The real cost of poor hot water performance

Written by Ben Hodges

The real cost of poor hot water performance

An architect recently asked me to formally review a tender submission that included an air source heat pump. Both the architect and the homeowner wanted reassurance that the heat pump solution would meet their requirements.  

A key concern was that the heat pump produced sufficient hot water. The property is a holiday home and hot water consumption is typically much higher than standard. There are more people staying in the house and they tend to want a shower/bath at the same time.  

All heat pumps become less efficient (and less powerful), the higher the water temperature they produce, and heat pump manufacturers take different approaches to deal with this. 

The tender bid met the general specification to provide all the heating and hot water for the property, it was just not clear on the limitations of how the system achieved this. 

The air source heat pump, from a major boiler manufacturer brand, can only produce hot water to approximately 45C. The system relies on a direct electrical immersion heater to raise the hot water cylinder temperature to good usable temperature of 55C.  

This feature means that when producing hot water twice a day, the homeowner will experience; 

  1. 16% increase in running cost and extra £109.50 of electricity per year 
  2. 33% increase in cylinder heat up time 

Assumptions: Air source heat pump operating at a COP efficiency of 2.5:1, 300l hot water cylinder, Electricity cost of 15p/kWh 


A minor change in tender specification wording to say “the heat pump must provide all heating and hot water without the need of additional immersion heaters” would have avoided this issue.  

Both the homeowner and architect were happy with our advice and ability to provide an alternative air source heat pump system for the project. 

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