From Gas Boiler to Heat Pump — An update on measured performance.

Project Overview

Location: Wanaka 
Original system: Gas boiler hydronic heating
Upgrade: Stiebel Eltron air-to-water heat pump with solar assist 
Focus: Reduced gas reliance, improved efficiency, future-proof comfort

Original Build & Heating Design

When the home was originally built, it was equipped with a gas boiler hydronic heating system, a common and proven solution at the time. The system delivered good comfort but relied heavily on gas during winter months.

Energy data from the pre-upgrade period shows:

• Annual gas consumption exceeding 14,000 kWh

• Peak winter gas use of 3,000–4,000 kWh per month

• Gas providing the majority of space heating demand

• Electricity playing a relatively minor role in winter comfort

While the home performed well thermally, the owners became increasingly conscious of:

• Rising gas usage during colder months

• Long-term exposure to fossil fuel pricing

• Limited ability to leverage on-site solar generation

Rather than waiting for the boiler to fail, the decision was made to proactively upgrade the system.

The Upgrade Strategy

Optum designed a system to work with the home — not against it.

The gas boiler was replaced with a Stiebel Eltron air-to-water heat pump, integrated into the existing hydronic distribution and paired with solar generation already on site.

The design intent was to:

• Maintain the same comfort level as the original system

• Dramatically reduce gas consumption

• Shift the majority of heating demand to electricity

• Maximise the use of self-generated solar energy

Measured Results — Before & After

Before Upgrade (Gas Boiler – 2022/23)

• Annual gas consumption: ~14,500 kWh

• Annual electricity consumption: ~7,200 kWh

• Gas was the dominant heating source, particularly in winter

After Upgrade (Heat Pump + Solar – 2024/25)

• Annual gas consumption reduced to ~3,100 kWh

• Gas reduction of approximately 75–80%

• Electricity became the primary energy source for heating

• Solar export regularly offset purchased electricity

Winter performance data showed that even in the coldest months:

• Heat demand was met predominantly by the heat pump

• Gas use was reduced to a secondary or backup role

• Several months ended with electricity usage fully offset by solar export

What Changed in Practice

The most telling outcome was not just cost — but energy behaviour.

Compared with the original gas boiler period:

• Winter heating was delivered with significantly lower total energy input

• Gas usage flattened instead of spiking

• Electricity use increased in a controlled and predictable way

• Solar generation actively contributed to heating rather than being exported unused

This demonstrates the real-world efficiency of modern heat pump systems when paired with a well-insulated home and solar generation.

Why the Heat Pump Performed Better

Unlike gas boilers, which are typically limited to ~80–90% efficiency, air-to-water heat pumps operate at 300–400% efficiency.
That means:

• Less energy is required to produce the same level of comfort

• Energy is used more steadily rather than in sharp winter peaks

• Solar electricity can be used directly for heating

In this case, the data clearly shows that the home’s heating demand did not increase — only the efficiency of how it was met.

Outcome

This upgrade transformed the home from a gas-reliant system to a predominantly electric, low-gas household, without sacrificing comfort.

~75–80% reduction in gas consumption
More stable seasonal energy use
Strong alignment with solar generation
Reduced exposure to fossil fuel pricing
A heating system aligned with long-term energy trends

Conclusion

This case study highlights how existing homes can evolve.

The original system wasn’t a mistake — it was simply built for a different energy landscape. By upgrading to a high-efficiency heat pump, the homeowners significantly reduced gas dependence and positioned their home for a more resilient future.

Thinking about upgrading from gas?

Optum designs systems based on real performance data — not assumptions.

Get in touch to discuss a tailored heat pump upgrade

Arrowtown residents experience distinct seasons, especially during the winter months when temperatures can drop significantly making heating a vital part of bringing comfort to local residents.  Having used Optum to service and maintain their ageing heating system these Arrowtown homeowners approached Optum needing a more reliable and efficient option.  Their heating system was frequently breaking down and was causing alot of frustration for Trevor and Liz.  They asked Optum to design a solution for them that would give them peace of mind and future proof their system for years to come.

