What is the exact form of hydronic heating and what is the structure of your extension. Why is your builder trying to steer you away from it? Yes, as a rule hydronic heating is the most effective and efficient form of heating, for any space. not to mention the most comfortable even form of heating available (aside from well executed passive solar design ;) ) That said however there are different forms of "hydronic heating" and there is a difference between hydronic heating executed in wall panels, and in-floor hydronic coil, either laid in a slab or within distribution"plates" fitted to a framed floor structure. If in the floor the heating effectiveness will be impacted by the surface so engineered floorboards will insulate to a certain degree vs tiles or exposed concrete finish.

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Yep, underfloor hydronic is the most effective form of heating you large-volume spaces with high ceilings. As hot air rises from the floor, heating the occupants, not the entire space. It's also a very comfortable form of heating, particularly for people who suffer from dust allergies or asthma.

But hydronic systems can be very expensive to install. It is also uncommon in Australia, so can be difficult to find a specialist who is experienced in designing these systems.

There are a few different ways to design an underfloor heating system.

In-slab underfloor heating, where the hydronic tubes are located within the concrete slab. The slab is a huge mass. This very large thermal mass creates thermal lag, leading to very, very slow reaction times. Meaning the floor takes a very long time to heat up when turned in, & a very long time to cool down when turned off. Think a day or two, for the full reaction time, not an hour or two. This slow reaction time is only appropriate in climates that are extremely cold, in Europe & North America, where the heating is left in 24/7, for the entire winter.

Unfortunately, due to the inexperience of many installers in Australia, this is often the method of underfloor heating installed here, & it's totally unsuitable to our variable winter climate. You have a warm winters day in Melbourne, & you're forced to open all the windows, as the floor is still radiating excess heat. This can often also be an inefficient system, as the floor losses heat into the surrounding earth. The slab must be very well insulated from the ground.

In-screed is a far better option. Where insulating boards are placed on top of the slab, the hydronic tubes are attached to the boards, & a topping screed is poured over the system. This system has some thermal mass, but it's much lower than the entire slab. So the reaction time of the system is reduced. It is also a better insulated system, so heat losses are minimised. It is a more expensive, but better system.

Thirdly, is hydronic systems for timber floors. which also uses aluminium defuser plates under the timber to spread the heat. Only certain engineered timbers should be used, as many timbers will warp. As Paul mentioned, timber is an insulator, so they'd is not an energy efficient system.

The system is only as efficient as it's heat source. The options are either a gas boiler, or a heat pump. Gas boilers are cheaper to install, but consume more energy. So heat pumps will be cheaper to run, & better for the environment.

My choice would be a heat pump. Make sure it's a heat pump specifically designed for hydronics, not a heat pump designed for domestic water, or the pool.

Thanks so much, Paul, for your very informative reply. In contrast to the original Victorian structure, the part we are renovating at the rear of the house sits on a concrete slab (laid in the 1980s, I believe). We decided a day or two ago to forget about adding timber flooring here and instead try to polish/burnish the existing slab. Perhaps it's possible to have the in-screed heating laid on top of that slab? Thanks also for the advice on boilers - will make sure to get a heat pump for hyrdronics. All the best, Gina

The floor levels between the 80's extension and the original will determine what your options are in regards to whether a screed is possible or whether you have to go panels. A screed with hydronic requires typically at least 60mm thickness, possibly more. Also be aware that screed layers can be prone to cracking as they are typically not reinforced, however with increased thickness this is possible to achieve. Screeds will give you control of concrete finish/colour too. This set-up however never a cheap option and needs to be understood in terms of cost. Sometimes it's the most logical option, particularly when wall space and/or floor area are limited. This is the sort of thing that we nut out and consider properly early on in design stage. It's not ideally something that you should be trying to sort out on the fly down the track with an engaged builder - best to utilise an experienced designer or architect for advice when at design/planning stages so you are making properly informed and balanced decisions in this regard. What we try and do is empower our clients with accurate information and options so they are super clear about why they are pursuing a particular design direction or detail vs the alternative.

Best of luck Gina! PD

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