Hot Water Cylinders for Heat Pumps Sizing Guide
Correctly sizing hot water cylinders for heat pumps is important because an air source or ground source heat pump normally heats domestic hot water at a lower flow temperature than a traditional gas or oil boiler. The cylinder therefore needs sufficient hot water storage capacity, an appropriately sized heat exchanger coil, and enough heat-transfer surface area to allow the heat pump to recover the stored domestic hot water (DHW) efficiently.
A heat pump cylinder should not be selected purely by its litre capacity. The number of occupants, showers and baths, peak hot water demand, heat pump output, flow temperature, cylinder coil size, recovery time and desired storage temperature all need to be considered.
What Size Hot Water Cylinder Do I Need for a Heat Pump?
The table below provides a useful starting point for a typical UK residential property.
| Occupancy | Typical Heat Pump Cylinder Size |
|---|---|
| 1 to 2 people | 150 to 180 litres |
| 2 to 3 people | 180 to 210 litres |
| 3 to 4 people | 210 to 250 litres |
| 4 to 5 people | 250 to 300 litres |
| 5 to 6 people | 300 to 350 litres |
| 6+ people | 350 litres+ |
These figures are indicative rather than a substitute for a proper domestic hot water demand calculation.
A four-person household taking short showers may comfortably use a smaller cylinder than another four-person household with two large baths, high-flow showers and significant simultaneous hot water demand.
Why Are Heat Pump Cylinders Often Larger?
Heat pumps generally produce domestic hot water differently from high-temperature boilers.
A conventional boiler may deliver a relatively high thermal output at flow temperatures of 70°C or more. An air source heat pump normally operates most efficiently at substantially lower water temperatures.
Because of this, heat pump systems often rely more heavily on:
Stored hot water volume
Heat exchanger surface area
Efficient heat transfer
Longer recovery periods
Correct cylinder stratification
A slightly larger heat pump hot water cylinder can provide the stored energy required to satisfy peak demand while allowing the heat pump to recover the cylinder gradually and efficiently.
Size the Cylinder According to Hot Water Demand
Occupancy is a useful starting point, but the most important factor is the property’s actual DHW demand.
Consider:
Number of occupants
Number of bathrooms
Number of showers
Shower flow rates
Number and size of baths
Kitchen hot water use
Simultaneous outlets
Morning and evening peak demand
How much recovery time is available between demand periods
For example, four people taking consecutive showers at 7:00 am creates a very different load from four people showering at different times throughout the day.
The heat pump has much more opportunity to recover stored hot water when demand is spread over several hours.
Allow for Shower Flow Rates
Shower flow rate can have a significant impact on the size of a hot water cylinder for a heat pump, particularly where several showers are used consecutively.
For example, a showerhead that delivers 8 litres per minute for 5 minutes will use:
8 litres/minute × 5 minutes = 40 litres of mixed water
This does not mean that 40 litres of hot water is drawn directly from the cylinder. The shower mixes stored hot water with incoming cold mains water to achieve the required outlet temperature.
For example, if the cylinder stores water at 55°C, the incoming cold water is 10°C, and the shower temperature is 40°C, approximately 27 litres of stored hot water would be required to provide the 40 litres of mixed shower water.
When sizing a heat pump hot water cylinder, it is therefore important to consider shower flow rate, shower duration, the number of consecutive showers, stored hot water temperature, incoming cold water temperature and the available cylinder recovery time.
Allow for Bath Usage
Baths can place a particularly high instantaneous demand on the hot water cylinder.
Depending on its size, a bath may require approximately 80 to 150 litres of mixed water.
A property with:
Four occupants
One standard shower
No regular baths
could therefore have considerably lower peak demand than a property with:
Four occupants
Two rainfall type showers
A large freestanding bath
This is why cylinder sizing should be based on usage rather than bedroom count alone.
Stored Water Temperature Changes the Usable Hot Water Volume
Cylinder capacity is not the same as the volume of usable mixed water available at the taps.
Hot water stored at a higher temperature can be blended with cold mains water through a thermostatic mixing valve or mixer outlet.
A simplified calculation is:
Mixed water volume = cylinder volume × (stored temperature − cold water temperature) ÷ (mixed water temperature − cold water temperature)
For example:
Cylinder capacity: 250 litres
Stored temperature: 55°C
Incoming cold water: 10°C
Required mixed water: 40°C
The theoretical mixed volume is:
250 × (55 − 10) ÷ (40 − 10)
= 375 litres
In real installations, the available volume will be affected by stratification, heat losses, incoming cold-water mixing and minimum usable delivery temperature.
