Wet Central Heating Systems
Central Heating · Plumbing
Most homes with a boiler heat their rooms with water. This topic follows that water round the circuit and explains the two common ways a system is kept full.
Water heated at the boiler is pumped out along a flow pipe, gives up its heat in the radiators and comes back cooler along a return pipe. Radiators warm a room mostly by setting up a slow loop of rising warm air and falling cool air.
An older open-vented layout is topped up from a small feed and expansion tank in the loft, and an open vent lets the system breathe. A sealed layout has no tank: it is filled through a temporary filling loop, an expansion vessel takes up the swell as the water warms, and a relief valve guards against too much pressure.
Zone valves and a pump send heat to the radiators, the cylinder or both, and named plans such as Y-plan and S-plan describe how those valves are arranged.
Follow the water round the circuit
Updated
How to reason about the system
Most heating faults make sense once you picture the loop: out hot along the flow, through each radiator, back cooler along the return. Then ask how the system is kept full, from a loft tank or through a sealed vessel and gauge.
Questions to take further
- How can I tell whether my heating is sealed or open-vented?
- A sealed system usually has a pressure gauge on or near the boiler and no small tank in the loft feeding the heating. An open-vented one has that loft tank and no pressure gauge to watch.
Sources
- Skills England: plumbing and domestic heating technician — The systems, materials, planning and customer responsibilities in the England occupational standard.
- HSE: domestic gas safety FAQs — Gas Safe registration and checking the categories of work on an engineer's ID card.
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Lessons on the plumbing path
- How a wet system moves heat
- Open-vented heating
- Sealed heating systems
- Pumps, zone valves and system plans
Terms to know
Before you start: some plumbing work is regulated or notifiable
Some water fittings work needs notice to, and consent from, the water supplier before it starts. Check the rules where you live.
Gas work is for competent workers covered by a Gas Safe registered business in Great Britain. Northern Ireland has separate gas regulations. Do not work on gas pipes or appliances yourself.
In England, installing an unvented hot-water system over 15 litres is notifiable building work: use a competent installer registered with a competent-person scheme, or tell building control before work starts. Leave this work to a suitably skilled person.
Wales, Scotland and Northern Ireland have their own water and building rules; check which apply before work starts. Electrical connections belong with an electrician.
This guide does not qualify you or make you competent, approved or registered for plumbing, gas or unvented work.