Hot water works but the radiators stay cold

Why the hot water works and the heating does not

Hot water at the taps proves the burner still lights. Heat is being made. What fails is the route from the boiler to your radiators.

In a combi boiler that route is set by a diverter valve. Vaillant describes the valve as the part that directs heated water to the radiators or to the taps. Stuck in one position, it leaves only one circuit warm. Glow-worm names this as the likely cause when every hot tap is shut and the heating still will not run.

System and regular boilers work differently. They heat a hot water cylinder and use motorised zone valves instead of a diverter valve, as Viessmann explains. A zone valve that fails to open produces the same picture: hot taps, cold radiators.

What you can safely check yourself

Four checks are yours to make. Everything else on this page belongs to a Gas Safe registered engineer.

• Room thermostat. Set it above the current room temperature. Ideal Heating suggests trying around 20 °C to see whether the boiler responds.
• Programmer or timer. Check the clock and the heating on-times. A power cut can wipe the settings, so the heating never gets asked for. Worcester Bosch sets out the difference between a timer, a programmer and a programmable room thermostat.
• Pressure gauge. Read it with the system cold and compare it with the figure in your boiler manual.
• Reset button. Press it once, following the steps in your boiler manual. Note any fault code on the display first.
• Bleeding radiators. If a radiator is cold at the top and warm at the bottom, air is trapped. Switch the heating off and let it cool before you use the bleed key.

If none of that changes anything, stop there. Opening the boiler casing, working on the flue or touching gas pipework is not a householder job.

Does the pressure gauge explain it?

Low system pressure can stop the heating on its own. Viessmann puts the usual cold reading between 1 and 1.5 bar. Your boiler manual gives the figure that applies to your appliance, and that figure wins.

Topping up now and then is normal. Vaillant warns that pressure dropping again soon after the filling loop is closed points to a leak or a faulty pressure relief valve. Repeated topping up only masks that. Our guide to low boiler pressure goes through what the needle is telling you.

Is the flow temperature set too low?

Flow temperature is the temperature of the water the boiler sends out to the radiators. Glow-worm puts roughly 60 to 70 °C as the working range for heating. Set well below that, radiators feel lukewarm rather than properly hot.

Most combi boilers have one control for hot water and a separate one for heating. That is why the taps can stay hot while the heating dial has been turned right down. Check your boiler manual before you change either setting.

Cold radiators while the boiler runs: air, sludge or valves

Where a radiator is cold tells you a lot. Cold at the top with a warm bottom means trapped air, and bleeding usually clears it. Cold at the bottom with a hot top points the other way, towards sludge settling in the radiator. Our guide on a radiator hot at the top and cold at the bottom covers that pattern.

Sludge is magnetite, a black iron oxide that builds up inside a heating system. Baxi lists the warning signs, including cold spots and blocked pipework, and notes the damage it does to components such as the pump and heat exchanger. Clearing it properly needs specialist equipment, not a DIY afternoon. Power flush or chemical flush explains which treatment suits which system.

A single cold radiator is often a valve. Viessmann describes the pin inside a thermostatic radiator valve seizing in the closed position, which keeps that radiator cold while the rest of the house warms up. An engineer can free or replace it.

When it has to be a Gas Safe registered engineer

In England, work on gas appliances may only be carried out by a business registered under the Gas Safety (Installation and Use) Regulations 1998, as amended. HSE states that this registration is a legal requirement and has to be renewed each year. Diverter valves, zone valves, pumps and heat exchangers all sit inside that boundary.

Ask to see the engineer's registration card when they arrive. HSE explains that the back of the card lists the categories of gas work the person is qualified to do. Check that boilers are on it.

If the house stays cold and someone in it is vulnerable, your energy supplier's Priority Services Register is worth knowing about. Ofgem describes the extra support it offers, including alternative heating and cooking facilities when a supply is interrupted.

Getting it looked at

FM Power is Gas Safe registered and works across London and the surrounding counties of Essex, Kent, Surrey, Hertfordshire and Middlesex. Appointments run seven days a week, including evenings and weekends. There is no monthly contract to sign.

You get a written fixed price before any work starts, so you know what the repair involves before you agree to it. The reverse problem is covered in our guide on no hot water but the heating still works. If your radiators are still cold after the checks above, our boiler service and repairs page shows how to arrange a visit.

Sources

• Glow-worm – Why is my boiler giving me hot water but no heating?
• Vaillant – How to fix a boiler diverter valve
• Viessmann – How to fix a diverter valve
• Ideal Heating – Why is my boiler not working?
• Worcester Bosch – Timers, programmers and programmable room thermostats
• Viessmann – How to read a boiler pressure gauge
• Vaillant – How to repressurise a boiler
• Baxi – How do I know if I've got sludge in my central heating system?
• Viessmann – Why is my radiator cold?
• Worcester Bosch – How to thaw a frozen boiler condensate pipe
• HSE – Gas Safe Register
• HSE – Check an engineer: are they Gas Safe registered?
• legislation.gov.uk – Gas Safety (Installation and Use) Regulations 1998, Regulation 36
• Ofgem – Join your supplier's Priority Services Register