The Heat Exchanger Coil Is Critical
One of the most important differences between a heat pump cylinder and a standard boiler cylinder is the heat exchanger.
A heat pump cylinder normally requires a large surface-area coil.
This is because heat transfer becomes more difficult as the temperature difference between the heat pump flow water and stored domestic water becomes smaller.
For example, transferring heat from water at 75°C into a cylinder at 50°C is comparatively straightforward.
Transferring heat from a heat pump operating at around 50°C to 55°C into domestic water at a similar temperature requires a much larger heat-transfer area.
A suitable cylinder therefore needs:
Large heat exchanger surface area
Good coil positioning
Low hydraulic resistance
Good thermal transfer
Compatibility with the heat pump output
Simply connecting a heat pump to an old boiler cylinder with a small coil can result in poor heat transfer, long recovery times and reduced system performance.
Match the Cylinder Coil to the Heat Pump Output
The cylinder heat exchanger must be capable of accepting the heat pump’s thermal output.
For example, if an air source heat pump can deliver 8 kW while producing domestic hot water, the cylinder coil should be capable of transferring that heat effectively at the manufacturer’s stated operating temperatures.
It is not enough to simply compare coil surface area between two cylinders.
You should ideally check the manufacturer’s:
Heat exchanger output
Coil surface area
Primary flow temperature
Primary flow rate
Pressure drop
Cylinder water temperature
Recovery performance
A large coil with poor flow characteristics can still limit system performance.
Calculate the Energy Needed to Heat the Cylinder
The energy required to heat stored water can be estimated using:
Energy required (kWh) = litres × temperature rise × 0.001163
For a 250 litre heat pump cylinder being heated from 10°C to 55°C:
Temperature rise:
55 – 10 = 45°C
Therefore:
250 × 45 × 0.001163 = 13.08 kWh
Approximately 13.1 kWh of thermal energy is required.
If the heat pump provides 5 kW of thermal output to the cylinder:
13.08 ÷ 5 = 2.62 hours
The theoretical full-cylinder recovery time would be approximately 2.6 hours.
Actual recovery time will depend on factors including heat exchanger performance, heat pump output, flow temperature and the temperature of the stored water.
Recovery Time Is as Important as Cylinder Capacity
A larger cylinder provides greater stored hot water capacity, but it also requires more energy to reheat.
Correct sizing therefore balances:
storage capacity + heat pump output + recovery time
For example, a 300 litre cylinder may be appropriate for a high-demand household, but fitting a very large cylinder to a relatively low-output heat pump could lead to excessively long recovery periods.
Conversely, selecting too small a cylinder may result in the household regularly exhausting the stored hot water before the heat pump has time to recover it.
Heat Pump Flow Temperature Matters
The domestic hot water flow temperature affects both efficiency and recovery performance.
As the required cylinder temperature increases, an air source heat pump generally has to work harder.
Higher water temperatures can result in:
Lower COP
Reduced heat pump output
Longer compressor run times
Increased electricity consumption
For this reason, the cylinder should be designed to achieve effective heat transfer at the relatively low primary temperatures associated with heat pumps.
Cylinder Stratification
Effective thermal stratification is beneficial in a heat pump hot water cylinder.
Hot water naturally rises to the upper part of the cylinder while cooler water remains lower down.
Maintaining this temperature layering can provide usable hot water at the cylinder outlet even while the lower portion of the cylinder is cooler.
Cylinder design, coil position, cold-water inlet arrangement and draw-off rates can all influence stratification.
Poor mixing characteristics can reduce the amount of useful hot water available from the nominal cylinder volume.
Direct or Indirect Heat Pump Cylinder?
Most cylinders connected to a separate air source heat pump are indirect hot water cylinders.
The heat pump circulates primary heating water through a heat exchanger coil inside the cylinder. Heat passes through the coil wall into the potable domestic water without the two water circuits mixing.
A typical indirect heat pump cylinder therefore contains:
Large surface-area heat exchanger coil
Cold-water inlet
Hot-water outlet
Temperature sensor pocket
Immersion heater connection
Cylinder thermostat or temperature sensor
Insulation
Where an unvented cylinder is used, additional safety and expansion components will also be required.
Unvented Heat Pump Cylinders
Many modern UK installations use an unvented hot water cylinder.
An unvented cylinder is supplied directly from the incoming mains water rather than relying on a cold-water storage cistern.
