Heating the home with air conditioning
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Goldilocks heating method

The more we investigated heating options, the more the experience reminded me of Goldilocks choosing a bowl of porridge.  Which method might be too hot? Which might be too cold?  Which type of heating would be just right for what we want?

As we have decided not to use natural gas connected to the property, we’d settled on using a heat pump as the energy source for a room heating method.  Heat pumps are an efficient use of electricity.  The numbers are quite impressive.  It’s not magic but the next best thing.  Check out the heat pump explanation for how the device can use 1 kWhr of electricity to put 5 kWhr of heating into a house.

We have three options under consideration.

Hydronic heating warms the concrete slab.

Hydronic heating coils laid before the slab is poured

Running water filled tubes through the concrete floor slab means it can be heated during cold weather.  When you need to heat the room, warm water is circulated through the tubes.

One advantage of hydronic heating is it produces a gentle flow of warm air rising from the floor.  Hence it’s possible to feel warm at lower air temperatures than you would with an air conditioner.  Blowing the air round makes you feel a bit cooler as the air flows past.  Using a lower set temperature on the hydronic heating system reduces the energy consumption.

Another positive is the possibility of combining the hydronic heating water tank with the potable water tank.  One system provides the hot water for the kitchen and showers as well as the heating coils.  However, we have seen comments about the need to be careful with this combination.  Potable water contains dissolved oxygen which can corrode the fittings in the hydronic system unless they are made from copper or plastic lined.

Some issues to think about.

On the other hand, running slab hydronic heating with a heat pump is the most expensive option for this particular heating method.  The ATA (Alternative Technology Association) noted there was a significant jump in installed price when the traditional gas fired boiler was replaced with a heat pump.  We heard the same thing when talking to installers at the Melbourne Home Show.  Cost could be an issue for us.

Maintenance might be another thing to consider.  Not saying there would be a failure in a well designed, professionally installed in-slab hydronic heating coil, but what happens if the pipe does spring a leak?

Discussions with other interested parties highlighted that the combination of large variations in Melbourne’s daily temperature, the long delay in response time for hydronic heating and a well-insulated home might cause problems.

Picture this.  It’s a cold, crisp autumn morning in Melbourne.  The hydronic heating was running overnight to keep the selected rooms at a comfortable temperature.  Knowing it takes hours to warm up the slab, this is likely to be the preferred mode of operation.  The super-insulated walls and ceiling are doing a great job of containing the warmth.

The day turns out to be one of those bright, sunny days where the temperature quickly climbs.  It’s not hot, but it’s not wintry.  Our passive design ensures the house soaks up the available solar energy.  Inside, the hydronic heating figures out that there’s no need to continue warming the slab.  It shuts off.  However, the slab continues to be warmed by the sun.  Towards the end of the day, could we need to ventilate the rooms in order to dump excess heat?

Reverse cycle (heat pump) air conditioner is an option.

Would we be better with a system that allowed us to give a quick burst of heat in the morning (if necessary) to warm up the rooms?  As the slab heated/cooled due to passive heating, the quick response system would provide the little bit of extra heat just when it was needed.

The reverse cycle air conditioner offers the potential to quickly heat and cool rooms.

Reverse cycle air conditioner for heating and cooling


As our climate changes to longer, hotter summers and shorter, warmer winters, cooling could be more important than heating.  This system offers both options in the one package.

The trick here is not to over-size the unit.  With the level of insulation we’re planning, the standard calculations based on room size could predict higher kW heat pumps than we’ll need.

Reverse cycle air conditioning is quick to warm the air in a room. However, there’s always a but.  It’s likely we’ll need to run a slightly higher set point on the unit (compared to hydronic heating) because of the air being blown around the room. Moving air produces a slight cooling effect so it will feel cooler than it actually is Also, the warmer air will be pumped out near the ceiling, leaving a cooler layer of air near the floor.

Hydronic heating using radiant panels is the third heating option.

Thermaskirt uses skirting boards for hydronic heating

Thermaskirt is a hydronic heating system that circulates the water through the aluminium skirting boards around the edge of the room.  This avoids the issue of standard hydronic heating radiators taking up wall space.

Warmth is transferred to the room as radiant heat which means there are minimal air currents.  The warmed air rises and is replaced by cooler air.  As a result, the entire room ends up feeling warm. Also, because the water is just heating the skirting boards, the system is quick to respond when switched on.

The information we’ve seen looks promising as it addresses concerns raised with an in-slab hydronic system.

Given the skirting boards are connected to water sourced from a heat pump, it’s possible to run the system “backwards” in summer.  Putting through chilled water might be one way to draw heat out of the rooms if the natural ventilation/fans can’t handle a particularly hot run of summer days.

No decision made yet.

making a decision about heating

The research has helped but the final decision is yet to be made.

We want to install a heating system that complements the home’s passive design and assists with providing a comfortable environment over a range of weather conditions.  It’s not about constantly using energy to create the desired room temperature.  Instead we want to use the minimum amount of power during weather extremes.

Installation and operating costs are important factors.  The ability of a single system to heat and cool would be beneficial.  Low maintenance is essential.

Next step is getting a few quotes to flesh out the costs.  The most likely outcome is a comparison between reverse cycle air conditioning and Thermaskirt.


  • Peter Taylor

    June 22, 06 2016 08:40:29

    Hello Libby & Howard…
    Just read your posts…
    Have you seen our Warmboard underfloor hydronic system, Warmboard is a fast heat, low mass, low energy consumption system. We have air to water Heat pumps that will heat and cool.


    • Libby & Howard

      June 22, 06 2016 09:29:18

      Peter – Thanks for the suggestion. I hadn’t heard about the product but will definitely investigate further.

  • Nat

    June 29, 06 2016 01:05:58

    Hi Howard, loving your blog! I was listening to a podcast last night that refers to a New Zealand model of a reverse cycle unit that you install near the floor so the warm air is being blown in at a lower level. See the Tim Forcey podcast on this page http://bze.org.au/media/radio . Also, having recently experienced the fun of a cracked copper hot water pipe in our slab I would be steering clear of any pipes in slabs! Looking foward to seeing what you decide. All the best. Nat

    • Libby & Howard

      June 29, 06 2016 06:27:07

      Thanks Nat. Appreciate the information about the floor level reverse cycle unit. Going to check that out as part of our decision making process. Stay tuned for the final decision.

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