How to specify GSHP

Sizing the GSHP
There are a lot of misunderstandings when it comes to sizing a heat pump.  Many suppliers will have you believe that for some magical reason the heat pump only needs a small output.  This may be because they only produce small heat pumps or more likely, they just don’t understand how to correctly size the unit.

Sizing the heat pump starts in just the same way as sizing a boiler; you have to work out the true heat loss of the building.  There are many ways to do this and if you need help then we can quote you for this.  Once you know the heat loss you can then look to match this to a heat pump. 

 

A quick guide to sizing

Location

South

Midlands

North

Building Type (thermal insulation)

 

 

 

 

 

 

 

New - High U Value

38 W/m2

42 W/m2

48 W/m2

 

 

 

 

New - 2009 Building Regs

48 W/m2

50 W/m2

58 W/m2

 

 

 

 

Retrofit - Upgrade/Improved

72 W/m2

79 W/m2

85 W/m2

 

 

 

 

Retrofit - Original (no upgrade)

96 W/m2

107 W/m2

115 W/m2

Guide note and example;
The best form of heat emitter is a low temperature delivery system such as UFH or fan coils (or fan assisted radiators).  For areas with high ceilings that are not using UFH then the total volume of the space needs to be factored in which will add extra kW capacity.

UFH should always be installed with the pipes embedded in screed or similar to give high
thermal mass.  The UFH pipework should be spaced at 150mm centres for current building regulations (and upgraded retrofit) and 200mm centres for high U value buildings.  For original retrofit buildings with poor insulation it is advisable to space the pipes at 100mm centres.

e.g. A 200m2 building in the midlands that has a high U value would have a 8.4kW heat loss.

Selecting the right heat pump
Once you have the total output requirement for your project you can select the Radiant ground source heat pump that is nearest to this.  Radiants single phase heat pumps have single compressors for outputs up to 11kW and twin compressors from 14 to 24kW.  The twin compressor units can operate on one or both compressors depending on the load.

 

kW Output

Heat only Model

kW Output

Heat/Cool Model

5

RBB5GH

5

RBB5GHC

7

RBB7GH

7

RBB7GHC

9

RBB9GH

9

RBB9GHC

11

RBB11GH

11

RBB11GHC

14

RBB14GH

14

RBB14GHC

16

RBB16GH

16

RBB16GHC

20

RBB20GH

20

RBB20GHC

24

RBB24GH

24

RBB24GHC

The above table is for ground temperature at 0oC and water temperature at 35oC.  The COP of these heat pumps over this temperature range is 4.0.

For the example we gave of the 200m2 building, the required heat only model would be the RBB9GH. 

Collector sizing
The energy we collect from the ground in the UK largely comes from absorbing the suns radiation.  In fact around 99% of the energy stored in the ground comes from the sun with just 1% coming from the earth’s molten core.  It is therefore important to have an understanding of how different ground conditions can affect the conductance of heat to the collector pipes.

When considering the design of the collector loop you must insure that there is a balance between the required load and the amount of heat (specific energy) that can be extracted.  If this situation becomes unbalanced then the ground will begin to freeze causing very serious problems.

The thermal conductivity of the ground for a horizontal collector loop can vary from as little as 8 W/m2  to as much as 40 W/m2.  This highlights the importance of getting the right amount of pipe in to the correct land area.

How much Land area do I need?
There is a simple calculation to give you the total ground area required for heat extraction.  The amount of energy required from the ground is the total output of the heat pump less the electrical energy to power it.  This is what’s called the refrigeration capacity of the machine and the ground collectors must be able to provide at least this much energy. 
As an example if we consider our 14kW (14,000W/hr) heat pump and an average ground thermal conductivity of 20 W/m2  then the calculation looks like this;

Total land/ground area = refrigeration capacity (10,500 W/hr) divided by the specific energy (20 W/m2 ) would = 525 m2  of required land.  This gives a clear indication of the potentially large area of ground required for energy extraction.

If you are not sure how to calculate the collector of the correct size of heat pump then use our design service.

 

What to do next
You can either but it now or for a full package, request a quote which may include such things as the buffer tank, the circulation pump and anti freeze etc.  If you want help adding the GSHP to your existing system or to be part of multi energy design then go to system integration.

  • buy it
  • get a quote
  • design service

Thermal mass (also called heat capacity) is the capacity of a body to store heat. This is achieved in a building by placing the UFH pipes in a concrete or gypsum screed. A further benefit is that this mass can have a lower temperature input without loss of performance. It is therefore ideal for low temperature UFH using heat pump systems.
In its simplest form a fan coil is a radiator with an electric fan to circulate air over the radiator. There are floor, wall and ceiling mounted units and these can be used for both heating and cooling applications. Because of the fan, the water temperature can be relatively low when used for heating and is therefore ideal for use with a heat pump.
Underfloor Heating is a warm water based heat distribution system of embedded pipes in a floor. The average temperature of the heating water is 35 - 40oC and is therefore ideal for using with a heat pump.
A buffer tank is a small water tank similar to a normal hot water tank in appearance. When using solar to integrate into the heating system the buffer tank has a heat exchanger coil fitted. The buffer tank is mainly used with a heat pump. The buffer tank acts as a heat store between the heat pump and heating circuits and maintains correct operation of the heat pump when radiators or UFH circuits become satisfied. The heat pump will shut down when the buffer tank reaches a normal running temperature (30 - 45ºC). Therefore the buffer tank prevents the heat pump from continually turning on and off when running small or partial heat loads.