Why does a heating system need a bypass?

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.

What is an automatic bypass valve (ABV)?

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.

Where should I install the bypass?

Install the bypass between the main flow and return pipes, as close to the boiler as possible but after the pump. It should be on the last circuit — not before the radiators. Use full-bore isolation valves either side for servicing. The bypass pipe should be the same size as the main flow pipe.

What ΔP setting should the ABV be set to?

Set the ABV to approximately 25% of the pump maximum head. For a pump with 4m head, set the bypass to about 1.0 bar ΔP. This ensures the bypass only opens when significant flow restriction occurs (TRVs closing). Too low a setting wastes energy; too high risks boiler overheating.

Do combi boilers need an external bypass?

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.

TradeCalculator

System Bypass Calculator — ABV Settings

Calculate automatic bypass valve settings, minimum boiler flow rate protection, and ΔP settings for heating systems with TRVs.

Boiler Type

Type of boiler installed

Boiler Output

Boiler rated heating output

System ΔT

Design flow-return temperature difference

Number of TRVs

Total TRVs on system

Bypass Type

Type of bypass arrangement

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: kW ÷ (4.186 × ΔT × 60) L/min
Minimum flow: ~30% of total system flow
Bypass ΔP: ~25% of pump maximum head
Pipe size: Based on bypass flow rate

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.