Hot Water Cylinder Not Heating

What's happening?

A cylinder heated by the boiler is warmed by a coil inside it. For that to happen, a chain has to work. The programmer calls for hot water, the cylinder thermostat, strapped to the side of the cylinder, says the water is cool, a motorised valve opens to send boiler water through the coil, the pump moves it and the boiler fires. If the radiators heat but the cylinder stays cold, the boiler and pump are fine and the break is in the hot water part of that chain. This page is about a cylinder fed by a boiler. If the cylinder has an electric immersion heater and that is what has failed, the immersion page is the right one, and if there is no hot water at all from a combi, start with the no-hot-water pages.

What usually causes it?

• The programmer's hot water channel off, set to the wrong times or left on heating only
• A cylinder thermostat set too low, failed or not touching the cylinder
• A motorised valve that is stuck, has a failed actuator or has lost its power
• An airlock or a blockage in the coil circuit
• A pump that is not running or is too slow
• A cylinder coil furred with scale, or a cylinder full of sludge
• On an unvented cylinder, a tripped thermal cut-out or a failed control

Is it dangerous?

Medium for an unvented cylinder, which stores hot water at mains pressure under safety controls. Never tamper with its relief valves or pipework, and never block the discharge pipe. A vented cylinder with a loft tank is lower risk. In both cases, a cold cylinder is a fault to fix, not an emergency.

Can I fix it myself?

Check the programmer is set to give hot water at the times you want and switch it to on for a test. Check the cylinder thermostat is set to about 60 degrees. Listen for the motorised valve clicking and the pump running, and check the cylinder pipes: the flow pipe to the coil should heat when hot water is called. If it does not, note whether the boiler fires. If you have an immersion heater, you can use it temporarily. Do not touch relief valves or the discharge pipe.

When should you call a heating engineer?

Call a heating engineer if the controls are set correctly and the cylinder stays cold, if the pipe to the coil does not heat, or if an unvented cylinder shows a tripped cut-out. Unvented cylinders must be worked on by someone qualified to do so, and the controls are not for homeowners.

What will the heating engineer actually do?

• Check the programmer output and wiring and the cylinder thermostat setting and contacts
• Test the motorised valve and its actuator and microswitch
• Check the pump and bleed the coil circuit for air
• Descale or replace the coil or cylinder if it is fouled or failed
• Test the safety controls on an unvented cylinder and reset or replace as needed
• Prove the cylinder reaches temperature and hold it for a full heat cycle

Who fixes this

• Hot water cylinder repair and replacement
• Heating not working

Questions people ask

Sources & References

1. Vaillant: what is a system boiler?
2. Vaillant: heating controls
3. Vaillant: how to fix a boiler diverter valve
4. GOV.UK: Approved Document G, sanitation, hot water safety and water efficiency