Boilers and Heat Pumps, a Quick Comparison

When it comes to heating systems, homeowners often face the dilemma of choosing between heat pumps and boilers. In this article, we’ll delve into the key differences between these two options and explore their respective advantages.

For the purpose of this comparison, let’s consider a standard gas-fired condensing boiler with a good efficiency rating of 90%. This means that 90% of the energy derived from the burned fuel is delivered as usable heat, with the remaining 10% lost. While other boilers may have different efficiencies, solid fuel boilers have specialised requirements that are beyond the scope of this article.

To showcase the impact on system performance, we’ll assume that both a heat pump and a boiler are connected to a 300-litre domestic hot water cylinder with an ideally sized coil. The starting water temperature in the cylinder is 10°C, and it needs to be raised to 50°C.

To estimate the time taken to heat the cylinder and the approximate cost, we’ll utilise data from the Sustaintable Energy Authority of Ireland (SEAI) website, considering the most current energy prices available.

Heat Pump

• Effective Heat Output: 9kW
• Max Flow Temperature: 55°C
• Return Temperature: 45°C
• Heat Exchanger Surface Area: 3m²
• Heat Exchanger Efficiency: Higher
• Maximum Storage Temperature: 50°C

Boiler

• Effective Heat Output: 25kW
• Max Flow Temperature: 70°C
• Return Temperature: 60°C
• Heat Exchanger Surface Area: 1m²
• Heat Exchanger Efficiency: Lower
• Maximum Storage Temperature: 62°C

The boiler, with its higher output temperatures and greater delivered energy, heats the water much faster. Consequently, hot water is available sooner from a boiler system when starting from cold. On the other hand, a heat pump takes longer to deliver the same amount of heat. However, the energy cost of a heat pump is significantly lower. In cases where night-rate electricity is available, the cost of heat pump operation can be halved. Unfortunately, there is no night rate available for gas.

Since a heat pump operates at a lower flow temperature, it requires a larger heat exchanger area within the cylinder to maintain good performance. In contrast, a boiler with its higher flow temperatures requires a smaller coil area to achieve similar results. However, this smaller coil is less efficient at transferring heat, resulting in a larger temperature difference and potential heat loss within the system during the return to the boiler.

It’s worth noting that the heat pump considered here is operating in its least efficient regime, as it’s being asked to operate at its maximum rated heat output. Beyond this point, reaching higher temperatures would require an electric immersion heater or an alternative heat source.

In reality, how often do we require hot water at 50°C? For most purposes, such as washing, water is used at more comfortable temperatures below 30°C. Thus, this store of hot water is usually blended with cold water to achieve the desired temperature. A heat pump operates far more efficiently if allowed to work a ta cooler temperature, such as 35°C.