Why is my Glow-worm not giving hot water?
A Glow-worm stops giving hot water because the boiler has been set not to, because it cannot fire, or because a part that switches heat to the taps has failed. The first two you can usually sort yourself. The third needs an engineer.
Glow-worm is part of the Vaillant Group, and its combis work in a similar way to a Vaillant ecoTEC. One burner heats both the radiators and the hot water. When you open a hot tap, a flow sensor tells the boiler, and a diverter valve sends the heat to a plate heat exchanger that warms the mains water on its way to the tap. A lockout stops both services. A settings problem or a diverter valve fault usually stops the hot water alone.
The Energy7 combi operating instructions include a troubleshooting list for a boiler giving no hot water. The table below is based on it, with the internal faults an engineer is likely to find added at the end.
| What you notice | Likely cause | What to do |
|---|---|---|
| No hot water or heating, display blank | No power or boiler switched off | Check the fuse and switch the boiler on |
| No hot water or heating, F22 shown | Water pressure too low | Top up the heating system |
| No hot water or heating, F28 shown | Ignition failure, often no gas | Check the gas stopcocks are open, then press reset |
| Heating works, taps cold, no code | Heating only mode or hot water set too low | Check the operating mode and temperature |
| Weak or no flow from the hot tap | Cold water isolation valve closed | Open the cold water isolation valve |
| Boiler fires on a hot tap, water stays cold | Diverter valve stuck | Gas Safe registered engineer |
| Boiler silent when a hot tap runs, heating fine | Flow sensor fault | Gas Safe registered engineer |
Check the operating mode
The operating mode is the first thing to check on a current Glow-worm, because one of the modes switches hot water off completely. The Energy7 combi operating instructions list four modes, which you step through by pressing the mode button repeatedly until the one you want shows on the display:
- Heating and domestic hot water. The normal setting for most homes.
- Heating only. The boiler heats the radiators but not the taps.
- Domestic hot water only. The usual summer setting, with the radiators off.
- Off. No heating or hot water.
If the display shows heating only, switch back to heating and hot water and try a hot tap again. It is an easy mode to select by accident while trying to turn the heating off for summer.
Check the hot water temperature and ECO mode
The Energy7 combi lets you set the hot water temperature on the boiler between 35 and 60 °C. If you have an external Glow-worm control, the temperature may be set there instead, so check both. A setting turned right down gives water that feels cool at the tap, especially in winter when the mains water is colder.
The Energy7 also has an ECO hot water mode. The operating instructions describe it as setting the hot water temperature to 50 °C and switching off the warm start for hot water on a combi. With warm start off, the boiler does not keep its hot water side warm between uses, so you wait a little longer for hot water to reach the tap. It is switched off as the factory setting. ECO mode never stops hot water altogether, so if you get none after running a tap for a minute, look elsewhere.
The Ultimate and Betacom ranges also set hot water from the display. Their operating instructions, which Glow-worm publishes on its website, show the controls for your exact model.
If hot water arrives but is only warm, the problem is different from having none at all. Our guide to hot water that is not hot enough covers scale, flow rate and temperature settings.
Check the display for F codes
If the heating has stopped as well, the boiler is not firing and the display normally shows a code. These are the most common:
- F22 means there is not enough water in the heating system. The Energy7 instructions give filling the heating installation as the fix. Our guide to Glow-worm boiler pressure shows how to top up to 1 to 1.5 bar.
- F28 means the boiler failed to light. The Energy7 instructions say it switches to fault mode after three unsuccessful ignition attempts. Check the gas supply and any prepayment meter first.
- F29 means the flame went out while the boiler was running, which can be a gas supply problem or a frozen condensate pipe in cold weather.
Check the gas, power and cold water valve
The Energy7 troubleshooting list includes three supply checks that are easy to miss after other work in the house:
- Gas stopcocks. Glow-worm lists both the gas stopcock on site and the one on the boiler. The handle is open when it lines up with the pipe.
- Power. Check the fuse in the building and the switched fused spur next to the boiler, and that the boiler itself is switched on.
- Cold water isolation valve. A combi takes its cold mains through an isolation valve under the boiler. If a plumber has closed it and not reopened it, the boiler has no water to heat. The Energy7 instructions give opening it as the fix.
Open the hot tap fully when testing, because a combi needs a minimum flow before it switches to hot water. If no water at all comes out of the hot tap while the cold works, see our guide to no water coming out of the hot tap.
Diverter valve and flow sensor faults
If the mode, settings and supplies are all correct, there is no code and the heating works, the fault is inside the boiler. Two parts account for most cases.
Diverter valve
The diverter valve decides whether heated water goes to the radiators or to the hot water heat exchanger. If its actuator fails or the valve sticks, the heat goes to the radiators every time. You will usually hear the boiler fire when you open a hot tap, but the water stays cold and the radiators warm up, even with the heating turned off. Our guide to diverter valve replacement cost explains the repair and what affects the price.
Flow sensor
The flow sensor detects water moving through the boiler when a tap opens. If debris or scale stops it working, or it fails electrically, the boiler never responds to the hot tap and stays silent, even though the heating works when the thermostat calls. An engineer can test the sensor and clean or replace it.
Resetting a Glow-worm after a fault
A reset only helps when the boiler has locked out with a fault code. The Energy7 instructions say to press the reset button for one second, after which the boiler makes another attempt to light. After three unsuccessful attempts, they tell you to consult a competent person. A reset does not change the operating mode or hot water setting, and it will not free a stuck diverter valve.
Never remove the boiler casing to look at the diverter valve, flow sensor or gas parts. They are for a Gas Safe registered engineer only. If you smell gas, turn off the gas at the meter if you can, open windows and call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe registered engineer
Book a Gas Safe registered engineer, or contact Glow-worm if the boiler is under guarantee, when the operating mode includes hot water, the temperature is set sensibly, there is no code, the pressure is right and the cold supply is good, but the taps still run cold. Also call if F22 or F28 keep returning after you have dealt with the obvious cause.
Tell the engineer your model, whether the heating works, whether the boiler fires when you open a hot tap and whether the radiators warm at the same time. For the same symptoms on any make, our guide to no hot water when the heating works covers combi and cylinder systems.