For homes already running a geothermal heat pump for heating, the same loop can pre-heat your domestic hot water — adding negligible running cost.
A geothermal hot water system uses a ground-source heat pump to heat domestic water from the stable 14–18 °C temperature found underground, reaching a COP of 4.5–5.5, the highest of any hot water source. It makes most sense as an add-on to a geothermal heating system, where the ground loop already exists. AHHAC designs ground-source hydronic and hot water systems across Sydney, Melbourne and Brisbane, and will tell you when air-source is the better buy.
Reviewed by the AHHAC Technical Team — licensed plumbers & hydronic engineers, NSW Lic 262528C.How we write & review
Geothermal hot water makes most sense as a tag-on to an existing geothermal heating system. The ground loop and heat pump are already there — adding domestic hot water is a small incremental capital cost for excellent ongoing efficiency.
Standalone geothermal hot water (without heating) is rarely cost-effective — air-source heat pump or solar thermal will give better return.
A geothermal hot water system is rarely designed on its own. In nearly every case we are adding domestic hot water to a ground-source heat pump that already heats the house, or designing both together. That changes the job in three important ways.
The loop itself depends on the block. Rural and acreage sites can often use horizontal trenches, which are cheaper to dig. Suburban Sydney and Melbourne lots usually need vertical bores, drilled by a licensed driller, and the rig needs access to the yard. Bores are the major cost, but they are also the part that lasts: a ground loop life of 50+ years against 15–25 years for the heat pump. When the heat pump is eventually replaced, the loop stays.
Who does it not suit? If you only want hot water, an air-source heat pump or solar system costs a fraction of a bore field and delivers most of the efficiency. Small households, homes you plan to sell soon and tight sites with no rig access are also poor candidates. Where the house is being heated by ground source anyway, adding hot water is a modest extra cost for a COP of 4.5–5.5.
Read how ground loops work on our geothermal hydronic heat pumps page, compare the air-source alternative, and see the cylinders and thermal stores in our hot water tanks range.
Written from on-site experience by the AHHAC Technical Team.
AHHAC's in-house design and service team — licensed plumbers and hydronic engineers (Lic 262528C) who have designed and installed 2,400+ hydronic heating, cooling and hot water systems across Australia since 1998.
Every plumbing and hot water job is assessed on site, carried out by our own licensed plumbers and gasfitters to AS/NZS 3500 (plumbing) and AS/NZS 5601 (gas), pressure-tested and documented, and backed by ongoing service. Nothing is subcontracted out.
Almost always as an add-on to an existing geothermal heating system. The ground loop and heat pump are already there — adding domestic hot water is a small incremental capital cost for excellent ongoing efficiency. Standalone geothermal hot water (without heating) is rarely cost-effective.
COP of 4.5–5.5 — the highest of any hot water source. Stable ground temperatures (14–18°C year-round at depth) mean efficiency stays consistent across seasons, unlike air-source which drops in winter cold.
50+ years for the buried pipework. The heat pump itself has a 15–25 year design life. Most cost is in the loop install — once it's in, it's there for the lifetime of the building.
"AHHAC delivered our 22-zone underfloor system on time, on budget and to spec. The crew were tidy, the documentation was thorough, and the commissioning was textbook."
"We've worked with AHHAC for 17 years. They've installed and serviced our hot water plant across three commercial buildings — and they pick up the phone on weekends."
"Three mobs quoted us; AHHAC's design was the only one based on a proper heat-loss calculation. The system runs beautifully — the others would've been oversized and expensive."