No hot water but heating works (or vice versa)? Here is the diagnosis

This is one of the most common phone calls I get, and one of the most useful from a diagnosis point of view. When a boiler partially fails, where it fails tells me almost exactly what is wrong before I have set foot in the property.

Here is what each version means.

"I have heating but no hot water"

On a combi boiler (the kind in most London flats and small houses) the diverter valve is the most likely culprit. This is a small motorised valve inside the boiler that switches water flow between the heating circuit and the hot water circuit. When it sticks in the heating position, you get heating but no hot water, even though the boiler is firing fine.

The diverter valve is a known weak point on most boilers. Worcester models, Vaillant models, Baxi, Ideal, all of them. The good news: it is a known repair, parts are cheap, and any competent gas engineer can swap one in 60 to 90 minutes.

Cost in 2026 London: typically £180 to £280 for a diverter valve replacement, parts and labour. Some boiler models have an integrated diverter that is more expensive (the part itself can be £150+). Worth checking your specific model before the visit.

Other less common causes:

• Faulty hot water flow sensor. The boiler is not detecting that you have opened a tap. Sometimes a clean fixes it, sometimes a replacement.
• Hot water temperature set wrong. Sounds silly, but I have driven across London for this twice. Check the dial on the front of the boiler.
• Domestic hot water pump failure (on system boilers with separate hot water circuits).

"I have hot water but no heating"

Less common, and the diagnosis is different.

On a combi: again, the diverter valve, but stuck in the hot water position. Same fix as above.

On a system or regular boiler with a hot water cylinder: the boiler heats the cylinder fine, but the heating circuit is not flowing. Usually:

• Heating pump failure. The pump that pushes heated water around the radiator circuit has stopped. You may hear a faint humming when the heating asks to come on. Pump replacement is £200 to £350 in London.
• Motorised valve stuck. Many older systems have a Y-plan or S-plan setup with motorised valves controlling which circuit is active. When one fails closed, you lose that circuit.
• Air locked in the heating circuit. Air trapped in the system can stop water flowing. Bleeding all radiators and re-pressurising sometimes clears it.
• Thermostat or programmer failure. The boiler is not being told to fire for heating. Check that your thermostat is calling for heat (turn it up well above room temperature and listen for a click).

"Both work but the hot water is lukewarm"

Different problem. This is usually one of:

1. Diverter valve passing water in both directions instead of switching cleanly. Heating water gets mixed with hot water, and the hot tap runs warm. Same diverter repair as above.
2. Plate heat exchanger blocked with limescale (combi boilers). The plate heat exchanger is where mains cold water gets heated for your taps. In hard water areas (which is most of central, south, and east London), it scales up over time. Symptoms include weak hot water pressure as well as lukewarm output. Fix: descale or replace the plate heat exchanger. Replacement is £200 to £350 typically.
3. Worn-out tap mixers or thermostatic shower valves. If only one tap or shower is lukewarm, the problem is at the tap, not the boiler.

"Both work but hot water has weak flow"

Almost always limescale on the plate heat exchanger. Hard water in London is unforgiving on combi boilers that have no scale protection.

I see this most in:

• N1, N5, N7 (Islington area)
• E2, E5, E8 (Hackney)
• SE5, SE15, SE17 (Camberwell, Peckham)
• SW9 (Brixton)
• SW1, W1 (central)

Less of an issue in west London (W4, W6, W12) where the water is slightly softer.

Fix is either descaling the existing exchanger (£150 to £250 if it works) or replacement. Long-term, a magnetic system filter plus annual servicing helps. A water softener or scale reducer on the mains is the only real prevention.

What to check before calling

Five-minute self-diagnosis:

1. Pressure gauge. Should read 1 to 1.5 bar cold. Below 0.8 and the boiler will refuse to fire fully.
2. Hot water temperature setting. Some boilers have separate hot water and heating dials. Check both.
3. Thermostat. Turn it up well above room temperature and listen for the boiler firing. If nothing happens, the thermostat or its wiring is the issue, not the boiler.
4. Run the hot tap for 60 seconds. If it goes from cold to lukewarm to cold again, it is the diverter or flow sensor. If it stays cold the whole time, the boiler is not firing for hot water at all.
5. Listen at the boiler when you open a tap. A combi should fire within a few seconds of you opening any hot tap. If you hear nothing, the boiler is not detecting the demand.

When to stop using it

If the boiler is making any unusual noise alongside the partial failure (banging, kettling, grinding) turn it off and call someone. A failing diverter is mechanical, but pump and motor failures sometimes come with overheating, and overheating boilers should not be left running.

If you have a partial failure and a small kid in the house in winter, that is a same-day call. I keep diverter valves and pumps for the most common boilers in the van, so a lot of these jobs finish in one visit.

What I bring on a no-hot-water visit

For a typical London property the parts I carry that resolve 80 percent of these calls:

• Diverter valves for Worcester, Vaillant, Baxi, and Ideal combis
• Common heating pumps (Grundfos, Wilo)
• Plate heat exchangers for the most common combi models
• Flow sensors

If your boiler is something less common (older Glow-Worm, Potterton, Main, etc.) tell me the model when you call so I can pick up the right part on the way.

Send a WhatsApp first

Before you book a visit, send me a quick message with:

1. Your boiler make and model (sticker on the front)
2. Which works (heating or hot water) and which does not
3. Any error code on the display
4. When the problem started

In about half of these cases I can confirm the likely cause from those four pieces of information and quote a fixed price for the repair. Saves you a diagnostic fee and gets the repair done faster.