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Zone valves

Checked against its sources: Updated

A zone valve in central heating is a motorised valve that opens or blocks hot water to one part of the system, so areas can be heated separately.1 Zoning cannot be done from the boiler alone, and fitting needs a qualified engineer.1

Covers
Wet central heating, UK. The building regulations points are England only
You
Check settings and radiator behaviour, as below. No fitting, wiring, or dismantling.
A professional
Fitting, wiring, testing the manual lever, repairing leaks, and replacing the valve or its thermostat.

1 What a zone is

Viessmann's example is upstairs and downstairs, each heated at its own time, and it also suggests leaving out warmer rooms such as a kitchen1. When the thermostat for a zone says the room is warm enough, the valve closes and hot water stops flowing to those radiators. As the room cools, it opens again1.

This differs from a thermostatic radiator valve, which controls just one radiator, and from a lockshield, which sets flow for balancing.

2 Names you may see

Worcester Bosch's wiring guide for engineers labels layouts by valve. S-Plan uses two 2-port valves, S-Plan Plus uses three, and Y-Plan uses one mid-position valve2. That document is only cited for these names, since wiring is not a homeowner task.

3 A check you can do

Viessmann suggests this test on its web page for a valve that may be stuck, using only the radiator controls. It is not from a boiler user manual. Radiators will be hot, so touch them carefully.

  1. With the heating on, turn all thermostatic valves in the zone up to full, usually number five (Viessmann)1
  2. Wait 30 minutes, then check each radiator by hand (Viessmann)1
  3. If all stay cold, the zone valve may be stuck closed. If only some do, the fault is probably at those radiator valves (Viessmann)1
  4. To test for stuck open, turn them to zero or the snowflake, wait about 30 minutes and check. If all stay warm, the valve is the likely cause (Viessmann)1

Check the programmer and room thermostat first, since a wrong setting can look like a faulty valve. Viessmann says most valves have a manual lever that can help with testing1; leave that to the engineer.

4 Common faults

Viessmann names three: a valve stuck open or closed, a faulty thermostat reading the wrong temperature, and a leak where the rubber seal has worn1. Any of these may mean a new seal or valve, fitted by a suitably qualified engineer. For when electrical work has to be notified, see Part P.

5 The rules in England

Approved Document L says new homes with a floor area of 150 m² or more, with wet heating, should have at least two independently controlled heating circuits3. When a boiler is replaced, rooms should have thermostatic control, and it can be justified to use zones in cases such as open-plan single-storey homes4.

Sources

  1. Adding a zone, Viessmann UK read 3 November 2026.
  2. Worcester wiring guide, Greenstar gas boilers engineer document, valve labels only, read 3 November 2026.
  3. Approved Document L, 2021 edition, para 5.14 read 3 November 2026.
  4. Approved Document L, paras 5.20 and 5.21 read 3 November 2026.