Electric vs Hydronic

In the past it was common to see electric heating in slabs – not any more, the cost is prohibitive. Hydronic inslab heating is now the only way to warm your new home. With hydronic heating, special PEX pipes are laid on top of the reiforcing mesh and warm water at 40 degrees circulates, warming the floor and making your house comfortable. This is the Rolls Royce of comfort!

First Steps

Ensure your architect takes into account the principles of good solar design.

Orientation. The long axis of the house should be East-West to maximise North facing window area.
Insulation. Roof, walls, under the slab, and around the edge of the slab
Thermal Mass. Try to get your concrete floor exposed to the sun on the North of the building.
Increase thickness of slab – see below
Next, try to work out whether the rooms on the south side of the house might need additional heating.
Do a heat loss calculation (see FAQ)

Design

Pipe size – 16mm typical, but larger diameter 18mm pipe has many advantages. (We scan supply either)

Each zone is 100m of pipe (max)

Coverage is 20m2 per zone

Pipe spacing – typically 200mm

Keep the length of each zone similar to get good hydraulic balance

Avoid internal walls ( to minimise the chance of damage from nails through the base plate)

Use reverse spiral for good heat distribution

try to make manifold central and in wet area

minimise run from heat source to manifold

Slab Thickness

“Australian Standard AS 2870 Residential slabs and footings — Construction, contains some requirements on heating cables and pipes within slab-on-ground construction. For hot water heating pipes embedded in the slab, to ensure that the required slab thickness is not reduced by the pipes, the slab thickness must be increased to at least 125 mm Figure 4 (a). Also, the reinforcement is to be increased by one level, from say SL72 to SL82 mesh. ”

Pipe Size

Pipe Size of 16mm is common. However in this article on Does Tube Size Matter? Siegenthaler makes the case for the larger diameter pipe. He compares the performance of a tube size (outside diameter) of various internal diameters. Based on his findings, we conclude that 18mm pipe provides 3% more heat into the house compared to 16mm pipe. “For systems using a condensing boiler, geothermal heat pump or solar energy collectors, it’s important to keep the water temperature as low as possible to maximize the efficiency of the heat source.” So the water temperature can be lower to get the same heat output – which is good for solar, and better for the pump which doesnt have to push so hard. Another bonus: The 18mm pipe we use is cheaper than Rehau 16mm pipe!

Slab Insulation

Check the slab design – does it have good insulation? Most downwards heat loss thru a slab is towards the edge. If you install hydronic heating it makes sense to prevent that heat from escaping, so good edge and perimeter insulation is essential. It is also a requirement of the BCA. There are a number of ways to do edge insulation. The pictures below show how it can bve done using thermacells. The diagrams in Perimeter Slab Insulation show three possible methods for typical installation in more conventional buildings.

Note: The BCA specifies that vertical edges of slabs on ground are required to be insulated when in-slab heating or cooling in installed within the slab. See 3.12.1.5 Floors in BCA 2010 Volume Two. Also see this fact sheet: 4.8 Insulation Installation

It is also essential to prevent downward losses. Common methods are to use extruded polystyrene boards, or waffle pods

Thermacell Footings

Slab Design Note how one edge is cut away ready for the slab.
Waffle pods will be used on this job to bring the finished floor to the right height. The Reo starter bars will be bent over and tied to the mesh. The builder on this job should have paid more attention to safety – yellow caps should have been put on each vertical reo bar. Its not worth the risk of a worker losing an eye.

Slab Design The finished slab

How can I work out how much heating I will need?

An engineer can work this out for you. The method involves making calculations for each room, making guestimates about the roof and wall insulation, the orientation of the windows, the type of windows, the local climate data, the desired temperature range in the house and so on. Doing these heat loss calculations is a very good first step, and one that is often overlooked.  It takes time and effort to do them, and that translates to an extra cost which will add to the expense of the end result.

Heat capacity of Concrete

The specific heat of concrete is about 0.9. The density of concrete is 2400kg/m3. So a slab 200m2 by 100mm thick would be 20m3 volume, or about 48 tonnes. To warm this slab by 5 degrees would require 20 * 2400000 * 0.9 * 5 = 216MJ.

So this is nearly twice as much as a 1000 litre tank of water warmed by 30 deg C (126MJ).

Flooring

The flooring manufacturers often play it safe and say their products are not suitable for use over a heated slab. There are some that are OK however.

Possible Products:

HM Walk
After the concrete is poured, it needs to be left for many weeks, then the boiler needs to be set on the lowest setting and run contuinuously for a week or so to dry it out sufficiently before the before the flooring can be put down. When using timber floor coverings, care should be taken to store the natural wood in the room with the UFH turned on, until the moisture content does not exceed 8%. This is necessary to reduce the effects of shrinkage.

The start up temperature on the flow header to the manifolds should be limited and maintained between 20ºC and 25ºC for 3 days.
The design flow temperature must then be maintained for 4 days without the system going into set-back mode.
The system then should be allowed to cool down.

Why use water – everyone else has forced-air?

Liquid is by its nature a much more effective heating or cooling method as the heat capacity of water is over 4,000 times that of air. Also, once the heat is transferred to the water, it can be handled more efficiently, warming you by radiation, which is a much more comfortable way to keep warm.