Automatic bypass valves on central heating: what they do and why

Somewhere near the boiler or the pump on many wet central heating systems there is a small brass valve linking the flow pipe to the return pipe, often with a numbered knob on top. That is usually an automatic bypass valve, sometimes shortened to ABV.

This guide explains what it does, why it is often there, and the signs that it may be set wrongly. It does not give a method for adjusting or replacing one. If you are not sure, that is a job for a heating engineer, and anything at the boiler itself is for a Gas Safe registered engineer.

The problem a bypass solves

A heating system needs water to move through the boiler while the boiler is firing. The circulator pump pushes that water round the radiators and, on many systems, the hot water cylinder coil.

Now imagine most of the radiators have thermostatic radiator valves. As rooms warm up, the TRVs close. At the same time, zone valves may shut off the heating or hot water circuit. If enough valves close, the pump is pushing against an almost closed system. Water barely moves through the boiler, pressure across the pump rises, and the system can get noisy or work poorly.

What an automatic bypass valve does

An automatic bypass valve is a spring-loaded valve on a short pipe between the flow and the return. While the radiators are open, the spring keeps it shut and water goes round the system as normal. As valves close and the pressure difference between flow and return rises, the bypass starts to open and lets some water go straight back to the return.

That keeps a minimum flow moving through the boiler and pump, and takes the strain off when most of the system is shut. The knob on top sets the point at which it starts to open.

It is called automatic because it opens and closes by itself in response to the system. An older approach was a fixed bypass, a permanently open pipe or one radiator left without a TRV, which lets some water round all the time.

Why one is not always there

Whether a system needs an external bypass depends on the boiler and the design. Some boilers have a bypass built in. Others ask for one in the installation instructions. Some systems use a radiator with no TRV, often in the bathroom or the room with the main thermostat, to do a similar job.

So do not assume a system is wrong because you cannot find a bypass valve. And do not add a TRV to the one radiator that was deliberately left without one until a heating engineer has confirmed the system does not rely on it.

Signs the bypass may be wrong

None of these prove the bypass is at fault, but they are worth mentioning to an engineer:

• Whistling, rushing or humming in pipes when most radiators are warm and their TRVs have closed. Noisy pipes and water hammer covers other causes.
• Radiators slow to warm while the pipe through the bypass gets hot. A bypass set too low can send a lot of water straight back, starving the radiators.
• The boiler switching on and off often when the house is nearly up to temperature.
• A bypass that has been changed. If someone has turned the knob, removed the valve or capped the pipe, tell the engineer.

Sludge can also make a bypass stick, open or shut. A system that is dirty enough to block valves usually has other symptoms; see heating inhibitor and sludge.

What you can safely do

• Find it and note its setting. A photo of the valve and the number on the knob is useful to an engineer.
• Leave the knob alone unless an engineer has told you otherwise. The right setting depends on the boiler, the pump and the system.
• Do not remove a valve, pipe or cap from the heating system. It is pressurised or full of water, and draining and refilling it properly is part of the engineer's job.
• Do not open the boiler or touch anything inside it. That is for a Gas Safe registered engineer.

Setting or replacing one is a heating engineer's job

Adjusting a bypass properly means looking at the boiler maker's requirements, the pump setting and how the system behaves with valves open and closed. Replacing one means isolating or draining part of the system, fitting the valve the right way round, refilling, venting and putting back inhibitor. If you are not sure, it is a heating engineer's job.

If the work involves the boiler, its controls or anything near its gas supply, the engineer must be Gas Safe registered. Our guide to hiring a plumber covers what to ask before booking.

Do I need an automatic bypass valve if all my radiators have TRVs?

Many systems with TRVs on most radiators have some form of bypass, either a separate valve, one built into the boiler, or a radiator left open. Which your system needs depends on the boiler maker's instructions and the design, so ask a heating engineer.

Can I turn the knob on the bypass valve to stop the noise?

We would not. A wrong setting can starve the radiators or let the boiler run with too little flow. Note the noise and the setting and ask a heating engineer to check it.

Is a bypass valve the same as a zone valve?

No. A zone valve opens and closes to send heat to the heating or the hot water when the controls call for it. A bypass valve opens by itself to keep a minimum flow when other valves have shut.