The standard way to ensure an efficient heat pump system is to use a weather compensation curve so that flow temperatures from the heat pump vary with the outside air temperature. This means that in warmer weather the heat pump can reduce its flow temps to ensure the maximum efficiency whist holding a property at the same steady temperature. Every 1 degree reduction in running temperatures equates to a 3% saving in running costs, making this highly desirable. It also ensures the efficiency of the heat pump by benchmarking performance requirements against the outside weather (a very large and therefore relatively stable environment) compared to the indoor air, which is relatively small and volatile environment (an open door or window will quickly reduce a warm room to a cold one, for example). As heat pumps are most efficient when running at constant low effort rather than turning on and off using high effort to reach the same temperature, weather compensation is a great way to control them.

Whilst this automatic adjustment of effort is smart and energy saving, it is not, however, the easiest system to set up. Every property and every owner will have its own ideal compensation curve based on such variables as location, the age and insulation values of the building, the type of emitters and a subjective definition of comfort. In particular, properties – or zones of properties – with radiators will typically have a very different curve to those with underfloor heating, as will new build extensions on older properties.

Exactly how these factors will combine to affect performance in a property can be difficult for an installer to gauge when commissioning a property. The first set up of a curve is by its nature an educated guess. Our Internet Gateway Service then allows remote adjustment of the curve in response to live performance data to improve the balance between efficiency and comfort. This is, however, not a process that consumers can easily engage with themselves to make immediate changes in response to their needs.

The temptation for heat pump installers is to use standard room thermostats to allow customers to have the simple controls with responses with which they are familiar. But there are number of issues with such an approach. Ordinary thermostats, which rely solely on room temperature to instruct the heat pump, tend to cause heat pumps to cycle like boilers, repeatedly going on, working hard and then going off in response to small changes in room temperature. This is bad for efficiency and system longevity. 

Our FET controls are designed to solve this problem and allow consumers more direct and immediate control of their own comfort, without accidentally sacrificing either efficiency or the longevity of their system. These control units are a remote control for the heat pump manager, offering room temperature control and a humidity sensor. They allow consumers to choose how much influence to give to outdoor air temperatures (weather compensation) vs the actual room temperature when deciding how hard to work. They can choose a position on a sliding scale between 100% weather compensation and 100% room temperature control that gives them the response times and comfort that they want whilst ideally allowing some smart modulation of the heat pump’s effort to maximize efficiency. 

FET controls are best sited in the core of a building to protect them from rapid and frequent temperature fluctuations such as you might see by a door to the outside. You can also have up to six FET controllers in a single property, which has the additional benefit of allowing different heat curves to be set for different zones (such as the new build extension). We’ve already mentioned that underfloor and radiator curves are typically different, but a common scenario is for a property to have both – with underfloor heating downstairs and radiators upstairs. In an underfloor zone it is beneficial to give more influence to the compensation curve and heat more slowly to desired temperatures because this results in less energy wasted by overshooting desired temperatures. Radiators, being much smaller in size compared to the space they heat, will need higher flow temperatures to hit the same room temperatures and thus benefit from more room temperature influence, but as they cool down quickly there is less risk of overshooting a set temperature and wasting energy. Wherever the highest heat requirement is detected (typically thus a zone with radiators) the FET in that zone will take precedence when telling the heat pump what flow temperature to produce. But The FET on the underfloor zone will then tell the underfloor mixing valve what temperature to mix the underfloor supply to, to ensure that the room does not overheat. 

FET controls are also useful in systems with cooling as they monitor humidity and alert the system to react to prevent condensation forming on the floor.

If you need any more information about our FET controls and how to design them into a system please don’t hesitate to contact technical@stiebel-eltron.co.uk or call 0151 346 2301