The first in an upcoming series of short articles discussing the basics of design for your heating system, a comparison between boilers and heat pumps. What can each heat-source actually achieve?

We’ve already discussed the technical and physical side of heat pumps before. In summary, the higher the output temperature required, the less efficient the Heat Pump becomes. The Coefficient of Performance is a measure of the Heat Pump’s efficiency, the higher the better. The Hitachi Yutaki series of Heat-Pumps offered by Greentherm offer an approximate Coefficient of Performance of between 2 and 4 under normal operating conditions.

For the purposes of this article, we shall consider a standard gas-fired condensing boiler with a good efficiency rating of 90%. This means that 90% of the energy in the burned fuel is delivered as useable heat, with the other 10% lost.  Other boilers are reasonably similar, though with different efficiencies. Solid Fuel Boilers have certain specialised requirements that are beyond the scope of this article for the time being.

In the table below, Flow temperature is the maximum output water temperature from the Heat Pump or Boiler. Return temperature is the temperature of water returning to the Heat Pump. The maximum storage temperature will be somewhere between these values – less a small amount for losses in the cylinder heat-exchanger.

To demonstrate the effect this on system performance we will assume that both Heat Pump and Boiler are each connected to a 300 Litre domestic hot water cylinder with an ideally sized coil. The water in the cylinder is starting at a cold temperature of 10°C and needs to be raised to 50°C.

The time taken to heat the cylinder to temperature can be estimated and the approximate cost to heat the cylinder is then calculated using data available on the SEAI website here:*

The higher output temperatures, and greater delivered energy from the boiler heat the water much faster. Therefore, hot water will be available sooner from a boiler system from cold. A Heat Pump will take longer to deliver the same amount of heat. However, the cost of that energy from the heat-pump is a good deal less.  If night-rate electricity are used, the Heat Pump costs are halved. There is, however, no night rate available for gas.

Because it operates at a lower flow temperature the Heat Pump requires a larger heat-exchanger area within the cylinder to give good performance. Because of its higher flow temperatures, a boiler requires less coil area to achieve the same level of performance. This smaller coil is less efficient at transferring heat, requiring a larger temperature difference to be effective, meaning more heat can be lost within the system on the return to the boiler.

It must be remembered also that the above Heat Pump is operating in its least-efficient regime. It’s being asked to operate at its maximum rated heat output, a point at which the effective coefficient of performance will be closer to the minimum. To get to temperatures beyond this, an electric immersion heater or alternative heat-source would be required.

In truth, how often is hot water required at 50°C? For most washing water will be used at more comfortable temperatures below 30°C, so this store of hot water will likely be blended down with some cold water to achieve a desired comfortable temperature. The Heat Pump will operate far more efficiently if we allow it to work at a cooler temperature – say, 35°C.

Is there a way to meet our hot water demands using cooler water? This leads nicely into what will be the next post in this series:

Water Storage Temperatures and Why.

*As of September 2014. Energy Prices may vary.