW needs special attention: Legionella bacteria proliferate between 20–45°C. A conventional boiler cylinder stored at 65°C is lethal to Legionella. A heat pump cylinder stored at 50°C is at the upper limit of the proliferation range — not ideal.

HSE L8 compliance for heat pump DHW:

1. Store at 50–55°C daily (acceptable: limits proliferation but not elimination)
2. Weekly pasteurisation to 60°C for 1 hour minimum — kills Legionella throughout the cylinder
3. The pasteurisation cycle is performed by the immersion heater (the heat pump cannot reliably reach 60°C at a useful COP — most heat pumps' COP falls sharply above 55°C)
4. Programme the pasteurisation to occur overnight or during off-peak electricity tariff hours to minimise running cost impact

Practical setup: Most heat pump controllers include a "legionella protection" schedule. Set to:

• Daily DHW setpoint: 50–55°C (heat pump heats cylinder to this temperature daily)
• Weekly legionella cycle: Tuesday or Wednesday, 02:00–04:00 (or any off-peak period); immersion heater heats to 60°C; held for 1 hour; hot water drawn through all outlets after pasteurisation (by normal morning use)

DHW Priority and COP Impact

Efficiency impact of DHW heating: Heating DHW to 50–55°C is less efficient than heating space heating circuits to 40–45°C. The additional temperature lift reduces the ASHP's COP:

• Space heating at 45°C: COP ~3.5
• DHW at 55°C: COP ~2.5–3.0

For accurate SCOP (Seasonal COP) calculation, the DHW contribution must be included. The proportion of the heat pump's annual output used for DHW typically represents 20–30% of total demand for a well-insulated property.

Minimising DHW running cost:

• Heat the DHW cylinder during the warmest part of the day (afternoon solar gain if applicable; milder outdoor temperature means higher ASHP COP)
• Or heat DHW overnight on an off-peak tariff (Octopus Agile, Go, or Economy 7) — the lower electricity rate partially offsets the lower COP
• Set the DHW setpoint at the minimum safe temperature (50°C if a weekly 60°C pasteurisation cycle is in place)

Frequently Asked Questions

Can the heat pump heat the DHW and the space heating simultaneously?

Most domestic ASHP controllers run in DHW priority mode — the heat pump dedicates its full output to DHW heating, then switches to space heating once the cylinder is satisfied. Simultaneous operation is possible on some models but less common. DHW priority heating for a 200L cylinder typically takes 2–3 hours; during this time, space heating is paused. On a well-insulated property, this is rarely noticeable.

Do I need a buffer tank if I have underfloor heating?

Usually not. Underfloor heating provides a large volume of water in the circuit (the UFH loops act as a thermal buffer) and operates continuously at low load. Short-cycling is rarely a problem with UFH systems. Buffer tanks are primarily needed in systems with small hydraulic volumes and aggressive zone control (multiple room thermostats shutting down the circuit).

Is a vented (open-vented) cylinder acceptable with a heat pump?

Technically yes, but open-vented cylinders operate at low pressure (gravity-fed), giving poor flow rates in multi-story properties. Unvented cylinders at mains pressure are the standard for heat pump DHW — they provide consistent pressure throughout the property and are compatible with modern showers and bathrooms. Open-vented systems are only specified where the building prohibits mains pressure (listed building restrictions, very old pipework) or where an existing vented system is being retained temporarily.

What is a "twin coil" cylinder and when is it needed?

A twin-coil cylinder has two heat exchanger coils: one connected to the heat pump, one connected to a secondary source (typically solar thermal panels or a solid fuel stove). For combined solar thermal + heat pump systems, a twin-coil cylinder allows both sources to heat the DHW without complex valving. The lower coil is connected to the solar thermal circuit (prioritised when solar is available); the upper coil is connected to the heat pump (tops up when solar output is insufficient).

Regulations & Standards

• G3 (Building Regulations Approved Document G) — unvented hot water storage; installation and annual inspection requirements

• HSE L8 — Legionella: The Control of Legionella Bacteria in Water Systems; pasteurisation requirements for DHW storage

• Water Regulations (Water Supply (Water Fittings) Regulations 1999) — thermostatic blending valve requirement, backflow prevention on mains pressure systems

• MCS 007 — heat pump installation standard; DHW cylinder sizing documentation requirement

• BS 853 — cylinders and tanks for water storage; relevant to pressure vessel construction

• HSE L8 — Legionella guidance — pasteurisation temperature and frequency requirements

• WRAS (Water Regulations Advisory Scheme) — TMV and water fitting requirements

• MCS 007 guidance — mcscertified.com — DHW sizing documentation

• Gledhill / Telford / Megaflo product technical data — heat pump cylinder coil area specifications

• air source heat pump installation — commissioning DHW setpoints and pasteurisation schedule

• mcs 007 heat pump standard — MCS documentation including DHW cylinder specification

• heat pump controls setup — DHW priority mode and legionella schedule configuration

• ground source heat pump installation — GSHP DHW requirements (identical cylinder spec)