No Hot Water Anywhere in the House? Combi and Cylinder Checks
When no tap in the house runs hot, the first question is what kind of system you have. A combi and a cylinder system fail in different ways, and a few minutes of safe checks usually shows which part to look at.
If there is no hot water anywhere in the house, start by working out which system you have, because the checks are different. With a combi boiler, look at the pressure gauge, the display for a fault code and whether gas is reaching the house. With a hot water cylinder, check the programmer's hot water setting, the cylinder thermostat and whether the boiler fires at all. Most of these take a few minutes and none of them involve opening the boiler.
Combi or Cylinder: Which System Do You Have?
If there is no hot water tank anywhere in the house, you almost certainly have a combi boiler, which heats water on demand. If there is a large insulated cylinder in an airing cupboard or loft, you have a system or regular boiler that heats a stored tank of water.
Some homes have no gas boiler for hot water at all and rely on an electric immersion heater in the cylinder. If that is you, a boiler engineer is the wrong call. The immersion element, its thermostat and its switch are electrical work for an electrician.
Combi Boiler With No Hot Water: Four Checks
With a combi, first check whether the heating works. If heating works but hot water does not, the fault is usually inside the boiler. If neither works, there is often a simple cause worth ruling out.
- Pressure: most combis stop if the gauge falls below about 1 bar. Topping up through the filling loop, following the manual, is a documented homeowner job.
- Fault code: note the code and the boiler make. Our boiler fault codes reference explains the common ones by brand.
- Gas supply: check the hob lights and a prepayment meter has credit.
- Condensate pipe: in freezing weather, a frozen condensate pipe outside is one of the most common reasons a combi stops.
If the heating works and only hot water has gone, the likely causes are the diverter valve, the flow sensor or a scaled plate heat exchanger, all inside the boiler. Our guide to no hot water while the heating still works goes through each.
Cylinder System With No Hot Water: Start at the Programmer
On a system with a cylinder, the most common reason for no hot water is the controls, not the boiler. The hot water channel on the programmer may be off, set to the wrong times, or showing the wrong clock after a power cut.
Set the hot water to constant or boost for an hour and see whether the boiler fires. Then check the cylinder thermostat, a small dial usually strapped to the side of the tank. If it has been turned right down, the cylinder will stop calling for heat. A setting of around 60°C is normal for stored hot water.
If you have an immersion heater as a backup, switching it on is a useful test. If the water heats on the immersion but not from the boiler, the cylinder is fine and the fault lies between the boiler and the tank.
What Usually Fails on a Cylinder System
When the controls are set correctly and the water still stays cold, the usual culprit is a motorised valve that has stuck, a failing pump or the boiler itself not firing. Those are parts an engineer needs to test.
- Motorised valve: a two-port or three-port valve directs hot water from the boiler to the cylinder. When its motor or switch fails, the boiler may not get the signal to fire, or may heat only the radiators.
- Central heating pump: if the pump has stopped, hot water from the boiler does not reach the cylinder coil. You may hear the boiler run briefly and cut out.
- Boiler not firing: if neither heating nor hot water works, the problem is likely the boiler, the gas supply or the wiring centre.
If your pump is the suspect, our central heating pump repair service covers the diagnosis.
No Water at All From the Hot Taps
If the hot taps give no water at all, rather than cold water, the problem is supply, not heat. On a gravity system, the cold tank in the loft that feeds the cylinder may have run dry because its float valve has stuck.
An airlock after work on the pipes can also stop flow from the hot taps. On an unvented cylinder, a closed or failed inlet valve can cut the supply. A tank or valve problem is plumbing work rather than gas work, and an unvented cylinder should only be worked on by someone qualified for that type of cylinder.
When to Stop and Call an Engineer
Call an engineer once the simple checks are done and the hot water has not come back, or if the boiler shows a fault code after one reset. Keep resetting a boiler that locks out and you risk turning an easy diagnosis into a harder one.
Stop straight away and call the National Gas Emergency Service on 0800 111 999 if you smell gas, see a yellow or orange flame instead of a blue one, or notice soot around the boiler. Any work on the gas side of the boiler must be done by a Gas Safe registered engineer. When you get in touch, tell us your system type, the boiler make and any code on the display, and we can usually say what we expect to be looking for before we arrive.
Frequently asked questions
Why do I suddenly have no hot water?
On a combi, the most common reasons are low pressure, a fault code, an interrupted gas supply or a frozen condensate pipe. On a cylinder system, check the programmer's hot water setting and the cylinder thermostat first, because a power cut or a knocked dial is often the cause.
My heating works but there is no hot water. Is that the boiler?
On a combi, yes, usually the diverter valve, flow sensor or plate heat exchanger. On a cylinder system, it is more often the motorised valve, the cylinder thermostat or the programmer, which are part of the controls rather than the boiler itself.
Can I use the immersion heater while I wait for an engineer?
If your cylinder has one, yes. It is a sensible stopgap and a useful test: if the immersion heats the water but the boiler does not, the fault lies between the boiler and the cylinder. Remember to switch it off once the boiler is repaired to avoid high electricity bills.