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To Buffer or not to Buffer

Buffer vessels, heat exchangers and low loss headers

Historically every heat pump had a buffer vessel, the reason for this is that heat pumps were once single speed machines. In low load conditions this meant the heat pump had to cycle on / off to match the demand. A buffer vessel allows the heat pump to operate and rest for longer periods.

Modulation and Inverters

With the introduction of variable speed units using Inverter technology, the buffer vessel is no longer essential. In a boiler you can alter the output by turning the gas up and down, this can be seen with the flame indicator changing in size on the front of the boiler, this is called modulation. Within the heat pump an inverter changes the speed of the compressor to give the same effect. Inverters are modulation for heat pumps. Samsung Inverter driven units can vary their output down to 10%, making cycling is a thing of the past!


In cold – wet conditions the outdoor unit can freeze, with a coating of ice visibly forming on the unit itself. However In extremely cold conditions (below -2℃) the coils will not freeze due to there being reduced levels of water in the air.

If the coil does become blocked with ice the unit will automatically enter defrost mode. At the most this will happen every hour and last around 6 minutes.

While in Defrost Mode the unit reverses its usual operation. It uses heat from the water in the house to warm the coil outside. This will result in radiator temperature temporarily dropping.

As the ice begins to melt off the back of the unit It may look like the unit has sprung a leak but it’s just condensation and is perfectly safe to drain it into the ground. Every time the water passes through the unit it loses 5℃, the flow rate will be 20l/ min. If the system volume is very small the temperature can drop dramatically. For example, if the system holds 20l, all the water will go through the unit 6 times in 6 minutes dropping 30℃ in temperature. If the system holds only a tiny Volume a buffer can be used to increase this.