Buffer vessels for boilers

Buffer Vessel Sizing for gas modular boilers

For commercial energy centres supplying multiple Heat Interface Units (HIUs), particularly when served by modular gas boilers, the correct buffer vessel sizing is crucial to:

  • Minimise short cycling at low load conditions.

  • Provide a thermal store to meet short-term peak demands exceeding boiler capacity.

Key Guidance on Buffer Vessel Sizing for Modular Boiler Systems

1. Minimising Boiler Cycling

To prevent frequent boiler cycling at low loads (which reduces boiler efficiency and increases wear), buffer vessels are used to increase system volume and thermal inertia.

CIBSE CP1 (2020): Heat Networks Code of Practice recommends: “Where boilers serve a variable load with many small terminal units (e.g. HIUs), a buffer should be provided to maintain minimum run times for the boilers.”

2. Sensible Minimum Buffer Volume

A rule of thumb often used is: 10–20 litres of buffer per kW of boiler capacity, depending on modulation capability and control strategy.

For a project such as this one 600 kW system, that would give:

  • Minimum: 600 kW × 10 litres = 6,000 litres

  • Maximum: 600 kW × 20 litres = 12,000 litres

A compromise size of 8,000–10,000 litres is often typical for systems with many HIUs and varying demand.

Additional Considerations

Store for Temporary Demand Above Boiler Capacity

If the buffer is also to handle peak demand that temporarily exceeds boiler capacity, its energy storage (kWh) must be sized accordingly.

Use the formula: Stored energy (kWh)} = (V x ΔT x C) / 3600

Where:

  • V = buffer volume in litres

  • ΔT = useful temperature swing (e.g. 80°C to 60°C → ΔT = 20K)

  • c = specific heat of water = 4.18 kJ/kg·K

Example: 10,000 litres, ΔT = 20°C

 

     (10,000 x 20 x 4.18) / 3600 =  23.2 kWh

This isn’t huge—so buffers for energy storage usually supplement but don’t replace capacity.

From Experience & Practice

  • Systems with frequent low-load conditions (e.g. hotels, residential blocks) benefit most from larger buffers to smooth out demand and avoid short cycling.

  • Modular boilers with good turndown ratios (e.g. 5:1 or higher) can sometimes use smaller buffers if properly controlled, but HIU systems with unpredictable loads still benefit from a decent volume.

  • Buffer location and integration with flow/return strategy (parallel or in-line) affects performance too.

Professional Sources to Consult

  • CIBSE CP1 2020 – Code of Practice for Heat Networks (soon to be replaced)

  • CIBSE AM12 – Combined Heat and Power for Buildings

  • BSRIA BG29 / BG50 – Water treatment and pre-commissioning

  • ADE Guidance – on demand-side management in decentralised systems

  • Manufacturers also provide specific sizing tools based on modulation, control setup, and application.

Recommendations Summary

System Recommended Buffer Size

  Cycling prevention only 6,000 to 8,000 litres

Storage for overshoot 8,000 to 12,000 litres

With CHP backup Smaller CHP store plus above

High-mass system May allow smaller buffer

 

For this project we can look at smaller buffers to stop cycling as the turndown is much greater using modular boilers

  • Boiler turndown of 5:1 means each 100 kW module can modulate down to 20 kW.

  • If you have multiple modules, the system can modulate even lower in steps (e.g. 3 × 100 kW can turn down to 20, 40, 60 kW, etc.).

  • However, short cycling can still occur if:

    • Heat demand drops below minimum combined output of the enabled boilers.

    • Control sequencing isn’t tight enough to delay firing.

    • Flow rates through HIUs vary rapidly.

Adjusted Buffer Sizing Approach with Modulation

If the minimum boiler output on this project is 20 kW , you can size the buffer to absorb that heat over a minimum acceptable run time — often 3 to 5 minutes is used to prevent cycling.

Sizing Formula Based on Run Time

Buffer Volume (litres) = (Qmin x t x 3600) / (c x ΔT)

Where:

  • Qmin = minimum boiler output (kW) = 20

  • t = desired run time (seconds) = 3 to 5 minutes = 180 to 300 s

  • c = specific heat = 4.18 kJ/kg·K

  • ΔT = usable temp difference (e.g. 20°C)

Example for 5-minute run:

Buffer Volume = (20 x 300 x 3600) / (4.18 x 20 x 1000) ≈  258litres

·       For 3 minutes, it’s around 155 litres.

So, per module, 200–300 litres of buffer volume is generally sufficient with good turndown and controls.

How It Applies to this 600 kW System

If the total boiler plant is 600 kW and is composed of modular boilers with 5:1 turndown:

·       Assume modules of 100–150 kW each, giving you around 4 to 6 modules.

  • If the minimum plant output is 20 kW, you could get away with 300–500 litres, in theory, just to prevent short cycling.

But in practice:

  • In HIU-based systems, loads can drop quickly.

  • Return temperatures may be low, extending boiler runtime.

  • You want some energy store to buffer against control lag and rapid load fluctuation.

Final Buffer Sizing Recommendation

 

Design Intent Recommended Volume

Prevent cycling at 20kW min load (5:1) 300 to 500 litres

Improved resilience, HIU smoothing 1,000 to 2,000 litres

Include energy store for short over-peak 3,000 to 5,000 litres