Is a Larger Hot Water Cylinder Always Better?
Is a Larger Hot Water Cylinder Always Better? No, a larger hot water cylinder is not always better. The best cylinder is one that stores enough domestic hot water to cover the property’s busiest periods without being unnecessarily oversized. A larger cylinder can provide more usable hot water for showers and baths, but it also costs more, occupies more space, takes more energy to heat from cold and can have greater total standing heat losses. With an air source heat pump water heater, cylinder capacity also needs to be considered alongside heat pump output and recovery time.
What are the benefits of a larger hot water cylinder?
The main advantage is straightforward: you have more stored hot water available.
This can be useful in properties with:
- Several occupants
- Multiple bathrooms
- Large baths
- High-flow showers
- Several showers used one after another
- High morning or evening hot water demand
A larger cylinder can also be useful with a heat pump water heater because heat pumps generally reheat stored water more gradually than high-output boilers.
Instead of relying on rapid recovery, the system can store enough hot water in advance to cover periods of peak demand.
At the compact end of our range is the 100 litre wall mounted air source hot water heater, intended for smaller domestic and selected light commercial hot water requirements.
For larger homes, compare the 200 litre home heat pump water heater with the 300 litre home heat pump water heater.
For applications requiring considerably greater hot water storage, consider our 400 litre high-capacity air source water heater or 500 litre high-capacity air source water heater.
Can a hot water cylinder be too large?
Yes.
If a household only uses a modest amount of domestic hot water, there is little benefit in heating and storing a much larger volume than required.
The cylinder itself will normally be more expensive and physically larger. This can be particularly important in homes where cupboard or plant room space is limited.
There are also heat losses to consider.
Even a well-insulated cylinder gradually loses heat to its surroundings. This is known as standing heat loss. A larger cylinder does not automatically have poor insulation, but its greater surface area and stored volume mean its total heat loss should be checked when comparing products.
Does a larger cylinder use more electricity?
Not necessarily in direct proportion to its size.
If you heat 300 litres of water but only use 100 litres, the remaining hot water is still stored for later use. You are not reheating the entire cylinder from cold every day.
However, a larger cylinder will require more energy for a complete heat-up from cold, and there may be greater standing heat losses over time.
How much electricity a heat pump water heater actually consumes will depend more broadly on hot water usage, storage temperature, COP, air temperature, heat loss and immersion heater use.
This is why cylinder size should be matched to demand rather than simply choosing the largest model available.
Does a bigger cylinder take longer to heat?
If the heat pump output remains the same, generally yes.
A 300 litre cylinder contains more water than a 200 litre cylinder, so raising the entire volume through the same temperature difference requires more energy.
This affects the recovery time.
For example, fitting a much larger cylinder while keeping the same heat pump heating capacity could result in a significantly longer full heat-up time.
That may not be a problem if the cylinder is heated overnight or during periods of low demand, but it needs to be considered when designing the system.
Can a larger cylinder improve heat pump efficiency?
Potentially, but not simply because it is larger.
Having enough stored water can allow the heat pump to operate steadily rather than relying on frequent high-temperature boost cycles or the electric immersion heater.
A larger cylinder may also make it easier to take advantage of off-peak electricity tariffs or solar PV. The heat pump can heat water when electricity is cheap or when surplus solar generation is available, storing that energy as hot water for use later.
In this situation, the cylinder acts as a form of thermal energy storage.
However, oversizing the cylinder excessively simply to store energy is unlikely to be worthwhile unless there is a clear reason for doing so.
Does storage temperature affect the cylinder size I need?
Yes.
The nominal cylinder capacity is not the same as the amount of usable mixed hot water it can provide.
Water stored at 60°C can be mixed with cold mains water to produce a larger volume at a typical shower temperature of around 38°C to 40°C.
A 200 litre cylinder can therefore provide more than 200 litres of usable mixed water.
However, running an air source heat pump at a higher water temperature generally increases the required temperature lift and can reduce its Coefficient of Performance (COP).
It is usually better to size the cylinder correctly than to rely on unnecessarily high storage temperatures to compensate for insufficient capacity.
What happens if the cylinder is too small?
Undersizing can be just as problematic as oversizing.
If the stored hot water is regularly exhausted, the heat pump may need to recover the cylinder during periods of high demand.
The immersion heater may also be brought into operation more frequently to provide faster recovery. This can increase electricity consumption and reduce the running cost advantage of the heat pump.
A little spare capacity can therefore be useful, particularly where hot water demand varies from day to day.
How do I choose the right hot water cylinder size?
Choose a cylinder based on the property’s peak hot water demand, not simply the maximum number of litres you can physically accommodate.
Consider the number of occupants, shower flow rates, bath volumes, number of bathrooms, storage temperature, usable mixed water volume, heat pump output and recovery time.
For a heat pump water heater, the ideal cylinder provides enough stored hot water to comfortably cover the busiest part of the day while still allowing the heat pump to recover it efficiently afterwards.
Bigger can be useful, but correctly sized is better.