Heat Pump Hot Water Cylinders: Oversized Coil Requirements, Buffer Tank Decision and Legionella Cycle

Summary

The hot water cylinder is the most performance-critical component in a heat pump system. Conventional indirect cylinders sized for gas boilers are incompatible with heat pump operation — their small coil surface areas require high primary flow temperatures (>60°C) to transfer heat quickly, which destroys heat pump efficiency. Specifying the wrong cylinder is one of the most common heat pump installation errors.

Heat pumps operate most efficiently at low temperature differentials between the heat source and the heat sink. For domestic hot water, this means heating a large volume of water slowly using a very large coil, rather than quickly heating a small volume through a small coil. The cylinder specification directly determines whether the system will achieve its rated Seasonal Coefficient of Performance (SCOP).

Buffer tanks add a secondary consideration. They protect the heat pump compressor from rapid on/off cycling when heat demand is low, but they add cost, space requirements and heat losses. The industry has shifted towards minimising or eliminating buffers through careful cylinder and emitter design. Understanding when a buffer is genuinely required versus when it is unnecessary cost is an important specification skill.

Key Facts

• Coil surface area — Heat pump cylinders require minimum 1.5m² coil; 2.0–3.0m² for systems ≤12kW; larger for higher-output heat pumps
• Primary flow temperature — Heat pump cylinder coils designed for 45–55°C primary; gas cylinder coils require 70–80°C
• Heat pump efficiency impact — Raising flow temperature by 1°C reduces SCOP by approximately 2–3%
• Cylinder sizing — 200–250 litres for 1–2 persons; 250–300 litres for 3–4 persons; apply manufacturer's sizing guide
• Immersion heater — Required for legionella pasteurisation; typically 3kW immersion rated for continuous duty
• Legionella risk — Legionella bacteria thrive between 20–45°C; minimum 60°C pasteurisation cycle required
• Pasteurisation temperature — Minimum 60°C throughout cylinder for 1 hour, or 65°C for 2 minutes per HSE L8 guidance
• MCS heat pump standard — MCS 007 covers heat pump installation; MCS 011 covers domestic hot water sizing
• Buffer tank volume rule of thumb — 10–20 litres per kW of heat pump output as minimum buffer
• Short-cycling limit — Most heat pump manufacturers specify minimum off-time of 3–10 minutes; verify in manufacturer documentation
• Unvented cylinders — Most heat pump cylinders are unvented; require G3 qualification to install, notify Building Control
• Smart controls — Many heat pump cylinders accept communication signals from the heat pump for coordinated DHW and space heating demand

Quick Reference Table

Detailed Guidance

Why Coil Size Matters — The Heat Transfer Equation

Heat transfer through the coil is governed by: Q = U × A × ΔT

Where:

• Q = heat transfer rate (kW)
• U = overall heat transfer coefficient (W/m²K) — typically 1,500–3,000 W/m²K for a clean coil
• A = coil surface area (m²)
• ΔT = temperature difference between primary and secondary fluid (K)

With a gas boiler operating at 70°C primary and 55°C stored water:

• ΔT = 15K
• Small coil (0.5m²) at U=2,000: Q = 2,000 × 0.5 × 15 = 15,000W = 15kW ✓ (adequate)

With a heat pump at 45°C primary and 40°C stored water:

• ΔT = 5K
• Small coil (0.5m²): Q = 2,000 × 0.5 × 5 = 5,000W = 5kW (insufficient for a 10kW HP)
• Large coil (2.5m²): Q = 2,000 × 2.5 × 5 = 25,000W = 25kW ✓ (adequate)

This is why the coil area must be multiplied approximately 4–5× when transitioning from gas to heat pump.

Buffer Tank Decision Framework

START: Heat pump specified
         |
         v
Is cylinder volume > 8 litres/kW output?
         |
        YES                NO
         |                  |
         v                  v
Do emitters have         Add buffer tank
good thermal mass?       (10-15L/kW)
(UFH, large rads)
         |
        YES
         |
         v
Buffer not required
(verify with manufacturer)

When a buffer IS required:

• All-electric radiator system with small heat pump (≤8kW) and small cylinders — cycling risk high
• Manufacturer documentation specifies minimum system volume exceeding cylinder + pipework
• System has zone valves that may cause all zones to close simultaneously (heat pump continues to run against closed valves)
• Property is very well-insulated with low heat demand — heat pump oversized relative to demand

When a buffer is typically NOT required:

• Dedicated heat pump cylinder with volume ≥200L (substantial thermal mass)
• Underfloor heating as primary emitter (high thermal mass)
• Variable speed compressor (inverter) heat pump — can modulate down to 20–30% output
• System volume (all pipework, emitters, cylinder) exceeds manufacturer's minimum system volume requirement

Sizing a buffer tank:

• Consult manufacturer's minimum system volume guide
• Calculate existing system volume: cylinder + pipework (approx. 6L per radiator, 7L per metre of 28mm pipe, 5L per metre of 22mm)
• Buffer = difference between required and actual
• Most residential systems need 60–150L buffer if required at all

Legionella Management in Heat Pump Systems

The conflict between heat pump efficiency and legionella management is fundamental. Heat pumps are most efficient when storing DHW at 45–50°C, but legionella bacteria survive below 60°C.

