Central Heating Not Working but Hot Water Is? What It Means and What to Check

If your central heating is not working but the hot water is, the boiler itself is almost certainly lighting and burning gas properly, because it is heating your taps. The fault sits somewhere between the boiler and the radiators: the controls are not asking for heat, a valve is not sending the hot water round the heating circuit, or the radiators cannot take it.

That is good news in one sense. It narrows things down a lot, and a few of the causes are things you can check yourself in five minutes. Below is the order our engineers work through it on repair visits across Deal, Dover, Sandwich and Thanet, starting with the simple stuff.

Why is my central heating not working when the hot water is?

A boiler has two jobs and switches between them. When you run a hot tap, it heats water for the tap. When the thermostat calls for heat, it heats water and pumps it round the radiators. If one job works and the other does not, the burner, the gas supply and the flue are doing their bit. What is failing is the part that decides where the hot water goes, or the part that tells the boiler the house is cold.

The most common causes we find, roughly in order of how often we see them:

1. The programmer or thermostat is not calling for heat
2. A wireless thermostat has flat batteries or has lost its connection to the boiler
3. The diverter valve (combi boilers) is stuck on hot water
4. A motorised zone valve (system and regular boilers) has failed on the heating side
5. The circulating pump has failed, or there is air or sludge in the system

Start with the controls

A good share of “no heating” calls in the first cold week of autumn turn out to be settings. Heating is often switched off for the summer and the controls have been sitting there for months.

• Check the programmer is set to heating, not hot water only. Many timers have separate channels for heating and hot water, and it is easy to leave heating on “off” after the summer.
• Check the clock and the on periods. A power cut can reset the time, so the heating comes on at 3am instead of 7am.
• Turn the room thermostat right up for a few minutes. You should hear a click and the boiler should fire. If the room stat is set to 18°C and the hallway where it lives is already 19°C, nothing will happen.
• Change the batteries in a wireless thermostat. Flat batteries are one of the most common causes we see. Some receivers also lose their pairing after a power cut, and the manual for your stat will show you how to pair it again.

If you are not sure whether the problem is the stat or the boiler, our guide on thermostat faults and heating controls walks through how to tell them apart.

Check the boiler display and pressure

Have a look at the front of the boiler. If there is a fault code showing, write it down, as it tells the engineer a lot before they arrive. You can look up what it means in our boiler fault codes library, which covers the brands we see most in East Kent, including the Glow-worm fault codes that come up on a lot of local boilers.

Check the pressure gauge too. A sealed system should normally read about 1 to 1.5 bar when cold. Low pressure usually stops both heating and hot water, but it is worth ruling out. If it has dropped, our guide to a boiler losing pressure explains how to top it up and why it might keep falling.

Combi boiler: hot water but no heating

On a combi boiler, the most likely mechanical cause is the diverter valve. This is a valve inside the boiler that sends the hot water either to your taps or round the radiators. When it sticks or its motor fails, it can stay in the hot water position. The result is exactly what you are seeing: lovely hot taps, cold radiators.

Signs that point to the diverter valve:

• The boiler fires when you turn up the thermostat, but the radiators stay cold
• The radiators only get warm while someone is running a hot tap
• The heating works on and off, then stops altogether

A diverter valve is inside the boiler casing, close to gas and electrical parts, so it is not a DIY job. Replacing it, or the motor head on top of it, must be done by a Gas Safe registered engineer.

System or regular boiler: hot water but no heating

If you have a hot water cylinder in an airing cupboard, you have a system or regular boiler, and the likely suspect is a motorised zone valve. Most homes have one valve for the heating and one for the hot water, or a single three-port valve that shares the job. If the heating valve fails shut, the boiler happily heats the cylinder but nothing goes to the radiators.

You can sometimes spot it. The valve usually sits near the cylinder and has a small motor head with a manual lever on the side. If the heating is calling and the lever moves freely with no resistance, the motor or its internal switch may have failed. Leave the repair to an engineer, but it is useful to mention when you ring.

In older homes with gravity-fed hot water, which we still find in some Victorian and Edwardian terraces around Ramsgate, Margate and Dover, the hot water can circulate by gravity while the heating relies entirely on the pump. In that setup, a failed pump gives you hot water but cold radiators. A pump that has stopped is often warm to the touch but silent, or it hums without turning.

When the boiler fires but the radiators stay cold

Sometimes the boiler clearly starts up on heating, the pipes near it get hot, but the radiators barely warm up. That points to the system rather than the controls:

• Air collects at the top of radiators, leaving them cold at the top and warm at the bottom. Bleeding them usually sorts it, but remember to check the pressure afterwards.
• Sludge settles at the bottom, leaving radiators cold at the bottom and warm at the top. That is a sign the system may need cleaning. Our guide to what a power flush involves explains when it is worth doing.
• Closed valves. If only one or two radiators are cold, check the valves at each end are open. Our article on one radiator not heating up covers balancing and stuck thermostatic valves.

If the opposite is happening in your home, with warm radiators but cold taps, read our guide on heating that works with no hot water instead.

What you can safely do, and where to stop

You can safely check and reset the programmer, change thermostat batteries, top up the pressure using the filling loop, bleed radiators and open radiator valves.

Stop there. Do not take the boiler casing off, do not try to free a diverter valve, and do not work on wiring at the zone valve or pump. Anything inside the boiler must be handled by a Gas Safe registered engineer. If you ever smell gas, turn the gas off at the emergency control valve, open the windows and ring the National Gas Emergency number on 0800 111 999 straight away.

What a repair visit costs

Repairs are charged by the hour: £90+VAT for the first hour, which is always charged, then £45+VAT per half hour after that, plus any parts. Many of these faults are found and fixed within the first hour, especially controls and zone valve faults where the part is on the van. Our pricing page sets out the rates in full.

Members of our BOYLA HomeCare plans pay no call-out fees, and paid plans get emergency response within 24 hours, Monday to Friday. It is also worth having the boiler looked at once a year, as a boiler service often catches a sticking diverter valve or a tired pump before it leaves you cold.

Get your heating back on

If you have worked through the checks above and the radiators are still cold, ring our team on 01304 381999. A real person answers the phone, and we will give you an honest answer on when we can get to you. We have been fixing boilers across East Kent for more than 25 years, and you can book a repair through our boiler breakdown and repairs page too.