Replacing a Gas Hydronic Boiler with a Heat Pump (2026 Guide)

Yes, and it's the most common upgrade question we get from owners of older gas systems. The boiler is only the heat source. The pipework, manifold and radiators or underfloor loops have a design life of 30+ years, and a hydronic heat pump can drive that same network on electricity, so a hydronic boiler replacement doesn't mean starting again.

What changes is the water temperature. An older gas boiler runs at 70 °C or more; a heat pump is most efficient at roughly 45–55 °C on radiators and 35–45 °C on underfloor. Moving to gas to electric hydronic heating comes down to four checks: can each radiator heat its room at that lower temperature, is the water clean, can the outdoor unit be sited and powered, and how will you make hot water once the boiler has gone.

Some symptoms point at the boiler; others point at the water or pipes, where a fix is far cheaper. The boiler itself is usually the problem when you see:

• Repeated lockouts or ignition faults every winter.
• Pressure loss that keeps needing topping up, which can mean a leaking heat exchanger or valve.
• Kettling or banging as it fires, often from scale or sludge.
• Corrosion or water marks around the casing or flue.
• Discontinued parts, so every breakdown raises the question of whether it can be fixed at all.

A yellow flame, soot or a fumes smell is different: turn the boiler off and call a licensed gasfitter. A radiator that's cold at the top usually has air in it, and one cold along the bottom often has sludge; those are system problems, not boiler problems.

Whether you're searching for a Raypak or Brivis hydronic boiler replacement or have another make, the decision is the same. We're not a dealer for those brands. We assess the whole system and recommend a condensing boiler, heat pump or hybrid on its merits.

This is the question that decides most conversions. Catalogue outputs are quoted at ΔT50, roughly 75 °C water in a 20 °C room, and output falls faster than temperature: at 50/45 °C a typical steel panel gives about 46% of its rating, and at 45/40 °C about 35%. Our hydronic radiator sizing guide sets out the maths.

Many older systems have slack, though. Radiators were often sized generously, and homes have since gained insulation, better glazing or draught sealing. Comparing each room's heat loss with its radiator's output at the planned flow temperature usually sorts a heat pump for existing radiators into three groups:

• Adequate at 50–55 °C, so they stay.
• Marginal, where a deeper panel on the same footprint, such as a K22 or K33 replacing a K11, closes the gap.
• Undersized, needing a longer panel, a second radiator or a fan coil.

Replacement panels from our IMAS radiators range are held in Australian stock. If your boiler's flow temperature is adjustable, a useful pre-check is to drop it to about 55 °C for a few cold days and note which rooms struggle; ask your service technician first if it's an older non-condensing model.

Heat pumps also run a smaller gap between flow and return, around 5 °C rather than a boiler's 10–20 °C, so they need more water flow for the same heat. We check primary pipe sizes and the circulating pump for that reason. See hydronic radiators Melbourne, or hydronic heating Camberwell for period homes with older radiator systems.

An older hydronic system usually carries some magnetite sludge, the black iron oxide that forms as steel radiators corrode. An old boiler tolerates it. A heat pump's plate heat exchanger has narrow passages and flow sensors, so dirty water means low-flow faults, lost efficiency and, at worst, a blocked exchanger.

So a conversion starts with the water. A central hydronic heating flush drives treated water through the circuit in both directions to lift sludge into a magnetic filter, typically in 4–8 hours. We then fit a permanent magnetic filter and strainer ahead of the heat pump and re-dose with inhibitor from our hydronic water treatment range. Where we find old plastic pipe without an oxygen barrier, separating the heat pump with a plate heat exchanger can be the safer design.

A heat pump works best running long and steady, and it needs a minimum water volume to do that. When most radiator valves close on a mild afternoon it can short-cycle, wasting energy and wearing the compressor, and it needs stored warmth to draw on while it defrosts its outdoor coil. A buffer tank solves both, adding volume and letting the heat pump and radiator circuit run at their own flow rates. We size it to the system, and supply 100 L and 200 L buffers among our hot water and buffer tanks.

If your boiler is a combi or heats a cylinder through a coil, removing it removes your hot water too. There are two common routes:

• One heat pump for both. The Maxa i-290 can heat a hot water cylinder as well as the radiators, with hot water up to 78 °C.
• A separate hot water heat pump such as the Viessmann Vitocal 161-A. It's a second appliance, but a listed hot water heat pump can attract incentives a space-heating unit can't.

A monoblock heat pump such as the Maxa i-290 or Immergas Magis keeps its refrigerant sealed in the outdoor unit; only water pipes enter the house. Where it goes matters:

• Airflow: clear space to draw air in and discharge it, not boxed into a narrow side passage.
• Noise: quiet but audible at night, so we keep it away from bedroom windows, neighbours' boundaries and reflective corners, within state noise rules and any council or strata conditions.
• Base and drainage: level, on anti-vibration feet, raised off the ground, with a path for defrost meltwater.
• Clearances and frost: R290 units have manufacturer clearance zones, and outdoor water pipes need insulation and freeze protection on frosty sites.

Smaller residential heat pumps generally run on single-phase 230 V, while larger models need three-phase; the Magis range comes in both. We check the switchboard, consumer mains and spare capacity at the survey. A dedicated circuit is standard, but a switchboard upgrade or three-phase is a real cost and can involve your network distributor. If hot water and cooking are leaving gas too, we assess the total load.

