Short answer: If you have no hot water, first work out your system. With a combi, check the pressure, the display for a fault code and, in frost, the condensate pipe. With a cylinder, check the hot water timer, the cylinder thermostat and the immersion switch. If heating works but hot water does not, suspect a valve.
No hot water is one of the most common calls a plumber gets, and a fair share of them can be fixed with a switch, a reset or a jug of warm water. Before you book anyone, it is worth ten minutes working through the basics. The checks depend on what kind of system you have.
Which system do you have?
- Combi boiler: no hot water cylinder anywhere, no cold tank in the loft. Hot water comes straight from the boiler when you open a tap. Common in Sheffield terraces and smaller semis.
- System boiler with a cylinder: a boiler plus a hot water cylinder, often in an airing cupboard. If the cylinder is unvented, there is no cold tank in the loft.
- Regular boiler with a cylinder and tanks: a boiler, a cylinder, and a cold water tank in the loft. Common in older and bigger houses, such as Victorian villas in Nether Edge and Broomhill.
Checks for a combi boiler
- Is the boiler on? Check the display, the switch next to it and the fuse spur.
- Is there a fault code? Note it down and look it up in the manual.
- Is the pressure OK? Around 1 to 1.5 bar when cold on most boilers. Below 1, it may lock out. Our post on boiler losing pressure explains how to top up.
- Is the gas on? Check other gas appliances, such as the hob.
- Is it freezing outside? Check the condensate pipe. If it is frozen, the boiler will stop. Our post on frozen condensate pipes shows how to thaw it.
- Is cold water flowing well? A combi needs a good flow of cold water to fire for hot water. If the cold tap is weak, the boiler may not switch on.
- Try a reset as the manual describes.
Low flow on high streets
That sixth point catches people out in Sheffield. A combi boiler only fires when enough water flows through it. Homes high up, such as on the Crookes ridge, can have lower mains pressure. Yorkshire Water’s water pressure guide explains that homes closer to the height of the storage tank get lower pressure.
If the cold flow is low, the combi may give lukewarm water, cut out when someone else turns on a tap, or not fire at all. Check that the stop tap in the cellar is fully open. Our Crookes area page has more on pressure on that ridge, and our post on low water pressure covers the other causes.
Heating works, hot water does not
On a combi, if the radiators heat but the taps run cold, the diverter valve inside the boiler may be stuck. It switches hot water between the radiators and the taps. In harder water areas, such as Handsworth and Rotherham, scale on the plate heat exchanger can also stop hot water heating properly. Both are jobs for a Gas Safe registered engineer.
Checks for a cylinder system
- Is the hot water switched on? Check the programmer or timer. Many have a separate hot water setting. Try “constant” or a boost.
- Is the boiler running? Check for a display, pressure and fault codes as above.
- Check the cylinder thermostat. It is a small dial strapped to the side of the cylinder. It should usually be around 60°C. If it has been knocked down, the water will be cool.
- Is the motorised valve opening? Near the cylinder there may be a valve with a small box on top and a manual lever. If the heating works but the cylinder stays cold, this valve may be stuck. An engineer can check it.
- Try the immersion heater. If there is an immersion switch near the cylinder, switch it on. If you get hot water after an hour or two, the immersion works and the boiler side is the problem. If you still get nothing, the immersion may have failed too.
Our post on an immersion heater not working goes into that in more detail.
Unvented cylinders
If you have an unvented cylinder and water is dripping from the tundish, a small funnel on the pipe below the cylinder, a safety device has opened. Turn off the boiler and immersion and get it checked. Do not ignore it. Our hot water cylinders page explains what those safety devices do.
Quick reference
| What you see | Likely cause |
|---|---|
| Combi, no hot water, fault code, freezing outside | Frozen condensate pipe |
| Combi, low pressure on the gauge | Pressure needs topping up, then find the leak |
| Combi, heating fine, taps cold | Diverter valve or scaled heat exchanger |
| Combi, lukewarm, weak cold flow | Low mains flow or part closed stop tap |
| Cylinder, heating fine, cylinder cold | Motorised valve, timer or cylinder thermostat |
| Cylinder, nothing from boiler or immersion | Boiler fault plus failed immersion |
| Water from the tundish | Unvented safety device opened, needs checking |
When to call
Call an engineer if:
- The boiler shows a fault code that will not clear after a reset.
- You smell gas. Leave the house and call 0800 111 999 instead.
- Water is leaking from the boiler or cylinder.
- The tundish is running.
- You have done the checks and still have nothing.
Any work inside a gas boiler must be done by a Gas Safe registered engineer. Ask to see their ID card. Our boiler repair page explains how a visit works.
Send us the boiler make, the fault code if there is one, and what you have checked. We will tell you whether it is likely a quick fix or a part.
Quick questions
My heating works but there is no hot water. Why?
On a combi, often the diverter valve or a scaled heat exchanger. On a cylinder system, often the motorised valve, the cylinder thermostat or the hot water timer.
Can I use the immersion heater if the boiler is broken?
Yes, if you have a cylinder with an immersion. It heats the water electrically while the boiler is repaired.
Why is my hot water only lukewarm?
On a combi, low mains flow or scale. On a cylinder, a thermostat set too low, a failed immersion or a valve not opening fully.
Do I need a Gas Safe engineer?
For any repair inside a gas boiler, yes. Checking settings, pressure and switches is fine to do yourself.
Last updated 23 October 2026. Written for homes in Sheffield. Rules can change, so check the linked sources for the latest position.