Heating
Hot water but no heating — what to check first
Hot taps and cold radiators is one of the calls we take most often between October and March. Four of the causes you can check yourself in ten minutes. The rest are inside the boiler, and those are ours.
Written by the ARA Services engineering team · Updated August 2026
The short answer
If the hot taps still run hot, the boiler is making heat — the problem is on the heating circuit. In the order we usually find it: the programmer or room thermostat is not calling for heat, system pressure has dropped too low, a radiator valve is shut or the system is full of air, or the condensate pipe has frozen. If all four are fine, the fault is inside the boiler — normally the diverter valve or the pump — and that is a job for a Gas Safe engineer.
1. Start at the controls, not the boiler
More cold-radiator calls come down to a control than to a broken part. Most boilers have a hot-water-only mode that is useful in summer and forgotten about in October, and any power cut can leave a programmer running on the wrong time or the wrong day.
Check the programmer is set to heating as well as hot water, and that the clock and the day are right.
Turn the room thermostat well above the room temperature and listen. If the boiler fires within a minute or so, the controls are talking to it.
Change the batteries in a wireless thermostat. A weak battery will still light the screen but stop it sending the call for heat.
With a smart thermostat, check it is still paired to its receiver. A router reboot or a power cut can drop the connection without any obvious sign.
Not sure what your controls are telling you? Describe it on the phone — advice costs nothing.
077 2715 47462. Read the pressure gauge
A sealed system needs pressure to push water round the radiators. Most combi and system boilers want roughly 1 to 1.5 bar when the system is cold, and many will shut the heating down below about 1 bar while still heating the taps — which is exactly the symptom you are looking at.
If the needle is in the red at the bottom, the system can be topped up through the filling loop. Your boiler manual sets out how to do it for that model, and it is worth reading before you touch anything: over-filling brings its own problems and the pressure relief valve will dump the excess outside.
One thing worth knowing. Pressure that falls again within days is not a top-up job. The water is going somewhere — usually a weeping radiator valve, a joint under a floor, or the expansion vessel losing its charge. Repeated topping up hides the leak and corrodes the system from the inside.
Pressure dropping every few days? We trace the leak rather than keep filling the system.
Leak detection and repair3. Work along the radiators
Thermostatic valves that have sat open all summer often seize shut the first time they are asked to close. Take the white head off the valve and look at the pin underneath: it should move freely. A gentle tap or a pair of pliers used carefully will usually free it.
Where the radiator gets warm tells you what is wrong. Cold at the top and warm at the bottom is trapped air, and bleeding it will fix it. Warm at the top and cold at the bottom is sludge sitting in the radiator, which needs the system cleaning rather than bleeding. Whole floors cold while others are hot points at balancing or a tired pump.
Cold at the bottom, or half the house not heating? That is a cleaning and balancing job.
Powerflushing and heating repairs4. In a cold snap, check the condensate pipe
Every modern boiler drains a small amount of acidic water through a plastic pipe, and where that pipe runs outside to a drain it can freeze. The boiler then locks out, usually showing a fault code, and it tends to happen after the first night below zero rather than in the middle of winter.
Pour warm water — not boiling — along the outside length of the pipe, particularly at bends and at the open end, then reset the boiler. If it thaws and runs, that was the fault. If it freezes again the following winter, the pipe wants lagging, re-routing or terminating internally, which is a short job we do while we are there.
Boiler locked out and showing a fault code? Read us the code and we will tell you what it means.
Call 077 2715 47465. When the fault is inside the boiler
If the controls, the pressure, the radiators and the condensate are all in order, the fault is behind the case. These are the four we find most, and none of them is a job for anyone but a registered engineer.
Diverter valve
On a combi, this valve decides whether the heat goes to the taps or the radiators, and it defaults to the taps. Stuck in that position you get exactly what you are seeing. It is the most common cause on boilers past eight or ten years old.
Circulation pump
A pump that has seized or lost its impeller will let the boiler fire without moving any water round the house. Often audible as a hum with no flow, and sometimes freeable, but usually replaced.
Motorised valve
On a system or heat-only boiler with a cylinder, a two-port or mid-position valve splits the flow. When the head fails it commonly parks on hot water, leaving the cylinder hot and the radiators cold.
Sludge and blockages
Years of corrosion settle in the heat exchanger and the bottom of radiators and choke the flow. The cure is chemical cleaning or a powerflush, plus a magnetic filter to stop it returning.
Do not take the case off a gas appliance. If you can smell gas, call the National Gas Emergency Service on 0800 111 999 before you call us.
What happens when we come out
The engineer tests the circuit rather than guessing at it: controls, pressure, flow and return temperatures, then the valve and pump. You get the diagnosis and a fixed price for the whole job before anything is dismantled, and the van carries the diverter valves, pumps and thermostats that fail most often, so most of these calls are finished on the first visit.
Cold radiators and out of things to check? We attend across every London borough, 24 hours a day.
Book an engineerQuestions we get asked on this call
Is it safe to keep using the hot water?
Yes, unless the boiler is leaking, making an unfamiliar noise or there is any smell of gas. A stuck diverter valve or a low-pressure lockout does not make the appliance dangerous, but it is worth getting looked at before it fails the other way and leaves you with neither.
Can a diverter valve be repaired instead of replaced?
Sometimes the actuator is at fault rather than the valve body, and that alone can be changed. On older boilers where the valve has worn, replacing it is the repair that lasts.
My boiler is fifteen years old. Is it worth repairing?
We quote the repair either way, and if the boiler is close to the end of its life we set out what a replacement would cost alongside it, so you can compare the two before deciding.
I am a landlord and my tenant has no heating. How fast can you attend?
Loss of heating is an emergency call-out and we attend around the clock, usually the same day. We can arrange access with the tenant directly and invoice the agency or the landlord.
Get the heating back on
Tell us the postcode and what the boiler is doing. The engineer diagnoses the fault and quotes the whole job before starting.