Heating Works But No Hot Water? What That Actually Means

If your radiators are getting hot but the taps run cold, the boiler itself is almost certainly still firing — the fault is in whatever decides where that heat goes. On a combi that usually points at the diverter valve; on a system or regular boiler it is more often the cylinder thermostat or a motorised valve that has stuck.

That distinction matters because it changes how urgent the job is and how much of it you can sensibly check yourself. Below is how our engineers work through it on a real call-out, in the order we actually do it.

Why do I have no hot water when the heating still works?

Every boiler produces heat in one place. What differs is how that heat is shared between the radiator circuit and the hot water.

A combi does both jobs from the same heat exchanger and switches between them with a diverter valve. When you open a hot tap, a flow sensor tells the boiler to stop sending heat to the radiators and send it to a second, smaller heat exchanger instead. If that valve sticks in the heating position, the heating carries on perfectly and the hot water never arrives.

A system or regular boiler heats a stored cylinder instead. There, the heat is shared by one or two motorised valves, told what to do by a programmer and two thermostats — one on the wall for the rooms, one strapped to the cylinder for the water. Heating working and water not usually means one half of that control chain has failed, not the boiler.

So “heating but no hot water” is rarely a dead boiler. It is nearly always a valve, a sensor or a control that has stopped doing its share.

No hot water on a combi boiler: the usual suspects

A sticking diverter valve

This is the single most common cause we find. The valve has a moving spindle and a rubber seat, and after years of hard East Kent water both scale up. It gets slow, then it gets lazy, then one morning it does not move at all.

The tell-tale signs are worth knowing:

• Hot water that is lukewarm rather than absent, because the valve is part-open.
• Hot water that only works while the heating is switched off.
• Hot water that arrives, then fades after twenty or thirty seconds.

A diverter valve is a repair, not a replacement boiler. On most makes it is either a new cartridge or a new valve body, and it is a job for a Gas Safe registered engineer — the boiler has to be drained down and the sealed system re-pressurised afterwards.

A failed flow sensor

Before the boiler diverts anything, it has to know a tap is open. That is a small flow sensor or flow switch in the cold inlet. When it silts up, the boiler simply never gets the message: the burner does not fire for hot water at all, and the display often shows nothing wrong. Heating, which is triggered by the room thermostat instead, keeps working normally.

Low pressure or a partly closed valve

Check the pressure gauge before anything else. If it is sitting below about 1 bar, the boiler may be running the heating on residual heat but locking out the hot water demand. Repressurising via the filling loop is a job most people can do from the boiler’s own instructions — but if the pressure keeps dropping, there is a leak somewhere and that needs finding rather than topping up week after week.

It is also worth checking the isolation valve on the cold inlet under the boiler has not been knocked shut, especially if anyone has been working nearby.

A fault code on the display

Modern combis will usually tell you something, even if it is cryptic. If there is a code showing, look it up in our boiler fault code guide before you do anything else — it will often narrow the problem down to the exact component and save you a diagnosis.

Boiler on but no hot water on a system or regular boiler

The cylinder thermostat

This is a small dial strapped to the side of the hot water cylinder under the insulation jacket. It tells the boiler when the stored water needs reheating. When it fails, it usually fails “satisfied” — meaning it permanently believes the cylinder is already hot, so the boiler is never asked to heat it. The heating side, on its own separate thermostat, carries on unaffected.

A stuck motorised valve

The two- and three-port valves that route hot water around the airing cupboard have a motor head and a lever. They fail often, and they nearly always fail in the heating position. Most have a small manual lever on the side; if the valve body is sound and only the motor head has gone, the head alone can be swapped without draining anything, which keeps the job short.

The programmer or timer

Before assuming a component has failed, check the programmer is actually calling for hot water. Timers lose their settings after a power cut, and the hot water channel can end up switched to “off” or on a schedule nobody remembers setting. We have driven to plenty of jobs that turned out to be exactly this, so it is worth two minutes of your own time first.

Simple checks you can do safely before you call

None of these involve removing a boiler case, which you should never do:

1. Check the programmer is calling for hot water, not just heating.
2. Check the pressure gauge on the boiler — around 1 to 1.5 bar when cold is normal.
3. Check for a fault code on the display and note it down.
4. Try the hot tap nearest the boiler — if that one is hot and the others are not, the problem is in the pipework or a tap, not the boiler.
5. Feel the cylinder if you have one. Hot at the top and cold at the bottom means it is heating but slowly; stone cold all over points at the thermostat or valve.

Anything beyond that — the boiler casing, the gas side, the sealed system — is Gas Safe territory by law. Call us on 01304 381999 and describe what you found; it genuinely shortens the visit.

When it is not the boiler at all

Two things catch people out. The first is a scaled-up hot tap or shower cartridge: if only one outlet is cold, the boiler is fine. The second is an unvented cylinder whose immersion element or expansion vessel has failed — the heating and the boiler both look healthy, but the stored water never gets there.

We see a third in older properties across the coast: a hot water circuit so choked with sludge that the cylinder coil can no longer transfer heat. That one shows up as water that is warm but never properly hot, and it is a system problem rather than a component failure.

What happens when we come out

Repairs and plumbing work are charged by the hour rather than at a fixed price — £80+VAT for the first hour, then £40+VAT per half hour after that. The first hour is always billable, and in most no-hot-water cases the diagnosis and the fix land inside it. We carry the common diverter cartridges, motorised valve heads and cylinder thermostats on the van for exactly this reason.

You can see the full picture on our boiler repairs page, or call and we will tell you honestly when we can get to you. A real person answers the phone, and same-day visits are available within working hours.

Stopping it happening again

Diverter valves, flow sensors and cylinder thermostats all fail faster in hard water, and most of East Kent is hard water. An annual service is the cheapest way to catch a valve that is getting sticky before it strands you — boiler servicing is £90+VAT and includes the checks that spot exactly this.

If you would rather have the service and any maintenance repairs handled on one monthly plan, BOYLA HomeCare starts at £18 a month for Home Essentials, with no call-out fees and an emergency response within 24 hours Monday to Friday.

Lost your hot water and want it sorted today? Call David Osborne Heating on 01304 381999. We are Gas Safe registered (number 2836), Deal-based, and have been looking after heating across East Kent for over 25 years.