Depending on the system design, associated components can include:
Expansion vessel
Inlet control group
Expansion relief valve
Temperature and pressure relief valve
Tundish
Motorised safety valve
Immersion heater
Thermostat and thermal cut-out
Installation of an unvented hot water storage system must comply with applicable UK Building Regulations, including the relevant requirements of Approved Document G and G3.
Immersion Heater Capacity
A heat pump cylinder will commonly include an electric immersion heater.
It can provide:
Emergency backup
Additional recovery capacity
High-temperature boost
Thermal disinfection
Anti-Legionella operation
The immersion heater should not normally be used as the primary heat source during everyday operation where the objective is to maximise heat pump efficiency.
Heat pump operation can achieve a Coefficient of Performance (COP) greater than 1, whereas a conventional resistance immersion heater effectively converts approximately one unit of electrical energy into one unit of heat.
Twin Coil Heat Pump Cylinders
A twin-coil hot water cylinder may be appropriate where two heat sources are connected.
For example:
Air source heat pump + solar thermal
Heat pump + boiler
Heat pump + another renewable heat source
The heat pump should be connected to a sufficiently large heat exchanger designed for low-temperature operation.
The position and size of each coil need to be considered carefully because coil arrangement can influence available storage capacity and cylinder stratification.
Do Not Oversize the Cylinder Unnecessarily
A larger cylinder is not automatically better.
Excessive cylinder capacity can increase:
Standing heat losses
Heat-up time
Installation cost
Required floor space
Stored water volume
System weight
A correctly sized cylinder should provide enough hot water for peak demand without storing substantially more hot water than the property requires.
Good cylinder insulation and a low standing heat loss are especially important with larger storage vessels.
Problems Caused by an Undersized Heat Pump Cylinder
Undersizing can be equally problematic.
Possible consequences include:
Running out of hot water
Increased immersion heater use
Poor bath and shower performance
Frequent reheating cycles
Insufficient recovery between demand periods
Increased electricity consumption
It is therefore important to consider the whole system rather than choosing the smallest cylinder that appears suitable for the number of occupants.
Heat Pump Cylinder Sizing Example
Consider a typical UK home with:
Four occupants
Two bathrooms
Two showers
One bath
Moderate shower flow rates
Main hot water demand in the morning and evening
A 210 to 250 litre heat pump cylinder could provide an appropriate starting range.
If the same property had two high-flow rainfall showers and a large bath that was regularly used, a 250 to 300 litre cylinder could be more appropriate.
The final selection should then be checked against the:
Heat pump output
Heat exchanger coil rating
Coil surface area
Primary flow temperature
DHW storage temperature
Recovery time
Standing heat loss
Immersion heater capacity
Required peak hot water volume
Heat Pump Cylinder Sizing for Commercial Applications
Commercial heat pump cylinder sizing should be based on a proper hot water demand profile rather than simply the number of occupants.
Applications such as:
Hotels
Care homes
HMOs
Student accommodation
Gyms
Leisure centres
Changing facilities
Restaurants
can experience concentrated periods of very high DHW demand.
The designer should establish:
Daily hot water consumption
Peak hourly demand
Maximum simultaneous demand
Required storage temperature
Incoming mains water temperature
Available cylinder volume
Heat pump thermal output
Heat exchanger capacity
Required recovery period
Backup heat source capacity
Large systems may use multiple hot water cylinders, multiple heat pumps or cylinders piped in parallel to achieve the required storage and recovery capacity.
Key Information Needed to Size a Heat Pump Cylinder
Before selecting a cylinder, establish:
Number of occupants
Number of bathrooms
Number of showers
Shower flow rates
Number and size of baths
Peak domestic hot water demand
Daily DHW consumption
Desired storage temperature
Incoming cold-water temperature
Required mixed water temperature
Heat pump heating capacity
Heat pump DHW flow temperature
Heat exchanger surface area
Heat exchanger output
Cylinder recovery time
Standing heat loss
Immersion heater output
Available installation space
What Is the Most Important Factor When Sizing Hot Water Cylinders for Heat Pumps?
The correct hot water cylinder for a heat pump needs enough storage capacity to satisfy the property’s peak domestic hot water demand while providing a sufficiently large heat exchanger coil to transfer the heat pump’s output efficiently at low primary water temperatures.
Cylinder volume alone should therefore never be the only selection criterion.
A properly designed system balances DHW demand, cylinder capacity, heat exchanger surface area, heat pump output, flow temperature, recovery time, stored water temperature and standing heat loss.