No Hot Water but Heating Works: Troubleshooting Guide
When the radiators are warming up nicely but the taps are still running cold, it usually points to a fault in the hot water side of the system rather than a total boiler failure. That is good news in one sense, because the boiler is at least still firing and circulating heat.
It can still be frustrating, of course. Showers become a problem, washing up turns into a chore, and it is not always obvious whether the issue is a quick settings fix or a repair job. The right starting point is to work out what type of heating system you have, then check the simple things first.
What this usually tells you
If heating works but hot water does not, the boiler is often doing part of its job properly. The missing piece is usually a valve, control, thermostat, sensor, or blocked component that affects domestic hot water only.
That narrows the fault down quite a bit.
With a combi boiler, the usual suspect is the diverter valve. This is the part that switches the boiler between heating the radiators and heating your tap water. If it sticks, the boiler may stay focused on the heating circuit and ignore hot water demand.
With a system boiler or a conventional boiler with a hot water cylinder, the issue is more often linked to a motorised valve, cylinder thermostat, or programmer setting. In those systems, the boiler has to send heat to the cylinder separately from the radiators. If the control side for the cylinder fails, you can end up with warm rooms and cold taps.
The most likely causes by system type
Not every home in Southampton or the New Forest has the same setup, so the likely cause depends on the boiler and hot water arrangement.
| System type | Most likely fault | What it often looks like |
|---|---|---|
| Combi boiler | Stuck or failed diverter valve | Radiators hot, taps cold or only lukewarm |
| Combi boiler | Plate heat exchanger scaled up or blocked | Boiler fires, but hot water is weak or not properly hot |
| System boiler with cylinder | Faulty 2-port or 3-port motorised valve | Heating works, cylinder does not heat up |
| System boiler with cylinder | Cylinder thermostat or hot water controls fault | Hot water never gets called for, even though heating does |
| Conventional boiler | Programmer issue or failed hot water valve | Heating comes on to schedule, hot water does not |
| Conventional boiler | Sludge or scale in cylinder coil or pipework | Poor or inconsistent hot water performance |
Low pressure, frozen condensate pipes, or a gas and power problem can also stop hot water, but those faults often affect heating as well. When only hot water is missing, it is more likely to be a hot-water-only control or flow issue.
Safe checks you can do first
Before touching anything, keep it to user checks only. Do not remove the boiler casing or interfere with gas components. If you smell gas, leave the property and call the gas emergency service straight away.
A few basic checks can save time and sometimes solve the problem without a repair visit.
Programmer and thermostat: make sure hot water is actually set to come on, and that the clock has not reset after a power cut
Cylinder thermostat: if you have a cylinder, check it is turned up to a sensible setting, usually around 60°C
Boiler pressure: look for around 1 to 1.5 bar when the system is cold, unless your boiler manual says otherwise
Reset function: if the boiler is showing a fault code or lockout, try one standard reset using the maker’s instructions
Hot tap flow: check whether water is flowing normally from the hot tap, even if it stays cold
If the pressure is low, topping up with the filling loop may bring the boiler back into normal operation. If it drops again quickly, that points to a leak or another system fault that needs an engineer.
Look at the controls before assuming the boiler is broken
A surprising number of no-hot-water call-outs come down to settings. Timers get knocked. Wireless thermostats lose connection. Hot water channels are turned off without anyone realising. Summer and winter modes can also cause confusion on some systems.
If you have a cylinder, check whether the hot water is scheduled separately from the heating. It is common to find the heating on, but the hot water programme off. On older systems, a faulty programmer can also stick on one channel and ignore the other.
Modern boilers can be very helpful here. If there is an error code on the screen, write it down before resetting anything. It can give a heating engineer a clearer picture of what is happening.
Combi boilers: the classic cause
With a combi, there is no separate hot water cylinder. The boiler heats water on demand when you open a tap. That means the boiler has to detect the water flow and switch itself from heating mode to hot water mode.
If the diverter valve sticks, the boiler may carry on heating the radiators while failing to send heat to the tap water. This is one of the most common reasons for this exact symptom.
Another common issue is scale or debris in the plate heat exchanger. In hard water areas, hot water performance can drop slowly over time. It may start as lukewarm water, then become inconsistent, then fail altogether. If the heating side still works well, that points more strongly to a domestic hot water component than a full system breakdown.
