Heating works but no hot water? Causes and checks

Some causes are easy to fix, like a timer setting or low pressure. Others are parts inside the boiler. If the checks below don't help, you'll need a boiler repair from a Gas Safe engineer. This guide is part of our common boiler problems series. The reverse problem, hot water but no heating, is covered further down.

Which type of boiler do you have?

The causes depend on how your home gets its hot water. A combi boiler heats water straight from the mains, while a system or regular boiler heats a tank called a cylinder. If you have a big tank in the airing cupboard, you have a cylinder.

A regular boiler also has a cold water tank in the loft.

What can I check before calling an engineer?

Start with the pressure gauge, any fault code, the hot water settings and the timer. These simple checks sometimes get the hot water back without a callout. Then try other hot taps, to see if the problem is the boiler or one tap.

• The pressure gauge: if it's below 1 bar, see our guide to a boiler losing pressure.
• A fault code or a flashing light: write it down with the boiler model. Your manual explains what it means.
• The hot water setting: make sure hot water is switched on, and the temperature isn't turned right down.
• The timer: on a system with a cylinder, the hot water has its own timer. Check it's on, and the clock is right after a power cut.
• Other hot taps: if only one tap runs cold, the problem may be the tap, not the boiler.
• A stronger flow: open the tap fully if the water stays cold. Some combis need a strong enough flow before they fire up for hot water.
• The heating: if it has stopped too and it's freezing outside, see our guide to a frozen condensate pipe. It stops heating and hot water together.
• One reset: if there's a reset button, try it once. If the fault comes back, call an engineer.

Why does my combi boiler have heating but no hot water?

On a combi boiler, the most common cause is a stuck diverter valve. This is the part that switches hot water between your radiators and your taps. A faulty flow sensor, limescale or low pressure can also stop the hot water.

• The diverter valve. When you turn on a hot tap, it should switch the hot water to the taps. If it sticks, the radiators get hot, but the taps stay cold.
• The flow sensor. This tells the boiler that a hot tap is running. If it fails, the boiler doesn't know to heat the water.
• The plate heat exchanger. This is the part that passes heat into your tap water. Limescale can build up inside it. Then the water may only get warm, or the boiler may cut out.
• Low pressure. Many combis stop making hot water when the pressure falls too low.

The first three parts are inside the boiler case. Only a Gas Safe engineer should look at them.

Why is my hot water cylinder not heating up?

If the heating works but the cylinder stays cold, check the hot water timer and the cylinder thermostat first.A stuck motorised valve is another common cause. Many cylinders have an immersion heater you can use until it's fixed.

• The timer or settings. Check the hot water timer is on. Check the thermostat on the cylinder isn't turned right down.
• A motorised valve. This is a valve that opens to send hot water from the boiler to the cylinder, the radiators, or both. If it sticks, the radiators may heat up while the cylinder stays cold.
• An immersion heater as a backup. It's an electric heater inside the tank, with its own switch nearby. You can use it for hot water until the fault is fixed.

Why is my hot water only lukewarm?

Lukewarm hot water often means the boiler's hot water setting is too low. On a combi, hot water can come out cooler if the tap is fully open. The boiler can only heat so much water at once.

First, check the hot water temperature on the boiler. On a combi, try running the tap a bit slower. If the water stays lukewarm, limescale or a worn part may be to blame.

Why do I have hot water but no heating?

If your taps run hot but the radiators stay cold, check the thermostat, the heating timer and the radiator valves first. If those are fine, a stuck valve or a faulty pump is a common cause, and that needs an engineer.

Try these checks first.

• The thermostat: turn it up a few degrees above the room temperature. If it runs on batteries, change them.
• The heating timer: check the heating is set to come on, and the clock is right.
• Radiator valves: check the valves on the radiators are open.
• The pressure: check the gauge reads about 1 to 1.5 bar with the heating off.

If those are fine, common causes are a stuck motorised valve, a faulty pump or a diverter valve stuck the other way. The pump is what pushes hot water round your radiators. We repair and replace heating pumps and valves. See our boiler repair page. Our guide to cold radiators covers more checks.

What if there's no heating and no hot water at all?

If nothing works, check the power to the boiler, the gas supply and the pressure gauge first. On a freezing day, a frozen condensate pipe is a common cause.

See our guide to a frozen condensate pipe for how to thaw it. Our common boiler problems page lists the other first things to check.

When to call a Gas Safe engineer

Call a Gas Safe engineer if the checks above haven't helped, or the same fault code comes back after a reset.The diverter valve, flow sensor and heat exchanger are inside the boiler case, so only an engineer should work on them.

• the checks above haven't helped
• the boiler shows the same fault code again after a reset
• you can hear the boiler fire up, but the taps stay cold
• the hot water keeps going hot and cold

The engineer first runs a thorough check of the boiler. That finds the fault and the parts needed. See our boiler repair page for the faults we fix.

A yearly boiler service can spot worn parts early, so they're less likely to fail in the middle of winter.

Other boiler guides

• Boiler losing pressure: what the gauge should read and how to top it up.
• Frozen condensate pipe: how to thaw it safely.
• Cold radiators: what cold at the top or bottom means.
• Leaking boiler: what to do first, and where the water is coming from.
• All common boiler problems.

Still no hot water? Book a boiler repair and we'll find the cause.