Hot Water Runs Out Quickly? Why Your Cylinder Goes Cold

Check how far down the cylinder actually gets hot

This is the single most useful test, and it needs nothing but your hand. After the cylinder has had its normal heating time, with nobody using hot water, open the airing cupboard and run your palm slowly down the side of the cylinder from top to bottom. If there is foam insulation, feel the pipework and any bare metal at the top and bottom, or press gently on the jacket.

A cylinder that is working properly is hot from the top to within a short distance of the bottom. If it is hot at the top and goes cold a third or halfway down, the tank is only part heated, and the hot water will run out at exactly the point you would expect. That result points at the heat source rather than at your usage. If the cylinder is hot all the way down and the hot still runs out quickly, the problem is more likely the size of the cylinder, the flow rate of what is drawing from it, or cold water getting into the hot side somewhere. If the cylinder is cold from top to bottom, that is not this problem at all; the guide to a hot water tank not heating covers it, and water that never gets properly hot at any tap is dealt with under water not getting hot.

The immersion heater is on the wrong setting

A lot of older cylinders have a dual immersion, either two separate elements or one head with a short element and a long one, with a switch marked Bath and Sink. Sink runs the short element near the top and heats only a small amount of water, enough for washing up. Bath runs the long element and heats the whole cylinder. If the switch has been left on Sink, or someone flicked it by mistake, you get a small amount of hot water and then nothing.

It sounds too simple, but it is common in rented flats and after a change of occupant. Switch it to Bath, allow a full heating period, and test again. If the Bath side does not heat the full cylinder, the long element or its thermostat may have failed, and the short element has been quietly doing all the work. That is a straightforward element replacement, set out on our immersion heater repair page.

A scaled coil only heats the top of the tank

If the cylinder is heated by the boiler, the heat reaches the water through a coil of pipe inside the tank. In hard water, and London water is hard, scale builds up on the outside of that coil over the years. Scale is an insulator, so the coil passes its heat on more slowly. The top of the cylinder warms because hot water rises, but the lower part never gets properly hot in the time the programmer allows.

The signs build slowly. The hot water lasts less than it used to, the cylinder takes longer to recover after a bath, and you may hear rumbling or ticking from the cylinder as it heats. A sludged heating system can do the same from the inside of the coil. Neither can really be cleaned out of a cylinder in place, so a badly scaled coil is often the point at which a replacement becomes worth considering, particularly on an older copper cylinder.

Cylinder thermostat, timer or boiler settings

The cylinder thermostat, a small dial strapped to the side of the tank, tells the boiler when the water is hot enough. If it has been turned down, or it has failed and switches off early, the cylinder stops charging before it is full of hot water. Stored water should be kept at 60 degrees or above, which is the temperature at which Legionella bacteria are controlled, so the dial should not be set lower to save money.

The programmer matters too. If the hot water channel only runs for half an hour in the morning, the cylinder may never get a full charge. And if the boiler flow temperature has been turned down for efficiency on the heating side, the coil may not be hot enough to bring the whole cylinder up to temperature. Try setting the hot water to constant for a few hours and the cylinder thermostat to around 60, then repeat the hand test. If the tank now heats all the way down, it was the settings.

The cylinder is too small for how you use it now

Sometimes nothing is broken. A cylinder that was sized for a couple is now serving a family, a second bathroom has been added, or a new shower head delivers far more water per minute than the old one. On an unvented cylinder in particular, where the showers run at mains pressure, a high flow shower can empty a smaller cylinder surprisingly quickly.

The test is to heat the cylinder fully, confirm with your hand that it is hot top to bottom, and then run one shower and time it. If a single ordinary shower drains a fully heated cylinder, the cylinder is undersized for the demand. Our guide to hot water cylinder sizes explains how size relates to bathrooms and occupants, and the answer may be a larger cylinder rather than a repair.

Cold water getting into the hot side

Less common, but easy to miss. If a mixer tap, a shower valve or a thermostatic blending valve has a failed internal check valve, mains cold water can cross into the hot pipework when a tap is open elsewhere. The hot at the tap then seems to cool quickly, even though the cylinder is full of hot water. The clue is that the cylinder is hot to the touch at the top while the tap runs lukewarm.

On some cylinders the cold water inlet at the bottom has a spreader fitted to stop the incoming water churning up the stored hot water. If that is missing or has come adrift, cold water mixes into the store as you draw off, and the usable hot water falls away faster than it should. Both need an engineer to confirm, because the symptom at the tap looks the same as a heating fault.

• Hot at the top, cold halfway down: the tank is only part heated, look at the immersion setting, coil or controls.
• Hot top to bottom, still runs out after one shower: the cylinder is too small for the demand.
• Cylinder hot but taps run lukewarm: cold crossing into the hot side, a mixer, shower valve or blending valve.
• Slower and noisier every year: scale on the coil or the element.

If you have a combi boiler instead

A combi does not store hot water, so it cannot run out. If the water from a combi goes cold after a few minutes, the boiler is cutting out or failing to keep up, which usually means a scaled plate heat exchanger, a diverter valve not holding its position, a flow problem or a fault the boiler is locking out on. That is covered on our page on no hot water from a combi boiler. If it is only the shower that goes cold, the no hot water in the shower guide deals with the valve and cartridge side of it.

When to call us

Call if the cylinder only heats part way down on the correct settings, if the immersion does not heat the full tank on the Bath setting, if the cylinder is noisy or much slower than it used to be, or if the hot is running lukewarm at the taps while the cylinder feels hot. It helps to tell us what you found with the hand test, whether you have loft tanks or a mains pressure unvented cylinder, and roughly how long the hot water lasts.

We have 20 engineers across London and hot water cylinders are our core work, so we can test the coil, the controls and the immersion on one visit and tell you whether it is a part, a setting, or a cylinder that no longer suits the house. Our rate is on the pricing page and agreed before we attend.