There's an article of that title that's appeared on the wiki
Bigger Tank
----------- The obvious option is a bigger tank. Thankfully there are easier ways 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 95C gives a lot more hot capacity, but high temperatures are a serious burn risk, and best avoided.
Drain Heat Exchanger
-------------------- A lot of heat goes down shower drains. Nearly the entire contents of the hot water tank in fact.
A Drain Heat Exchanger recovers a good percentage of this heat, returning it to the shower cold feed. So less hot water is needed to bring it upto temp, and the tankful lasts longer.
By reducing energy use these exchangers can pay back their cost in under a year in some cases. For information see the main Drain Heat Exchanger article.
Move Thermostat Lower
--------------------- HW tank thermostats are typically 2/3 the way down the tank, but are sometimes higher up. HW tanks heat up from the top 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 tank. 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 exchanger or element, heating that extra bottom zone of the HW tank will take longer, and the top water will get hotter than the stat setting.
Solar Preheater
--------------- A solar preheater produces a batch of warm water which is fed into the tank when hot water is used, rather than the HW tank drawing cold water in as happens with most HW tanks. 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 varying significantly from one design to another.
With any solar thermal equipment, it is strongly recommended 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.
Use CH Circuit Heat
------------------- The Central Heating radiator circuit contains hot water in winter and tepid Water in summer. It is possible to harvest this heat and add it to the HW, thereby giving greater HW capacity.
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 output for a while.
- With a conventional HW tank, the cold feed to the HW tank goes through the CH exchanger.
Performance
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.
The conventional tank 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. Although capacity is not improved as much, HW recovery times will be improved all year round, and more so in winter.
Electric Boost
-------------- This one is just an idea for discussion, and has not been tried by the author. You should not try it without finding out whether it would work ok first.
Add a 3kW heater to the cold water feed at the shower & bath taps, along with a low temperature thermostat to avoid uncomfortable temperature rise.
If heater flow is restricted, it may be possible to split the feed into 2 parallel pipes, one that goes through the heater and one that contains an isolating valve. The 2 feeds are then reunited. Adjustment of the iso valve determines the flow sharing.
By prewarming the cold feed, less HW is needed, so the tanked HW lasts longer.
Another variant is to add a 3kW heater to the combi heated water output (or possibly cold water input), and trigger it by a flow switch on the shower & bath hot water feed.
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I have some questions which I posted to the article's discussion page: