Would a Legalett in slab radiant heat system be better than a heat pump?
I am looking at having Legalett engineer a raft slab foundation for our new build. My question is regarding also installing their in slab radiant air heating system instead of a ducted heat pump for the heating needs of our home.
The plan is to have a well insulated and air tight envelope...would the Legalett heating system be worth the cost when considering that the cold weather heat pumps available today can be up to 3 times more efficient per watt than radiant heat generated by an electric heater,which is the heat source for their radiant air system. Also the heat pump can perform as an air conditioner for the warmer months. Have you run the numbers to compare the installation/operational cost of those two methods for home heating?
I would think of it more as a comfort and design issue, and whether or not you want a heated floor. Just like hydronic radiant floor systems, the Legalett air heated radiant floor can be powered with a boiler, in which case you wouldn’t need to choose.
Legalett circulates air through the floor rather than liquid (see the pros and cons of air heated floors vs hydronic here) but they can supply heat to the heater box in the floor using either water or electric coils. So you could sort of ‘have your cake and eat it too’ by installing an air-to-water heat pump. That way you can have radiant heat, heat pump efficiency, and you can still run it with electricity instead of fossil fuels.
As for cooling, I know this isn’t what you’re asking but I’m covering it anyway for others who may be reading along – it is never wise to use a radiant floor system for cooling, as humid air can condense and leave mildew. Not to mention your feet would be frigid so it would not be a comfortable system. But that’s not a reason not to have radiant floor heat - which is extremely comfortable particularly with slabs - you would just need a separate system, which is often the case with other home heating systems anyway.
Something to consider however – there is so much thermal mass in the slab that it regulates heat during heat waves as the concrete will absorb heat from the air. And simply by opening windows at night you can drastically reduce (or possibly eliminate) the need for main floor cooling during a heat wave.
And as a cooling design option, you could get a ventilation system with a cooling coil so you are using the same duct work, or you could get a mini-split heat pump. Depending on your climate and how well-insulated your house is of course, but a small heat pump may be all you need to maintain comfortable temperatures.
As an example, we built a demo house in Quebec Canada that has a radiant floor, and despite very hot and humid summers it can be kept cool using a single head mini split heat pump (see here). That is possible largely for the reasons you mentioned – it's airtight and very well-insulated, not to mention well-shaded on south facing windows and has a lot of thermal mass inside the building envelope.