Optum are preferred installers and partners of Stiebel Eltron and designed a new system based around Stiebel Eltron Geothermal Hydronic Heating Heat Pumps for this project.  Optum recommend Stiebel Eltron heating products to our customers as they are market leaders who offer reliable, energy efficient, sustainable products to New Zealand home owners.  Installed into Trevor and Liz’s home were the following heating products:

• Stiebel Eltron WPE-I h 230 Premium Geothermal Heat Pump
• Stiebel Eltron DHW Cylinder 400L
• Stiebel Eltron Internet Service Gateway (ISG)

Read on to find out how our customers, Trevor and Liz from Arrowtown, got on with the installation and their new Stiebel Eltron heating system:

Client’s Primary Goal
The primary goal for installing a new heating system was future-proofing their home to ensure consistent heating and hot water availability, while reducing breakdowns and operational costs.

Why Optum?
The client chose Optum Plumbing and Heating because of their reputation as leaders in the field.  Their expertise and strong recommendations stood out as key reasons for trusting Optum with this project.

Pre-Project Challenges
Before the installation, the client faced significant challenges with their outdated heating system.  High running costs and frequent breakdowns caused constant frustration, particularly due to their need for consistent hot water for heating.

Impact on Daily Life
The unreliable system created ongoing stress, making it difficult to maintain a comfortable and functional household.

Key Requirements
The client prioritised a future-proof solution with minimal risk of breakdowns.  They also sought improved efficiency and reliability in their new system.

Collaboration with Optum
The Optum team maintained excellent communication throughout the project.  Their friendly and professional approach ensured the client felt supported at every stage.

The Solution
The newly installed heating system delivered on all the client’s initial goals, providing consistent and reliable performance.

Client’s Satisfaction
The client particularly appreciated the peace of mind that came with knowing their new system would perform more efficiently than the previous one.

Measurable Benefits
Since the installation, the client has experienced reduced energy bills and improved comfort levels throughout their home.

Overall Impact
The new system eliminated frustrations caused by breakdowns, resulting in a much-improved household environment.

Customer Experience
The client described their overall experience with Optum as exceptional.  “From start to finish, the team’s expertise, communication, and professionalism exceeded expectations.”

Recommendation
When asked for advice to others considering a similar project, the client stated:

“We would have no hesitation recommending Optum Plumbing and Heating to install a heating system in your residence.”

Testimonial
Right from the start, when we were having issues with our original system, Optum Plumbing and Heating provided us with the option that would best suit our existing house layout.  Once we accepted their quote for a new heating option, they were on the mark straight away and kept us in the loop regarding the delivery of units.  They promptly installed the new units in a very short time span so that we could still have hot water during the installation.  The staff and other contractors that installed our units were very friendly and neat in their work.  Overall, Optum Plumbing and Heating provided the answer to our heating issues.

If you are looking for expertise and a thoughtful approach to help you deliver a future-proof, cost-effective, and reliable heating solution tailored to your needs, contact us at Optum Plumbing and Heating for advice and support.

Call Us: 03 443 5718

Email: admin@optum.co.nz

Faced with rising energy costs and the uncertainty of relying on dwindling gas supplies for heating, these Wanaka homeowners sought a solution that would provide long-term security, comfort, and consistent warmth into the future.  They approached Optum to help them find a solution that would reduce energy costs, future proof their system and increase comfort.  Kevin from Optum met with the homeowner, David, and designed a tailored heating system upgrade from a gas boiler system to an air to water heat pump system to meet their specific needs.

