No Hot Water but Heating Works? Causes and Fixes

Warm radiators and a stone-cold hot tap is one of the odder faults a boiler can throw at you. If there is no hot water but heating works fine, the good news is that the boiler itself is clearly firing, the gas supply is live and the pump is running. The fault almost always sits in the part of the system that decides where the heat goes, and that narrows things down quickly.

This guide walks through the likely causes for each type of system, tells you which checks are safe to do yourself, and is honest about the point where you should put the screwdriver down and call a Gas Safe engineer.

First, work out which type of boiler you have

The cause depends heavily on your setup, so start here.

Combi boiler: one wall-hung unit, no hot water cylinder, no tanks in the loft. Hot water is heated on demand when you open a tap.
System or conventional boiler: you have a hot water cylinder, usually in an airing cupboard, and possibly tanks in the loft. Water is heated in batches and stored.

If you are not sure, open the airing cupboard. A large copper or white insulated cylinder means you are in the second camp.

No hot water but heating works on a combi boiler

On a combi, the prime suspect is the diverter valve. This is a small motorised component inside the boiler that switches the flow of hot water between your radiators and your taps. Hot water is supposed to take priority: open a tap and the valve should swing across and send everything to the tap.

Diverter valves wear, and when they stick they tend to stick in the heating position. The classic symptom is exactly what you are seeing: radiators heat perfectly, but the hot tap runs cold or lukewarm. Sometimes you get a brief burst of warm water that quickly fades, which is the valve partially moving and giving up.

Two other combi possibilities worth knowing about:

Low water pressure: many combis will run the heating but refuse hot water, or behave erratically, if system pressure has dropped. Check the pressure gauge on the front of the boiler. It should typically sit between 1 and 1.5 bar when cold. If it is well below 1, topping up via the filling loop is a job most homeowners can do, and your boiler manual explains how.
A failed flow sensor: the boiler relies on a small sensor to detect that a tap has been opened. If it fails, the boiler never realises you want hot water. This is an engineer job.

Rule of thumb: on a combi, heating fine but taps cold usually means a stuck diverter valve. It is a common, well-understood repair, not a sign you need a new boiler.

No hot water on a system or conventional boiler

With a cylinder in the mix, there are more moving parts, but the logic is the same: the boiler is making heat, and something is failing to route it to your hot water.

The motorised valve

Most cylinder systems use one or two motorised valves, usually mounted on the pipework near the cylinder or boiler. A three-port valve directs heat to the radiators, the cylinder or both. If it jams in the heating position, the radiators get everything and the cylinder gets nothing. A faint humming from the valve, or a valve body that is hot on one side and cold on the other, points this way. Replacement is a job for a heating engineer, though it is routine and usually done in a single visit.

The cylinder thermostat

Strapped to the side of the cylinder is a thermostat that tells the system when the stored water is hot enough. If it fails, or if it has been knocked to a very low setting, the system may simply believe the water is already hot. Check the dial: around 60 degrees is the standard setting. Do not set it lower than 60, because stored hot water below that temperature can allow legionella bacteria to grow.

The programmer

Less dramatic but surprisingly common: the timer or programmer has hot water switched off or on the wrong schedule. Power cuts, a change of clocks or a curious child can all knock settings out. Check that hot water is set to On or Timed with a period that covers now, then give the cylinder 30 to 45 minutes to heat.

Checks that are safe to do yourself

Before calling anyone, run through this list. None of it involves opening the boiler casing, and any one of them can save you a call-out.

Check the programmer or timer has hot water enabled, not just heating.
Check the boiler pressure gauge and top up to around 1 to 1.5 bar if low.
Check the cylinder thermostat is set to about 60 degrees, if you have one.
Look for an error code on the boiler display and note it down, along with the boiler make and model. This genuinely speeds up any engineer visit.
Try one reset using the boiler's reset button. If the fault returns, stop resetting. Repeated resets can mask a fault the boiler is trying to protect you from.
Run the hot tap nearest the boiler for a minute or two. On a combi, a tap opened too gently sometimes fails to trigger the boiler at all.

Where DIY stops

The line is simple and it is a legal one as well as a safety one.

Anything behind the boiler casing, and anything involving gas, is Gas Safe territory. No exceptions.

Diverter valves, flow sensors, printed circuit boards and heat exchangers all live inside the sealed case, and working on them without registration is illegal for good reason. The same goes for motorised valves on the pipework in most cases: while the valve itself is outside the boiler, it is wired into the mains and connected to a pressurised system, so it is not a sensible weekend project.

If your checks have not restored hot water, describe the symptoms to the engineer when you call: which type of boiler, what the taps do, any error code, and what you have already tried. A clear description often means the right part arrives on the first visit.

Worth knowing: if the same fault keeps returning on a boiler that has not been serviced in years, book a service at the same time as the repair. Many of these faults start as wear that an annual service would have caught.

Is it ever a sign you need a new boiler?

Usually not. A stuck diverter valve or a failed motorised valve is a repair, not a death sentence. The calculation changes if the boiler is over 12 to 15 years old, parts are becoming hard to source, or this is the third or fourth fault in a couple of years. At that point it is fair to ask an engineer for an honest view on repair versus replacement, and a decent firm will give you both prices and let you decide.

If you are in Bristol or the surrounding villages and the checks above have not brought your hot water back, Bedminster Plumbing & Heating has been diagnosing exactly this fault since 2004. Call us or use the callback form on our website, tell us your boiler make and what you have tried, and we will quote a fixed price after taking a look.