No hot water but heating works: what to check

Radiators piping hot, shower running cold. It is a strange combination, and it confuses people because the boiler is obviously working, you can hear it, the heating proves it, yet the taps disagree. The upside of that strangeness is that it tells us a lot. If the boiler can heat the radiators, then the burner, the pump and the gas supply are all fine. The fault is almost always in the part of the system that decides where the heat goes.

Which part depends on what type of system you have, so start there.

First, work out what system you have

A combi boiler heats water on demand, the moment you open a hot tap. There is no hot water cylinder anywhere in the house. Most Manchester flats and a large share of terraces converted in the last twenty years run combis.

A system or conventional boiler heats a hot water cylinder, usually in an airing cupboard, and the taps draw from that stored supply. Conventional (heat-only) systems also have tanks in the loft.

The diagnosis is completely different for each, so pick your section below.

Combi boiler: heating works, taps run cold

On a combi, one component sits at the centre of this fault: the diverter valve. Inside the boiler, this valve switches the heat between two jobs. Heating on, it sends hot water round the radiators. Hot tap open, it is supposed to snap across and send everything to a small heat exchanger that heats your tap water instead. Hot water always takes priority, which is why the radiators pause when someone showers.

When a diverter valve sticks, wears or tears its diaphragm, the boiler keeps doing whichever job the valve can still reach. Heating but no hot water is the classic symptom. Interestingly, so is the reverse, and so is hot water only when the heating happens to be on, because the valve is jammed on the heating side and heat is sneaking across.

The other regular offender is the flow sensor, a small turbine that spins when you open a hot tap and tells the boiler to fire. If it is stuck or scaled up, you open the tap and the boiler simply never gets the message. The tell here: the boiler does not even attempt to fire when you run the hot tap. With a diverter fault, you can often hear the boiler fire and then the water still runs cold or lukewarm.

Before assuming either, run these safe checks:

1. Check the pressure gauge. Some boilers will run heating on low pressure but refuse hot water, and some lock hot water out first. Below 1 bar, top up via the filling loop and retest.
2. Check the display for a fault code and note it down, it saves diagnostic time when you call.
3. Check the hot water temperature setting on the boiler. It sounds daft, but dials get knocked. Make sure the tap symbol setting is not turned to minimum.
4. Try more than one hot tap. If the kitchen runs cold but the bathroom runs hot, the problem is that tap or its pipework, not the boiler.
5. Listen when you open a hot tap. Boiler fires but water stays cold: diverter territory. Boiler does nothing at all: flow sensor territory.

Both parts live inside the boiler casing, and that is where your involvement rightly ends. All work inside a gas boiler is carried out by Gas Safe registered engineers. The repair itself is routine for our boiler repair manchester team, diverter valves and flow sensors are among the most common parts we replace, and it is a repair, not a death sentence for the boiler.

System boiler with a cylinder: heating works, cylinder stays cold

With a cylinder, the switching happens outside the boiler, which puts more of the diagnosis within your safe reach.

Check the programmer first. Heating and hot water are separate channels on the timer. It is surprisingly common after a power cut, a clock change or a fiddled schedule for the hot water channel to be off or set to strange hours. Set hot water to constant as a test.

Check the cylinder thermostat. A dial strapped to the side of the cylinder, usually behind the insulation jacket, normally set around 60 to 65 degrees. If it has been knocked down low, or has failed, it never calls for heat. Turn it up and listen for a click and the boiler firing.

Check the motorised valve. This is the big one. On the pipework near the cylinder you will find one or two boxes with a small lever, either a three-port (Y-plan) valve that shares heat between radiators and cylinder, or separate two-port (S-plan) valves. When the hot water side sticks shut, you get exactly this fault: radiators fine, cylinder cold. With hot water calling, feel the pipe after the valve, if the boiler is running but that pipe stays cold, the valve is not opening. Often it is just the motorised head that has failed, which is a quick, inexpensive swap for an engineer, done without draining the system.

Use the immersion heater in the meantime. Most cylinders have an electric immersion element as a backup, switched from a labelled fused spur nearby. Turn it on and you will have hot water within the hour regardless of the fault. If the immersion heats the water fine, you have also just proved the cylinder and taps are healthy, which narrows the fault to the valve, stat or programmer.

One honest caveat on older cylinders: if yours is decades old, uninsulated or already weeping at the fittings, it may be worth thinking bigger than the immediate fault. Our hot water cylinder replacement manchester page covers the options, including modern unvented cylinders that give mains-pressure showers, which is a genuine upgrade in a lot of the older semis we work in across south Manchester.

When it is an emergency and when it can wait

Be honest with yourself about urgency, because it changes what you should do. A family with a working immersion heater can book a repair for a convenient day. A cold house is fine here too, because by definition your heating still works.

But no hot water at all, no immersion backup, and small children or someone vulnerable in the house is a different matter, especially midwinter. That is what our no hot water emergency service exists for, and it runs 24 hours across Greater Manchester.

What to tell us when you ring

You will get a faster, cheaper fix if you can answer these: combi or cylinder, any fault code on the display, does the boiler fire when you open a hot tap, does the immersion work, fully cold or lukewarm. Two minutes of checks on your side regularly saves half an hour of ours, and since we are honest about pricing, that saving is yours. Ring us on 0161 533 0201 and we will get your hot water back.