Heating won't come on after summer? Safe checks to try first

The first cold evening of autumn, you turn the thermostat up and nothing happens. A heating system that has sat unused since spring often won't start first time, and the cause is usually simple: a timer still on its summer setting, flat thermostat batteries, low system pressure, or a part that has stuck from months of standing still.

This post walks through why heating struggles after a summer off, the checks you can safely make yourself, and the signs that mean it's a job for a Gas Safe registered engineer. You can look at settings, pressure and radiators, but never remove the boiler casing or touch any gas parts.

Why heating struggles after a summer off

Over summer, many homes only use the boiler for hot water, so the heating side can sit still for four or five months.

That rest causes a few common problems. Water pressure can creep down slowly through tiny weeps at valves. Air collects at the top of radiators. Moving parts that rely on regular use, such as the pump and the valves on each radiator, can stick. And settings get changed or forgotten, especially if there's been a power cut or a battery has run flat.

Check the controls first

Many heating problems in October turn out to be settings, so start here.

• Summer mode. Many boilers have a switch or button that turns the heating off and leaves only hot water. Check the front panel or your manual for a radiator symbol and make sure heating is switched on.
• The timer or programmer. This is the clock that tells the heating when to come on. If it was set to hot water only over summer, or lost its settings in a power cut, the heating won't start. Clocks also go back at the end of October, so check the time on any older timer that doesn't change itself.
• The thermostat. Set it a few degrees above the room temperature. If it's wireless or battery powered and the screen is blank or faint, fit new batteries.
• Smart controls. If you use an app, check that a holiday or away mode isn't still on from the summer.

Check the boiler pressure

On most combi and system boilers there's a gauge or a number on the display showing the water pressure in the heating system. With the heating off and cold, it usually sits around 1 to 1.5 bar, but your boiler manual gives the right figure for your model.

If it has dropped below 1 bar, the boiler may refuse to fire to protect itself. Many boilers can be topped up using the filling loop, a short hose or built-in lever underneath, following the steps in the manual. If the pressure drops again within days, there's a leak or a faulty part somewhere, and our guide on what to do when your boiler pressure keeps dropping explains the likely causes.

Check the radiators

If the boiler fires but some radiators stay cold, the problem is usually in the radiators rather than the boiler.

A radiator that's warm at the bottom but cold at the top has trapped air. Letting it out is a simple job with a radiator key, and our step-by-step guide on how to bleed a radiator safely shows you how. Check the pressure again afterwards, as bleeding can lower it.

A radiator that stays completely cold may have a stuck thermostatic radiator valve, or TRV, the numbered knob on the side that controls how hot that radiator gets. The small pin under the head often seizes over summer. Our guide on freeing a stuck TRV pin covers the safe way to do it.

Try a reset, once

If the boiler shows a fault code or a warning light, write the code down and look it up in your manual. Most boilers have a reset button that clears a minor lockout. Press it once and give the boiler a few minutes to go through its start-up.

If it locks out again, don't keep resetting it. Repeated lockouts mean the boiler has found a fault it can't clear on its own.

When it's a job for an engineer

Some faults that show up after summer can't be fixed from the outside of the boiler. The pump, which pushes hot water around the system, can seize after months of not running. The diverter valve, the part in a combi boiler that switches hot water between your taps and your radiators, can stick so you get hot water but no heating. Ignition and sensor faults can also stop the boiler lighting.

All of these sit inside the boiler casing, close to gas and electrical parts, so they need a Gas Safe registered engineer. Book one if:

• the boiler locks out again after one reset
• you hear the boiler trying to light but it never does
• the pump hums or the boiler makes banging or rumbling noises
• the pressure keeps dropping after you've topped it up
• you have hot water but no heating, or the other way round

Signs to treat as an emergency

If you smell gas, turn off the gas at the meter if you can do so safely, open doors and windows, don't use electrical switches, and call the National Gas Emergency number on 0800 111 999.

Switch the boiler off and get it checked if you see a lazy yellow or orange flame instead of a crisp blue one, sooty marks on or around the boiler, or if people in the home get headaches, dizziness or nausea that ease when they go outside. These can be signs of carbon monoxide, a poisonous gas you can't see or smell, and every home with a gas boiler should have a working carbon monoxide alarm.

How to avoid it next autumn

Two small habits help. Run the heating for 15 minutes or so once a month through the summer, so the pump and valves keep moving. And book your boiler service for late summer or early autumn, before the first cold spell, when an engineer can spot a sticking part before you need the heat.

If your heating still won't come on after these checks, TwinSun Heating can help. Our Gas Safe registered engineers will find the cause and explain what's needed to get your heating running again. Book a visit through our boiler repair service, or get ahead of next winter with an annual boiler service.