Optum are preferred installers and partners of Stiebel Eltron and designed a new system based around Stiebel Eltron Air Source Hydronic Heating Heat Pumps for this project.  Optum recommend Stiebel Eltron heating products to our customers as they are market leaders who offer reliable, energy efficient, sustainable products to New Zealand home owners.  Installed into David’s home were the following heating products:

• Stiebel Eltron WPL 25 ACS 14kW Air-to-water Heat Pump for Heating/Cooling/Hot Water
• Stiebel Eltron Heat Pump Manager/Controller (WPM)
• Stiebel Eltron Heating Buffer Tank 100L
• Stiebel Eltron Internet Service Gateway (ISG)

Read on to find out how our customer, David from Wanaka, has got on with his new Stiebel Eltron heating system:

Primary Goal:
‘Our primary motivation for installing a new heating system was to address rising heating costs associated with gas and the anticipated future supply issues.  We became increasingly concerned about the dwindling national gas supplies, the reliance on imported gas, and the likelihood of rising costs.  This led us to explore alternative energy sources that could utilise our existing underfloor and radiator infrastructure.’

Why Optum?
‘Optum was the natural choice for this project.  They had been involved in the original build and installation of our heating and plumbing system, so they were already familiar with our infrastructure and had access to the necessary records and layout.  Given their past involvement and expertise, it made sense to trust them with this project.’

Pre-Project Challenges:
‘We weren’t facing any specific operational problems with the old heating system.  However, we were looking to reduce our reliance on gas and future-proof our heating setup against rising energy costs.  We were eager to hear from Kevin about a feasible, cost-effective proposal.’

Impact on Household:
‘Although our old system worked adequately, the uncertainty of rising costs and gas supply shortages left us wanting a more sustainable and financially viable solution for the future.’

Key Features Desired:
‘Our primary focus was on a heating system that was simple to operate, “set and forget,” and would provide more consistent heat at a lower cost.  We also wanted the system to be future-proof, ensuring that we wouldn’t have to deal with further upgrades down the road.’

Collaboration with Optum:
‘The Optum team was fantastic throughout the project.  Communication was smooth, and we were regularly updated on progress.  They worked closely with us to ensure the system would meet our needs, with attention to detail at every stage.’

The Solution:
‘The new heating system has met all of our expectations.  We now experience consistent heat output, even during the weather inversion layers we experience in Wanaka in Winter, and our home remains comfortable throughout.  The system’s ease of operation, particularly after mastering the WiFi technology, has also been a huge plus.’

Key Features:
‘We are particularly satisfied with the improved heat output and the consistent warmth.  The fact that the system is so simple to use has greatly improved our day-to-day comfort.’

Measurable Outcomes:
‘Since the installation of the new electric air-to-water heat pump, we’ve noticed significant changes in our energy usage.  During the winter of 2024 (April to September), our electricity consumption increased from 5,111 kWh to 7,900 kWh, while our gas consumption dropped from 13,455 kWh to just 2,260 kWh.  This shift resulted in significant savings, though we’re still coordinating the data to provide a more concrete figure.’

Overall Impact on Household:
The new heating system has had a positive impact on our household.  We are enjoying increased comfort, and the peace of mind that comes with knowing we are using a more sustainable and cost-effective energy source.

Experience with the Optum Team:
My wife and I couldn’t be happier with the quality of workmanship and professionalism displayed by the team from Optum.  The gasfitter, heating specialist, and electrician were incredibly neat and personable.  They caused minimal disruption, and it was a pleasure having them in the house.  From the Managing Director to the on-site team, everyone was helpful and approachable, and they remain on hand to offer advice if needed.

Advice for Others Considering a Similar Project:
We would recommend anyone considering a similar project to thoroughly research their options.  If you can incorporate existing infrastructure, it can make the process much smoother.  A full retrofit might be more challenging, so working with what you already have can save time and effort.

If you are looking for expertise and a thoughtful approach to help you deliver a future-proof, cost-effective, and reliable heating solution tailored to your needs, contact us at Optum Plumbing and Heating for advice and support.

