What are the disadvantages of a heat pump water heater?
What are the disadvantages of a heat pump water heater? The main downside of a heat pump water heater is that it usually costs more to buy than a conventional electric water heater and can take longer to reheat the stored domestic hot water. It also needs a suitable air source, produces some fan and compressor noise, requires condensate drainage, and can lose efficiency when the surrounding air temperature is low or when very high hot water temperatures are required. For many UK properties, however, these disadvantages can be offset by significantly lower electricity consumption compared with direct electric resistance heating.
Higher Initial Purchase Cost
A heat pump water heater is more complex than a traditional electric hot water cylinder.
Instead of relying only on an immersion heater, it typically contains a compressor, evaporator, condenser, refrigerant circuit, fan, expansion device and electronic controls.
This additional technology generally means a higher purchase price.
Installation costs may also be greater because the system can require:
- Air ducting
- Condensate drainage
- Additional electrical connections
- Suitable airflow around the appliance
- More careful positioning
The higher capital cost should therefore be compared against the potential reduction in long-term energy consumption.
For further information about our air source heat pump water heater range, please visit the following product pages:
Wall mounted air source heat pump water heaters
Domestic air source heat pump water heaters
Commercial air source heat pump water heaters
Slower Hot Water Recovery
One of the most important considerations is recovery time.
A conventional immersion heater commonly provides around 3 kW of direct electric heating. A heat pump water heater normally heats the cylinder more gradually by transferring heat from the surrounding air into the water.
This is highly efficient, but it may take longer to recover after a large volume of hot water has been used.
This makes correct hot water cylinder sizing particularly important.
A household with several showers, baths, or periods of high simultaneous domestic hot water demand may need a larger cylinder or a system with an electric immersion heater that can provide additional heating during peak periods.
The Unit Needs a Suitable Air Source
An air source heat pump water heater extracts low-grade thermal energy from the air passing across its evaporator.
If the unit takes air directly from the room in which it is installed, that room will usually become cooler while the heat pump is operating.
This can be useful in a warm utility room, plant room, or garage, but it may be less desirable if the unit is installed inside the heated envelope of a property.
During winter, removing heat from an already heated room can mean that the space-heating system has to replace some of that energy.
A ducted heat pump water heater can often avoid this issue by drawing air from outside or another suitable location, provided the arrangement complies with the manufacturer’s installation requirements.
Heat Pump Water Heaters Produce Noise
Unlike a standard hot water cylinder, a heat pump water heater contains a compressor and fan.
These create operational noise, around the same level as a refrigerator operating: circa 50 dBA.
Modern units can be relatively quiet, but positioning still matters, especially in flats, apartments, or homes where the cylinder is close to bedrooms or living areas.
The manufacturer’s sound power level and sound pressure data should therefore be considered when selecting a unit.
Condensate Drainage Is Required
As warm air passes across the cold evaporator, moisture in the air can condense.
The appliance therefore produces condensate, which must be safely discharged to a suitable drain.
This adds another installation requirement compared with a conventional electric cylinder.
Poor condensate drainage can lead to leaks or water collecting around the appliance, so the drain arrangement should be considered during the initial system design.
Efficiency Changes With Air Temperature
Heat pump water heater performance is commonly measured using the Coefficient of Performance, or COP.
A COP of 3, for example, means the heat pump can theoretically deliver around 3 kWh of heat for every 1 kWh of electricity consumed under the specified test conditions.
However, COP is not constant.
Performance is influenced by:
- Air temperature
- Hot water temperature
- Refrigerant
- Cylinder temperature
- Compressor operating conditions
- Heat losses
When the source air becomes colder, the heat pump generally has to work harder to extract useful energy.
Likewise, producing very high hot water temperatures increases the temperature lift required from the refrigerant circuit and can reduce efficiency.
High Water Temperatures Can Reduce COP
Heat pumps normally operate most efficiently at relatively moderate water temperatures.
Domestic hot water systems must also consider hygiene and Legionella control, so many heat pump water heaters incorporate a periodic high-temperature cycle.
Depending on the appliance, this may be achieved using the heat pump compressor, an electric immersion heater, or a combination of both.
Using direct electric resistance heating reduces the overall system efficiency while it is operating, although it may only be needed occasionally.
Is a Heat Pump Water Heater Worth It?
For many UK homes, the disadvantages are manageable if the system is correctly specified.
The main issues are the higher initial cost, slower recovery, noise, condensate drainage, installation requirements, and varying efficiency at different air and water temperatures.
In return, a properly selected heat pump water heater can use considerably less electricity than a conventional direct electric hot water cylinder.
The key is correct system design. Cylinder capacity, household occupancy, shower flow rates, daily domestic hot water demand, recovery time, available air temperature, storage temperature, and installation location should all be considered before choosing a heat pump water heater.