Increase Hot Water Capacity

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Does the hot water run out too soon? 6 ways to increase hot water capacity are discussed.

This page represents the ideas of one individual editor of this Wiki
There has been a discussion of these ideas on the uk.d-i-y newsgroup

Bigger Hot Water Cylinder

The obvious option is a bigger cylinder. However this isn't always a suitable option, eg when renting accommodation, or the best option, eg when budget is lacking. There are other options too.

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 your HW lasts longer.

Taking this method to extremes, setting the stat to 95°C would give a lot more hot capacity, but such high temperatures are a burn risk, and best avoided.

A lot of electric immersion heaters can be set to any desired temperature, even non-recommended temperatures such as 95°C. Newer immersion heater thermostats can not be set to these elevated temperatures.

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

The downsides are the scald risk from 100% hot water, increased scaling in hard water areas, and increased standing heat loss from the cylinder.

Turn down booster pump

Shower boost pumps increase hot water use. Slowing them down thus reduces use and prolongs available shower time.

Single speed booster pumps have no built in speed adjustment. The speed of these may be reduced with a series capacitor, but correct calculation and ratings are required for the capacitor. News:sci.electronics.basics would be a good place to ask if you need to work out what value & voltage capacitor to use, quoting the pump power, VA or power factor and dc resistance.

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 much 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.

Move Thermostat Lower

HW cylinder thermostats are typically 2/3 the way down the cylinder on gas heated systems, but are sometimes higher up. 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, and this could be heated to gain more HW capacity.

Moving the thermostat further down will increase the quantity of hot water 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 much 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 much lower than the exchanger in a gas powered system, the water will not overheat, but the circulation pump will run 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 in as happens with most HW cylinders. The result is less energy use and to some extent increased HW output. The amount of output increase will depend on the temperature of the preheat water.

There are several designs of solar preheater, with performance, cost, payback and ease of fitting varying widely from one design to another.

With any solar thermal equipment, it is wise to assess any proposed system properly before construction, since many designs are unable to pay back their cost. Professionally supplied systems are worse than DIY ones in this respect.

Solar Thermal

Use CH Circuit Heat

The Central Heating radiator circuit contains hot water in winter and tepid water in summer. It is possible to harvest the heat stored in this water and add it to the HW feed, thereby giving greater HW capacity. How much capacity increase can be achieved depends on the volume and temperature of the water in the central heating circuit, thus this gives greater return with larger houses.

Ways to arrange this:

  • With a heat bank or Thermal Store, The cold water supply goes through a heat exchanger on the CH circuit before going to the HW heat exchanger.
  • With a combi, the cold water supply to the combi goes through the CH exchanger, thus boosting the heat heat output for a while and enabling quicker bath filling. Only some combis will accept pre-heated water.
  • With a conventional HW cylinder, the cold feed to the cylinder goes through the CH exchanger.


The Heat bank option uses all the CH heat. It is the most effective option.

How much boost the combi option gives depends on system design and boiler characteristics. It may prove to be advantageous to restrict the CH heat exchanger size to ensure the boiler does not modulate at first during HW heating. This option speeds up bath filling by using the CH system's stored heat.

The conventional cylinder variant can only benefit from part of the CH circuit's heat capacity. Once the CH circuit drops below usable HW temp, the remaining heat will not be used in a way that increases HW output at the tap. Hence there will be no HW boost in summer. HW recovery times are improved in winter.

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