Call Us: 03 443 5718

Email: admin@optum.co.nz

The owners of the Henderson Home were, quite frankly, fed up with cold draughty homes and ever-increasing power bills. They wanted to invest in a home that provided the comfort they wanted, with minimal power costs. 

They have now been in the home for 12 months. The home has far exceeded their expectations with the high level of health and comfort:  

“Once you have lived in a home like this, it would be a struggle to go back to living in a standard NZ built home.” 

Read on for the full spec description and details on just how this fantastic home achieved its goals. 

The Insulation 

Insulation is a key component for any home, as it adds significant value in keeping the home warm in the winter and cool in the summer.  

The new standards for the NZ Building code from May 2023 increased the insulation values for the roof/ceiling to R6.6 and the walls to R2.0 (Double what the old building code standard was for the ceiling.) In our case study home, the Ceilings are R8.0 and R4.8 in the walls (at double or more the NZ building code standards). This is made up of two layers of R4.0 in the ceilings, one layer of R3.6 in the timber frame wall and a further R1.2.  

The whole house is highly insulated with Terra Lana WOOL insulation for sustainability and performance. Terra Lana wool insulation was used for many reasons, but mainly because it is local, and sustainable and all scraps head back to the factory to be reused. The garage is also highly insulated in the same way, with insulated garage doors.  

Generally, floors have a lot of heat loss, so minimising that was carried out using 150mm insulated panels and polyblock on the foundations. This made it simple to keep the heating or cooling in the floor and maintain the high efficiency of the home. 

The Building Wrap 

The House is entirely wrapped with Pro Clima Intello. This is a building paper which is taped to completely seal the home from draughts or any air from the outside. This provides minimal air changes within the home- so the heat generated within the home stays in the home (and minimal heating is required).   

The outcome is a home that is easier to keep warm, lower energy bills, no draughts, more comfort, a healthier home with less condensation and mould. Mechanical ventilation can be used as well as natural ventilation (doors and windows being open) to achieve the desired comfort in the home.

The Windows 

All windows and doors are triple glazed, thermally broken and argon filled. The windows provide excellent insulation and also allow minimal noise from the outside. The glazing is also set up to minimise UV intrusion into the home. The windows chosen were from E13 Performance Windows.  

Triple glazing is gaining traction in NZ, with the energy efficiency provided keeping the home warmer or cooler. The extra pane of glass allows another surface to apply low E coatings to help control the sunlight into the home (which adds excess heat into the rooms). Another important concept can be noise reduction from the outside, keeping the home quieter. “Thermally broken” windows significantly reduce the heat transfer through the window frames creating better efficiency and no condensation forming on the inside of the window joinery. It all adds to the comfort and efficiency of the home. 

The Front door is also thermally broken to minimise heat loss. There was no point having triple glazing if the door was not thermally broken and airtight. E13 Performance Windows took care of that too. 

The Solar Shades 

On the north side of the house there are “solar shades” above the windows. The shades are set to minimise the summer sun getting into the home and overheating but allow maximum sun in the winter to heat the home naturally without heating being required (or at least a smaller amount of heating).  

The Underfloor Heating 

There is underfloor heating in the concrete and tile floor areas and “active ceiling” panels in the bedrooms. The ceiling heating provides a faster reaction time (similar to radiators). High performance homes do not always require a lot of heating. However, with the concrete and tile floors in this home, it adds to the comfort, to take the chill off the floor and maintain a constant warmth in the days of inversion – which can be bone chilling.  

The home has underfloor pipework in all the areas with concrete and tiles. The bedrooms are carpeted and as the rooms barely get below 16 degrees, having the Messana active ceiling panels supplied and designed by Waterware placed in the ceiling, using the giboard ceiling to act as a radiator takes the rooms quickly to the temperature the occupants find most comfortable. The Active ceilings react much quicker than the underfloor, so don’t have the “lag time” to wait. The panels use the warm water from the heat pump to produce the warmth needed in the room. The Active ceiling panels were predominantly put in place for cooling in the summer months, with the heat pump working in reverse to create the perfect sleeping temperature. The concrete floors were also cooled by the heat pump, to maintain a constant comfortable temperature in the hot Central Otago Summers.

