Do you still need a large hot water cylinder with a heat pump?
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Anyone switching to a heat pump will sooner or later face the same question: how large does the hot water cylinder actually need to be?
While a traditional gas combi boiler can heat domestic hot water rapidly as it is needed, a heat pump works differently. Domestic hot water is usually stored in a cylinder, which means the cylinder's capacity plays an important role in determining the level of comfort in the home.
But does that automatically mean bigger is always better? Or are there smarter ways to ensure sufficient hot water while using a smaller cylinder?
Why do heat pumps need a hot water cylinder?
Heat pumps can produce hot water very efficiently, but they generally heat it more slowly than a gas combi boiler.
For this reason, domestic hot water is usually stored in a hot water cylinder. This provides a reserve of heated water for showers, baths and other everyday uses.
The required cylinder size depends on several factors, including:
- the number of occupants;
- showering habits;
- the type and flow rate of the shower;
- the desired level of comfort;
- whether the home has a bath.
How large is a typical heat pump hot water cylinder?
There is no universal cylinder size for a particular household. As a broad indication, commonly used capacities are approximately:
| Household | Indicative cylinder capacity |
|---|---|
| 1–2 people | 120–150 litres |
| 3–4 people | 170–250 litres |
| 5+ people | 210–300 litres or more |
These figures should only be regarded as a starting point. The actual requirement depends heavily on how hot water is used, as well as the cylinder temperature, heat pump performance and recovery time.
A family of four taking short, efficient showers may require less stored hot water than a two-person household where both occupants take long showers using a high-flow rain shower.
The impact of modern showers
Rain showers are popular because of the comfort they provide, but they can use considerably more water than water-saving shower heads.
A water-saving shower head typically has a flow rate of no more than around 7.2 litres per minute. Large rain showers can have significantly higher flow rates, with some using around 12–15 litres per minute or more.
This directly affects:
- hot water consumption;
- the required cylinder capacity;
- the amount of energy needed to produce hot water.
The greater the demand for hot water, the greater the required storage capacity will generally be.
A larger cylinder also has disadvantages
It can be tempting to choose the largest possible cylinder simply to make sure there is always enough hot water available.
However, larger cylinders also have disadvantages.
Higher purchase and installation costs
Larger cylinders are generally more expensive to purchase and install.
More space required
Not every utility or plant room has enough space for a large hot water cylinder.
Potentially higher standing heat losses
Even a well-insulated hot water cylinder gradually loses some heat to its surroundings. A larger store of hot water can mean greater total standing losses, although the actual losses depend strongly on the cylinder's insulation, construction and operating temperature.
A larger cylinder is therefore not automatically the most energy-efficient solution.
Using hot water more intelligently
Instead of simply increasing storage capacity, there are several ways to reduce hot water demand or use available energy more efficiently.
Examples include:
- water-saving shower heads;
- smart hot water controls;
- demand-based heat pump controls;
- shower wastewater heat recovery.
Shower heat recovery is particularly interesting because it can reduce both energy demand and the amount of stored hot water required.
How does shower heat recovery work?
A shower heat recovery system uses heat from warm wastewater leaving the shower to preheat incoming cold mains water.
The two water streams remain completely separate. Heat is simply transferred from the outgoing shower water to the incoming cold water through a heat exchanger.
As a result, less additional energy is required to bring the incoming water up to the desired shower temperature.
This offers several benefits:
- lower energy consumption;
- reduced demand on the heat pump;
- reduced demand on the hot water cylinder;
- more usable shower time from the same amount of stored hot water.
Depending on the type of system and installation, shower heat recovery can reduce the energy required for showering by roughly 40–50%.
In practice, this can also mean that a household may be able to use a smaller hot water cylinder than would otherwise be required.
More comfort without larger installations
For many homeowners, comfort is just as important as reducing energy consumption.
Nobody wants to run out of hot water halfway through a shower.
This is where heat recovery provides an interesting advantage. Because some of the heat that would normally disappear down the drain is immediately reused, less stored hot water is required to achieve the same shower temperature.
The available hot water supply can therefore effectively last longer.
This can be particularly useful for:
- families with several children;
- homes with high-flow or rain showers;
- homes with limited plant-room space;
- apartments and renovation projects.
The energy transition requires greater efficiency
The Netherlands is gradually moving towards electric heating systems and heat pumps.
As a result, using domestic hot water efficiently is becoming increasingly important.
Simply installing ever-larger hot water cylinders does not solve every challenge. In many situations, making better use of energy that is already available can be a more efficient approach.
Shower heat recovery fits well within this development because it captures heat that would otherwise simply disappear down the drain.
What is the best choice?
There is no single answer to the question of how large a hot water cylinder should be.
The right choice depends on:
- the number of occupants;
- daily showering habits;
- shower flow rates;
- the desired level of comfort;
- the available installation space;
- the heat pump and cylinder specifications;
- additional energy-saving measures.
Households that reduce hot water demand or recover heat from shower wastewater may, in some situations, be able to choose a smaller storage cylinder without sacrificing comfort.
Conclusion
A larger hot water cylinder is not always the best solution when installing a heat pump. Using hot water intelligently can be just as important as adding more storage capacity.
Technologies such as shower wastewater heat recovery reduce the amount of energy required for showering and can significantly reduce the demand placed on both the heat pump and the hot water cylinder.
This makes it possible to achieve a better balance between comfort, sustainability, space requirements and energy efficiency.
In modern homes, after all, making a property more sustainable is not simply about installing more capacity. It is increasingly about making smarter use of the energy that is already available.
