Heat pumps can be cost-effective to run (cheaper than oil or gas). But sadly, due to less-than-optimal installation, they often work out more expensive to run. This article outlines why most heat pumps systems aren’t as efficient as they could be. But first, let’s see the impact that a suboptimal design has on running costs of an air-source heat pump, assuming an electricity unit price of €0.33:
Small-Medium Home
| Design | Annual Heat Demand | Real System Efficiency | Annual Electricity Consumption | Annual Electricity Cost |
|---|---|---|---|---|
| Poor | 12000 kWh | 250% | 4800 kWh | €1584 |
| Mediocre* | 12000 kWh | 300% | 4000 kWh | €1320 |
| Good | 12000 kWh | 360% | 3333 kWh | €1100 |
| Excellent | 12000 kWh | 440% | 2727 kWh | €900 |
Large Home
| Design | Heat Demand | System Efficiency | Electricity Consumption | Electricity Cost |
|---|---|---|---|---|
| Poor | 24000 kWh | 250% | 9600 kWh | €3168 |
| Mediocre* | 24000 kWh | 300% | 8000 kWh | €2640 |
| Good | 24000 kWh | 360% | 6667 kWh | €2200 |
| Excellent | 24000 kWh | 440% | 5455 kWh | €1800 |
As you can see, the difference between a well-designed heat pump system and a poorly-designed one could easily be €1000 per year.
Now on to the most common specific reasons heat pumps don’t perform excellently:
Reason 1: Inadequate heat emitters
Heat pumps need to operate at relatively low temperatures (ideally under 45°C) to be most efficient. But 45°C is not enough to heat all houses or rooms using the existing radiators. If your radiators are not adequate, these ought to be upgraded before installing a heat pump. Failure to upgrade radiators means the heat pump will have to run hotter, thus consuming more energy.
Reason 2: Heating zones and closed valves
When valves stop or restrict the flow to several radiators, the remaining radiators have to run hotter in order to heat a home. This makes for lower efficiency. It also restricts the flow rate from the pump, which can cause the heat pump to throw and error and shut down.
The most efficient solution is to ensure that a sufficient number of heat emitters remain fully open at all times (usually those in common areas and bathrooms). These should be appropriately sized and not have any occupant-control valves fitted.
However, many heat pump installers use an alternative solution, which stops the heat pump from throwing an error, but doesn’t address the reduced efficiency: Buffer tanks and low loss headers…
Reason 3: Buffer tanks and low loss headers
Buffer tanks and low loss headers should be avoided where possible, as they result in lower efficiency. There are two reasons for this:
Problem 1: Higher flow temperature on the heat pump side. Due to the mixing that occurs inside the tank or header, your heat pump will end up running hotter than the radiators. This is bad for efficiency, as shown in an experiment directly comparing various heat pump configurations.
Problem 2: Energy consumed by the extra pump. Having a low loss header or buffer tank means you’ll be using an extra pump to circulate water through the heat emitters. This may only require, say, 200 Watts, which doesn’t sound like a lot. But as it runs constantly while your heating is on, the cost seriously adds up over the years and decades.
Buffer tanks can reduce on-off cycling in the heat pump. But simply ensuring that enough radiators are open at all times also prevents short cycling, without the need for a buffer tank. Correct heat pump sizing and using an inverter-driven (variable-speed) heat pump also help to avoid short cycling.
Reason 4: Controls not correctly set
Heat pump controllers should be set up to take advantage of features like weather compensation, providing an extra boost to efficiency. However, in many cases, heat pump installers neglect to do optimise the system controls. While it may be tedious, it’s generally worthwhile reading your heat pump’s manual and personally verifying that the controls are set up optimally, as this could save you thousands over the lifetime of the heat pump.
Reason 5: Oversized heat pump
An oversized heat pump will have to throttle down more frequently than a correctly-sized one, resulting in lower efficiency. Oversizing also increases the risk of short-cycling, reducing efficiency further, and potentially reducing lifespan.
Reason 6: Poor insulation
This one is not unique to heat pumps, but still worth a mention. Poor insulation obviously means you’ll more heat. But less obvious is that it also results in your heat pump running hotter and less efficiently. That makes for a double-whammy effect on the heat pump’s efficiency. So it’s always recommended to get your insulation in order before installing a heat pump.
Why heat pump problems are so common, and how to avoid them
Heat pumps are a relatively new technology. They’re also more complex than oil or gas boilers. So while they have the potential to perform great, in practice this is not always the case, as many installers don’t know how to get the best from them.
Moreover, the interests of the homeowner and the installer are not always aligned. For example, upgrading radiators costs the installers extra. But it’s the homeowner who reaps the rewards of a more efficient system that radiator upgrades can bring.
Here are some tips that can help you ensure that the heat pump system you purchase is designed for high efficiency:
- Choose an installer with good reviews
- Ask for references from previous clients in your area
- Ask what the flow temperature will be (lower is generally better; preferably 45°C or lower)
- Ask what the “SCOP” will be, what it means, and how the installer calculated it. Probe further on this point to make sure their installation will live up to the promise.
- Research the heat pump brand that the installer is offering. Does it have a good reputation?
- Don’t deal with pushy salesmen
As a bonus, we recommend carrying out a credit check on your installer before placing a deposit. This is primarily to manage financial risk. But there also tends to be a strong overlap between companies that do the best work and companies that have the strongest balance sheets.
