Heat pump hydronic heating & all electric homes

This is a story of how to retrofit heating in a Victorian weatherboard house as part of an all-electric home.

Heating and cooling accounts for around 40% of energy consumption at home. It is a big deal and has a significant impact on our comfort at home. And in Melbourne, heating is a bigger factor than cooling, with up to 5 times as much energy used to keep a home warm in winter, as is used to keep a home cool in summer.

This blog documents how we transitioned from a gas ducted heating system to a hydronic heat pump system at our place, to maximise use of our solar and battery system and help us move towards a zero emissions house.

Why change and why hydronic?

There are a lot of different types of heating and cooling systems on the market. The operation and capital cost of these systems isn’t cheap. You’ll spend far more on your heating and cooling (the appliances and in running them), than you will on your fridge, oven or tv.

We had a gas ducted heating system and decided to change the gas ducted system due to the following issues:

  • The ducts were decaying (you only know that after crawling around under the house) which means you are essentially heating the air under the floorboards

  • It uses gas – a fossil fuel – which we know we need to stop burning for climate reasons

  • It blows small particulates through the vents – so not a great indoor air quality outcome

  • The fan uses quite a bit of electricity (so gas and electricity use both peaked in winter)

  • It is hard to zone or scale. While you can close vents in some rooms, that actually increases the pressure on the underfloor duct system, as the pressure remains the same but there are less locations for the hot air to flow into the house

  • By not using any gas at all (meaning we are not using gas for hot water heating or kitchen cook tops) we could save $400 a year just through not paying for the connection fee.

Some say gas is cheap to run. I disagree. I distinctly remember getting a $600 winter gas bill for just a two month period when gas was cheap (which doesn’t factor in the increased electricity bill that month too).

We chose hydronic to replace the heating system on the basis that it provides a nice comfortable (silent) heat, and with a heat pump system can be achieved very (energy) efficiently, and doesn’t require lots of heads to be located high in each room. I am not against reverse cycle air con (we actually have one in our lounge room to use on most days instead of the evaporative system), but this is a more comfortable heating system.

We chose a wall mounted hydronic system, meaning the heat travels through a dedicated (and closed loop) pipe network under the floor, to each panel in each room, with cooler return flow water flowing back to the buffer tank. This is a different solution to a hydronic in-slab system, which wasn’t possible unless you can rip the flooring up and lay a new concrete slab with hydronic coils running through it!

The system design

We considered several systems and chose a Stiebel air to water heat pump system, purchased through Hydrosol. We needed 10 outlets: 8 rooms with wall mounted panels and 2 towel rails in bathrooms. We estimated the heat load based on a thermally efficient house. We found that most suppliers tend to overestimate the load, and are more interested in the load of the panel- not the appropriate temperature of the room or how to reduce the overall energy demand to achieve a comfortable house.

We selected a Stiebel Eltron WPL 17 ACS classic, with an output of 8.5 kW, and a Coefficient of Performance of 4.86 (which is the eratio of energy inputed to energy generated – more than 1 is good!). The system includes:

· 1 x Air to Water heat pump WPL 17 ACS Classic

· 1 x Hydraulic module HM Trend

· 1 x FEK Relative humidity remote control unit

· 1 x Safety temperature controller STB-FB

· 1 x Buffer cylinder SBP 100 E

· 1 x Internet-Service Gateway ISG web

As this is a heat pump, it can heat and cool in reverse cycle. More on cooling below.


There is a difference between using a heat pump boiler compared to a gas boiler. A gas boiler will produce hot water at 70 to 80 degrees, that then circulates through the panels. Heat pumps produce hot water at 50 to 55 degrees, and some suppliers therefore look to upsize the wall mounted panels to provide the equivalent radiation. I think instead it requires better thermal efficiency in the room and / or accepting that the system may take longer to heat the room. With an automatic controller, I set the system to start heating at 6am – whereas with a gas system I may have delayed this to be 6:30 or 7am.

We have designed this system with 6 out of the 10 panels to include a thermostatic head – meaning they regulate flow and heating (and turn off) at the individual panel and room scale. With a buffer tank the heat pump then regulates (i.e. scales up or down) how much hot water is required.

We installed a small (single fan 600mm) hydronic panel in the laundry, enabling us to hang clothes in that room in winter, and avoid the need to purchase a dryer.

The system is set up to run cold water through the panels in summer, creating a cold thermal mass in each room. While there is no fan forced element to circulate cool air, it does have a small impact on cooling each room, and is very nice to touch on a hot day. As this is running off our solar and battery system, I’m not concerned about additional energy loads in the house. You do have to be conscious of not setting the temperature set point too low in summer, to prevent condensation from forming on the panels, and that condensed water then dropping on to the floor. The Stiebel Eltron heat pump does have a humidity control feature which can manage this issue.


Some simple sustainability initiatives we added into the project

  • Firstly, we actually bought and reused six wall mounted hydronic panels – that reduces the materials required (i.e. life cycle impacts). We purchased two new panels and two new towel rails.

  • We insulated every single exposed section of pipe – going back after they were connected to ensure any movement that exposed some of the Rehau was then also insulated

  • We minimised the bends to reduce the energy required to pump the water through the loop

  • We included thermostatic heads in the bedrooms

  • We tapered the underfloor pipe network to ensure we get even distribution of hot water through all rooms – rather than most heat being lost in the first panel in the circuit.

  • We installed insulated bubble wrap behind each panel – to reduce the possibility of heating external walls rather than the room where it is intended

  • We have insulation in the roof the walls and under the floor

  • We have drafted proofed the windows

  • We haven’t double glazed the windows – yet!

  • We have the ability to run the heating at set times – i.e. change the set point and hence not require the system to run over night or at times we are not at home

  • We can maximise the use of solar (even in winter) by scheduling the system to start heating at 12pm or 1pm – and reduce the exporting of solar power.

  • We reused the packaging: craft projects for kids and firewood.


The Stiebel Eltron heat pump has a heat curve feature, which takes account of the outside and inside air temperature, and room temperature settings to continually monitor and adjust the hydronic water flow temperature to optimise the Coefficient of Performance (heat output (kW) divided by power used (kW), also known as COP). I will be interested to see how well this works during the coming winter.

The fit out

I have to be honest here and say retrofitting is hard! It would have been a lot easier to do this in the new build stage, but we eventually completed the install and set up. The fit out can be broken down into 4 tasks: rough in of insulated pipe network, plumbing, electrical, and commissioning. I was crawling around under the house to connect and lay the insulated pipes. I actually had to tunnel under a few joists to get to one area of the house, and got stuck a few times in between stormwater pipes, sewer, old ducts and the joists. I had already been under the house for a while putting in the underfloor insulation, so I knew what to expect under there!

I was very conscious of insulating every little bit of pipe (to not lose any heat), and creating an efficient loop system. We created two branches of return loops (approximately 50 metres of Rehau in total), and placed every inlet to a panel as closest to the heat pump as possible – hoping that the hot water had to travel that tiny bit less distance to get to the panels.

Ideally the hydraulic controller would have been installed inside – but this wasn’t possible as were retrofitting the system and didn’t have the space. It is outside but with an awning to protect it from the weather.

The system is complicated, the plumber and electrician were happy to remind me of that! We had some issues in commissioning, to do with making sure the sensors were all connected properly. But now I can’t wait for winter! I am hoping to do another blog when I have data on energy use and comfort.

Thanks to Chris Siddons at Hydrosol and Steffen Reich and Touie Smith at Stiebel Eltron – for getting this set up and commissioned. More info at

Let us know if you want Wave Consulting to help with your retrofit or new build.