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Heat Transfer Methods
There are three methods of heat transfer – Conduction, Convection, and Radiation.
Conduction occurs when two objects at different temperatures are in contact with each other. Heat moves from the warmer to the cooler object. Soldering irons and hot water bottles are examples of Conduction.
Convection is the transfer of heat via the movement of a liquid or gas between areas of different temperature. Commonly understood by "heat rises". Air based heating and cooling systems use convection and the wind itself is a convection current.
Radiation is heat emitted because of the internal energy that an object has. All objects radiate heat but the hotter something is the more radiant energy it emits. When you hold your hands up and feel the warmth 'coming off' something; that is radiant heat. The heat coming off a car engine and the suns heat are both forms of radiant heat transfer.
Central Heating Systems
Most central heating systems are either:
- Exclusively convection based (air to air heat pumps and ducted air)
- Exclusively radiant based (underfloor and radiant ceiling based systems)
- Hybrid convection radiation system (radiator based systems)
Radiant based systems condition the entire mass of the home, not just the air inside. The walls, floor, and all the furnishings inside are brought to the desired temperature. This presents clear advantages over convection based systems that heat and cool the air only:
- The comfort does not disappear immediately when the system is suspended as the energy is held in the mass of the house
- More uniform temperature distribution without hot or cold spots
- Allergy friendly with forced air movement and dust circulation
- No discomfort from forced air circulation
- Almost completely silent operation
Most Common Forms of Traditional Central Heating
The two most common forms of traditional central heating are Underfloor & Radiator based systems.
Underfloor systems are fast becoming the gold standard for luxury in new homes in New Zealand. Oxygen barrier pipe is laid in circuits inside a concrete slab, heating the entire floor turning the slab itself into the heating element that emits purely radiant heat. Each room will have its own circuit, sometimes more if the room is large, and all circuits connect back to a centrally located manifold.
Underfloor systems are best suited to concrete finishes but can also heat through carpet and even wooden overlays. Slabs have a large thermal mass so remain warm under foot and provide comfort even after the system has turned off. Zone control is achieved by installing thermostats in bedrooms, media rooms, and other less occupied spaces. This allows different running hours and temperatures to suit the spaces. The thermostats operate actuators on the manifold that start and stop water flow to the related circuits.
Underfloor systems can be heated by Natural Gas, LPG, Diesel Boilers, and air to water Heat Pumps. A heat pump system has a higher initial installation cost but the lowest ongoing running cost which can be further offset by Solar Photovoltaic (PV). Using an air to water heat pump also allows for slab cooling. A home cooling effect that can drop the indoor temperature of the house by up to 4°C.
Radiators are present in the vast majority of homes in the UK and Europe.They are a popular choice in both renovations and new homes in New Zealand. Radiator systems use a network of pipes in the ceiling space or below the floor to deliver heated water to radiators in each room. Usually these systems are heated via Natural Gas, LPG, or Diesel Boilers. Heat pumps produce lower temperature water and are not well suited to radiator applications.
Using radiation and convection heating, radiators have an extremely fast response time. Systems begin heating up within minutes and are able to heat the entire home from 10°C right up to 20°C in as little as 15 minutes.This is an advantage of the hybrid convection and radiation based systems.
Each radiator has its own control which turns the radiator on and off thermostatically. This allows each room to be set to its own temperature, and never overheat.
Most commonly installed are the typical steel panel style but a large range of indent designer radiators is available. Typical sizing for a bedroom of 3.5m x 4m would require a radiator size of 100mm thick x 600mm high x 800mm wide, whereas Living areas such as a 6m x 5m space require larger panels, 100mm thick x 600mm high x 1200mm wide.
Active Ceiling - Radiant Ceiling System
The latest type of central heating is a radiant ceiling based system. “Active Ceiling” is a concept introduced to the NZ market by Waterware in the middle of 2019. Though new to NZ, Europe has used ceiling based heating and cooling systems for almost two decades.
Active Ceiling is a purely radiant based energy system like underfloor but from inside your ceiling. At first glance, this may seem odd but there are clear benefits over the more common underfloor installation. The ceiling has less thermal mass than the slab, meaning faster response times in heating or cooling. A higher wattage per square meter means not only does your house warmup quickly, but the entire cooling load of a space can be met by the ceiling system alone for up to an 8°C drop in internal temperature, and by dropping the ceiling height by 50-60mm it’s possible to retrofit this system. Each Active Ceiling zone is piped back to a centrally located manifold, and the zone control operates in the same way as with underfloor systems.
Article written by Darren Yearsley.