Underfloor Heating

Underfloor heating is recognised as the most luxurious & comfortable heat for the human body because it warms your feet and is cooler up near your head. Radiant under floor heating uses low temperature water and can be heated by any heat source.

 

Advantages of warm water underfloor heating

New Zealanders have been familiar with underfloor heating for some years – in the form of electric cables running underneath tiled areas of the home. Underfloor water heating is based on the same radiant heating principle, but has a number of key advantages:

■ It has a much faster response time. This is because the pipes are 16mm in diameter which gives them a much greater surface area than their electric equivalent at 6 – 8mm. This allows faster heat transfer into the floor.

■ It is more economical – particularly at times of the day when electricity costs are highest. It is possible to warm the house at times convenient to the occupants without exorbitant costs, because the prices of fuels used for combusting boilers don’t tend to vary during the day. They are also generally cheaper energy sources than electricity.
■ Underfloor heating is particularly well suited to buildings with high ceilings. The radiant effect of heat from the floor is effective up to a height of about two metres, so the problem of heat gathering at the top of the room is less than with other systems.

■ Because the heat source is at the feet, the perceived comfort level is greater. As a result, actual room temperatures can possibly be one or two degrees below those required with other systems – resulting in energy cost savings. However, if the system is used for only short periods of time and at infrequent intervals, radiator systems may return a lower running cost.

NB: Care must be taken with the choice of floor coverings. In general, underfloor heating works best with tiled or concrete floor surfaces, although it still works very well with wooden and carpeted floors. Home owners should advise which floor coverings they intend to use at the time the heating system is being designed. 

 

 

Installation and design

Central Heating New Zealand uses pipes designed in Europe specifically for warm water systems. They are strong and durable, enabling them to be laid in a concrete slab without risk of damage. The piping is laid in loops and secured to the reinforcing mesh in the slab during construction (or an insulation layer within the slab – see below). Each loop runs back to a central point and connects to a central manifold, enabling different heating “zones” to be established within the house. Each zone can be controlled individually using a thermostat and timer.

When the heating system is designed, the distance between pipe loops is calculated according to the area of each zone – ensuring optimum heat levels are achieved. The client’s personal comfort preferences are also taken into account. 

The European Method

In New Zealand, underfloor heating (both electric and water) has traditionally been laid within a 100mm thick concrete floor slab, as this is how most houses are constructed. This is referred to as the ‘hangi’ method. However, in the rest of the world a rough floor slab is laid first, followed by a layer of polystyrene (or latterly polyurethane) insulation. The pipes or cables are then attached to the insulation, and a 30mm screed is poured on top. This means the heated floor is much thinner and has a far quicker reaction time up and down. It is also completely insulated around the perimeter of the slab. Energy costs and wastage from heat loss into the ground and through the perimeter are reduced considerably.

 

This method is catching on in New Zealand, although the construction design of the house has to be considered. Energy savings of up to 30 percent are achievable over standard in-floor designs. 

Design Considerations

■ Primary pipework runs from the boiler to the underfloor manifold. It can be run in a lagged and sheathed conduit in the tailings under the floor slab, or in the framing timber. Primary pipes are usually 25 – 35mm in diameter. Once lagged and sheathed, they are typically 65mm in diameter.

■ The underfloor manifold is generally best positioned in the centre of the house, as all underfloor pipes emanate from it. Very large houses may require two manifolds. A small (typically 750mm x 1000mm x 150mm), flat cabinet is installed to house the manifold and underfloor pump. This can be recessed into a wall in the same manner as an electrical circuit board. Typical locations would be the back of a hall cupboard, laundry or equipment room.

■ All houses with underfloor heating should employ the best possible insulation under the slab, to limit downwards heat loss. We recommend at least 50mm of high density polystyrene be laid under the slab. We also recommend 25mm of edge insulation, as up to 15% of the heat can be lost through the edge of the slab. This can be polystyrene or treated wood, laid on its edge.

■ The underfloor pipes are laid and attached to the reinforcing mesh, which is supported by chairs. The recommended distance of pipes below the surface of the concrete slab is 30 – 40mm. This is to ensure an acceptable response time.

■ Because of the proximity of the pipes to the surface, we recommend no saw cuts in the slab. There are several crack-inducing products on the market that eliminate the need for cutting.

■ As a rule of thumb, the maximum length of any loop is 100 metres. Loops are continuous (ie, no joins at all). The maximum number of loops per manifold is 12.

■ Three variables are used in the design, to determine output:
1. Water temperature. 2. Water flow. 3. Pipe spacing.

■ Depending on the design, the temperature of water used for underfloor heating is typically 40°C to 60°C. The maximum temperature the floor slab should reach is 29°C. On average it is around 26°C. The air temperature produced in the room is approximately 5°C lower than the floor temperature.

■ The output requirements and overall design of the system determine the number of loops emanating from the manifold.

■ A pressure test on the pipes is undertaken after the slab has been laid. If a pipe within the slab is damaged during construction, it is possible to chip away the concrete and repair it.

Download a brochure on Underfloor Heating systems.

Go back to the Heat Distribution page.

Learn about Radiator & Underfloor combinations.

Read about the Multitubo Pipe & Fittings used in underfloor installations by clicking here to access a  PDF file on Multitubo.