Why is my unvented cylinder running out of hot water?
If an unvented cylinder runs out of hot water quickly, it’s usually because the stored hot water is used up faster than it can be heated or topped up. When you drain off the hot water from an unvented cylinder, it takes time to recover and heat more water.
A common cause is that the cylinder is undersized for the property. If the cylinder was sized for a single bathroom but is now being used for multiple showers, baths or higher occupancy, it might not be able to keep up. Using large baths, high-flow showers, and several outlets close together can empty the cylinder much quicker than expected.
Another reason is a heating fault. The boiler, motorised valve, cylinder thermostat, programmer or heating coil may not be heating the water properly with an indirect unvented cylinder. With a direct unvented cylinder, one of the immersion heaters could have failed, the thermostat may be set too low, or the timer may not be allowing sufficient heating time.
The hot water temperature setting can also help. If the cylinder is only heating to a low temperature, then there will be less hot water available to use. However, the temperature cannot be increased without taking into account safety, the risk of scalding, and the system design. Thermostatic mixing valves may also influence the temperature of the delivered water.
It could be a faulty blending valve, incorrect pipework, a passing mixer shower or tap or cold water entering the hot water pipework. Such issues can give the impression that the cylinder has no hot water left when the problem is in fact temperature mixing.
If your unvented cylinder is regularly running out of hot water, a competent engineer should check the system. They can confirm the size of the cylinder, recovery rate, controls, immersion heaters, boiler operation, thermostats, valves, and hot water usage. Sometimes all that is needed is a change in timings or a replacement of a faulty component, but in some properties a bigger cylinder may be required.
