Glow-worm Boiler No Hot Water: Causes and What to Check
A Glow-worm boiler with no hot water has either locked out or lost its hot water circuit. If the display shows an F code, such as F.22 for low pressure or F.28 for a failure to light, the boiler has stopped completely and a reset or a repair is needed before anything works. If there is no code and the radiators still heat, the boiler is fine as a heat source and the fault is on the hot water side. On a Glow-worm combi, such as a Betacom, Energy or Ultimate, that is almost always the diverter valve, the hot water flow sensor or a scaled plate heat exchanger. On a Glow-worm system or regular boiler, it is usually the motorised valve, the cylinder thermostat or the programmer.
Glow-worm and Vaillant: why the advice overlaps
Glow-worm is part of the same group as Vaillant, and its boilers use the same style of display codes and a similar layout inside the case. If you have read about a Vaillant with no hot water, much of it applies here, and our guide to a Vaillant boiler no hot water covers the shared ground. The differences that matter are in the ranges themselves.
The Betacom is Glow-worm's long running compact combi and one of the most common boilers in London flats. The Energy and Ultimate ranges come as combi, system and regular boilers. The model name on the front, or the data badge underneath, tells you which you have. If there is a hot water cylinder anywhere in the property, the boiler is not a combi.
F codes on a Glow-worm: what to note and when to reset
Codes starting with F are faults, and the boiler has stopped until the cause is cleared. Write the code down before you do anything else, because a reset clears it and it is the first thing an engineer will ask for.
F.22 usually means the water pressure in the heating system is too low. Check the gauge, and if it reads well under 1 bar with the boiler cold, top it up through the filling loop as the user manual shows. F.28 means the boiler tried to light and failed, and F.29 means it lit and then lost the flame. For those, check other gas appliances work and that the condensate pipe outside is not frozen. Press reset once or twice at most. A code that keeps coming back is the boiler protecting itself, and repeated resets do not fix the cause.
Quick checks before you call
These take a few minutes and are worth doing first.
- Pressure gauge: roughly 1 to 1.5 bar cold is typical. If you have to keep topping it up, there is a leak to find.
- Gas: light a hob, if you have one. No gas there is a supply or meter issue.
- Hot water dial: make sure the hot water temperature on the boiler has not been turned right down or switched off.
- Programmer: on a system or regular boiler, check the hot water channel is on as well as the heating.
- Condensate pipe: in a frost, thaw the plastic pipe outside with warm, not boiling, water.
- Another tap: hot at one tap but not the next is a tap or valve fault, not the boiler.
Glow-worm combi: radiators hot, taps cold
On a Betacom, Energy or Ultimate combi with no code showing, the fault is nearly always in one of three parts. The diverter valve switches the heat from the radiators to the hot water when a tap opens. If it sticks on heating, the radiators stay hot and the taps run cold. If it sticks partway, the radiators warm up whenever you run a hot tap and the water never gets properly hot. What that repair involves is covered in our guide to diverter valve replacement cost.
The flow sensor on the cold inlet tells the boiler that a tap is open. When it fails, nothing happens when you run the hot tap, yet the boiler fires normally for the heating. A blocked filter on the cold inlet can cause the same symptom.
The plate heat exchanger passes heat from the boiler's own water into the tap water. On compact combis working hard in hard water areas, it scales up and the hot water becomes lukewarm, or the boiler fires and cuts out while the tap runs. The three parts are the same across most combis, and our guide to no hot water from a combi boiler covers them in more detail.
No hot water from a Glow-worm system or regular boiler
With a cylinder, the Glow-worm may be working perfectly and simply never asked to heat it. The programmer calls for hot water, the cylinder thermostat agrees the water is cold, and a motorised valve opens the route to the cylinder coil and tells the boiler to fire. If any part of that chain fails, you get heating and no hot water.
The motorised valve is the usual failure. If the cylinder has an immersion heater, switching it on is a useful test, because hot water from the immersion proves the cylinder is fine and points at the boiler side. Our guide to no hot water but the heating works goes through the valve and thermostat checks on vented and unvented cylinders.
Repair or replace an older Glow-worm?
Diverter valves, flow sensors, plate heat exchangers and motorised valves are standard parts, and most of these calls end in a repair. The picture changes on an older boiler that has had several parts already, has scaled up badly, or needs a part that is getting hard to find. In that case a repair can still be the right call, but it is worth knowing the boiler's overall condition before spending on it.
A boiler diagnosis visit is how that gets decided. We read the fault history, test the parts rather than guess, and tell you plainly whether the boiler is worth repairing. If it is not, our boiler replacement page explains what that involves.
Opening the case of a gas boiler is gas work and must be done by a Gas Safe registered engineer, including the diverter valve, the flow sensor and the plate heat exchanger. If you smell gas, turn it off at the meter, open windows and call the gas emergency line on 0800 111 999.
When to call us
Call us on 020 4652 5111 or send an enquiry if an F code returns after a reset, if the pressure will not stay up, if water is dripping from the boiler, or if the taps still run cold after the checks above. Tell us the model, such as Betacom or Energy, and any code on the display. We have 20 engineers across London, and our rate is on the pricing page, agreed before anyone attends.
Questions we get asked
- Why does my Glow-worm boiler have heating but no hot water?
- On a Glow-worm combi it is usually a diverter valve stuck on heating, a flow sensor that does not register the tap, or a scaled plate heat exchanger. On a system or regular boiler it is usually the motorised valve, the cylinder thermostat or the hot water channel on the programmer.
- How do I reset a Glow-worm boiler?
- Press the reset button on the control panel and give the boiler a minute to restart. Do this once or twice at most. If the same F code comes back, the cause needs an engineer.
- What does F22 mean on a Glow-worm boiler?
- It usually means the water pressure in the heating system is too low to run safely. Top it up to around 1 to 1.5 bar with the boiler cold, using the filling loop as your manual shows, then reset. If it keeps dropping, there is a leak to trace.
- Are Glow-worm and Vaillant boilers the same?
- They are made by the same group and share a similar code system and internal layout, but they are separate ranges with different models and parts. The fault finding approach is much the same.
- Is it worth repairing a Glow-worm boiler with no hot water?
- Usually, because the parts that cause it are standard. On an older boiler that has had repeated failures, a diagnosis first tells you whether the repair is money well spent.