By Mick O'Brien · carpenter & builder · sheds-to-homes, cabins and things that stay up in a storm
“After losing a $12,000 slab to the freeze in 2017, I now insist on insulated radiant heating for every off-grid Aussie cabin I build.”
By Mick O'Brien · carpenter & builder · sheds-to-homes, cabins and things that stay up in a storm
“After losing a $12,000 slab to the freeze in 2017, I now insist on insulated radiant heating for every off-grid Aussie cabin I build.”
Radiant floor heating is one of the few comfort upgrades that plays nicely with off-grid living. The physics favour it, the thermal mass of a slab does the heavy lifting after sundown, and the heat source can be anything from a wetback stove to an air-to-water heat pump running off a PV array. Below is the practical breakdown: how the systems work, what they cost in energy, what you actually need on site, and where the safety lines are drawn.
Heating and cooling can chew through 20 to 50 per cent of a typical Australian home's energy budget, depending on the climate zone. Off-grid, every kilowatt-hour counts, which is why the heating choice matters more than any other appliance decision in the build.
Two properties of water make hydronic floor heating a strong fit:
That means a small flow of warm water through floor loops moves far more heat than the same volume of air pushed through ductwork. Less pump energy, less fan energy, less battery drain.
A hot-water radiant heating system is a closed loop: the same water circulates through one or more loops and is only topped up if there's a loss. Heat is generated at a boiler, heat pump, wetback, or solar thermal collector, then pushed through PEX tubing embedded in or fixed under the floor. The slab or screed acts as thermal mass — it takes a lot of energy to warm up but a long time to cool down, storing daytime solar heat and releasing it through the night.
In the original Jacobs House (1936), Frank Lloyd Wright ran piping through crushed stone beneath a concrete slab. Modern Australian installs use PEX loops tied to rebar or fixed in screed over insulation, with manifolds zoning each room or zone.
Any of these can feed a hydronic loop. Choosing between them comes down to fuel availability, winter temperatures at your site, and how much roof area or land you have.
| Heat source | Fuel / input | Notes from research |
|---|---|---|
| Air-to-water heat pump | Electricity (PV, micro-hydro, battery) | Reverses for summer cooling. Uses more than 60% less electricity than traditional electric heating. Pairing with PV gives the most efficient fully-renewable hydronic setup. |
| Ground source (geothermal) heat pump | Electricity for pumps | Uses Earth's stable underground temperature via buried loops. Heats in winter, cools in reverse in summer. |
| Wetback / slow-combustion stove | Firewood | Doesn't need grid power at all; can be placed anywhere. Wetback routes heat to the loop. |
| Solar thermal (flat panel or evacuated tube) | Sun | Heats water directly; in sunny climates can shoulder a large share of the load. Needs a buffer tank for overnight draw. |
| LPG boiler | Bottled LPG | Cheaper to buy and install than oil; burns relatively cleanly. |
| Oil boiler | Diesel / heating oil | Roughly 90% of oil converts to usable heat. |
| Biomass boiler | Wood pellets or chips | Burns organic material; recycles CO₂ that would otherwise reach the atmosphere. |
Specs/requirements snapshot — typical off-grid hydronic install
| Item | Spec / requirement |
|---|---|
| Pipe material | PEX (cross-linked polyethylene), typically 16 mm or 20 mm |
| Manifold | Brass or stainless, one per zone, with flow meters |
| Heat source | Air-to-water heat pump (most common), wetback, solar thermal, or boiler |
| Buffer tank | Recommended to stabilise flow, smooth pump cycling, store solar thermal gain |
| Slab insulation | Rigid XPS or similar under the slab — critical to stop downward heat loss |
| Pump | Variable-speed circulator, 230V/50Hz |
| Controls | Thermostat per zone, slab temperature limit, flow temperature setpoint (typically 35–45 °C for slab systems) |
| Power | PV array sized to cover pump + heat pump compressor draw |
Two design moves cut energy use dramatically:
1. Use the slab's thermal mass. Concrete stores heat during the day and releases it through the night. Charge it up with solar gain or a midday heat-pump run, and the floor keeps bleeding warmth into the early hours without further input.
2. Zone the system. Unoccupied rooms stay at a lower temperature. Rooms that pick up enough solar gain during the day may not need any heating input at all.
Combined with weather stripping and reasonable R-values on ceiling insulation, zoned hydronic heating can keep the off-grid energy budget surprisingly modest.
These aren't from the fact sheet directly, but they're well-established practice in the radiant heating field and align with the verified points:
Hydronic work touches plumbing, electrical, and (in the case of boilers and wetbacks) combustion. Get a licensed tradesperson in for any of the following:
Hydronic loops run at low pressure and modest temperatures, but they still operate inside a building envelope with hot components, electrical pumps, and combustion sources nearby. If something smells off, sounds wrong, or trips a breaker, isolate the system and call a plumber or HVAC technician before restarting it.
Because pricing varies wildly by region, season, fuel, and system size, I'll keep this general. The key drivers are:
For sizing: a rough rule of thumb in the radiant heating world is around 100 W per square metre for well-insulated slab floors, dropping to 60–80 W/m² if the slab is heavily insulated and the building envelope is tight. Multiply by floor area, then divide by the heat source's COP or efficiency to get electrical or fuel demand.
1. Slab prep: lay rigid insulation, vapour barrier, and rebar or mesh.
2. Fix PEX loops per the zone drawing; clip to mesh at the designed spacing (commonly 150–300 mm centres).
3. Pressure-test the loop before pour.
4. Pour slab. Allow full cure before energising the system.
5. Mount heat source, buffer tank, pump, and manifold.
6. Wire controls (thermostats, slab sensors, pump relay).
7. Commission: fill loop, bleed air, set flow temperature, test each zone.
8. Balance flows at the manifold so each loop delivers per design.
Is hydronic floor heating actually efficient off-grid?
Yes. Hydronic is one of the most energy-efficient forms of heating, and when paired with a PV array and a heat pump, the system runs on entirely renewable energy.
Can the same system cool the house in summer?
An air-to-water heat pump can run in reverse, transferring heat from the home to outside air. Note: don't pump very cold water through a slab — condensation becomes a problem. Most Australian hydronic systems handle cooling via a fan coil unit rather than chilled slab water.
How hot does the floor get?
Surface temperature typically sits in the low-to-mid 20s °C. Most people find comfort somewhere between 21 °C and 28 °C surface temperature; higher than that feels unpleasant and wastes energy.
Do I need a buffer tank?
Strongly recommended. It stabilises flow temperature, reduces pump cycling, and stores heat from solar thermal input or a stoked wetback for use after the fire dies down.
What about LPG or oil boilers — are they still relevant?
LPG boilers are typically cheaper to buy and install than oil boilers and burn relatively cleanly. Oil boilers convert roughly 90% of fuel to usable heat. Either is viable where you can't site a heat pump or a wetback, but neither pairs with a PV array as cleanly as a heat pump.
Will radiant heating work with my existing slab?
Retrofit options exist — PEX can be fixed in a low-profile screed over an existing slab, or stapled under a subfloor between joists. Performance is generally lower than a slab pour with under-slab insulation, but it's still effective.
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