Step 1: Check the basics
Start with the simple things. Check that the thermostat is set above the room temperature and that the timer is on. Replace the batteries in the thermostat if it has them, because flat batteries are a common cause.
Make sure the boiler has power, because a tripped switch is easy to miss. Look at the fused spur switch near it. Also check that the gas supply is on, although you should not open up the boiler yourself.
Step 2: Is the boiler firing?
Listen for the boiler starting when you turn up the heating. Some boilers have a display that shows a fault code. Look up the code in your manual, since it often points to the problem.
If the boiler does not light, try the reset button once, since a single reset is safe. Do not keep pressing it. If the fault returns, stop and call a Gas Safe registered engineer.
Step 3: Boiler pressure and the filling loop
Check the pressure gauge. Many boilers need about 1 to 1.5 bar when cold, but your manual gives the right figure. Low pressure can stop a boiler from firing.
If the pressure is low, you can top it up using the filling loop. Follow the maker’s steps and close the valves when done. Our guide to how to bleed a radiator also explains pressure after bleeding.
Step 4: Are any radiators heating?
Feel each radiator, because the pattern of cold spots tells you a lot. If some are warm and others are cold, the system may have air or sludge in it. A radiator that is cold at the top needs bleeding, while one that is cold at the bottom may have sludge.
If every radiator is cold but the boiler runs, water may not be moving round the system. That points to a pump or valve fault. In that case, you should call a heating engineer.
Step 5: The pump and the diverter valve
The pump pushes hot water round the pipes, so a weak pump leaves radiators cold. If it fails, the boiler may heat up but the radiators stay cold. You may hear no hum from the pump, or feel no warmth in the pipes leaving the boiler.
A diverter valve switches heat between hot water and radiators. If it sticks, you may get hot water but no heating. Both are jobs for an engineer, since they need specialist tools.
Step 6: Check the condensate pipe
A condensing boiler drains acid water through a small plastic pipe. In freezing weather, this pipe can freeze and block. The boiler then shuts down for safety.
If the pipe runs outside, check whether it is frozen. Pour warm, not boiling, water along it. Never use boiling water, because it can crack the pipe.
When you need a plumber
Call a heating engineer if the boiler keeps locking out or the pressure keeps dropping, because these faults need testing. The same goes for leaks, strange noises or fault codes you cannot clear. These faults need testing, and some are unsafe to fix yourself.
You can also ask for help through our site. We take a free quote request and pass it on to a local engineer.
Gas safety note
If you smell gas, do not touch any switches. Open the windows and leave the house. Then call the National Gas Emergency Service on 0800 111 999.
Only a Gas Safe registered engineer should work on a gas boiler. Ask to see their ID card first.
Related guides and local help
If you want to know more, read How to Bleed a Radiator: A Simple Step-by-Step UK Guide. You can also read Boiler Service: What Happens, How Often and Why It Matters and How to Drain and Flush a Central Heating System at Home, because they cover closely related problems.
These guides help too: Boiler Pressure: Normal Range, Low Fixes and High Pressure, New Boiler Guide: How to Choose the Right Boiler for You, When to Replace Drain Pipes: Signs, Lifespan and What to Do, Powerflush Cost Guide: What Affects the Price You Pay and Victorian and Edwardian Drainage: Common Problems to Expect. Each one explains a related topic in plain English.
Need help near you? Read our powerflush page first, or see Powerflush in Antrim and Powerflush in Arbroath. You can also see Plumbing Repairs in Caerphilly, since every service page has a free quote form. We pass each request to a local drainage engineer.