You don't have to go fully electric in one step. In a hybrid, the heat pump is the main heat source and the existing boiler stays connected as a top-up for the coldest mornings, or for when a room needs hotter water than the heat pump makes efficiently. A buffer tank or low-loss header keeps the two hydraulically separate. It makes sense when:

• the boiler is still sound;
• some radiators are undersized but can't be replaced yet, for example in a heritage room or ahead of a renovation;
• a single-phase supply can't carry a full-size unit; or
• you're on a very cold site and want backup.

The trade-offs: you keep paying the gas supply charge and servicing the boiler, and when it fails you face the full decision anyway. We design hybrids so that last step is simple, with the heat pump, buffer and controls staying and only the boiler coming out.

These are the three realistic options when a gas hydronic boiler reaches the end of its life. For brand-level detail, see Viessmann Vitocal vs Vitodens and our Maxa vs Viessmann vs Immergas comparison.

Costs are site-wide guides; yours is quoted after a site visit.

We can't price a conversion from a photo of the boiler, because the big cost drivers sit elsewhere in the house. As a site-wide guide, a heat pump swap onto an existing hydronic system, reusing the pipework and most radiators, runs roughly $12,000–$22,000. If many radiators need replacing, the job looks more like a full radiator system at $12,000–$30,000 installed. What moves your figure:

• Heat pump size, set by the heat-loss calculation, not the old boiler's rating.
• Radiator upsizing and any fan coils.
• Buffer tank and hot water route.
• Electrical work, from a dedicated circuit to three-phase.
• Flushing and water treatment.
• Siting, frost protection and decommissioning the old boiler.

We quote every conversion after a site visit and heat-loss calculation, with a fixed-fee design and quote back within 24 hours. Weighing up a gas boiler replacement in Melbourne against a heat pump? We'll price both.

What happens to running costs

Heat pump heating typically costs 60–75% less to run than gas, because it delivers 3.5–5.5 units of heat per unit of electricity. Where you land depends on flow temperature (every upsized radiator lets it run cooler), your tariffs, how much load solar covers, and whether you drop the gas supply charge entirely. Our hydronic heating running costs guide has the numbers. For Melbourne's long season of around 1,500 heating degree-days, see heat pump hydronic heating Melbourne.

Rebates

As at October 2026, no program we checked funds an air-to-water heat pump used for hydronic space heating. STCs cover only registered heat pump and solar water heaters, and the Victorian Energy Upgrades and ACT heating incentives cover air-to-air reverse-cycle units. A listed hot water heat pump can still attract STCs and state discounts; our heat pump rebates guide has the detail. The case for converting rests on running cost, comfort and getting off gas.

Victoria's electrification direction

• The Victorian Government's guidance for existing homes says there's no requirement to replace existing gas space heating, and gas appliances can still be used and repaired. The November 2025 notice of decision on the Building and Plumbing Amendment Regulations 2025 records that a proposed ban on replacing gas space heating in existing homes was removed.
• From 1 March 2027, gas water heaters can't be installed, or replaced like-for-like, in existing homes, with limited exemptions such as where the main switchboard would need altering.
• From 1 January 2027, new homes must be built all-electric.

On that guidance, a heat-only gas hydronic boiler can still be replaced with another gas boiler. The grey area is a combination boiler that also heats your taps: we haven't seen specific official guidance on combis, so check before ordering a like-for-like unit. Landlords should also check the separate rental minimum standards.

This is how we run a conversion, from first visit to first winter:

1. Survey the boiler type and condition, hot water, radiators, pipe sizes and switchboard.
2. Calculate room-by-room heat loss at your local winter design temperature.
3. Check every radiator at heat-pump temperatures and list what stays, goes deeper or is replaced.
4. Flush and treat the system, adding a magnetic filter and strainer.
5. Design the plant: heat pump, buffer, hot water, weather-compensated controls, full or hybrid.
6. Prepare the site and complete any electrical upgrade.
7. Install and commission before a licensed gasfitter disconnects the boiler and caps the gas, so you're never without heat for long.
8. Fine-tune the weather-compensation curve through the first winter.

Our Canberra cold-climate heat pump retrofit followed this sequence: correctly sized radiators were kept, undersized ones replaced with larger panels, and the heat pump was running before the gas plant came out. See hydronic heating Canberra for local design conditions.

We'd rather tell you to wait than install a heat pump that disappoints. Hold off, or consider a hybrid, if:

• Your boiler is young and reliable. Get the heat-loss calculation and radiator survey done now, so you aren't deciding during a mid-winter breakdown.
• A major renovation is coming. Insulation, new windows or an extension change the heat loss; do them first and you'll need a smaller heat pump.
• Radiators can't be enlarged and rooms need very hot water. A heat pump can run hotter, but most of the saving disappears.
• There's nowhere sensible for the outdoor unit, or your owners corporation won't approve one.
• The electrical upgrade isn't affordable yet.

• Victoria Government Gazette No. S 654 (26 November 2025): notice of decision on the Building and Plumbing Amendment Regulations 2025, read October 2026.
• Victorian Government guidance for existing residential buildings (energy.vic.gov.au), checked October 2026.
• Federal and state rebate pages, as summarised in our heat pump rebates guide (October 2026).
• The EN 442 radiator output method and IMAS data used in our radiator sizing guide, plus Maxa, Immergas and Viessmann manufacturer documentation.
• AHHAC conversion, flushing and water treatment practice.

Written and reviewed by the AHHAC Technical Team, licensed plumbers and hydronic engineers (NSW Lic 262528C) installing to AS/NZS 3500 and AS/NZS 5601. Next review: early 2027, after Victoria's 1 March 2027 hot water rule takes effect.