• Mike Casey

Electrifying our underfloor heating and hot water

Earlier this year, we eliminated all fossil fuels from both our orchard and our family home. When we first moved in, our home was equipped with a diesel furnace and tank for underfloor heating, and gas bottles used for both hot water and cooking.


After a lot of shopping around we found Heat Central Wanaka who provided us with a few options to achieve our goal. The TLDR is: We replaced all our gas and diesel heating and cooking with a heat pump, hot water cylinder and induction oven. The total bill was a little over $30,000 which actually doesn't seem that much when the diesel bill was approaching $5000 per year!



It's early days yet on figuring out the energy costs and effectively contrasting them against the savings from fossil fuels. We had this installed for winter, and we were happy with the performance after some early tweaks were made to get it working efficiently as a retrofitted system.


Unfortunately, we did not have our Power Radar monitoring system running in winter so we cannot yet provide clear data on winter costs yet. However, here is the data for the month of October.


Bar Graph showing high solar production and moderate grid consumption with the consumption used for hot water and underfloor heating. Used to demonstrate the power consumption in August is small compared to total electricity supply available
Hot water / underfloor heating compared to solar production and grid consumption.

The above graph shows our heat pump consumed around about 25 kWh a day in the month of October. At a standard 30c per kWh that would cost about $7.50 per day, however in our case our solar and spot price contract, we can bring that down considerably to an estimated 5c per kWh or $1.25 a day.


You can also see on the graph below that our heating and hot water makes up a significant portion of our household power consumption in October, appearing to range from 10-30% depending, which is largely weather dependent.


Bar graph showing the days of the week and how hot water and underfloor power consumption compares to total household consumption. Used to illustrate 10-30% of total household power bill in October
Hot water / under floor power consumption vs total household consumption consumption

I thought it best to let Steve from Heat Central Wanaka explain the process of retro-fitting a modern electric alternative into an existing house. Hopefully this blog post helps others decide that this path forward is indeed possible, and we can begin to remove fossil fuels from within our homes.


A guest post from Steve at Heat Central Wanaka.


Retrofitting a cutting edge heating system into Kiwi homes comes with its own challenges, but when Forest Lodge came to us wanting to move away from fossil fuels, we were excited to look at some options with them. The brief was to replace the diesel boiler heating the house and the gas califont heating the hot water with a more energy efficient and environmentally friendly solution.


Luckily, the underfloor heating system was a good match to the Stiebel Eltron premium air to water heat pumps, and this coupled to the smart HSBC300 combined hot water cylinder and buffer tank can also provide the domestic hot water for the house.


The key to installing an efficient air to water heat pump system is to ensure that the heat pump is correctly sized for the house. If you put in a heat pump that is too big it will keep you warm, but it will use more power. If it’s too small, it will struggle to keep up once it gets cold. In this case, we had a good set of plans from when the house was built, and with the help of our design consultant, we were able to calculate the heat loss of the house, which was well matched to Stiebel Eltron’s

WPL25 heat pump.


With the design work taken care of, the fun part begins, out with the old and in with the new. The installation work at Forest Lodge went smoothly, and we’re able to get the system up and running before removing the gas califont and switching the domestic hot water production to the heat pump.


Air to water heat pumps have been around for a long time, and technology has come on a lot over the years. Now with the latest refrigerants, they are even more efficient and can achieve much higher water temperatures even at very low outdoor temperatures. They work by harvesting energy from the outside air and turning it into comfortable warmth in your home. The Heat pump sends this

heat to the combined hot water cylinder and buffer tank, where it is either diverted to the bottom to heat the underfloor or to the top to heat the domestic hot water.


With these systems, we can achieve a seasonal COP of 5 meaning that for every kilowatt of electricity used, you get five kilowatts of heat. With everything up and running, then the fine tuning begins. A great feature of the Stiebel Eltron

setup is that it is equipped with an internet service gateway that allows you to monitor the performance and adjust settings remotely. This is great in a retrofit situation, as with older houses, it can take a bit of experimentation to get the best out of the system.


Now, in the spring, it’s time to turn the underfloor heating off. Another bonus with this heat pump is that it can run chilled water through the underfloor pipework to help cool the house in the summer. We’re looking forward to getting this going for Forest Lodge once the weather warms up a bit.

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