Radiators and Heating Controls
Central Heating · Plumbing
Radiators only heat evenly when their valves and controls are set up well. This topic covers the valves on each radiator, the routine jobs of bleeding and balancing, and the controls that run the whole system.
A thermostatic radiator valve senses the air around it and throttles one radiator, while the lockshield at the other end is set once to give that radiator its share of the flow. Where a radiator is cold tells you a lot: cold at the top is usually air, cold along the bottom is usually sludge.
Balancing trims the lockshields so the radiators nearest the boiler stop taking most of the water, judged by the drop in temperature from flow to return. A programmer sets the times, a room thermostat sets the overall temperature, and interlock keeps the boiler off when nothing is asking for heat.
In England, Approved Document L guidance describes the controls expected when a boiler is replaced. Swapping a boiler counts as notifiable building work in England and Wales, either through an installer in a competent person scheme or by telling building control before work begins; Wales publishes its own Part L, and Scotland and Northern Ireland have their own standards.
Read the radiator before touching the controls
Updated
How to reason about the system
Where a radiator is cold points to the fix: air at the top, sludge at the bottom, a valve when it is cold all over, and balancing when only the far rooms lag. Controls then decide when the heat runs and how warm each room gets.
Questions to take further
- Is replacing a boiler notifiable work?
- In England and Wales, yes. An installer registered with a competent person scheme notifies it, or building control is told before work starts. Scotland and Northern Ireland run their own systems, and the gas work goes to a Gas Safe registered engineer.
Sources
- GOV.UK: Approved Document L — Guidance for England on energy efficiency, including heating controls when a boiler is replaced; guidance is not itself the legislation.
- 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
- Radiator valves and TRVs
- Bleeding a radiator
- Balancing a system
- Programmers, thermostats and Part L
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.