The rule this tool uses
ASHP minimum = rated output (kW) × 25 L/kW (BS EN 14511 defrost-cycle requirement). GSHP minimum = rated output (kW) × 10 L/kW (cycling-prevention baseline).
Minimum run-time check:
V (L) = Q (kW) × 10 min × 60 ÷ (ΔT × 4.18), where ΔT is the design temperature difference across the buffer (typically 5–10°C).
The recommended volume is the larger of the two results, rounded up to the next available Heatlyt tank — never down.
Full explanation and worked examples in the buffer tank sizing guide.
Your heat pump
ASHP needs defrost-cycle energy (25 L/kW); GSHP does not (10 L/kW).
Use the rated heating capacity at the design condition (A7/W35 for ASHP, W0/W35 for GSHP) from the data sheet.
Typically 5–10°C for heat pump systems. Lower ΔT gives a larger, more conservative result — 5°C is the default.
Result
Minimum buffer volume for your heat pump
–
litres minimum
Enter a heat pump output above.
| L/kW minimum (BS EN 14511) | – |
|---|---|
| Run-time check (10 min, your ΔT) | – |
| Governing figure | – |
| Recommended Heatlyt tank | – |
Why the run-time check applies to GSHP as well as ASHP:
the run-time formula guards against short-cycling independently of heat pump type, so the
buffer tank sizing guide and this
calculator both apply it alongside the L/kW rule and take whichever gives the larger volume —
never the smaller. On outputs from about 8 kW that governing volume passes 200 L, which is
why both put those systems on two HB-200 vessels in series or a custom order rather than any single
standard tank. Confirm with the Heatlyt technical team before placing an order against a specification.
Heatlyt HB buffer tank range
The calculator only recommends standard-catalogue Heatlyt HB tanks. Above 200 L, Heatlyt supplies custom buffer tanks up to 500 L under its OEM programme — contact the team for lead times and a specification sheet.
| Model | Capacity | Typical fit |
|---|---|---|
| HB-100 | 100 L | Governing volume up to 100 L |
| HB-150 | 150 L | Governing volume 101–150 L |
| HB-200 | 200 L | Governing volume 151–200 L |
| Custom OEM order | Up to 500 L | ASHP >6–8 kW, or where the run-time check exceeds 200 L |
Check it by hand: for an 8 kW ASHP at ΔT = 5°C —
L/kW minimum = 8 × 25 = 200 L. Run-time check = 8 × 10 × 60 ÷ (5 × 4.18)
= 4800 ÷ 20.9 = 229.7 L. The larger figure (229.7 L) governs, which exceeds every
standard HB tank, so the result is a custom order — matching the sizing guide's own note that
ASHP installations above 6 kW where the run-time formula exceeds 200 L need a custom tank.