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

Quick Answer: Heat pump hot water cylinders require an oversized heat exchanger coil — minimum 2.0–3.0m² coil surface area is typical for domestic systems, compared to 0.5–0.8m² in a standard indirect cylinder. Buffer tanks are required by most manufacturers when the cylinder volume is insufficient to prevent short-cycling, but modern dedicated heat pump cylinders often eliminate this need. A legionella pasteurisation cycle must run at 60°C minimum, requiring the heat pump to boost with an immersion heater.

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

Quick Reference Table

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Cylinder Type Coil Area Compatible with Heat Pump Notes
Standard indirect (vented) 0.5–0.8m² No Requires >70°C primary, inefficient
Standard indirect (unvented) 0.5–1.0m² No Same coil limitation
HP-dedicated cylinder (entry) 1.5–2.0m² Yes (≤8kW HP) Adequate for smaller systems
HP-dedicated cylinder (standard) 2.0–3.0m² Yes (8–14kW HP) Most common domestic choice
HP-dedicated cylinder (large coil) 3.0–4.0m² Yes (>14kW HP) Required for larger systems
Thermal store (combined) 4.0m²+ Yes Integrates buffer and DHW functions

Detailed Guidance

Why Coil Size Matters — The Heat Transfer Equation

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

Where:

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

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

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:

When a buffer is typically NOT required:

Sizing a buffer tank:

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:

Heat pump DHW cycle options:

Option 1 — Immersion boost (most common):

Option 2 — High-temperature HP mode:

Option 3 — Thermal store arrangement:

Recommended approach for domestic installations:

Cylinder Siting and Installation

Physical requirements:

Pipework connections for heat pump:

Insulation:

Common Installation Errors

Error Consequence Fix
Standard indirect cylinder used Heat pump cannot charge efficiently; high electricity bills Replace with HP-dedicated cylinder
Undersized buffer (or none where needed) Short-cycling; compressor wear; error codes Add buffer tank of correct volume
No immersion heater fitted Cannot achieve legionella pasteurisation Fit correct immersion; re-commission
Cylinder charged to 60°C continuously COP 1.0–1.5 (no better than direct electric) Lower DHW setpoint to 45–48°C; weekly legionella cycle
Expansion vessel undersized Pressure relief valve lifts; water wasted Replace with correctly sized vessel

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