Heat pumps have been used in continental Europe for a number of years, and are a proven technology. However, because the process of collecting the heat will vary according to each site, the soil condition, depth of pipe, coverage area and required temperature are important factors in determining how much heat which will be made available to the pump, and thus dictate the overall efficiency of the system.

Giving Back More Than You Get?

As a general rule of thumb, a typical oil boiler gives back less than 1 unit of heat for each unit of oil we buy. But a well designed heat pump can give back up to 4 and sometimes up to 5 units of heat when compared to the energy used to drive the pump. So that means it's money for nothing, right? Well, no. Sadly, heat pumps themselves require a huge amount of electricity to handle both the pumping of the refrigerant through the pipes and the condensing of the heat given back from the pipes to create the heat source. Typically, heat pumps have a standard power rating of between 2 and 5Kwh. This means that, when running, they are adding a significant amount to the electricity bill. The question is: does the cost of running the heat pump for a year offer savings over the cost of using physical fuel during the year?

Let's do a quick calculation to determine this:

• 1900sq feet (my house) = 176 sq metres x 75watts (sizing cost per sq metre) = 13200, or 13Kwh.
• So heat pump required is 12-13Kwh model
• Typical power requirement for this size of heat pump would be 3Kwh per hour
• Maximum running cost: 3000 x 10hrs x 30days x .000163 = EUR147, based on an "on" time of 10 hours per day.
• Average running cost: 3000 x 7hrs x 20days x .000163 = EUR68.

So I would expect running costs within the range of EUR68 to EUR147 to run the heat pump. If I use the average running cost and annualise it I get a figure of EUR816 per year, which is two thirds of the cost of my oil per year (EUR1200). So that appears to be in line with comparisons that other people have made. However, the savings are based on electricity prices remaining competitive.   

Different Types Of GSHP

There are 2 different approaches to heat pump design, both in terms of the type of refrigerant that is used and the type of pipe layout that is used. The main difference to the homeowner would be whether the pipes, or collector area, are laid horizontally or vertically. This is usually dictated by ground conditions and available space. Vertical digging is not the preferred route because the further you dig down, the more difficult it is to extract the earth and more specialist equipment is required. Thus the costs for installing vertical boreholes for laying the collector pipes can be high. This is also reflected in a slightly bigger grant being made available for homeowners electing to install a vertical collector. 

The second, and more typical solution, is to lay the pipes in horizontal fashion. This is ideal if the homeowner has a large surface area which can be used for this purpose. The area would have to be kept free of large tress and bushes as the roots could damage and eventually crack the buried pipe work, which are laid only a few feet below the surface. Also, due to fluctuations in soil temperatures though the year, the horizontal collector area is often oversized to compensate for this, so the final collector area could be as much as half an acre or more. If you are lucky you can get away with a footprint equal to your total floor area. 

There are also methods employed to directly circulate the refrigerant and capture the evaporate, which is termed direct evaporation. This means that the heat exchanger itself is not required as that function has already been done, thus leading to higher overall efficiency. This contrasts with a less efficient water/brine circulating solution which requires both pumping and heat exchanging, leading to lesser efficiencies.

Components & Usage:

The typical components of the heat pump are the collector area, the heat pump itself and the distribution system for the heat, such as underfloor pipes. The picture above shows a typical horizontal collector area. One main factor in determining the suitability of the heat pump is whether it is viable in a retrofit or upgrade of the heating system. Often parts of the system, especially where direct refrigerant is used, can be sited outside and this frees up more space in the boiler room.

Due to the indirect nature of how the water is heated, many manufacturers quote temperatures no higher than 30-40° Celsius. This will not work for a system like mine, where radiators are used as the primary heat distribution system. This is why most heat pumps are specified for use in under floor systems, where more of the water is distributed at lower temperatures to achieve the same effect as less water delivered at high temperatures to radiators. So if I were to use this system to heat my house, I would probably need to remove the radiators and install under floor pipes instead.

Which Ground Source Heat Pump?

In preparing this sample list, I have merely selected some random heat pumps available in the market, to get an idea of the costs. As the footprint of most heat pumps tends to be similar to that of an oil boiler, I have assumed any version will fit within the boiler room dimensions. The price shown below is simply a guide price, but factors in the cost of the pump, collector pipes, grant of EUR4,300 and typical installation in a horizontal layout. I have not included the cost of the heat distribution changes, which I would estimate to be in the region of EUR10,500 to decommission the radiators, dig up the floors and lay the under floor pipes. For those contemplating new builds these costs will be substantially less.  

It was also interesting to note the different co-efficient of performance (COP) of each pump, in theory those with the highest COP will give more energy back in the form of heat for less electricity, although in reality, the final COP will be determined, as we mentioned earlier, by many factors such as installation, usage, heat requirement and method of heat distribution.  

MANUFACTURER / MODEL	PICTURE	COMMENTS	PRICE
Dimplex GSHP		5-25Kw power COP of 4.0 quietness of pump: < 50dB life expectancy: ~ 25years single and three phase	EUR10,200
Heliotherm GSHP		cop of over 5.5 internet controllable d/x and brine options highest efficiency at 30-40°C	EUR10,000
Waterfurnace GSHP AHAC		cop of up to 5 up to 20Kw power can be linked to other fuel boilers water heated to over 50°C	EUR12,000
Neura GSHP ecoheat		cop of up to 4.5 Direct Evaporation Discreet design silent running web control	EUR<soon>
Nordic GSHP nordic-geothermal		cop of up to 4.5 no additional pump required lower collector areas required	EUR10,500

Ground Collector Area Excavation:

I was saying earlier that there are 2 types of collector layout: horizontal or vertical. Well, it turns out that's not strictly true. There is a third type of collector layout which can be described as shallow vertical. Here the ground is excavated to a depth of three metres, and the collectors, which come in panel form, are slotted into the ground vertically. The collectors themselves are 2 metres in depth, so they sit about 1 metre below ground. The trenching line is increased horizontally to cater for additional heating requirement. This, in theory, would reduce excavation costs so is an option for homeowners who are not keen on allowing large horizontal collector areas, but want a less expensive solution than a true vertical collector.

And, if digging up the garden is simply not an option, then perhaps a heat pump which relies on air would be a better solution? Let's take a look on the next page.