Increase Hot Water Capacity

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Does the hot water run out too soon? Some ways to increase capacity:

Bigger Hot Water Cylinder

The obvious option is a bigger cylinder. However this isn't always a good option, eg when renting, or if budget is lacking. There are other options.

Higher Thermostat Temperature

The simplest way to increase capacity is to turn the HW thermostat up. This means the hot water is used more slowly, as less of it is needed to bring the shower or bath upto the required temp. Thus the HW lasts longer.

If incoming water is 10C, and HW temps are 55C and 75C before and after adjustment, the capacity increase is (75-10)/(55-10) = 1.44 or 44% more.

Avoid temperatures that are a burn risk.

A lot of electric immersion heaters can be set to any desired temperature, even burn risk temperatures such as 95°C. Newer immersion heater thermostats can't be set to these elevated temperatures.

Gas boiler water heating works a bit differently, and the cylinder stat should be set to a temp below the primary circuit temp. If its set higher, the circulation pump will run continuously, using power for no gain. Primary temp varies according to system design and setting, but if the pump runs all day, the cylinder stat is set too high.

The possible downsides are the scald risk if too hot, increased scaling in hard water areas, and increased standing heat loss from the cylinder.


Aeration of shower and taps reduces water use, making a given amount of hot water last longer. Aerators are cheap and easy to fit to showers.

Slower booster pump

A Shower boost pumps with no speed adjustment can have max flowrate & pressure reduced with a valve, if one isn't already fitted, giving longer shower time.

Drain Heat Exchanger

A lot of heat goes down shower drains. Nearly the entire contents of the hot water cylinder in fact.

A Drain Heat Exchanger recovers a percentage of this heat, returning it to the shower cold feed. So less hot water is needed to bring it upto temp, and the hot water lasts longer.

By reducing energy use, these exchangers can pay back their cost several times over in some cases. Actual payback depends on system design and use, and only sometimes justifies their use.

Move Thermostat Lower

HW cylinder thermostats are typically 2/3 the way down the cylinder on gas heated systems, but are sometimes higher up. If higher up, moving it down can increase HW capacity.

HW cylinders heat up from the top section downwards, and water below the stat will be at lower temp than the stat setting. Often this cooler water is no more than lukewarm.

Moving the thermostat down increases HW quantity in the cylinder. The implications depend on where the stat is in relation to the heating element or built in exchanger.

  • If the stat was high up and is moved to 2/3 down, things will behave normally after moving
  • If the stat is moved lower than the electric heating element, the water may overheat. This needs to be checked for when the stat is moved, and if it occurs the stat needs to be moved back up. Electrically heated hot water must not be left in an overheating condition.
  • If the stat is moved lower than the exchanger in a gas powered system, the water doesn't overheat, but the circulation pump runs continuously. If this occurs, move the stat further up to avoid any energy waste.

Solar Preheater

A solar preheater produces a batch of warm water which is fed into the cylinder when hot water is used, rather than the HW cylinder drawing cold water. Whether this increases effective HW capacity depends on the temp of the incoming solar preheated water, and what the HW is used for.

  • Hot preheated water increases effective capacity
  • Warm preheated water increases effective capacity with baths, but not other uses

There are several designs of solar preheater, with performance, cost, payback and ease of fitting varying widely.

With any solar thermal equipment, any proposed system needs proper assessment before construction, as many designs never pay back their cost. Professionally supplied systems are worse than properly designed DIY ones in this respect.

See Solar Thermal

See Also