Sometimes the flow sensor also causes trouble. If the boiler does not detect that a tap has been opened, it may not fire for hot water at all.
System and conventional boilers: check the cylinder side
If your home has a hot water cylinder, the problem is often outside the boiler rather than inside it. The boiler may be running perfectly, but the heat is not being directed to the cylinder.
That usually comes back to one of three things: the motorised valve, the cylinder thermostat, or the programmer.
A stuck 2-port or 3-port valve can leave the heating circuit open while keeping the cylinder circuit closed. In plain terms, the boiler sends heat to the radiators and bypasses the stored hot water completely. A failed actuator on the valve can do the same thing.
If the cylinder thermostat is faulty or set too low, the system may never get the signal to heat the stored water. The radiators can still run because they are controlled separately.
Clues that can point to the fault
You do not need to diagnose every part yourself, but a few signs can help narrow things down before you call.
Lukewarm water rather than fully cold water
Radiators heating whenever you run a hot tap
A boiler that fires for heating but not for hot water
Clicking from a motorised valve, but no hot water afterwards
Pressure dropping below the normal range
If you notice the radiators warming up when someone opens a hot tap on a combi system, that is often a strong sign that the diverter valve is not switching properly.
When a blocked system is part of the problem
Not all no-hot-water faults come from electrical or mechanical parts. Sludge, rust, and limescale can also cause trouble.
On combi boilers, scale can restrict the plate heat exchanger. On systems with cylinders, sludge in the primary circuit or scale in the coil can reduce heat transfer badly enough to leave you with poor hot water. In older systems, years of dirty water in the heating circuit can affect valves and sensors as well.
This is one reason powerflushing can be useful in the right situation. It is not a cure for every fault, and it will not fix a broken valve motor or failed thermostat. What it can do is clear sludge and debris from the system, improve circulation, and help prevent repeat problems linked to dirty water in the pipework.
When to stop and call a Gas Safe engineer
If the easy checks do not solve it, it is time to get professional help. Anything involving the inside of a gas boiler, live electrical testing, replacement valves, sensors, pumps, or printed circuit boards should be left to a qualified engineer.
Call for help sooner rather than later if any of the following apply:
The boiler is locking out repeatedly: one reset is reasonable, repeated resets are not a fix
Pressure keeps dropping: this may point to a leak, failed expansion vessel, or another system problem
You have a cylinder and no obvious control issue: the fault may be in the motorised valve or thermostat wiring
The hot water is intermittent: this often points to a sticking part rather than a simple setting
There are unusual noises or signs of leaking: shut the system down and get it checked
If you are in Southampton, the New Forest, or nearby postcode areas, this is the kind of fault a local heating engineer sees regularly. A quick visit with the right test equipment can usually confirm whether the issue is a diverter valve, zone valve, thermostat, heat exchanger, sensor, or blockage.
What a heating engineer will usually check
A proper fault-finding visit should start with the basics: system type, control settings, boiler pressure, fault codes, and whether the boiler is receiving a demand for hot water.
That methodical, step-by-step approach mirrors how Roofing.work sets out a homebuyer roof inspection checklist, where following an ordered sequence prevents missed faults and saves time.
From there, the engineer may test the hot water controls, cylinder thermostat, motorised valve operation, or diverter valve behaviour, depending on the setup. They may also check for poor flow, blocked components, or signs of sludge in the system.
For homes and landlords locally, KJP Plumbing & Heating – Powerflush Specialists covers hot water and heating repairs, boiler servicing, cylinder work, central heating repairs, and 24/7 emergency call-outs. That means there is a clear route whether the fix turns out to be a control issue, a failed valve, a blocked heat exchanger, or a wider system-cleanliness problem that needs powerflushing.
Preventing the same fault from coming back
Annual boiler servicing helps catch hot water faults before they become a full loss of service. Valves, sensors, pressure issues, and early signs of scale or sludge are often spotted during routine maintenance.
It also helps to keep the heating water clean and properly treated. In older systems, sludge can slowly build up for years before the symptoms become obvious. If radiators have cold spots, the boiler is noisy, or hot water performance has been slipping, it may be worth asking whether the system water needs cleaning or inhibitor treatment.
A small problem on the hot water side rarely stays small forever. If your heating is working but your taps are cold, a few quick checks are sensible, but a prompt repair is usually the best way to get things back to normal before the fault gets more expensive.