TradeCalculator.co.uk
System Bypass Calculator — ABV Settings
Calculate automatic bypass valve settings, minimum boiler flow rate protection, and ΔP settings for heating systems with TRVs.
Boiler rated heating output
Design flow-return temperature difference
Total TRVs on system
Minimum flow as % of total. ~30% is a rough default — use the actual l/min figure from your boiler's install manual
Pump maximum head in metres (a typical domestic pump is 4–6 m ≈ 0.4–0.6 bar)
Next up: related pages
How We Calculate This
This calculator determines the bypass requirements for a heating system with TRVs. It calculates the minimum flow rate the boiler needs for safe operation and the correct ABV pressure differential setting.
Method
- System flow (hydronic heat balance, CIBSE Guide G): (kW × 60) ÷ (4.186 × ΔT) L/min, where 4.186 kJ/kgK is the specific heat of water
- Minimum flow: taken as a default ~30% of total system flow. This is only a rough estimate — the real minimum circulation flow is boiler-specific, so use the l/min figure in your boiler’s installation manual
- Bypass ΔP (rule of thumb, not a standard): a starting point of roughly half the pump maximum head, using 1 m water ≈ 0.098 bar. Always fine-tune on commissioning per the ABV maker’s instructions (ESI / Banico / Caleffi) — open until the bypass leg just warms with TRVs closed, then back off a quarter turn
- Pipe size: sized on the bypass flow rate
The flow formula is standard CIBSE/hydronic engineering. The bypass ΔP starting value is a practical rule of thumb, not a figure from a British Standard — the authoritative procedure is to commission the valve to the actual system, keeping the setting below the pump’s maximum head so it can still open.
Frequently Asked Questions
When thermostatic radiator valves (TRVs) close as rooms reach temperature, the flow through the system reduces. If all TRVs close simultaneously, the boiler has no water flow — causing it to overheat and lock out. A bypass provides a path for minimum water flow to protect the boiler. Use our TRV Sizing Calculator to correctly size the thermostatic valves.
An ABV is a pressure-operated valve that opens when the system differential pressure increases (as TRVs close). It automatically passes enough flow to protect the boiler. It’s installed between the flow and return pipes, typically near the boiler. The ΔP setting determines when it starts to open.
Install the bypass between the main flow and return pipes, immediately after the pump and as close to the boiler as practical, on the last circuit — not before the radiators. Use full-bore isolation valves either side for servicing. As a conservative rule of thumb the bypass is often run in the same size as the boiler tails (commonly 22mm), but it only has to carry the minimum bypass flow, so a smaller pipe sized on that flow rate is acceptable. Always check the boiler manufacturer’s instructions for any specified bypass arrangement.
As a starting point, set the ABV to roughly half the pump maximum head. Pressure converts at about 1 metre of head ≈ 0.098 bar, so a 4 m pump (≈ 0.39 bar total) suggests a starting setting of around 0.2 bar — well inside the typical 0.05–0.6 bar ABV adjustable range. Most domestic systems sit comfortably at 0.2–0.3 bar. Then fine-tune on commissioning: with the TRVs and zone valves closed, open the valve until the bypass leg just warms, then back off about a quarter turn. The setting must stay below the pump’s maximum head, otherwise the valve can never open. Too low wastes energy; too high risks the boiler losing flow and overheating.
Many modern combi boilers have an internal bypass valve. Check the manufacturer’s instructions — some combis still require an external bypass, especially on systems with many TRVs. The internal bypass may not have adequate capacity for larger systems. When in doubt, fit an external ABV.
Was this calculator helpful?
Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.