The Heating & Hot Water 

All heating and hot water is powered by an air to water heat pump for a renewable energy generation source.  The Stiebel Eltron air to water heat pump has been calculated and sized for the demand of the heating/cooling and hot water of the home, in this case the WPL A07 or 7 kw high temperature heat pump was all that was required.  

The Heat pump can heat the domestic hot water to 75 degrees if required, but for this home the hot water is heated to 50 degrees and topped up every two days to 60 degrees for legionella. The 400L Stiebel Eltron HWC also has a large heat exchanger so the heat pump can heat the hot water quickly and efficiently.  

The heating and cooling water is stored in a buffer tank, so the heat pump only has to provide the energy required for the supply of the buffer tank, keeping the heat pump energy efficient with longer run times, so the compressor/inverter has less on/off cycles.  

The whole heating/cooling system is run using an outside sensor: the heat pump only delivers the heating or cooling needed at any given time of the day. If the outside temperature is -5 and the indoor temperature inside is required to be 22 degrees, then the heat pump is working hard to deliver the temperature difference needed, however, if the outside temperature is 10 degrees and the internal temperature is 22 degrees the heat pump adjusts and compensates for the temperature difference required, delivering a much lower temperature heated water to the underfloor. Most of the time in this home, the buffer tank will run at 30 degrees to maintain the 22 degrees in the living/kitchen area, as the temperature rises outside the buffer tank will run at 25 degrees. It is an extremely efficient and comfortable way to run the heating/cooling system. 

Fire 

The fire in the home is a Biofuel fire which is flueless (to maintain air tightness of the home) and uses organic plant-based biofuel to operate. Biofuel fires are a relatively new concept to the New Zealand market yet have become quite popular in Europe and other parts of the world that have smoke-controlled zones similar to Central Otago.  

The Biofuel fire chosen was from Naked Flame and produces a surprising amount of heat for the room. Bioethanol is pure alcohol- the combustion of bioethanol is clean, which makes it an efficient and effective source of heat because there is no soot, ash or harmful emissions that need to be extracted through a flue – meaning all the heat stays in your room.  

Bioethanol – or simply ‘ethanol’ is a renewable energy source made by fermenting the sugar and starch components of plant by-products – mainly sugarcane and crops like grain, using yeast. It is also made from corn, potatoes, milk, rice, beetroot and recently grapes, banana and dates depending on the country’s agricultural strength. 

Cooling 

Cooling is also provided by the same heat pump using the underfloor pipework and the ceiling panels. There are days when the home will overheat, and the cooling maintains a comfortable temperature without opening doors and windows. 

Ventilation 

The whole house had mechanical heat recovery ventilation. This ventilation provides clean filtered air, helps control some of the temperature in the home, removes moisture from the home to maintain balanced air without the need to open doors and windows. There are two Stiebel Eltron heat recovery ventilation units in the home. This has provided a dust-free environment, with dry, clean fresh air provided at all times. All moisture is removed by the ventilation system from the home.  

The dry environment makes the home easy to heat or cool, and the heat recovery system integrates well with the heating and cooling of the home, without the need to open windows or doors for fresh air. The whole system is designed to be balanced for the supply and extraction of air within the home.  

For cooking, a recirculating range hood has been used to maintain the air balance in the home. To maintain the air balance in the home the range hood is a recirculating model with charcoal filters to extract and food moisture, fats or other cooking emissions before being removed by the ventilation system.

Fire Sprinklers 

For peace of mind, the whole house has fire sprinklers fitted throughout (including the garage). There is a 25000L in-ground water tank which has a designated 7000L of water specifically for the fire sprinklers.  

