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 this calculator always applies the run-time check, including for GSHP:
the sizing guide's own methodology (Step 3) states the run-time formula
“guards against short-cycling independently of heat pump type” and that you should
“always apply both steps 2 and 3 — use whichever gives the larger volume.”
The guide's illustrative reference table, however, recommends GSHP tanks (e.g. HB-100 for a 10 kW
GSHP) using only the 10 L/kW figure, without applying the run-time check even where it is larger.
This calculator follows the stated step-by-step rule rather than the table's shortcut, because the
guide separately warns “never size down” — so for higher-output GSHP systems this
tool may recommend a larger tank (or a custom order) than the reference table's GSHP row. Confirm
with the Heatlyt technical team for a specification you plan to order.
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 | GSHP to 10 kW; ASHP to 4 kW (mild climates) |
| HB-150 | 150 L | ASHP 4–5 kW; GSHP to 15 kW |
| HB-200 | 200 L | ASHP to 6–8 kW; GSHP to 20 kW |
| 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.