HSE L8 (Legionella: The Control of Legionella Bacteria in Water Systems) requirements:

• Hot water must reach ≥60°C within 1 hour of start of heating
• Cold water stored or distributed at ≤20°C
• Hot water distributed at ≥50°C at outlets

Heat pump DHW cycle options:

Option 1 — Immersion boost (most common):

• Heat pump charges cylinder to 45–50°C
• Timer triggers immersion heater (3kW) weekly or as required
• Immersion raises full cylinder to 60°C+
• Energy cost: approx. 8–10 kWh per legionella cycle at 3kW for 2.5–3 hours
• SCOP penalty: slight, as most of the work is done by the heat pump

Option 2 — High-temperature HP mode:

• Some heat pumps can operate in a high-temperature mode (55–65°C flow)
• Enabled once weekly for legionella cycle
• SCOP during this mode drops significantly (HP operates outside optimum range)
• Preferred where immersion is unavailable or heating time is critical

Option 3 — Thermal store arrangement:

• Thermal store at 65–70°C draws cold mains water through a plate heat exchanger
• DHW outlet temperature controlled by blending
• HP charges primary volume; legionella managed passively by storage temperature
• Higher upfront cost but eliminates cycling problem

Recommended approach for domestic installations:

• Immersion heater on weekly timed legionella cycle (minimum)
• Set heat pump DHW charge temperature to 45–48°C for efficiency
• Log legionella cycle completion (landlord properties require documented records under L8)
• Inform homeowner of WHY the immersion heater is programmed

Cylinder Siting and Installation

Physical requirements:

• Unvented cylinder: must be installed by a qualified person holding a G3 assessment (Part L of Part G, Building Regulations Approved Document G3)
• Building Control notification required for every unvented cylinder installation
• Expansion vessel must be matched to cylinder volume and system pressure
• Discharge pipe must run to safe location per G3: minimum 500mm above drain level, terminate outside if possible

Pipework connections for heat pump:

• Primary flow/return: match to heat pump primary circuit (typically 28mm for ≤12kW, 35mm for larger)
• Immersion connections: 1¾" BSP or 2¼" BSP (confirm with cylinder manufacturer)
• Cold mains: 22mm minimum
• Hot draw-off: 22mm minimum; 28mm if multiple simultaneous outlets in larger properties

Insulation:

• Minimum 50mm factory-fitted insulation or equivalent on site
• Primary pipework: 25mm wall insulation on primary flow/return within the heating zone
• Any uninsulated pipework in the cylinder cupboard contributes to standing heat losses

Common Installation Errors

Frequently Asked Questions

Can I use my existing cylinder when retrofitting a heat pump?

Only if the existing cylinder has an adequate coil area AND the coil material is suitable for the flow temperatures involved. In most retrofit situations, the existing cylinder must be replaced. The cost of a correct heat pump cylinder (£600–£1,200 typically) is small relative to the efficiency penalty of incorrect cylinder selection. Many MCS heat pump installers specify cylinder replacement as mandatory.

My heat pump keeps short-cycling. Is it always a buffer tank problem?

Not always. Short-cycling can also result from: oversized heat pump relative to demand; zone valve closing while HP runs; heat pump set to maintain a narrow temperature band; flow rate too low across the HP (dirty filter, undersized pipework, closed isolators). Diagnose the cause before adding a buffer. Adding a buffer to an oversized heat pump on a warm day does not address the underlying sizing issue.

How often should the legionella pasteurisation cycle run?

HSE L8 does not specify a mandatory weekly interval for domestic properties, but weekly is the standard industry practice and recommended by most heat pump manufacturers. For landlord properties (HMOs, rental properties), legionella risk assessment and records are a statutory requirement under the Health and Safety at Work Act and associated ACOP L8. In practice, weekly 60°C cycles are the minimum defensible schedule.

What is the difference between a buffer tank and a thermal store?

A buffer tank is a plain water tank with no coil, used purely to add thermal mass to the heating circuit. A thermal store is a vessel designed to hold all the heat energy for the system and acts as both a buffer and a DHW source (via an internal plate heat exchanger or coil). Thermal stores are more expensive but can eliminate the conventional cylinder, providing all heat from one vessel.

Regulations & Standards

• Approved Document G3 — G3 Unvented Hot Water Storage Systems: qualification, safety devices, Building Control notification

• HSE L8 — Legionella: The Control of Legionella Bacteria in Water Systems (ACOP)

• MCS 007 — Installer standard for heat pumps; includes cylinder and buffer requirements

• BS 1566-1 — Indirect hot water storage cylinders: specification

• BS EN 12897 — Water supply: specification for indirectly heated unvented hot water storage systems

• BS 7593:2019 — Treatment of water in heating systems: code of practice

• HSE L8 ACOP Legionella — Authoritative legionella control guidance

• MCS Installation Standard MCS 007 — Heat pump installer standard including DHW requirements

• CIBSE Heat Pump Design Guide — CIBSE CP1: Heat Pump Design Guide

• boiler selection — Cylinder sizing comparison with conventional boilers

• low temperature design — Low temperature system design context for heat pump cylinders

• unvented cylinders — G3 qualification and installation requirements for unvented cylinders

• biomass heating — Alternative low-carbon heating comparison