A buffer tank is sized for a specific operating problem. The useful volume depends on surplus output and usable temperature swing, not on floor area alone.
1. Define the operating problem first
Record which heat source cycles, which zones are open, the outdoor condition and the observed on and off times. A buffer can lengthen a short cycle, but it cannot correct every control, flow or commissioning fault.
Check minimum flow, sensor placement, pump operation, bypasses and control settings before assuming that more water is the answer.
2. Calculate only the surplus heat
During a cycle, the building and active emitters continue to accept heat. Storage absorbs the difference between delivered heat-source output and the smallest active load.
Use output at the actual water and outdoor condition. A modulating source may have a much lower minimum output than its nameplate maximum, while a fixed-output source may not.
3. Choose a genuinely usable temperature swing
The theoretical tank range is not automatically usable. The lower limit must still serve the emitters, and the upper limit must remain compatible with the source, controls, materials and safety devices.
A larger usable swing stores more heat in the same volume, but it can affect comfort, efficiency and control behaviour. Document the temperatures behind the number.
4. Credit only water that participates
Existing pipework, heat exchangers, emitters and vessels may contribute water volume, but only when that water is hydraulically connected during the short-cycle condition.
Tank connection arrangement matters. A volume that is bypassed or poorly mixed may not provide the assumed storage, while stratification can change useful operation in either direction.
5. Verify the complete hydraulic design
After choosing a preliminary volume, check manufacturer minimum system volume, minimum flow, pump duties, hydraulic separation, expansion capacity, relief protection, insulation and sensor locations.
Commission the system and log cycle time, supply and return temperatures, tank temperatures and active zones. Adjust controls only within approved limits and compare the result with the original problem.
Inputs that change useful buffer volume
| Input | Why it matters | Evidence to use |
|---|---|---|
| Source output | Sets heat entering the circuit | Performance data at the operating condition |
| Minimum active load | Reduces the heat sent to storage | Room loads and active emitter output |
| Minimum run time | Sets the storage interval | Manufacturer guidance and observed cycling |
| Usable temperature swing | Sets energy stored per litre | Control and emitter temperature limits |
| Participating water | Reduces added tank volume | Measured or documented connected volume |
Worked example
Screen a source that cycles on one small zone
A 12 kW source serves a 4 kW active load. The target run is 20 minutes, the usable swing is 10°C and 80 L already participates.
Surplus = 12 − 4 = 8 kW Stored heat = 8 × 20 / 60 = 2.67 kWh Required water = 2.67 × 3,600 / (4.186 × 10) = 229 L Additional volume = 229 − 80 = 149 L
About 149 L is an early thermal-volume estimate. The final tank and connection layout still depend on equipment limits, flow, controls, expansion and safety design.
What to have ready
- Observed cycle times and active zones
- Heat-source output and minimum modulation at the operating condition
- Usable supply and return temperature limits
- Participating system water volume and proposed hydraulic arrangement
- Manufacturer, expansion and safety requirements
Common mistakes
- Sizing a tank from building floor area alone.
- Using full source output instead of the surplus above active load.
- Claiming a large temperature swing that the emitters or controls cannot use.
- Counting isolated or bypassed water as active storage.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.