Heat pumps and water chemistry

Typically, in the UK heat pumps are paired with wet heating systems filled with tap water and chemical inhibitor designed to prevent corrosion in the system.

For customers seeking a green heating system, there are, however, some greener options to consider. These are common in Europe and are regulated by standards that we can learn from.

Option one is simply to fill the system with softened water as long as meets the criteria given in your heat pump manual for such things as PH, conductivity and various measures of water hardness. In theory this could be soft local tap water, but in either case, you’ll need a good water testing kit to verify your water – treated or not - meets the required criteria. As will be no surprise to those from a plumbing background, it is not a good idea to wing this. I saw a very highly corroded new system only a few weeks ago in which debris was causing the heat pump to stop working for this very reason. This method reduces limescale but has some potential corrosion.

This was power flushed out of a Heat Exchanger from a poorly maintained system. Accumulated debris can damage a heat pump past repair.

Option two – which has better all around reduction of limescale and corrosion- is to fill the system via a commercial water filter which purifies and demineralises the water completely. This has the benefit of balancing PH (reducing the potential for corrosion). A smaller, domestic plumbed-in resin cartridge unit is then put in place on site for topping up the system. The offer of this service for filling the system is something that the enterprising installer can consider as an add-on sell.

Water quality requirements as specified by the manual of one of our air source heat pumps with criteria for softened water (our option1) or desalinated water (our option 2)

Both methods will have a limited amount of oxygen in the system that will allow some – although minimal- corrosion. Once that has oxidized the corrosion from oxygen will cease – but remember to take care to minimise aeration of the water as you fill the system and bear in mind that every top up will add a further small amount, so any additional safeguards you can add to the system to prevent the need to refill or drain may be worthwhile. 

For either of these approaches there are a couple of key actions to take:

1. Remember always to flush a system before filling – whatever method you use – to get rid of any accumulated debris. Don’t forget that things like cylinders can start to corrode before you even fill the system if left sitting around on site for a while. You also want the system to be as clean as possible to prevent the growth of organic life which can then alter the water chemistry through its biological processes as well as being and a possible cause of 

You will need to calculate the required volume of water before filling and how long it will take to fill based on the output of any water purification plant you are using.