The key components of a heat pump hot water unit

Key Takeaways

• The compressor is the engine of your unit
• Tank material doesn’t really matter too much - but be aware different materials require different levels of maintenance.
• Some view a backup element as a positive or feature of a unit, some view it as a flaw. I wouldn’t make it the basis of your decision. 

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Here is what you need to know about the key components in a heat pump hot water unit.

Compressor

As outlined in How does Heat Pump Hot Water work, the compressor compresses the refrigerant in the units. This process of compression heats up the refrigerant which in turn heats the water. Common compressors found in heat pump hot water units are manufactured by Panasonic, Toshiba and other Japanese brands.

The “Compressor Rated Input Power” is a measure of the power of a compressor. This tends to be between 500 and 1000 Watts for units in the market. 

A compressor will draw more power at higher temperatures to compress and heat the refrigerant. For example, it might draw 500W of power at 15 degrees celsius, but 900W of power at 55 degrees celsius. Heating water from 55 to 60 degrees requires a great deal more power than from 15 to 20 degrees.

Some compressors on the market are “variable speed compressors” or “Inverters.” Inverters operate differently from traditional fixed-speed compressors by adjusting their speed to match demand. They are equipped with sensors that monitor various parameters such as temperature, pressure and humidity. The compressor uses these inputs to then adjust the speed of the compressor motor. Variable speed compressors typically operate more efficiently than fixed-speed compressors because they avoid the frequent cycling on and off that fixed-speed compressors experience. By continuously adjusting their speed to match demand, variable speed compressors maintain consistent temperatures with minimal energy consumption. Inverter compressors generally last longer than fixed-speed compressors as the gradual ramp up and ramp down of the units put less stress on the units.

Refrigerant

Please see my article dedicated to Refrigerants

Tank Material

You don’t need to stress too much about what your tank is made from. The cost of the unit and how long it’s likely to last is more important, but it pays to be aware as tank material will affect your warranty and ongoing maintenance. 

Most heat pump hot water tanks in the Australian market are usually made from either vitreous enamel or stainless steel.

Vitreous enamel is the most common. Vitreous enamel is a glass coating chemically bonded to steel at high temperatures. The glass lining protects the steel tank a little, but still needs further protection (see below). The warranty on glass lined tanks is typically between 5 and 10 years.

Stainless steel tanks are found in premium heat pump hot water units. They don’t need further protection but can not be used in all water types. The stainless steel tank offered by Reclaim comes with a 15 year warranty.

Tank Protection

There are two common ways to protect your hot water tank:

1. Sacrificial anode - a long metal rod, usually made of aluminium or magnesium, that sacrifices itself to corrosion instead of the tank. It does so by attracting minerals in the water. Most manufacturers recommend inspecting the condition of the sacrificial anode every 3 years or so, and replacing it every 5 years (more frequently for ‘hard water’ which has a higher concentration of minerals). The sacrificial anode usually breaks down into the tank, so it’s a good idea to flush the tank to clear it out and stop the residue clogging filters. Also keep in mind where the anode is in the tank when choosing the location of your hot water unit. Your plumber will need access, so don’t block it off!
   
   
2. Impressed current anode - an impressed current cathodic protection (ICCP) system consists of anodes that are connected to a power source that provides a protective electrical current. Unlike the sacrificial anode rod, the impressed current anode does not sacrifice its structure. The flow of electrons prevents the formation of corrosion by making the metal surface more cathodic (negatively charged) compared to the surrounding environment. 

Developments in heat pump technology

Heat pump technology continues to improve in multiple ways:

• New compressors can get refrigerants to higher pressures using less power
• Variable speed compressors allow heat pumps to ramp power up and down
• Heat exchangers are getting bigger and more efficient

Element

Well here’s a contentious topic - to element or not to element?

Many heat pump hot water units come with a ‘backup element.’ The heat pump is designed to do the bulk of the work and the element is either a backup or used to heat the water, say upwards of 60 degrees.

Traditional electric hot water units heat water through an element, a metal rod (usually made from copper, stainless steel or nickel alloy) that heats up when it conducts electricity.  A kettle works the same way - as electricity flows through the element, it encounters resistance with the conductive material. This resistance generates heat which is transferred into the surrounding water. 

Electric resistance hot water units are horribly inefficient - they are the second most costly way to heat water for the home (behind bottled gas $$$).

Some view the presence of an element as a positive, some view it as a negative.

Positive	Negative
Element is a backup and will buy people time when the heat pump fails, rather than be stranded with no hot water	They are ‘faux’ heat pumps - how do we know if the heat pump is doing the work or the element?
Element is only used as a backup or in special situations, so there’s no downside
Element can help heat water to high temperatures which the heat pump may not reach (eg: up to 70 degrees)

My suggestion: I wouldn’t get carried away with whether your unit has an element or not - base your decision on other criteria, notably the cost and reliability of the unit.