What Affects the Efficiency of a Heat Pump Water Heater?
What Affects the Efficiency of a Heat Pump Water Heater? The efficiency of a heat pump water heater is actually pretty heavily influenced by a handful of key factors: the temperature of the air it uses, the kind of hot water temperature you need, how much hot water your household gets through, the amount of heat that leaks out of the cylinder and how often you need to break in the immersion heater.
Generally, a heat pump is going to work most efficiently when it’s pulling in relatively warm air and doesn’t have to crank out water that’s way too hot. Getting the sizing right, ensuring decent airflow, and adjusting the settings sensibly can all make a noticeable difference to how much electricity the heat pump water heater uses.
For modest hot water requirements, see our 100 litre compact air source water heater, which can be suitable for smaller residential properties and selected light commercial uses.
For larger homes, compare our 200 litre residential air source hot water heater and 300 litre residential air source hot water heater.
Where hot water demand is considerably higher, see our 400 litre commercial air source DHW system or 500 litre commercial air source DHW system.
Air Temperature Has a Big Say
A heat pump water heater is essentially a device that hoovers up heat from the air around it and transfers it into the water stored in the cylinder.
The warmer the air it’s got to work with, the easier that job becomes.
Take a unit that’s pulling in air from a utility room at 18 degrees Celsius, for instance. That’s usually a lot easier for it to heat the water up to 50 degrees than one that’s sucking in outside air at 5 degrees.
As the air gets colder and colder, the compressor has to work that much harder to get the water to the same temperature – which usually ends up reducing its Coefficient of Performance (COP).
So, where the unit draws its air from is actually a pretty key part of getting the installation right.
How Hot You Need the Water Matters Too
The temperature you ask the heat pump to produce is just as important.
Heat pumps generally get less efficient as you ask them to crank out hotter water. The difference between the temperature of the heat source and the water you want is known as the temperature lift.
A unit churning out water at 50 degrees will usually do a bit better on the COP front than the same unit producing water at 60 degrees under similar conditions.
That doesn’t mean you should just turn the temperature down to the lowest setting – you still need enough hot water and a strategy for keeping Legionella under control. You just don’t want to be running unnecessarily high temperatures.
Sizing the Cylinder and Hot Water Demand
Getting the cylinder size right also affects how the system actually performs.
If the cylinder is too small, the heat pump will be running up and down all the time to meet demand – or breaking in the immersion heater.
If the cylinder is too big, that can cause its own issues, like higher standing heat losses and unnecessarily heating up water that’s never used.
So, you should match the cylinder size to the household’s actual hot water demand – taking into account the number of people in the household, the number of showers and baths they take, and the times when they need the most hot water.
Immersion Heater Can Be a Real Power Hog
Most heat pump water heaters come with an immersion heater or direct electric backup of some sort.
This is useful in case of emergencies or if you need to crank out high-temperature water for hygiene reasons. But it’s a lot less efficient than using the heat pump itself.
An immersion heater is around 1 kWh of heat for every 1 kWh of electricity it uses at point of use, whereas a heat pump operating at a COP of 3 will give you around 3 kWh of useful heat for the same amount of electricity.
If you find yourself needing to use the immersion heater every day, it’s worth checking the settings, cylinder size and hot water demand.
Airflow Matters
The evaporator needs a steady supply of air to do its job properly.
Putting a heat pump water heater in a small cupboard can cause problems because the air gets gradually colder as the unit works – then it has to pull heat out of the cold air, which can reduce its efficiency.
Ducted systems need decent airflow too. If the ducts are undersized, or there are too many bends, or the grilles are restrictive, that can restrict airflow and reduce efficiency.
You should follow the manufacturer’s guidelines on minimum room volume, duct size and airflow requirements.
Cylinder Insulation and Standing Heat Loss
Once the heat pump has heated the water, you want to keep that heat in the cylinder for as long as possible.
Hot water cylinders will always leak some heat to their surroundings – that’s standing heat loss.
Better insulation reduces those losses, so the heat pump doesn’t have to run so often just to keep the water temp up.
Standing heat loss is worth paying attention to when you’re choosing a heat pump water heater – especially if you’ve got a larger cylinder where the water might be stored for a long time.
Installation and Controls Matter
Even a pretty efficient heat pump can underperform if the installation or controls are wrong.
If the temperature settings are out, the ducting is badly designed, the airflow is poor, or the immersion heater is being used too much, all those things can increase electricity consumption.
Timing hot water production can also be useful – like in homes with solar PV, where you can use spare solar electricity to heat the cylinder.
So, How Do You Get the Best Out of a Heat Pump Water Heater?
The key to getting a good performance is to keep the temperature lift as low as you need to, make sure there’s plenty of airflow, get the cylinder sizing right and let the heat pump do most of the work.When comparing units, look beyond the flashy headline COP – take a closer look at what really matters – the COP at what people actually use – that’s right normal air and water temperatures, and the heat loss that happens when the cylinder just stands there all day, plus how the immersion heater controls work, the operating temperature range of the system, and just how fast you can recover hot water.
A well-chosen heat pump water heater that’s installed to suit the job in hand can easily use a fraction of the electricity you’d need with a direct electric water heater, but dont get too caught up on the efficiency rating on the box – good system design is almost as important as that ‘efficiency figure’ the manufacturers want you to see.