System layout

The Greenstore series of heat pumps has the versatility to be used in a variety of different system designs.

Greenstore ground source heat pump and hot water cylinders

Hot water cylinders used with heat pumps must have sufficient surface area for heat transfer. This ensures that the lower flow temperatures and small temperature differential do not adversely affect the re-heat time. The heat pump operates on hot water priority so it is important to minimise the heat uptime to lessen the effect on heating. Cylinder designs with coil heat exchangers are not usually suitable for use with heat pumps.

With this in mind Worcester offers two Greenstore solar compatible cylinders, in sizes of 180 and 280 litres and with a tank-in-tank design for use with Greenstore System series heat pumps.

This system shows the advantage of the Worcester Greenstore heat pump is providing temperatures of around 65°C. In this system the solar coil is not used.

Greenstore ground source heat pump with solar support

This system uses the same products as the system above but with the solar coil in the cylinder linked to a Worcester Greenskies solar system. The system allows each of the technologies involved to work together with the fewest compromises. The advantage is that the heat pump is not required to be the sole provider of the domestic hot water and therefore will have the opportunity to run at lower flow temperatures when the solar is producing the hot water.

As well as providing the heating the heat pump provides any additional heat that might be required over that provided by the solar system.

System design requirements

The heating system should be designed to maintain 70% of the nominal flow of the system across the heat pump at all times. Where TRVs are fitted this could be achieved by maintaining a fully open position on the valves. At least half of the coils in an underfloor heating circuit must remain open at all times. Alternatively, a bypass may be fitted in order to maintain the flow around the system if TRVs start to close down.

Where it is not possible, due to the design of the heat emitters, to maintain this flow rate a low loss header or a Worcester primary store of around 100 litres should be fitted.

Heat pump sizing

Although the sizing of the heat pump can only be accurately carried out by taking all factors into consideration this section offers some explanation of the principles behind the sizing of heat pumps according to the energy requirement of the property.

The following examples are for demonstration purposes only:

A heat pump is typically sized to provide around 75-80% of the peak load of the house on the coldest day. Since the number of days that this requirement occurs is relatively low, taken as a proportion of the total year, the heat pump is typically sized to provide 95% of the total heating requirement for a property over the year as a whole. The remaining energy is provided by the builtin electrical heater.

The benefit of sizing the heat pumps below the peak load requirement is that the pump, for the majority of the year, is able to remain on and deliver a ‘trickle charge’ of heat to the property, rather than being oversized and constantly cycling in and out of operation. This helps the heat pumps to offer better efficiency.

The graph here is intended to show the principle behind the sizing of heat pumps to take into account the small number of hours where the peak heating load of the house is required. The dotted vertical line shows that the there have, in the example shown, only been 100 hours of outdoor temperature below 0ºC in the given 8,760 hours.

There are significant climatic differences across the UK and the example above is not intended to provide information on any particular installation. The Worcester system design service is able to provide information on an individual basis.