 The sprinklers are not the sprinklers you would normally see in a commercial building, they are hidden in the ceiling behind small coloured discs. Leap industries provided the design and calculations needed for the home with a simple step by step installation document. This was a “no brainer” for the peace of mind to protect the occupants of the home and to protect the home if the house was unoccupied. Part of the cost was offset by only requiring one water tank for domestic water and firefighting. 

Water Filtration 

All domestic water is filtered and run through UV. The water delivered to the water tank is clean, however, a Davey Microlene Smart UV filtration unit was added to maintain a clean and fresh water supply at all times. 

Recently featured in Abode Magazine, this new- build overlooking Coronet Peak gave us the opportunity to fully showcase our culture of innovation when designing & installing their plumbing, heating and ventilation systems.

The large 323m2 home, known as “Wings over Water”,  has a double twisted plane roofline, beautifully resembling a butterfly. The house is entirely clad in cedar, a major focus for builders Shore Construction, and looks out over an artificial pond to the mountains beyond. 

The design of the house, initially conceived by Kamermans Architects, is all about making efficient use of the sun. Homeowners Bob and Wendy wanted something energy efficient – and different. The house is built to a very high insulation standard and it reaches far above the Building Code: pushing on the edge of being Passive House Certified without quite going to that level.

Why Optum?

Bob and Wendy’s vision for an integrated system that was both renewable and efficient to run aligned perfectly with our commitment to cutting-edge plumbing solutions, so we were more than happy to design their system when approached by them- they had been following our work for some time.

First steps- the calculations. 

The home’s extensive use of glass necessitated really accurate heat loss calculations. Here we instilled confidence that we could not only meet, but surpass their comfort expectations- important in the extremes of temperatures experienced in Central Otago. Our choice of a Stiebel Eltron air-to-water hydronic heat pump as the primary heating and hot water energy source was a decision driven by its exceptional efficiency and remarkably quiet operation. This aspect was particularly crucial since the home resides in a serene rural setting, where tranquility ranks high on the priority list.

The Heart of the System: The Rotex Sanicube

Central to the system is the Rotex Sanicube, a 500L highly insulated thermal store designed to accommodate all energy sources required for heating and hot water. With the heat pump, an electric backup element, and a Rinnai gas water heater interconnected, the Rotex provides a versatile hub that ensures a continuous supply of warmth and comfort.

The intelligent design of the Rotex allows it to deliver hot water at higher temperatures while utilizing a heat exchanger positioned lower in the tank for extracting lower temperature water. This water is directed towards the underfloor heating system, offering a remarkably efficient way of heating using lower temperature water. The Rinnai water heater serves as both a top-up solution on colder days and a backup option when all power sources are unavailable.

Harnessing the Power of Solar and Efficiency

The integration of PV solar panels brings an added layer of efficiency and sustainability. By directly connecting the PV solar to the heat pump, we unlocked a high-efficiency solution where solar energy powers the heat pump. This synergy yields exceptional efficiency, often exceeding 400%, meaning that for every unit of energy used to run the heat pump, up to four units of energy are produced for heating and hot water.

Our Innovative Approach: Manifold System

In this project we had the opportunity to address the challenge of fluctuating water temperatures during simultaneous water usage, a common issue in many households, when everyone wants to take a shower at the same time. To eliminate uncomfortable scenarios, we implemented a “manifold system,” similar to underfloor heating. This ingenious setup involves separate pipework for each bathroom, preventing temperature fluctuations.

Furthermore, to minimize waiting times for hot water to reach distant bathrooms, we also introduced a hot water ring main pump. This strategic addition ensures swift delivery of hot water to even the farthest corners of the home, enhancing convenience and conserving energy.

Would we change anything?

Frankly- no. We’re proud of this work, and it stands as a testament to our commitment to innovation, efficiency, and client satisfaction. At Optum we create comfort and consistency for our clients, through designing heating and ventilation solutions that work for life.