Ground-source heat pumps

The heat pumps most commonly used for home heating are air-to-air models (see our Heat pumps report). They extract heat from the outside air and shift it to inside the house. But when the outside air temperature is between 7˚C and 0˚C the outdoor unit may begin to freeze up and need to defrost regularly. Below zero this problem is reduced as the amount of moisture in the air drops significantly.

Heating efficiency (the amount of heat delivered indoors for the amount of electricity consumed) is also reduced when the outside air temperature drops. So using an air-to-air heat pump in areas where the winter temperatures are close to zero (or below) is less effective than in areas with a milder climate.

How do they work?

On a cold frosty day, take a spade and start digging a hole. You’ll get warm but not just from the exercise: as you dig down, the earth gets warmer. This is the clue to how ground-source heat pumps work.

Once you get below half a metre, the winter temperature of the ground is likely to be higher than the air temperature and at between 1 to 1.5 metres down soil temperatures will have a year-round average of 12 to 15˚C. So in cold climates extracting heat from the ground is more efficient than extracting it from the air.

Types of system

There are two main types of ground-source heating systems: “closed loop” and “open loop”.

Closed loop systems

Here the heat pump extracts heat from water and antifreeze (or sometimes refrigerant fluid) circulating in a large loop of buried pipe.

Creating a closed loop system involves burying a loop of pipe. The length of buried pipe required depends on the size of the heat pump – which in turn depends on how much heat is required for the building.

The pipes can be laid out in one of 3 ways (the site determines which is the most suitable):

• By digging a hole typically 1.2 to 1.5 times the heated area of the house and laying coils of pipes in the bottom of the hole.
• By digging a series of trenches and laying the coils of pipe vertically in the trenches.
• By drilling a series of bore holes and dropping loops of pipe into the holes.

Open loop

This system pipes water from under the ground and passes it through a heat exchanger. The heat pump then extracts the heat from this piped water … and the cooled water is returned to the source some distance from where it was removed.

This method is suitable only where there’s an aquifer close by and you can get resource consent to take the water. Other options include using sources such as a lake, stream, or pond.

Heating the house

The most common way of distributing the heat inside the house is to circulate heated water (hydronic heating). This can be through underfloor pipes set in an insulated concrete slab, or through water-filled radiators. Either system should give excellent results if designed well.

Retrofitting

You can retrofit an existing house as long as there’s enough ground space to bury the pipes. Indoor heating would then normally be through water-filled radiators. Although underfloor heating can be retrofitted to existing floors, it involves adding a layer of insulation that has the water pipes clipped into it and putting a new floor surface (usually thin wood or laminate) on top of the insulation.

Not having the water pipes embedded in concrete means that you won’t get quite the same evenness of temperature and heat storage benefits.

Life expectancy

Heat pumps are mechanical devices that wear out. We’d expect a life of about 15 years, which is similar to that of a domestic fridge. The pipes buried in the ground and in the concrete slab should last much longer (at least 50 years). Because those pipes can be re-used, replacing the heat pump only would be considerably cheaper than the installation cost for the whole system.

Costs

If you think burying pipes in the ground outside and in concrete slabs sounds expensive – you’re right. Ground-source heat pumps are more expensive to install than air-to-air heat pumps. The likely costs for a typical 3-bedroom 150m² house are $20,000 to $25,000. For a ground-source heat pump system to be economic, you need to be in a part of the country where the heating season is long enough and the air temperatures low enough for the savings over other heating methods to outweigh the extra costs.

In some of these regions you may also be able to benefit from using the system to provide cheaper more efficient cooling in summer.

Don’t forget that you need to compare ground-source heat pump systems against other “whole of house” heating systems.

Benefits

• Underfloor heating is regarded as the best there is. The thermal mass of the warm floor means the whole house stays evenly warm – even if there’s a power cut for a few hours.
• Ground-source heat pumps can also be set up to heat hot water as well as the house.
• Ground-source heat pumps have a high co-efficient of performance (COP), typically around 4. Air-to-air models might only manage 2.5 in colder climates. This means running costs are low.
• Ground-source heat pumps have no on-going maintenance requirements.
• The heating performance is much less dependent on the season, the weather, or the time of day.

More information

EECA and GNS Science have published a free introductory guide to geothermal heat pumps in New Zealand, and it is available on the GNS website.

by Bill Whitley