27 Jul 2017

The beginner’s guide to ground source heat pumps

Everyone has to start somewhere. As a reader of i-build, you are clearly an ambassador for the self-build industry and innovative approaches to building, but for some self-builders renewable technology like ground source heat pumps remain an enigma. Here, Kensa talks through the ins and outs of ground source heat pumps.

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With ground source heat pumps attracting 19.64p/kWh under the Domestic Renewable Heat Incentive (RHI) (versus 7.63p/kWh for air source and 4.28p/kWh for biomass), and innovative micro-district ground source heat networks attracting the non-domestic RHI in retrofit and new-build schemes, there is a huge financial carrot for the specification of ground source heat pumps.

Add to this their carbon credentials, efficiency savings and the Committee for Climate Change’s Fourth Carbon Budget calling for four million heat pumps to be installed at residential properties by 2030, the market for ground source can only go one way. But to fulfil this ambition, self-builders need to be on board and share the vision.

Of course, as Kensa is a dedicated manufacturer of ground source heat pumps, and one of the confirmed largest suppliers in the UK (source: BSRIA 2017), we are bound to say that. So the aim of this article is to demystify and briefly explain, in simple terms, the typical process of installing a ground source heat pump, to demonstrate that ground source is a technology that can be easily integrated into your project to great reward.

Know the load

Arguably, the first part of planning a ground source heat pump (GSHP) installation is the most important – calculating the property’s heat load. Get this part wrong and everything that follows will be wrong. You can’t design a GSHP system without knowing two vital bits of information, the peak heating load in kW and the annual heating load in kWh.

If the building is being upgraded in terms of insulation at the same time, it is perfectly fine to factor this upgrade work into the heat loss calculations, but it is equally important to ensure that the planned work actually gets done before the heating system is used in anger!

Typically, an installer or experienced self-builder will either do this heat loss calculation work themselves, or subcontract it out to a qualified specialist. If the heat pump is also supplying domestic hot water, then this load also needs to be calculated and added to the numbers.

Room by room

While the ground source heat pump itself will most likely be a single unit with a stated kW output, each room in the building is going to have a different heat demand, so needs its heat emitter (the term emitter covers radiators, underfloor heating [UFH] and other less common types) individually calculating to meet that heat demand.

When using a heat pump instead of a boiler, the rule is always to size the emitter so that it is capable of heating the room using the lowest possible water temperature as this maximises energy efficiency.

Practical limits for radiator sizes usually mean getting below 45°C flow temperature is rare, but UFH can sometimes work as low as 30°C. The usual practice is to work out which room needs the highest water temperature due to practical limits and then size all the other emitters to the same temperature.

In the ground

Once the peak heating load and heating flow temperatures are known, then a suitable heat pump model can be selected.

By referencing the manufacturer’s data, the efficiency (also called Coefficient of Performance or CoP) of that heat pump at the heating water temperature can be determined. It is this efficiency rating combined with the annual kWh demand of the building that determines the size of the ground array.

One of the ironies of using the newer generation ultra-efficient heat pumps, like the Kensa Evo, is that a higher percentage of the total heat output comes from the ground, not the electrical input, meaning that ground array sizing has to increase slightly.

Ground arrays typically take one of two forms, either plastic pipe laid in horizontal trenches (pipe can be straight or looped, known as ‘Slinkies’) or long, vertical drilled boreholes containing a U tube of plastic pipe.

Designing the ground array itself can be supported by your manufacturer or installer. The Microgeneration Certification Scheme (MCS) does provide design guidance or, again, specialist sub-contractors exist to do this work, especially for larger projects. What no ground array design can do though is cope with inaccurate load calculations, hence getting the initial heat losses right is vital.

Plumbing up

Ground source heat pumps have two separate wet systems with no connection between them. Flow rates and hence the sizes of pipes used in the plumbing are very important, with low flow rates being one of the most common faults that crop up.

Competent installers should know how to use a pipe sizing nomogram and calculate pressure drops in a system.

Design of both the ground array manifolding and heating distribution system should aim for even distribution and high flow rates. Tichelmann, a.k.a ‘reverse return’ plumbing designs, should be used on both systems if practically possible. Pipe sizes will usually be one size larger than typically expected on a boiler system in a similar-sized property, apart from this, standard good practice in plumbing is all that should be required.

Ground arrays are invariably filled with an antifreeze and water mixture. Filling and purging requires a separate pump and container, this is just about the only special tool needed to install a ground source heat pump (Kensa has a video on this process). It is essential that this antifreeze mixture is circulated through the heat pump ground side before power is ever applied to it – it is just about impossible to damage a GSHP, except by running it without antifreeze!

Hot water

Hot water systems are usually very similar to a common Y-plan design with a separate hot water cylinder, although the three-port valve will not have a mid-position in operation.

Getting good domestic hot water performance relies on having matched components. The internal diameter of the coil within a cylinder affects the flow rate; this is just as important as getting the surface area of the coil right.

Thermal stores can be very difficult to set up and get working well with a heat pump, this is mostly due to the lower operating temperature compared to a boiler. The simple solution is to use a cylinder approved by the ground source heat pump manufacturer.

Some systems will need a separate smaller storage tank known as a buffer tank, these can often be avoided by using a few open zones and ample pipe sizes, the heat pump manufacturer should again be able to provide guidance on the minimum flow rates and the minimum volume of water in the circuit, but don’t be tempted to try using a thermal store to provide buffering.

Control

Heat pumps can be controlled by any means the end-user wants; at the end of the day it is just a box that makes hot water on demand. Unfortunately, some manufacturers like to complicate things and try to have the building controlled by the heating appliance; in Kensa’s opinion, it should always be set up the other way around!

The current trend is for internet-connected heating controls, these are all well and good but quite how quickly they will go out of date and stop working is anyone’s guess; a ground source heat pump will operate trouble-free for 25 years, as will simple room thermostats.

Commissioning

If good plumbing practice has been followed, all the air bled out, systems carefully filled and valves opened, then the heat pump should start right away and be giving full heat output within a couple of minutes.

Building warm-up will usually take longer than with a boiler, but only because most boilers are grossly oversized compared to the true heat load, not because heat pumps take time to settle in or any other urban myths.

MCS

To claim Renewable Heat Incentive (RHI), all installations need to be MCS accredited, but for a new or infrequent installer becoming MCS registered can be prohibitive, both in terms of cost and time commitment for training. Some manufacturers offer ‘umbrella’ schemes where, for a fee, they will do all the installation calculations, design and MCS paperwork submissions for the self-builder or their preferred non-MCS installer.

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