The brief
The body corporate of a 48-apartment residential building in Sydney's Inner West came to AHHAC with a problem that will be familiar to a lot of strata committees. Their centralised hot water plant — a bank of ageing gas boilers feeding every apartment — was at the end of its life. Breakdowns were becoming frequent, emergency call-outs were eating the sinking fund, and the gas bill for the common plant had climbed year after year. Residents were increasingly frustrated by intermittent hot water, and the committee was facing a large, unplanned capital spend. They needed a solution that was reliable for the next 20 years, dramatically cheaper to run, and — ideally — aligned with where energy is heading.
The challenge
Commercial and strata hot water is a different discipline to a single home. This building had three hard constraints. First, continuity: 48 occupied apartments cannot lose hot water, so any changeover had to happen with the existing plant kept running until the new system was live. Second, plant-room space: the existing boiler room was compact, and whatever replaced the boilers had to fit the footprint and the access. Third, compliance: centralised hot water carries Legionella-risk management, tempering and AS/NZS obligations that have to be engineered in, not bolted on. On top of that, the committee wanted the operating cost and the carbon footprint down, and a service arrangement that would keep the plant reliable for the long term.
The design
AHHAC re-engineered the plant around a cascade of MAXA HWA2 reversible heat pumps — the same industrial R290 range we specify for hotels, aged care and multi-residential buildings. Instead of burning gas, the HWA2 units draw heat from the ambient air and deliver domestic hot water up to 78 °C at a fraction of the energy input, cascading so that each unit runs in its most efficient band and the building keeps hot water even if one module is taken offline for service. We paired the heat pumps with correctly sized buffer and DHW storage tanks to carry the building's morning and evening peaks, and built in dew-point-free tempering, Legionella management and full AS/NZS 3500 and 3000 compliance. The design was modelled to the building's real hot water demand profile — 48 apartments' worth of showers, kitchens and laundries — rather than a rule of thumb, which is the only way to size cascaded plant correctly.
Because the HWA2 is reversible, the system also gives the building common-area heating and cooling capability from the same plant — recovering heat between duties and keeping the whole installation all-electric and ready for rooftop solar or a green-power agreement. It's the commercial sibling of the residential hydronic heat pumps we install every week, scaled to plant-room duty.
Why cascaded heat pumps, not a like-for-like swap
The obvious option — and what a lot of contractors would have quoted — was to simply drop in new gas boilers. It's cheaper on day one and familiar. But it locks the building into another two decades of gas: rising tariffs, a standing charge on the common meter, combustion servicing and flue compliance, and a carbon footprint that only gets more expensive as the market prices emissions. For a body corporate that has to think in 20-year horizons, that's a poor bet. Cascaded heat pumps flip the economics. Each MAXA HWA2 module carries a share of the load and modulates to demand, so the plant runs at high efficiency across the day rather than firing a big boiler on and off. Sizing several smaller units instead of one large one also builds in redundancy — the single most valuable feature for a building that cannot tolerate a hot water outage. If a module needs service, the others carry the building; there is no single point of failure. And because the units are reversible and heat-recovering, the same capital does three jobs (hot water, heating and cooling) rather than one.
Sizing the plant to real demand
Getting cascaded plant right is an exercise in demand modelling, not guesswork. We profiled the building's actual hot water usage — the sharp morning and evening peaks of 48 households showering, cooking and doing laundry — and sized the heat pump cascade and the buffer and DHW storage together so the system meets peak draw without oversizing the compressors. The storage does the heavy lifting at peak; the heat pumps recharge it efficiently through the day. This is the discipline that separates a plant that quietly works from one that runs out of hot water at 7am or short-cycles itself to an early grave, and it is exactly the same heat-loss and demand modelling we apply to every residential job, scaled up. The result is a plant that is neither starved nor wastefully large — right-sized to how the building actually lives.
Compliance, monitoring and long-term service
Centralised hot water carries obligations that a single home doesn't. We engineered in Legionella-risk management and thermostatic tempering so water is stored hot enough to be safe and delivered at a safe temperature to every apartment, and delivered the installation to AS/NZS 3500 (plumbing) and AS/NZS 3000 (electrical) with full documentation for the body corporate's records. The plant is tied into the building management system over a standard protocol, so the building manager can see performance, energy use and alarms at a glance and act before a small issue becomes a call-out. Finally, we put the building on a scheduled AHHAC service contract — planned maintenance that keeps the manufacturer warranties valid and the plant running at its rated efficiency for its full 15–20 year life, rather than the reactive, expensive break-fix cycle the old boilers had fallen into.
The install
The changeover was staged so that residents never lost hot water. We installed and commissioned the new MAXA HWA2 cascade and storage alongside the running gas plant, tested it under load, and only then transferred the building across and decommissioned the old boilers. Our in-house commercial crew handled the mechanical engineering, refrigerant pipework, electrical works and the BMS integration (so the building manager can monitor and control the plant, with energy metering and alarms), and delivered the full commissioning records and compliance certificates the body corporate needs for its documentation. The physical changeover to the new system was completed without a single day of hot water outage for the 48 apartments.
The result
The building now runs entirely on the electric MAXA HWA2 plant — zero gas boilers remaining. Because the heat pumps deliver several units of heat for every unit of electricity, the common hot water energy use fell by roughly 55% against the old gas boilers, and with no gas standing charge the committee's operating cost dropped further still. Reliability is transformed: cascaded plant means built-in redundancy, and the emergency call-outs that were draining the sinking fund have stopped. Residents have consistent hot water to 78 °C, the building has cut its carbon footprint substantially, and the body corporate has a plant with a 15–20 year design life and a scheduled AHHAC service contract keeping it that way.
For the residents, the change is felt as reliability: hot water that is simply always there, without the intermittent failures and cold-shower complaints that had become routine. For the body corporate, the win is financial and strategic. The relentless emergency call-outs that were draining the sinking fund have stopped, the common-property energy bill has fallen sharply, and the committee has removed a looming capital risk from its ten-year plan by replacing end-of-life plant on its own terms rather than in a crisis. Just as importantly, the building has taken a large, credible step on decarbonisation — going all-electric on its biggest common-property energy load — which increasingly matters to buyers, valuers and lenders assessing a strata scheme. It is the kind of upgrade that quietly protects and lifts the value of every apartment in the building.
A repeatable template for ageing strata
This project isn't a one-off — it's a repeatable playbook for the many Sydney strata buildings and commercial sites still running on ageing, centralised gas hot water. The pattern is the same every time: profile the building's real demand, design a right-sized cascade of reversible heat pumps with matched buffer and DHW storage, stage the changeover so residents never lose hot water, and back the plant with full compliance documentation and a planned service contract. The outcome is the same too — a large cut in energy use and carbon, an end to reactive breakdowns and the call-outs that drain a sinking fund, and a plant that's future-proofed for an electrifying grid. Hotels, aged-care facilities, schools, gyms and older apartment blocks all face the same end-of-life-gas decision, and the same solution applies at each. For committees and building managers weighing the switch, the first step is simple and low-commitment: a plant assessment and a demand profile, from which we model the system, the staged changeover and the savings.
It's a template we're now rolling out across Sydney strata: get off failing, expensive gas plant, onto cascaded heat pumps, with no disruption to residents. For similar buildings see our commercial hot water (MAXA HWA2) page, our commercial plumbing service, or the equivalent Melbourne solution on our commercial hot water systems Melbourne page.